Astronomy
Astronomy, study of the universe and the celestial
bodies, gas, and dust within it. Astronomy includes observations and theories
about the solar system, the stars, the galaxies, and the general structure of
space. Astronomy also includes cosmology, the study of the universe and its past
and future. People who study astronomy are called astronomers, and they use a
wide variety of methods to perform their research. These methods usually involve
ideas of physics, so most astronomers are also astrophysicists, and the terms
astronomer and astrophysicist are basically identical. Some areas
of astronomy also use techniques of chemistry, geology, and biology.
Astronomy is the oldest science, dating back
thousands of years to when primitive people noticed objects in the sky overhead
and watched the way the objects moved. In ancient Egypt, the first appearance of
certain stars each year marked the onset of the seasonal flood, an important
event for agriculture. In 17th-century England, astronomy provided methods of
keeping track of time that were especially useful for accurate navigation.
Astronomy has a long tradition of practical results, such as our current
understanding of the stars, day and night, the seasons, and the phases of the
Moon. Much of today's research in astronomy does not address immediate practical
problems. Instead, it involves basic research to satisfy our curiosity about the
universe and the objects in it. One day such knowledge may well be of practical
use to humans. See also History of Astronomy.
Astronomers use tools such as telescopes,
cameras, spectrographs, and computers to analyze the light that astronomical
objects emit. Amateur astronomers observe the sky as a hobby, while professional
astronomers are paid for their research and usually work for large institutions
such as colleges, universities, observatories, and government research
institutes. Amateur astronomers make valuable observations, but are often
limited by lack of access to the powerful and expensive equipment of
professional astronomers.
A wide range of astronomical objects is
accessible to amateur astronomers. Many solar system objects—such as planets,
moons, and comets—are bright enough to be visible through binoculars and small
telescopes. Small telescopes are also sufficient to reveal some of the beautiful
detail in nebulas—clouds of gas and dust in our Milky Way Galaxy. Many amateur
astronomers observe and photograph these objects. The increasing availability of
sophisticated electronic instruments and computers over the past few decades has
made powerful equipment more affordable and allowed amateur astronomers to
expand their observations to much fainter objects. Amateur astronomers sometimes
share their observations by posting their photographs on the World Wide Web, a
network of information based on connections between computers.
Amateurs often undertake projects that
require numerous observations over days, weeks, months, or even years. By
searching the sky over a long period of time, amateur astronomers may observe
things in the sky that represent sudden change, such as new comets or novas
(stars that brighten suddenly). This type of consistent observation is also
useful for studying objects that change slowly over time, such as variable stars
and double stars. Amateur astronomers observe meteor showers, sunspots, and
groupings of planets and the Moon in the sky. They also participate in
expeditions to places in which special astronomical events—such as solar
eclipses and meteor showers—are most visible. Several organizations, such as the
Astronomical League and the American Association of Variable Star Observers,
provide meetings and publications through which amateur astronomers can
communicate and share their observations.
Professional astronomers usually have access
to powerful telescopes, detectors, and computers. Most work in astronomy
includes three parts, or phases. Astronomers first observe astronomical objects
by guiding telescopes and instruments to collect the appropriate information.
Astronomers then analyze the images and data. After the analysis, they compare
their results with existing theories to determine whether their observations
match with what theories predict, or whether the theories can be improved. Some
astronomers work solely on observation and analysis, and some work solely on
developing new theories.
Astronomy is such a broad topic that
astronomers specialize in one or more parts of the field. For example, the study
of the solar system is a different area of specialization than the study of
stars. Astronomers who study our galaxy, the Milky Way, often use techniques
different from those used by astronomers who study distant galaxies. Many
planetary astronomers, such as scientists who study Mars, may have geology
backgrounds and not consider themselves astronomers at all. Solar astronomers
use different telescopes than nighttime astronomers use, because the Sun is so
bright. Theoretical astronomers may never use telescopes at all. Instead, these
astronomers use existing data or sometimes only previous theoretical results to
develop and test theories. An increasing field of astronomy is computational
astronomy, in which astronomers use computers to simulate astronomical events.
Examples of events for which simulations are useful include the formation of the
earliest galaxies of the universe or the explosion of a star to make a
supernova.
Astronomers learn about astronomical
objects by observing the energy they emit. These objects emit energy in the form
of electromagnetic radiation. This radiation travels throughout the universe in
the form of waves and can range from gamma rays, which have extremely short
wavelengths, to visible light, to radio waves, which are very long. The entire
range of these different wavelengths makes up the electromagnetic spectrum.
Astronomers gather different wavelengths of
electromagnetic radiation depending on the objects that are being studied. The
techniques of astronomy are often very different for studying different
wavelengths. Conventional telescopes work only for visible light and the parts
of the spectrum near visible light, such as the shortest infrared wavelengths
and the longest ultraviolet wavelengths. Earth’s atmosphere complicates studies
by absorbing many wavelengths of the electromagnetic spectrum. Gamma-ray
astronomy, X-ray astronomy, infrared astronomy, ultraviolet astronomy, radio
astronomy, visible-light astronomy, cosmic-ray astronomy, gravitational-wave
astronomy, and neutrino astronomy all use different instruments and
techniques.
Observational astronomers use telescopes
or other instruments to observe the heavens. The astronomers who do the most
observing, however, probably spend more time using computers than they do using
telescopes. A few nights of observing with a telescope often provide enough data
to keep astronomers busy for months analyzing the data.
Until the 20th century, all
observational astronomers studied the visible light that astronomical objects
emit. Such astronomers are called optical astronomers, because they observe the
same part of the electromagnetic spectrum that the human eye sees. Optical
astronomers use telescopes and imaging equipment to study light from objects.
Professional astronomers today hardly ever actually look through telescopes.
Instead, a telescope sends an object’s light to a photographic plate or to an
electronic light-sensitive computer chip called a charge-coupled device, or CCD.
CCDs are about 50 times more sensitive than film, so today's astronomers can
record in a minute an image that would have taken about an hour to record on
film.
Telescopes may use either lenses or
mirrors to gather visible light, permitting direct observation or photographic
recording of distant objects. Those that use lenses are called refracting
telescopes, since they use the property of refraction, or bending, of light
(see Optics: Reflection and Refraction). The largest refracting
telescope is the 40-in (1-m) telescope at the Yerkes Observatory in Williams
Bay, Wisconsin, founded in the late 19th century. Lenses bend different colors
of light by different amounts, so different colors focus slightly differently.
Images produced by large lenses can be tinged with color, often limiting the
observations to those made through filters. Filters limit the image to one color
of light, so the lens bends all of the light in the image the same amount and
makes the image more accurate than an image that includes all colors of light.
Also, because light must pass through lenses, lenses can only be supported at
the very edges. Large, heavy lenses are so thick that all the large telescopes
in current use are made with other techniques.
Reflecting telescopes, which use
mirrors, are easier to make than refracting telescopes and reflect all colors of
light equally. All the largest telescopes today are reflecting telescopes. The
largest single telescopes are the Keck telescopes at Mauna Kea Observatory in
Hawaii. The Keck telescope mirrors are 394 in (10.0 m) in diameter. Mauna Kea
Observatory, at an altitude of 4,205 m (13,796 ft), is especially high. The air
at the observatory is very clear, so many major telescope projects are located
there.
The Hubble Space Telescope (HST), a
reflecting telescope that orbits Earth, has returned the clearest images of any
optical telescope. The main mirror of the HST is only 94 in (2.4 m) across, far
smaller than that of the largest ground-based reflecting telescopes. Turbulence
in the atmosphere makes observing objects as clearly as the HST can see
impossible for ground-based telescopes. HST images of visible light are about
five times finer than any produced by ground-based telescopes. Giant telescopes
on Earth, however, collect much more light than the HST can. Examples of such
giant telescopes include the twin 32-ft (10-m) Keck telescopes in Hawaii and the
four 26-ft (8-m) telescopes in the Very Large Telescope array in the Atacama
Desert in northern Chile (the nearest city is Antofagasta, Chile). Often
astronomers use space- and ground-based telescopes in conjunction. See also
Space Telescope.
Astronomers usually share telescopes.
Many institutions with large telescopes accept applications from any astronomer
who wishes to use the instruments, though others have limited sets of eligible
applicants. The institution then divides the available time among successful
applicants and assigns each astronomer an observing period. Astronomers can
collect data from telescopes remotely. Data from Earth-based telescopes can be
sent electronically over computer networks. Data from space-based telescopes
reach Earth through radio waves collected by antennas on the ground.
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Gamma-Ray and X-Ray
Astronomy |
Gamma rays have the shortest
wavelengths. Special telescopes in orbit around Earth, such as the National
Aeronautics and Space Administration’s (NASA’s) Compton Gamma-Ray Observatory,
gather gamma rays before Earth’s atmosphere absorbs them. X rays, the next
shortest wavelengths, also must be observed from space. NASA’s Chandra X-Ray
Observatory (CXO) is a school-bus-sized spacecraft that began studying X rays
from orbit in 1999. See also Gamma-Ray Astronomy; X-Ray Astronomy.
Ultraviolet light has wavelengths
longer than X rays, but shorter than visible light. Ultraviolet telescopes are
similar to visible-light telescopes in the way they gather light, but the
atmosphere blocks most ultraviolet radiation. Most ultraviolet observations,
therefore, must also take place in space. Most of the instruments on the Hubble
Space Telescope (HST) are sensitive to ultraviolet radiation (see
Ultraviolet Astronomy). Humans cannot see ultraviolet radiation, but
astronomers can create visual images from ultraviolet light by assigning
particular colors or shades to different intensities of radiation.
Infrared astronomers study parts of
the infrared spectrum, which consists of electromagnetic waves with wavelengths
ranging from just longer than visible light to 1,000 times longer than visible
light. Earth’s atmosphere absorbs infrared radiation, so astronomers must
collect infrared radiation from places where the atmosphere is very thin, or
from above the atmosphere. Observatories for these wavelengths are located on
certain high mountaintops or in space (see Infrared Astronomy). Most
infrared wavelengths can be observed only from space. Every warm object emits
some infrared radiation. Infrared astronomy is useful because objects that are
not hot enough to emit visible or ultraviolet radiation may still emit infrared
radiation. Infrared radiation also passes through interstellar and intergalactic
gas and dust more easily than radiation with shorter wavelengths. Further, the
brightest part of the spectrum from the farthest galaxies in the universe is
shifted into the infrared.
Radio waves have the longest
wavelengths. Radio astronomers use giant dish antennas to collect and focus
signals in the radio part of the spectrum (see Radio Astronomy). These
celestial radio signals, often from hot bodies in space or from objects with
strong magnetic fields, come through Earth's atmosphere to the ground. Radio
waves penetrate dust clouds, allowing astronomers to see into the center of our
galaxy and into the cocoons of dust that surround forming stars.
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Study of Other
Emissions |
Sometimes astronomers study emissions
from space that are not electromagnetic radiation. Some of the particles of
interest to astronomers are neutrinos, cosmic rays, and gravitational waves.
Neutrinos are tiny particles with no electric charge and very little or no mass.
All stars emit neutrinos, but neutrino detectors on Earth receive neutrinos only
from the Sun and supernovas. Most neutrino telescopes consist of huge
underground tanks of liquid. These tanks capture a few of the many neutrinos
that strike them, while the vast majority of neutrinos pass right through the
tanks.
Cosmic rays are electrically charged
particles that come to Earth from outer space at almost the speed of light. They
are made up of negatively charged particles called electrons and positively
charged nuclei of atoms. Astronomers do not know where most cosmic rays come
from, but they use cosmic-ray detectors to study the particles. Cosmic-ray
detectors are usually grids of wires that produce an electrical signal when a
cosmic ray passes close to them.
Gravitational waves are a predicted
consequence of the general theory of relativity developed by German-born
American physicist Albert Einstein. Since the 1960s astronomers have been
building detectors for gravitational waves. Older gravitational-wave detectors
were huge instruments that surrounded a carefully measured and positioned
massive object suspended from the top of the instrument. Lasers trained on the
object were designed to measure the object’s movement, which theoretically would
occur when a gravitational wave hit the object. No gravitational waves have yet
been detected. Gravitational waves should be very weak, and the instruments are
probably not yet sensitive enough to register them. In the 1970s and 1980s
American physicists Joseph Taylor and Russell Hulse observed indirect evidence
of gravitational waves by studying systems of double pulsars. A new generation
of gravitational-wave detectors, developed in the 1990s, uses interferometers to
measure distortions of space that would be caused by passing gravitational
waves.
Some objects emit radiation more
strongly in one wavelength than in another, but a set of data across the entire
spectrum of electromagnetic radiation is much more useful than observations in
any one wavelength. For example, the supernova remnant known as the Crab Nebula
has been observed in every part of the spectrum, and astronomers have used all
the discoveries together to make a complete picture of how the Crab Nebula is
evolving.
Whether astronomers take data from a
ground-based telescope or have data radioed to them from space, they must then
analyze the data. Usually the data are handled with the aid of a computer, which
can carry out various manipulations the astronomer requests. For example, some
of the individual picture elements, or pixels, of a CCD may be slightly more
sensitive than others. Consequently, astronomers sometimes take images of blank
sky to measure which pixels appear brighter. They can then take these variations
into account when interpreting the actual celestial images. Astronomers may
write their own computer programs to analyze data or, as is increasingly the
case, use certain standard computer programs developed at national observatories
or elsewhere.
Often an astronomer uses observations to
test a specific theory. Sometimes, a new experimental capability allows
astronomers to study a new part of the electromagnetic spectrum or to see
objects in greater detail or through special filters. If the observations do not
verify the predictions of a theory, the theory must be discarded or, if
possible, modified.
Up to about 3,000 stars are visible at a time
from Earth with the unaided eye, far away from city lights, on a clear night. A
view at night may also show several planets and perhaps a comet or a meteor
shower. Increasingly, human-made light pollution is making the sky less dark,
limiting the number of visible astronomical objects. During the daytime the Sun
shines brightly. The Moon and bright planets are sometimes visible early or late
in the day but are rarely seen at midday.
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Earth's Relative
Motion |
Earth moves in two basic ways: It turns in
place, and it revolves around the Sun. Earth turns around its axis, an imaginary
line that runs down its center through its North and South poles. The Moon also
revolves around Earth. All of these motions produce day and night, the seasons,
the phases of the Moon, and solar and lunar eclipses.
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Night, Day, and
Seasons |
Earth is about 12,000 km (about 7,000
mi) in diameter. As it revolves, or moves in a circle, around the Sun, Earth
spins on its axis. This spinning movement is called rotation. Earth’s axis is
tilted 23.5° with respect to the plane of its orbit. Each time Earth rotates on
its axis, it goes through one day, a cycle of light and dark. Humans
artificially divide the day into 24 hours and then divide the hours into 60
minutes and the minutes into 60 seconds.
Earth revolves around the Sun once every
year, or 365.25 days (most people use a 365-day calendar and take care of the
extra 0.25 day by adding a day to the calendar every four years, creating a leap
year). The orbit of Earth is almost, but not quite, a circle, so Earth is
sometimes a little closer to the Sun than at other times. If Earth were upright
as it revolved around the Sun, each point on Earth would have exactly 12 hours
of light and 12 hours of dark each day. Because Earth is tilted, however, the
northern hemisphere sometimes points toward the Sun and sometimes points away
from the Sun. This tilt is responsible for the seasons. When the northern
hemisphere points toward the Sun, the northernmost regions of Earth see the Sun
24 hours a day. The whole northern hemisphere gets more sunlight and gets it at
a more direct angle than the southern hemisphere does during this period, which
lasts for half of the year. The second half of this period, when the northern
hemisphere points most directly at the Sun, is the northern hemisphere's summer,
which corresponds to winter in the southern hemisphere. During the other half of
the year, the southern hemisphere points more directly toward the Sun, so it is
spring and summer in the southern hemisphere and fall and winter in the northern
hemisphere.
One revolution of the Moon around Earth
takes a little over 27 days 7 hours. The Moon rotates on its axis in this same
period of time, so the same face of the Moon is always presented to Earth. Over
a period a little longer than 29 days 12 hours, the Moon goes through a series
of phases, in which the amount of the lighted half of the Moon we see from Earth
changes. These phases are caused by the changing angle of sunlight hitting the
Moon. (The period of phases is longer than the period of revolution of the Moon,
because the motion of Earth around the Sun changes the angle at which the Sun’s
light hits the Moon from night to night.)
The Moon’s orbit around Earth is tilted
5° from the plane of Earth’s orbit. Because of this tilt, when the Moon is at
the point in its orbit when it is between Earth and the Sun, the Moon is usually
a little above or below the Sun. At that time, the Sun lights the side of the
Moon facing away from Earth, and the side of the Moon facing toward Earth is
dark. This point in the Moon’s orbit corresponds to a phase of the Moon called
the new moon. A quarter moon occurs when the Moon is at right angles to the line
formed by the Sun and Earth. The Sun lights the side of the Moon closest to it,
and half of that side is visible from Earth, forming a bright half-circle. When
the Moon is on the opposite side of Earth from the Sun, the face of the Moon
visible from Earth is lit, showing the full moon in the sky.
Because of the tilt of the Moon's orbit,
the Moon usually passes above or below the Sun at new moon and above or below
Earth's shadow at full moon. Sometimes, though, the full moon or new moon
crosses the plane of Earth's orbit. By a coincidence of nature, even though the
Moon is about 400 times smaller than the Sun, it is also about 400 times closer
to Earth than the Sun is, so the Moon and Sun look almost exactly the same size
from Earth. If the Moon lines up with the Sun and Earth at new moon (when the
Moon is between Earth and the Sun), it blocks the Sun’s light from Earth,
creating a solar eclipse. If the Moon lines up with Earth and the Sun at the
full moon (when Earth is between the Moon and the Sun), Earth’s shadow covers
the Moon, making a lunar eclipse.
A total solar eclipse is visible from
only a small region of Earth. During a solar eclipse, the complete shadow of the
Moon that falls on Earth is only about 160 km (about 100 mi) wide. As Earth, the
Sun, and the Moon move, however, the Moon’s shadow sweeps out a path up to
16,000 km (10,000 mi) long. The total eclipse can only be seen from within this
path. A total solar eclipse occurs about every 18 months. Off to the sides of
the path of a total eclipse, a partial eclipse, in which the Sun is only partly
covered, is visible. Partial eclipses are much less dramatic than total
eclipses. The Moon’s orbit around Earth is slightly elliptical, or egg-shaped.
The distance between Earth and the Moon varies slightly as the Moon orbits
Earth. When the Moon is farther from Earth than usual, it appears smaller and
may not cover the entire Sun during an eclipse. A ring, or annulus, of sunlight
remains visible, making an annular eclipse. An annular solar eclipse also occurs
about every 18 months. Additional partial solar eclipses are also visible from
Earth in between.
At a lunar eclipse, the Moon is actually
in Earth's shadow. When the Moon is completely in the shadow, the total lunar
eclipse is visible from everywhere on the half of Earth from which the Moon is
visible at that time. As a result, more people see total lunar eclipses than see
total solar eclipses.
In an open place on a clear dark night,
streaks of light may appear in a random part of the sky about once every 10
minutes. These streaks are meteors—bits of rock—burning up in Earth's
atmosphere. The bits of rock are called meteoroids, and when these bits survive
Earth’s atmosphere intact and land on Earth, they are known as meteorites.
Every month or so, Earth passes through
the orbit of a comet. Dust from the comet remains in the comet's orbit. When
Earth passes through the band of dust, the dust and bits of rock burn up in the
atmosphere, creating a meteor shower. Many more meteors are visible during a
meteor shower than on an ordinary night. The most observed meteor shower is the
Perseid shower (see Perseids), which occurs each year on August 11th or
12th.
Humans have picked out landmarks in the
sky and mapped the heavens for thousands of years. Maps of the sky helped people
navigate, measure time, and track celestial events. Now astronomers methodically
map the sky to produce a universal format for the addresses of stars, galaxies,
and other objects of interest.
Some of the stars in the sky are
brighter and more noticeable than others are, and some of these bright stars
appear to the eye to be grouped together. Ancient civilizations imagined that
groups of stars represented figures in the sky. The oldest known representations
of these groups of stars, called constellations, are from ancient Sumer (now
Iraq) from about 4000 bc. The
constellations recorded by ancient Greeks and Chinese resemble the Sumerian
constellations. The northern hemisphere constellations that astronomers
recognize today are based on the Greek constellations. Explorers and astronomers
developed and recorded the official constellations of the southern hemisphere in
the 16th and 17th centuries. The International Astronomical Union (IAU)
officially recognizes 88 constellations. The IAU defined the boundaries of each
constellation, so the 88 constellations divide the sky without overlapping.
A familiar group of stars in the
northern hemisphere is called the Big Dipper. The Big Dipper is actually part of
an official constellation—Ursa Major, or the Great Bear. Groups of stars that
are not official constellations, such as the Big Dipper, are called asterisms.
While the stars in the Big Dipper appear in approximately the same part of the
sky, they vary greatly in their distance from Earth. This is true for the stars
in all constellations or asterisms—the stars making up the group do not really
occur close to each other in space; they merely appear together as seen from
Earth. The patterns of the constellations are figments of humans’ imagination,
and different artists may connect the stars of a constellation in different
ways, even when illustrating the same myth.
Astronomers use coordinate systems to
label the positions of objects in the sky, just as geographers use longitude and
latitude to label the positions of objects on Earth. Astronomers use several
different coordinate systems. The two most widely used are the altazimuth system
and the equatorial system. The altazimuth system gives an object’s coordinates
with respect to the sky visible above the observer. The equatorial coordinate
system designates an object’s location with respect to Earth’s entire night sky,
or the celestial sphere.
One of the ways astronomers give the
position of a celestial object is by specifying its altitude and its
azimuth. This coordinate system is called the altazimuth system. The
altitude of an object is equal to its angle, in degrees, above the horizon. An
object at the horizon would have an altitude of 0°, and an object directly
overhead would have an altitude of 90°. The azimuth of an object is equal to its
angle in the horizontal direction, with north at 0°, east at 90°, south at 180°,
and west at 270°. For example, if an astronomer were looking for an object at
23° altitude and 87° azimuth, the astronomer would know to look fairly low in
the sky and almost directly east.
As Earth rotates, astronomical objects
appear to rise and set, so their altitudes and azimuths are constantly changing.
An object’s altitude and azimuth also vary according to an observer’s location
on Earth. Therefore, astronomers almost never use altazimuth coordinates to
record an object’s position. Instead, astronomers with altazimuth telescopes
translate coordinates from equatorial coordinates to find an object. Telescopes
that use an altazimuth mounting system may be simple to set up, but they require
many calculated movements to keep them pointed at an object as it moves across
the sky. These telescopes fell out of use with the development of the equatorial
coordinate and mounting system in the early 1800s. However, computers have made
the return to popularity possible for altazimuth systems. Altazimuth mounting
systems are simple and inexpensive, and—with computers to do the required
calculations and control the motor that moves the telescope—they are
practical.
The equatorial coordinate system is a
coordinate system fixed on the sky. In this system, a star keeps the same
coordinates no matter what the time is or where the observer is located. The
equatorial coordinate system is based on the celestial sphere. The celestial
sphere is a giant imaginary globe surrounding Earth. This sphere has north and
south celestial poles directly above Earth’s North and South poles. It has a
celestial equator, directly above Earth’s equator. Another important part of the
celestial sphere is the line that marks the movement of the Sun with respect to
the stars throughout the year. This path is called the ecliptic. Because Earth
is tilted with respect to its orbit around the Sun, the ecliptic is not the same
as the celestial equator. The ecliptic is tilted 23.5° to the celestial equator
and crosses the celestial equator at two points on opposite sides of the
celestial sphere. The crossing points are called the vernal (or spring) equinox
and the autumnal equinox. The vernal equinox and autumnal equinox mark the
beginning of spring and fall, respectively. The points at which the ecliptic and
celestial equator are farthest apart are called the summer solstice and the
winter solstice, which mark the beginning of summer and winter,
respectively.
As Earth rotates on its axis each day,
the stars and other distant astronomical objects appear to rise in the eastern
part of the sky and set in the west. They seem to travel in circles around
Earth’s North or South poles. In the equatorial coordinate system, the celestial
sphere turns with the stars (but this movement is really caused by the rotation
of Earth). The celestial sphere makes one complete rotation every 23 hours 56
minutes, which is four minutes shorter than a day measured by the movement of
the Sun. A complete rotation of the celestial sphere is called a sidereal day.
Because the sidereal day is slightly shorter than a solar day, the stars that an
observer sees from any location on Earth change slightly from night to night.
The difference between a sidereal day and a solar day occurs because of Earth’s
motion around the Sun.
The equivalent of longitude on the
celestial sphere is called right ascension and the equivalent of latitude is
declination. Specifying the right ascension of a star is equivalent to measuring
the east-west distance from a line called the prime meridian that runs through
Greenwich, England, for a place on Earth. Right ascension starts at the vernal
equinox. Longitude on Earth is given in degrees, but right ascension is given in
units of time—hours, minutes, and seconds. This is because the celestial equator
is divided into 24 equal parts—each called an hour of right ascension instead of
15°. Each hour is made up of 60 minutes, each of which is equal to 60 seconds.
Measuring right ascension in units of time makes determining when will be the
best time for observing an object easier for astronomers. A particular line of
right ascension will be at its highest point in the sky above a particular place
on Earth four minutes earlier each day, so keeping track of the movement of the
celestial sphere with an ordinary clock would be complicated. Astronomers have
special clocks that keep sidereal time (24 sidereal hours are equal to 23 hours
56 minutes of familiar solar time). Astronomers compare the current sidereal
time to the right ascension of the object they wish to view. The object will be
highest in the sky when the sidereal time equals the right ascension of the
object.
The direction perpendicular to right
ascension—and the equivalent to latitude on Earth—is declination. Declination is
measured in degrees. These degrees are divided into arcminutes and arcseconds.
One arcminute is equal to 1/60 of a degree, and one arcsecond is equal to 1/60
of an arcminute, or 1/360 of a degree. The celestial equator is at declination
0°, the north celestial pole is at declination 90°, and the south celestial pole
has a declination of –90°. Each star has a right ascension and a declination
that mark its position in the sky. The brightest star, Sirius, for example, has
right ascension 6 hours 45 minutes (abbreviated as 6h 45m) and declination -16
degrees 43 arcminutes (written –16° 43').
Stars are so far away from Earth that
the main star motion we see results from Earth’s rotation. Stars do move in
space, however, and these proper motions slightly change the coordinates
of the nearest stars over time. The effects of the Sun and the Moon on Earth
also cause slight changes in Earth’s axis of rotation. These changes, called
precession, cause a slow drift in right ascension and declination. To account
for precession, astronomers redefine the celestial coordinates every 50 years or
so.
Solar systems, both our own and those located
around other stars, are a major area of research for astronomers. A solar system
consists of a central star orbited by planets or smaller rocky bodies. The
gravitational force of the star holds the system together. In our solar system,
the central star is the Sun. It holds all the planets, including Earth, in their
orbits and provides light and energy necessary for life. Our solar system is
just one of many. Astronomers are just beginning to be able to study other solar
systems. See also Extrasolar Planets.
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Objects in Our Solar
System |
Our solar system contains the Sun, planets
(of which Earth is third from the Sun), and the planets’ satellites. It also
contains asteroids, comets, and interplanetary dust and gas.
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Planets and Their
Satellites |
Until the end of the 18th century, humans
knew of five planets—Mercury, Venus, Mars, Jupiter, and Saturn—in addition to
Earth. When viewed without a telescope, planets appear to be dots of light in
the sky. They shine steadily, while stars seem to twinkle. Twinkling results
from turbulence in Earth's atmosphere. Stars are so far away that they appear as
tiny points of light. A moment of turbulence can change that light for a
fraction of a second. Even though they look the same size as stars to unaided
human eyes, planets are close enough that they take up more space in the sky
than stars do. The disks of planets are big enough to average out variations in
light caused by turbulence and therefore do not twinkle.
Between 1781 and 1930, astronomers found
three more planets—Uranus, Neptune, and Pluto. This brought the total number of
planets in our solar system to nine. However, in 2006 the International
Astronomical Union (IAU)—the official body that names objects in the solar
system—reclassified Pluto as a dwarf planet. The IAU rulings reduced the number
of official planets in the solar system to eight. In order of increasing
distance from the Sun, the planets in our solar system are Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
Astronomers call the inner
planets—Mercury, Venus, Earth, and Mars—the terrestrial planets. Terrestrial
(from the Latin word terra, meaning “Earth”) planets are Earthlike in
that they have solid, rocky surfaces. The next group of planets—Jupiter, Saturn,
Uranus, and Neptune—is called the Jovian planets, or the giant planets. The word
Jovian has the same Latin root as the word Jupiter. Astronomers call these
planets the Jovian planets because they resemble Jupiter in that they are giant,
massive planets made almost entirely of gas. The mass of Jupiter, for example,
is 318 times the mass of Earth. The Jovian planets have no solid surfaces,
although they probably have rocky cores several times more massive than Earth.
Rings of chunks of ice and rock surround each of the Jovian planets. The rings
around Saturn are the most familiar. See also Planetary Science.
Pluto is tiny, with a mass about one
five-hundredth the mass of Earth. Pluto seems out of place, with its tiny, solid
body out beyond the giant planets. Many astronomers believe that Pluto is just
one of a group of icy objects in the outer solar system. These objects orbit in
a part of the solar system called the Kuiper Belt. In 2006 the International
Astronomical Union (IAU) reclassified Pluto as a dwarf planet because it had a
rounded shape from effects of its own gravity but it was not massive enough to
have cleared the region of its orbit of other bodies. Other dwarf planets in the
solar system include Eris, an icy body slightly larger than Pluto that also
orbits in part of the Kuiper Belt, and Ceres, a rocky body that orbits in the
asteroid belt.
Most of the planets have moons, or
satellites. Earth’s Moon has a diameter about one-fourth the diameter of Earth.
Mars has two tiny chunks of rock, Phobos and Deimos, each only about 10 km
(about 6 mi) across. Jupiter has more than 60 satellites. The largest four,
known as the Galilean satellites, are Io, Europa, Ganymede, and Callisto.
Ganymede is even larger than the planet Mercury. Saturn has more than 50
satellites. Saturn’s largest moon, Titan, is also larger than the planet Mercury
and is enshrouded by a thick, opaque, smoggy atmosphere. Uranus has nearly 30
known moons, and Neptune has at least 13 moons. Some of the dwarf planets also
have satellites. Pluto has three moons; the largest is called Charon. Charon is
more than half as big as Pluto. Eris has a small moon named Dysnomia.
Comets and asteroids are rocky and icy
bodies that are smaller than planets. The distinction between comets, asteroids,
and other small bodies in the solar system is a little fuzzy, but generally a
comet is icier than an asteroid and has a more elongated orbit. The orbit of a
comet takes it close to the Sun, then back into the outer solar system. When
comets near the Sun, some of their ice turns from solid material into gas,
releasing some of their dust. Comets have long tails of glowing gas and dust
when they are near the Sun. Asteroids are rockier bodies and usually have orbits
that keep them at always about the same distance from the Sun.
Both comets and asteroids have their
origins in the early solar system. While the solar system was forming, many
small, rocky objects called planetesimals condensed from the gas and dust of the
early solar system. Millions of planetesimals remain in orbit around the Sun. A
large spherical cloud of such objects out beyond Pluto forms the Oort cloud. The
objects in the Oort cloud are considered comets. When our solar system passes
close to another star or drifts closer than usual to the center of our galaxy,
the change in gravitational pull may disturb the orbit of one of the icy comets
in the Oort cloud. As this comet falls toward the Sun, the ice turns into vapor,
freeing dust from the object. The gas and dust form the tail or tails of the
comet. The gravitational pull of large planets such as Jupiter or Saturn may
swerve the comet into an orbit closer to the Sun. The time needed for a comet to
make a complete orbit around the Sun is called the comet’s period. Astronomers
believe that comets with periods longer than about 200 years come from the Oort
Cloud. Short-period comets, those with periods less than about 200 years,
probably come from the Kuiper Belt, a ring of planetesimals beyond Neptune. The
material in comets is probably from the very early solar system, so astronomers
study comets to find out more about our solar system’s formation.
When the solar system was forming, some
of the planetesimals came together more toward the center of the solar system.
Gravitational forces from the giant planet Jupiter prevented these planetesimals
from forming full-fledged planets. Instead, the planetesimals broke up to create
thousands of minor planets, or asteroids, that orbit the Sun. Most of them are
in the asteroid belt, between the orbits of Mars and Jupiter, but thousands are
in orbits that come closer to Earth or even cross Earth's orbit. Scientists are
increasingly aware of potential catastrophes if any of the largest of these
asteroids hits Earth. Perhaps 2,000 asteroids larger than 1 km (0.6 mi) in
diameter are potential hazards.
The Sun is the nearest star to Earth and
is the center of the solar system. It is only 8 light-minutes away from Earth,
meaning light takes only eight minutes to travel from the Sun to Earth. The next
nearest star is 4 light-years away, so light from this star, Proxima Centauri
(part of the triple star Alpha Centauri), takes four years to reach Earth. The
Sun's closeness means that the light and other energy we get from the Sun
dominate Earth’s environment and life. The Sun also provides a way for
astronomers to study stars. They can see details and layers of the Sun that are
impossible to see on more distant stars. In addition, the Sun provides a
laboratory for studying hot gases held in place by magnetic fields. Scientists
would like to create similar conditions (hot gases contained by magnetic fields)
on Earth. Creating such environments could be useful for studying basic physics.
The Sun produces its energy by fusing
hydrogen into helium in a process called nuclear fusion. In nuclear fusion, two
atoms merge to form a heavier atom and release energy (see Nuclear
Energy: Nuclear Fusion). The Sun and stars of similar mass start off with
enough hydrogen to shine for about 10 billion years. The Sun is less than
halfway through its lifetime.
| B |
|
Studying the Solar
System |
Although most telescopes are used mainly to
collect the light of faint objects so that they can be studied, telescopes for
planetary and other solar system studies are also used to magnify images.
Astronomers use some of the observing time of several important telescopes for
planetary studies. In general, planetary astronomers must apply and compete for
observing time on telescopes with astronomers seeking to study other objects.
Some planetary objects can be studied as they pass in front of, or occult,
distant stars. The atmosphere of Neptune's moon Triton and the shapes of
asteroids can be investigated in this way, for example. The fields of radio and
infrared astronomy are useful for measuring the temperatures of planets and
satellites. Ultraviolet astronomy can help astronomers study the magnetic fields
of planets.
During the space age, scientists have
developed telescopes and other devices, such as instruments to measure magnetic
fields or space dust, that can leave Earth's surface and travel close to other
objects in the solar system. Robotic spacecraft have visited all of the planets
in the solar system except Pluto. Some missions have targeted specific planets
and spent much time studying a single planet, and some spacecraft have flown
past a number of planets.
Astronomers use different telescopes to
study the Sun than they use for nighttime studies because of the extreme
brightness of the Sun. Telescopes in space, such as the Solar and Heliospheric
Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE), are
able to study the Sun in regions of the spectrum other than visible light. X
rays, ultraviolet, and radio waves from the Sun are especially interesting to
astronomers. Studies in various parts of the spectrum give insight into giant
flows of gas in the Sun, into how the Sun's energy leaves the Sun to travel to
Earth, and into what the interior of the Sun is like. Astronomers also study
solar-terrestrial relations—the relation of activity on the Sun with magnetic
storms and other effects on Earth. Some of these storms and effects can affect
radio reception, cause electrical blackouts, or damage satellites in orbit.
Our solar system began forming about 5
billion years ago, when a cloud of gas and dust between the stars in our Milky
Way Galaxy began contracting. A nearby supernova—an exploding star—may have
started the contraction, but most astronomers believe a random change in density
in the cloud caused the contraction. Once the cloud—known as the solar
nebula—began to contract, the contraction occurred faster and faster. The
gravitational energy caused by this contraction heated the solar nebula. As the
cloud became smaller, it began to spin faster, much as a spinning skater will
spin faster by pulling in his or her arms. This spin kept the nebula from
forming a sphere; instead, it settled into a disk of gas and dust.
In this disk, small regions of gas and dust
began to draw closer and stick together. The objects that resulted, which were
usually less than 500 km (300 mi) across, are the planetesimals. Eventually,
some planetesimals stuck together and grew to form the planets. Scientists have
made computer models of how they believe the early solar system behaved. The
models show that for a solar system to produce one or two huge planets like
Jupiter and several other, much smaller planets is not unusual.
The largest region of gas and dust wound up
in the center of the nebula and formed the protosun (proto is Greek for
“before” and is used to distinguish between an object and its forerunner). The
increasing temperature and pressure in the middle of the protosun vaporized the
dust and eventually allowed nuclear fusion to begin, marking the formation of
the Sun. The young Sun gave off a strong solar wind that drove off most of the
lighter elements, such as hydrogen and helium, from the inner planets. The inner
planets then solidified and formed rocky surfaces. The solar wind lost strength.
Jupiter’s gravitational pull was strong enough to keep its shroud of hydrogen
and helium gas. Saturn, Uranus, and Neptune also kept their layers of light
gases.
The theory of solar system formation
described above accounts for the appearance of the solar system as we know it.
Examples of this appearance include the fact that the planets all orbit the Sun
in the same direction and that almost all the planets rotate on their axes in
the same direction. The recent discoveries of distant solar systems with
different properties could lead to modifications in the theory, however.
Studies in the visible, the infrared, and
the shortest radio wavelengths have revealed disks around several young stars in
our galaxy. One such object, Beta Pictoris (about 62 light-years from Earth),
has revealed a warp in the disk that could be a sign of planets in orbit.
Astronomers are hopeful that, in the cases of these young stars, they are
studying the early stages of solar system formation.
| D |
|
Detecting Other Solar
Systems |
Although astronomers have long assumed that
many other stars have planets, they have been unable to detect these other solar
systems until recently. Planets orbiting around stars other than the Sun are
called extrasolar planets. Planets are small and dim compared to stars, so they
are lost in the glare of their parent stars and are invisible to direct
observation with telescopes.
Astronomers have tried to detect other
solar systems by searching for the way a planet affects the movement of its
parent star. The gravitational attraction between a planet and its star pulls
the star slightly toward the planet, so the star wobbles slightly as the planet
orbits it. Throughout the mid- and late 1900s, several observatories tried to
detect wobbles in the nearest stars by watching the stars’ movement across the
sky. Wobbles were reported in several stars, but later observations showed that
the results were false.
In the early 1990s, studies of a pulsar
revealed at least two planets orbiting it. Pulsars are compact stars that give
off pulses of radio waves at very regular intervals. The pulsar, designated PSR
1257+12, is about 1,000 light-years from Earth. This pulsar's pulses sometimes
came a little early and sometimes a little late in a periodic pattern, revealing
that an unseen object was pulling the pulsar toward and away from Earth. The
environment of a pulsar, which emits X rays and other strong radiation that
would be harmful to life on Earth, is so extreme that these objects would have
little resemblance to planets in our solar system.
The wobbling of a star changes the star’s
light that reaches Earth. When the star moves away from Earth, even slightly,
each wave of light must travel farther to Earth than the wave before it. This
increases the distance between waves (called the wavelength) as the waves reach
Earth. When a star’s planet pulls the star closer to Earth, each successive
wavefront has less distance to travel to reach Earth. This shortens the
wavelength of the light that reaches Earth. This effect is called the Doppler
effect. No star moves fast enough for the change in wavelength to result in a
noticeable change in color, which depends on wavelength, but the changes in
wavelength can be measured with precise instruments. Because the planet’s effect
on the star is very small, astronomers must analyze the starlight carefully to
detect a shift in wavelength. They do this by first using a technique called
spectroscopy to separate the white starlight into its component colors, as water
vapor does to sunlight in a rainbow. Stars emit light in a continuous range. The
range of wavelengths a star emits is called the star’s spectrum. This spectrum
has dark lines, called absorption lines, at wavelengths at which atoms in the
outermost layers of the star absorb light.
Astronomers know what the exact wavelength
of each absorption line is for a star that is not moving. By seeing how far the
movement of a star shifts the absorption lines in its spectrum, astronomers can
calculate how fast the star is moving. If the motion fits the model of the
effect of a planet, astronomers can calculate the mass of the planet and how
close it is to the star. These calculations can only provide the lower limit to
the planet’s mass, because it is impossible for astronomers to tell at what
angle the planet orbits the star. Astronomers need to know the angle at which
the planet orbits the star to calculate the planet’s mass accurately. Because of
this uncertainty, some of the giant extrasolar planets may actually be a type of
failed star called a brown dwarf instead of planets. Most astronomers believe
that many of the suspected planets are true planets.
Since 1995 astronomers have discovered more
than 160 extrasolar planets. Astronomers now know of far more planets outside
our solar system than inside our solar system. Most of these planets,
surprisingly, are more massive than Jupiter and are orbiting so close to their
parent stars that some of them have years (the time it takes to orbit the parent
star once) as long as only a few days on Earth. These solar systems are so
different from our solar system that astronomers are still trying to reconcile
them with the current theory of solar system formation. Some astronomers suggest
that the giant extrasolar planets formed much farther away from their stars and
were later thrown into the inner solar systems by some gravitational
interaction.
Stars are an important topic of astronomical
research. Stars are balls of gas that shine or used to shine because of nuclear
fusion in their cores. The most familiar star is the Sun. The nuclear fusion in
stars produces a force that pushes the material in a star outward. However, the
gravitational attraction of the star’s material for itself pulls the material
inward. A star can remain stable as long as the outward pressure and
gravitational force balance. The properties of a star depend on its mass, its
temperature, and its stage in evolution.
Astronomers study stars by measuring their
brightness or, with more difficulty, their distances from Earth. They measure
the “color” of a star—the differences in the star’s brightness from one part of
the spectrum to another—to determine its temperature. They also study the
spectrum of a star’s light to determine not only the temperature, but also the
chemical makeup of the star’s outer layers.
Many different types of stars exist. Some
types of stars are really just different stages of a star’s evolution. Some
types are different because the stars formed with much more or much less mass
than other stars, or because they formed close to other stars. The Sun is a type
of star known as a main-sequence star. Eventually, main-sequence stars such as
the Sun swell into giant stars and then evolve into tiny, dense, white dwarf
stars. Main-sequence stars and giants have a role in the behavior of most
variable stars and novas. A star much more massive than the Sun will become a
supergiant star, then explode as a supernova. A supernova may leave behind a
neutron star or a black hole.
In about 1910 Danish astronomer Ejnar
Hertzsprung and American astronomer Henry Norris Russell independently worked
out a way to graph basic properties of stars. On the horizontal axis of their
graphs, they plotted the temperatures of stars. On the vertical axis, they
plotted the brightness of stars in a way that allowed the stars to be compared.
(One plotted the absolute brightness, or absolute magnitude, of a star, a
measurement of brightness that takes into account the distance of the star from
Earth. The other plotted stars in a nearby galaxy, all about the same distance
from Earth.) The resulting Hertzsprung-Russell diagram, also called an H-R
diagram or a color-magnitude diagram (where color relates to temperature), is a
basic tool of astronomers.
On an H-R diagram, the brightest stars
are at the top and the hottest stars are at the left. Hertzsprung and Russell
found that most stars fell on a diagonal line across the H-R diagram from upper
left to lower right. This line is called the main sequence. The diagonal line of
main-sequence stars indicates that temperature and brightness of these stars are
directly related. The hotter a main-sequence star is, the brighter it is. The
Sun is a main-sequence star, located in about the middle of the graph. More
faint, cool stars exist than hot, bright ones, so the Sun is brighter and hotter
than most of the stars in the universe.
| A2 |
|
Giant and Supergiant
Stars |
At the upper right of the H-R diagram,
above the main sequence, stars are brighter than main-sequence stars of the same
color. The only way stars of a certain color can be brighter than other stars of
the same color is if the brighter stars are also bigger. Bigger stars are not
necessarily more massive, but they do have larger diameters. Stars that fall in
the upper right of the H-R diagram are known as giant stars or, for even
brighter stars, supergiant stars. Supergiant stars have both larger diameters
and larger masses than giant stars.
Giant and supergiant stars represent
stages in the lives of stars after they have burned most of their internal
hydrogen fuel. Stars swell as they move off the main sequence, becoming giants
and—for more massive stars—supergiants.
A few stars fall in the lower left
portion of the H-R diagram, below the main sequence. Just as giant stars are
larger and brighter than main-sequences stars, these stars are smaller and
dimmer. These smaller, dimmer stars are hot enough to be white or blue-white in
color and are known as white dwarfs.
White dwarf stars are only about the
size of Earth. They represent stars with about the mass of the Sun that have
burned as much hydrogen as they can. The gravitational force of a white dwarf’s
mass is pulling the star inward, but electrons in the star resist being pushed
together. The gravitational force is able to pull the star into a much denser
form than it was in when the star was burning hydrogen. The final stage of life
for all stars like the Sun is the white dwarf stage.
Many stars vary in brightness over time.
These variable stars come in a variety of types. One important type is called a
Cepheid variable, named after the star delta Cephei, which is a prime example of
a Cepheid variable. These stars vary in brightness as they swell and contract
over a period of weeks or months. Their average brightness depends on how long
the period of variation takes. Thus astronomers can determine how bright the
star is merely by measuring the length of the period. By comparing how
intrinsically bright these variable stars are with how bright they look from
Earth, astronomers can calculate how far away these stars are from Earth. Since
they are giant stars and are very bright, Cepheid variables in other galaxies
are visible from Earth. Studies of Cepheid variables tell astronomers how far
away these galaxies are and are very useful for determining the distance scale
of the universe. The Hubble Space Telescope (HST) can determine the periods of
Cepheid stars in galaxies farther away than ground-based telescopes can see.
Astronomers are developing a more accurate idea of the distance scale of the
universe with HST data.
Cepheid variables are only one type of
variable star. Stars called long-period variables vary in brightness as they
contract and expand, but these stars are not as regular as Cepheid variables.
Mira, a star in the constellation Cetus (the whale), is a prime example of a
long-period variable star. Variable stars called eclipsing binary stars are
really pairs of stars. Their brightness varies because one member of the pair
appears to pass in front of the other, as seen from Earth. A type of variable
star called R Coronae Borealis stars varies because they occasionally give off
clouds of carbon dust that dim these stars.
Sometimes stars brighten drastically,
becoming as much as 100 times brighter than they were. These stars are called
novas (Latin for 'new stars'). They are not really new, just much brighter than
they were earlier. A nova is a binary, or double, star in which one member is a
white dwarf and the other is a giant or supergiant. Matter from the large star
falls onto the small star. After a thick layer of the large star’s atmosphere
has collected on the white dwarf, the layer burns off in a nuclear fusion
reaction. The fusion produces a huge amount of energy, which, from Earth,
appears as the brightening of the nova. The nova gradually returns to its
original state, and material from the large star again begins to collect on the
white dwarf.
Sometimes stars brighten many times more
drastically than novas do. A star that had been too dim to see can become one of
the brightest stars in the sky. These stars are called supernovas. Sometimes
supernovas that occur in other galaxies are so bright that, from Earth, they
appear as bright as their host galaxy.
There are two types of supernova. One
type is an extreme case of a nova, in which matter falls from a giant or
supergiant companion onto a white dwarf. In the case of a supernova, the white
dwarf gains so much fuel from its companion that the star increases in mass
until strong gravitational forces cause it to become unstable. The star
collapses and the core explodes, vaporizing much of the white dwarf and
producing an immense amount of light. Only bits of the white dwarf remain after
this type of supernova occurs.
The other type of supernova occurs when
a supergiant star uses up all its nuclear fuel in nuclear fusion reactions. The
star uses up its hydrogen fuel, but the core is hot enough that it provides the
initial energy necessary for the star to begin “burning” helium, then carbon,
and then heavier elements through nuclear fusion. The process stops when the
core is mostly iron, which is too heavy for the star to “burn” in a way that
gives off energy. With no such fuel left, the inward gravitational attraction of
the star’s material for itself has no outward balancing force, and the core
collapses. As it collapses, the core releases a shock wave that tears apart the
star’s atmosphere. The core continues collapsing until it forms either a neutron
star or a black hole, depending on its mass.
Only a handful of supernovas are known
in our galaxy. The last Milky Way supernova seen from Earth was observed in
1604. In 1987 astronomers observed a supernova in the Large Magellanic Cloud,
one of the Milky Way’s satellite galaxies (see Magellanic Clouds). This
supernova became bright enough to be visible to the unaided eye and is still
under careful study from telescopes on Earth and from the Hubble Space
Telescope. A supernova in the process of exploding emits radiation in the X-ray
range and ultraviolet and radio radiation studies in this part of the spectrum
are especially useful for astronomers studying supernova remnants.
| A7 |
|
Neutron Stars and
Pulsars |
Neutron stars are the collapsed cores
sometimes left behind by supernova explosions. Pulsars are a special type of
neutron star. Pulsars and neutron stars form when the remnant of a star left
after a supernova explosion collapses until it is about 10 km (about 6 mi) in
radius. At that point, the neutrons—electrically neutral atomic particles—of the
star resist being pressed together further. When the force produced by the
neutrons balances the gravitational force, the core stops collapsing. At that
point, the star is so dense that a teaspoonful has the mass of a billion metric
tons.
Neutron stars become pulsars when the
magnetic field of a neutron star directs a beam of radio waves out into space.
The star is so small that it rotates from one to a few hundred times per second.
As the star rotates, the beam of radio waves sweeps out a path in space. If
Earth is in the path of the beam, radio astronomers see the rotating beam as
periodic pulses of radio waves. This pulsing is the reason these stars are
called pulsars.
Some neutron stars are in binary systems
with an ordinary star neighbor. The gravitational pull of a neutron star pulls
material off its neighbor. The rotation of the neutron star heats the material,
causing it to emit X rays. The neutron star’s magnetic field directs the X rays
into a beam that sweeps into space and may be detected from Earth. Astronomers
call these stars X-ray pulsars.
Gamma-ray spacecraft detect bursts of
gamma rays about once a day. The bursts come from sources in distant galaxies,
so they must be extremely powerful for us to be able to detect them. A leading
model used to explain the bursts is the merger of two neutron stars in a distant
galaxy with a resulting hot fireball. A few such explosions have been seen and
studied with the Hubble and Keck telescopes.
Black holes are objects that are so
massive and dense that their immense gravitational pull does not even let light
escape. If the core left over after a supernova explosion has a mass of more
than about five times that of the Sun, the force holding up the neutrons in the
core is not large enough to balance the inward gravitational force. No outward
force is large enough to resist the gravitational force. The core of the star
continues to collapse. When the core's mass is sufficiently concentrated, the
gravitational force of the core is so strong that nothing, not even light, can
escape it. The gravitational force is so strong that classical physics no longer
applies, and astronomers use Einstein’s general theory of relativity to explain
the behavior of light and matter under such strong gravitational forces.
According to general relativity, space around the core becomes so warped that
nothing can escape, creating a black hole. A star with a mass ten times the mass
of the Sun would become a black hole if it were compressed to 90 km (60 mi) or
less in diameter.
Astronomers have various ways of
detecting black holes. When a black hole is in a binary system, matter from the
companion star spirals into the black hole, forming a disk of gas around it. The
disk becomes so hot that it gives off X rays that astronomers can detect from
Earth. Astronomers use X-ray telescopes in space to find X-ray sources, and then
they look for signs that an unseen object of more than about five times the mass
of the Sun is causing gravitational tugs on a visible object.
The basic method that astronomers use to
find the distance of a star from Earth uses parallax. Parallax is the change in
apparent position of a distant object when viewed from different places. For
example, imagine a tree standing in the center of a field, with a row of
buildings at the edge of the field behind the tree. If two observers stand at
the two front corners of the field, the tree will appear in front of a different
building for each observer. Similarly, a nearby star's position appears slightly
different when seen from different angles.
Parallax also allows human eyes to judge
distance. Each eye sees an object from a slightly different angle. The brain
compares the two pictures to judge the distance to the object. Astronomers use
the same idea to calculate the distance to a star. Stars are very far away, so
astronomers must look at a star from two locations as far apart as possible to
get a measurement. The movement of Earth around the Sun makes this possible. By
taking measurements six months apart from the same place on Earth, astronomers
take measurements from locations separated by the diameter of Earth’s orbit.
That is a separation of about 300 million km (186 million mi). The nearest stars
will appear to shift slightly with respect to the background of more distant
stars. Even so, the greatest stellar parallax is only about 0.77 seconds of arc,
an amount 4,600 times smaller than a single degree. Astronomers calculate a
star’s distance by dividing 1 by the parallax. Distances of stars are usually
measured in parsecs. A parsec is 3.26 light-years, and a light-year is the
distance that light travels in a year, or about 9.5 trillion km (5.9 trillion
mi). Proxima Centauri, the Sun’s nearest neighbor, has a parallax of 0.77
seconds of arc. This measurement indicates that Proxima Centauri’s distance from
Earth is about 1.3 parsecs, or 4.2 light-years. Because Proxima Centauri is the
Sun’s nearest neighbor, it has a larger parallax than any other star.
Astronomers can measure stellar parallaxes
for stars up to about 500 light-years away, which is only about 2 percent of the
distance to the center of our galaxy. Beyond that distance, the parallax angle
is too small to measure.
A European Space Agency spacecraft named
Hipparcos (an acronym for High Precision Parallax
Collecting Satellite), launched in 1989, gave a set of accurate
parallaxes across the sky that was released in 1997. This set of measurements
has provided a uniform database of stellar distances for over 100,000 stars and
a somewhat less accurate database of over 1 million stars. These parallax
measurements provide the base for measurements of the distance scale of the
universe. Hipparcos data are leading to more accurate age calculations for the
universe and for objects in it, especially globular clusters of stars.
Astronomers use a star’s light to
determine the star’s temperature, composition, and motion. Astronomers analyze a
star’s light by looking at its intensity at different wavelengths. Blue light
has the shortest visible wavelengths, at about 400 nanometers. (A nanometer,
abbreviated nm, is one billionth of a meter, or about one forty-thousandth of an
inch.) Red light has the longest visible wavelengths, at about 650 nm. A law of
radiation known as Wien's displacement law (developed by German physicist
Wilhelm Wien) links the wavelength at which the most energy is given out by an
object and its temperature. A star like the Sun, whose surface temperature is
about 6000 K (about 5730°C or about 10,350°F), gives off the most radiation in
yellow-green wavelengths, with decreasing amounts in shorter and longer
wavelengths. Astronomers put filters of different standard colors on telescopes
to allow only light of a particular color from a star to pass. In this way,
astronomers determine the brightness of a star at particular wavelengths. From
this information, astronomers can use Wien’s law to determine the star’s surface
temperature.
Astronomers can see the different
wavelengths of light of a star in more detail by looking at its spectrum. The
continuous rainbow of color of a star's spectrum is crossed by dark lines, or
spectral lines. In the early 19th century, German physicist Josef Fraunhofer
identified such lines in the Sun's spectrum, and they are still known as
Fraunhofer lines. American astronomer Annie Jump Cannon divided stars into
several categories by the appearance of their spectra. She labeled them with
capital letters according to how dark their hydrogen spectral lines were. Later
astronomers reordered these categories according to decreasing temperature. The
categories are O, B, A, F, G, K, and M, where O stars are the hottest and M
stars are the coolest. The Sun is a G star. An additional spectral type, L
stars, was suggested in 1998 to accommodate some cool stars studied using new
infrared observational capabilities. Detailed study of spectral lines shows the
physical conditions in the atmospheres of stars. Careful study of spectral lines
shows that some stars have broader lines than others of the same spectral type.
The broad lines indicate that the outer layers of these stars are more diffuse,
meaning that these layers are larger, but spread more thinly, than the outer
layers of other stars. Stars with large diffuse atmospheres are called giants.
Giant stars are not necessarily more massive than other stars—the outer layers
of giant stars are just more spread out.
Many stars have thousands of spectral
lines from iron and other elements near iron in the periodic table. Other stars
of the same temperature have relatively few spectral lines from such elements.
Astronomers interpret these findings to mean that two different populations of
stars exist. Some formed long ago, before supernovas produced the heavy
elements, and others formed more recently and incorporated some heavy elements.
The Sun is one of the more recent stars.
Spectral lines can also be studied to see
if they change in wavelength or are different in wavelength from sources of the
same lines on Earth. These studies tell us, according to the Doppler effect, how
much the star is moving toward or away from us. Such studies of starlight can
tell us about the orbits of stars in binary systems or about the pulsations of
variable stars, for example.
Astronomers study galaxies to learn about
the structure of the universe. Galaxies are huge collections of billions of
stars. Our Sun is part of the Milky Way Galaxy. Galaxies also contain dark
strips of dust and may contain huge black holes at their centers. Galaxies exist
in different shapes and sizes. Some galaxies are spirals, some are oval, or
elliptical, and some are irregular. The Milky Way is a spiral galaxy. Galaxies
tend to group together in clusters.
Our Sun is only one of about 400 billion
stars in our home galaxy, the Milky Way. On a dark night, far from outdoor
lighting, a faint, hazy, whitish band spans the sky. This band is the Milky Way
Galaxy as it appears from Earth. The Milky Way looks splotchy, with darker
regions interspersed with lighter ones.
The Milky Way Galaxy is a pinwheel-shaped
flattened disk about 75,000 light-years in diameter. The Sun is located on a
spiral arm about two-thirds of the way out from the center. The galaxy spins,
but the center spins faster than the arms. At Earth’s position, the galaxy makes
a complete rotation about every 200 million years.
When observers on Earth look toward the
brightest part of the Milky Way, which is in the constellation Sagittarius, they
look through the galaxy’s disk toward its center. This disk is composed of the
stars, gas, and dust between Earth and the galactic center. When observers look
in the sky in other directions, they do not see as much of the galaxy’s gas and
dust, and so can see objects beyond the galaxy more clearly.
The Milky Way Galaxy has a core
surrounded by its spiral arms. A spherical cloud containing about 100 examples
of a type of star cluster known as a globular cluster surrounds the galaxy.
Still farther out is a galactic corona. Astronomers are not sure what types of
particles or objects occupy the corona, but these objects do exert a measurable
gravitational force on the rest of the galaxy.
| B |
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Characteristics of
Galaxies |
Galaxies contain billions of stars, but
the space between stars is not empty. Astronomers believe that almost every
galaxy probably has a huge black hole at its center.
The space between stars in a galaxy
consists of low-density gas and dust. The dust is largely carbon given off by
red-giant stars. The gas is largely hydrogen, which accounts for 90 percent of
the atoms in the universe. Hydrogen exists in two main forms in the universe.
Astronomers give complete hydrogen atoms, with a nucleus and an electron, a
designation of the Roman numeral I, or HI. Ionized hydrogen, hydrogen made up of
atoms missing their electrons, is given the designation II, or HII. Clouds, or
regions, of both types of hydrogen exist between the stars. HI regions are too
cold to produce visible radiation, but they do emit radio waves that are useful
in measuring the movement of gas in our own galaxy and in distant galaxies. The
HII regions form around hot stars. These regions emit diffuse radiation in the
visual range, as well as in the radio, infrared, and ultraviolet ranges. The
cloudy light from such regions forms beautiful nebulas such as the Great Orion
Nebula.
Astronomers have located over 100
types of molecules in interstellar space. These molecules occur only in trace
amounts among the hydrogen. Still, astronomers can use these molecules to map
galaxies. By measuring the density of the molecules throughout a galaxy,
astronomers can get an idea of the galaxy’s structure.
Interstellar dust sometimes gathers to
form dark nebulae, which appear in silhouette against background gas or stars
from Earth. The Horsehead Nebula, for example, is the silhouette of interstellar
dust against a background HI region. See also Interstellar Matter.
The first known black holes were the
collapsed cores of supernova stars, but astronomers have since discovered signs
of much larger black holes at the centers of galaxies. These galactic black
holes contain millions of times as much mass as the Sun. Astronomers believe
that huge black holes such as these provide the energy of mysterious objects
called quasars. Quasars are very distant objects that are moving away from Earth
at high speed. The first ones discovered were very powerful radio sources, but
scientists have since discovered quasars that don’t strongly emit radio waves.
Astronomers believe that almost every galaxy, whether spiral or elliptical, has
a huge black hole at its center.
Astronomers look for galactic black
holes by studying the movement of galaxies. By studying the spectrum of a
galaxy, astronomers can tell if gas near the center of the galaxy is rotating
rapidly. By measuring the speed of rotation and the distance from various points
in the galaxy to the center of the galaxy, astronomers can determine the amount
of mass in the center of the galaxy. Measurements of many galaxies show that gas
near the center is moving so quickly that only a black hole could be dense
enough to concentrate so much mass in such a small space. Astronomers suspect
that a significant black hole occupies even the center of the Milky Way. The
clear images from the Hubble Space Telescope have allowed measurements of
motions closer to the centers of galaxies than previously possible, and have led
to the confirmation in several cases that giant black holes are present.
Galaxies are classified by shape. The
three types are spiral, elliptical, and irregular. Spiral galaxies consist of a
central mass with one, two, or three arms that spiral around the center. An
elliptical galaxy is oval, with a bright center that gradually, evenly dims to
the edges. Irregular galaxies are not symmetrical and do not look like spiral or
elliptical galaxies. Irregular galaxies vary widely in appearance. A galaxy that
has a regular spiral or elliptical shape but has some special oddity is known as
a peculiar galaxy. For example, some peculiar galaxies are stretched and
distorted from the gravitational pull of a nearby galaxy.
Spiral galaxies are flattened
pinwheels in shape. They can have from one to three spiral arms coming from a
central core. The Great Andromeda Spiral Galaxy is a good example of a spiral
galaxy. The shape of the Milky Way is not visible from Earth, but astronomers
have measured that the Milky Way is also a spiral galaxy. American astronomer
Edwin Hubble further classified spiral galaxies by the tightness of their
spirals. In order of increasingly open arms, Hubble’s types are Sa, Sb, and
Sc.
Some galaxies have a straight, bright,
bar-shaped feature across their center, with the spiral arms coming off the bar
or off a ring around the bar. With a capital B for the bar, the Hubble types of
these galaxies are SBa, SBb, and SBc.
Many clusters of galaxies have giant
elliptical galaxies at their centers. Smaller elliptical galaxies, called dwarf
elliptical galaxies, are much more common than giant ones. Most of the two dozen
galaxies in the Milky Way’s Local Group of galaxies are dwarf elliptical
galaxies.
Astronomers classify elliptical
galaxies by how oval they look, ranging from E0 for very round to E3 for
intermediately oval to E7 for extremely elongated. The galaxy class E7 is also
called S0, which is also known as a lenticular galaxy, a shape with an elongated
disk but no spiral arms. Because astronomers can see other galaxies only from
the perspective of Earth, the shape astronomers see is not necessarily the exact
shape of a galaxy. For instance, they may be viewing it from an end, and not
from above or below.
Some galaxies have no structure, while
others have some trace of structure but do not fit the spiral or elliptical
classes. All of these galaxies are called irregular galaxies. The two small
galaxies that are satellites to the Milky Way Galaxy are both irregular. They
are known as the Magellanic Clouds. The Large Magellanic Cloud shows signs of
having a bar in its center. The Small Magellanic Cloud is more formless. Studies
of stars in the Large and Small Magellanic Clouds have been fundamental for
astronomers’ understanding of the universe. Each of these galaxies provides
groups of stars that are all at the same distance from Earth, allowing
astronomers to compare the absolute brightness of these stars.
In the late 1920s American astronomer
Edwin Hubble discovered that all but the nearest galaxies to us are receding, or
moving away from us. Further, he found that the farther away from Earth a galaxy
is, the faster it is receding. He made his discovery by taking spectra of
galaxies and measuring the amount by which the wavelengths of spectral lines
were shifted. He measured distance in a separate way, usually from studies of
Cepheid variable stars. Hubble discovered that essentially all the spectra of
all the galaxies were shifted toward the red, or had redshifts. The redshifts of
galaxies increased with increasing distance from Earth. After Hubble’s work,
other astronomers made the connection between redshift and velocity, showing
that the farther a galaxy is from Earth, the faster it moves away from Earth.
This idea is called Hubble’s law and is the basis for the belief that the
universe is fairly uniformly expanding. Other uniformly expanding
three-dimensional objects, such as a rising cake with raisins in the batter,
also demonstrate the consequence that the more distant objects (such as the
other raisins with respect to any given raisin) appear to recede more rapidly
than nearer ones. This consequence is the result of the increased amount of
material expanding between these more distant objects.
Hubble's law states that there is a
straight-line, or linear, relationship between the speed at which an object is
moving away from Earth and the distance between the object and Earth. The speed
at which an object is moving away from Earth is called the object’s velocity of
recession. Hubble’s law indicates that as velocity of recession increases,
distance increases by the same proportion. Using this law, astronomers can
calculate the distance to the most distant galaxies, given only measurements of
their velocities calculated by observing how much their light is shifted.
Astronomers can accurately measure the redshifts of objects so distant that the
distance between Earth and the objects cannot be measured by other means.
The constant of proportionality that
relates velocity to distance in Hubble's law is called Hubble's constant, or H.
Hubble's law is often written v=Hd, or velocity equals Hubble's constant
multiplied by distance. Thus determining Hubble's constant will give the speed
of the universe's expansion. The inverse of Hubble’s constant, or 1/H,
theoretically provides an estimate of the age of the universe. Astronomers now
believe that Hubble’s constant has changed over the lifetime of the universe,
however, so estimates of expansion and age must be adjusted accordingly.
The value of Hubble’s constant probably
falls between 64 and 78 kilometers per second per megaparsec (between 40 and 48
miles per second per megaparsec). A megaparsec is 1 million parsecs and a parsec
is 3.26 light-years. Astronomers used the Hubble Space Telescope to study
Cepheid variables in distant galaxies to get an accurate measurement of the
distance between the stars and Earth to refine the value of Hubble’s constant.
The value these astronomers found was 72 kilometers per second per megaparsec
(45 miles per second per megaparsec), with an uncertainty of only 10
percent.
The actual age of the universe depends
not only on Hubble's constant but also on how much the gravitational pull of the
mass in the universe slows the universe’s expansion. Some data from studies that
use the brightness of distant supernovas to assess distance indicate that the
universe's expansion is speeding up instead of slowing down. Astronomers
invented the term “dark energy” for the unknown cause of this accelerating
expansion and are actively investigating these topics.
The ultimate goal of astronomers is to
understand the structure, behavior, and evolution of all of the matter and
energy that exists. Astronomers call the set of all matter and energy the
universe. The universe is infinite in space, but astronomers believe it does
have a finite age. Astronomers accept the theory that about 14 billion years ago
the universe began as an explosive event resulting in a hot, dense, expanding
sea of matter and energy. This event is known as the big bang (see Big
Bang Theory). Astronomers cannot observe that far back in time. Many astronomers
believe, however, that within the first fraction of a second after the big bang,
the universe went through a tremendous inflation, expanding many times in size,
before it resumed a slower expansion (see Inflationary Theory).
As the universe expanded and cooled,
various forms of elementary particles of matter formed. By the time the universe
was one second old, protons had formed. For approximately the next 1,000
seconds, in the era of nucleosynthesis, all the nuclei of deuterium (hydrogen
with both a proton and neutron in the nucleus) that are present in the universe
today formed. During this brief period, some nuclei of lithium, beryllium, and
helium formed as well.
When the universe was about 1 million
years old, it had cooled to about 3000 K (about 3300°C or about 5900°F). At that
temperature, the protons and heavier nuclei formed during nucleosynthesis could
combine with electrons to form atoms. Before electrons combined with nuclei, the
travel of radiation through space was very difficult. Radiation in the form of
photons (packets of light energy) could not travel very far without colliding
with electrons. Once protons and electrons combined to form hydrogen, photons
became able to travel through space. The radiation carried by the photons had
the characteristic spectrum of a hot gas. Since the time this radiation was
first released, it has cooled and is now 3 K (-270°C or –450°F). It is called
the primeval background radiation and has been definitively detected and
studied, first by radio telescopes and then by the Cosmic Background Explorer
(COBE) and Wilkinson Microwave Anisotropy Probe (WMAP) spacecrafts. COBE, WMAP,
and ground-based radio telescopes detected tiny deviations from uniformity in
the primeval background radiation; these deviations may be the seeds from which
clusters of galaxies grew.
The gravitational force from invisible
matter, known as dark matter, may have helped speed the formation of structure
in the universe. Observations from the Hubble Space Telescope have revealed
galaxies older than astronomers expected, reducing the interval between the big
bang and the formation of galaxies or clusters of galaxies.
From about 2 billion years after the big
bang for another 2 billion years, quasars formed as active giant black holes in
the cores of galaxies. These quasars gave off radiation as they consumed matter
from nearby galaxies. Few quasars appear close to Earth, so quasars must be a
feature of the earlier universe.
A population of stars formed out of the
interstellar gas and dust that contracted to form galaxies. This first
population, known as Population II, was made up almost entirely of hydrogen and
helium. The stars that formed evolved and gave out heavier elements that were
made through fusion in the stars’ cores or that were formed as the stars
exploded as supernovas. The later generation of stars, to which the Sun belongs,
is known as Population I and contains heavy elements formed by the earlier
population. The Sun formed about 5 billion years ago and is almost halfway
through its 11-billion-year lifetime.
About 4.6 billion years ago, our solar
system formed. The oldest fossils of a living organism date from about 3.5
billion years ago and represent cyanobacteria. Life evolved, and 65 million
years ago, the dinosaurs and many other species were extinguished, probably from
a catastrophic meteor impact. Modern humans evolved no earlier than a few
hundred thousand years ago, a blink of an eye on the cosmic timescale.
Contributed By:
Jay M.
Pasachoff.
