Astronomy, study of the universe and the celestial bodies,
gas, and dust within it.
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 India "Rishis" i.e. learned people used to study them in
deltail and mentioned them in "Vedas", they successfully applied astronomy to
make solar calendar for use in agriculture and many other uses like making
calender, astrology etc. In fact for Astrology knowing Astronomy is a must.
In ancient Egypt, the visibility of certain stars for the
first time each year marked the onset of the seasonal flood, an important event
for agriculture. As early as the 1300's B.C., Chinese astronomers charted the
positions of the stars and recorded eclipses of the sun and moon. By about 700
B.C., the Babylonians could predict when planets would appear closest to and
farthest from the sun. They also predicted when various astronomical objects
would be visible for the first or last time in a year. The ancient Egyptians
determined the beginning of springtime by noting the position of Sirius, the
brightest star in the sky. They also used their astronomical knowledge to build
temples whose walls lined up with certain heavenly bodies.In 17th-century
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.
A new view of the universe emerged during the early 1900's, chiefly from the
work of the famous German-born physicist Albert Einstein. In 1905, Einstein
proposed his special theory of relativity. According to this theory, nothing can
travel faster than the speed of light. From this theory comes the idea that mass
and energy are interchangeable and are related by the equation E equals m times
c-squared. In this equation, E stands for energy, m for mass, and c-squared for
the speed of light multiplied by itself. During the 1930's, astronomers
discovered that stars get their energy through the transformation of mass into
energy as described by Einstein's equation.
In 1916, Einstein presented his theory of gravitation, called
the general theory of relativity. This theory links the three dimensions of
space with a fourth dimension, time. In most cases, the results obtained by
using Einstein's theory do not differ significantly from those obtained by using
Newton's theories. However, the general theory of relativity must be used in
studies of the universe as a whole or of events that occur in extremely strong
gravitational fields. For example, the general theory of relativity predicts the
existence of black holes. It explains how the mass present in a black hole can
affect space in such a way that not even light can escape.
The general theory of relativity implies that the universe is
expanding. But in 1916, Einstein had no observational evidence to support this
idea. He therefore changed his equations to describe a universe of constant
size. In 1929, however, the American astronomer Edwin Hubble demonstrated that
the universe is expanding. As a result, Einstein restored his original
equations. Modern theories of cosmology are based on solutions to these
equations.
The development of radio astronomy. In 1931, Karl Jansky, an
American engineer at the Bell Telephone Laboratories in New Jersey, studied
static that was interfering with short-wave communication systems. He found that
the static appeared four minutes earlier each day. Jansky knew that the stars
rise four minutes earlier daily, and so he concluded that the static must be
coming from beyond the solar system. He was actually receiving radio waves from
the centre of our galaxy.
Astronomers also search for life on other planets. Some
astronomers use radio telescopes to listen for signals from intelligent beings
from far-off civilizations. Most prominent of these is SETI .
Back to top
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.
Amateur astronomers sometimes share their observations by posting their
photographs on the World Wide Web, a network of information based on connections
between computers.
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.
Back to top
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.
A Observation 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.
i Using Optical Instruments 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.
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 Hubble Space Telescope , a reflecting telescope that orbits Earth,
has returned the clearest images of any optical telescope. The main mirror of
the Hubble 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 Hubble can see impossible for ground-based telescopes.
Hubble 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 Hubble can.
ii Using 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 scheduled to begin
studying X rays from orbit in 1999. It is designed to make high-resolution
images. See also Gamma-Ray Astronomy; X-Ray Astronomy.
iii Using Ultraviolet 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 are sensitive to
ultraviolet radiation .
iii Using Infrared 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 . 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. The Next Generation Space Telescope, which NASA plans to launch in
2006, will operate especially in the infrared.
iv Using Radio Waves Radio waves have the longest wavelengths.
Radio astronomers use giant dish antennas to collect and focus signals in the
radio part of the spectrum. Most of the HAM are capable of doing radio
astronomy.
v Observing 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.
The Sun and supernovas emit neutrinos. 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.
B Analysis and Theory Usually the data are handled with the aid of a
computer, which can carry out various manipulations the astronomer requests.
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.
Back to top
A Mapping the Sky 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.
i The Constellations 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 Indians, Greeks and Chinese
resemble the Sumerian constellations. The northern hemisphere constellations
that astronomers recognize today are based on the Greek constellations. 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.
ii Coordinate Systems 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.
Back to top
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.
Our solar system contains the Sun, nine planets (of which Earth is third from
the Sun), and the planets' satellites. It also contains asteroids, comets, and
interplanetary dust and gas.
i Planets and Their Satellites
In order of increasing distance from the Sun, the planets in our solar system
are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.
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.
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 at least 17
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 at least 18 satellites. Saturn's largest moon, Titan, is also larger than
the planet Mercury and is enshrouded by a thick, opaque, smoggy atmosphere.
Uranus has at least 17 moons, and Neptune has at least 8 moons. Pluto has one
moon, called Charon. Charon is more than half as big as Pluto.
ii Comets and Asteroids
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.
Most of the asteroids 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. Perhaps 2,000 asteroids larger than 1 km (0.6 mi) in diameter are
potential hazards.
iii The Sun 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.
Back to top
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.
For classification and measuring distance we use 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.
i Normal Stars Main Sequence Stars 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.
ii 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.
iii White Dwarf Stars 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.
iv Variable Stars 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. 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 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 Hubble 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.
v Novas 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.
vi Supernovas 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. In 1987 astronomers
observed a supernova in the Large Magellanic Cloud, one of the Milky Way's
satellite galaxies . 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.
vii 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.
vii Black Holes 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
fives 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. By 1999 astronomers had found about a dozen potential black
holes.
Locations of Stars 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.
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), 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.
Starlight 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 links the wavelength at which the most energy is
given out by an object and its temperature.
Astronomers can see the different wavelengths of light of a star in more detail
by looking at its spectrum. Using spectrometry categories of star 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.
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.
Back to top
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.
The Milky Way Our Sun is only one of a trillion 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.
Galactic Black Holes 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.
Types of Galaxies 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.
Movement of Galaxies Our Universe is expanding. 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. 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, corrected for the effect of gravitation,
theoretically provides the age of the universe.
The value of Hubble's constant probably falls between 55 and 75 kilometers per
second per megaparsec. A megaparsec is one million parsecs and a parsec is 3.26
light-years. The Hubble Space Telescope studied 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 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 even seem to indicate that the universe's expansion may be
speeding up instead of slowing down. Astronomers were actively investigating
these topics at the end of the 20th century.
Back to top
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 some 10 or 15 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 . Astronomers cannot
observe that far back in time. Many astronomers believe, however, the theory
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.
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 ). 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 ). 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) spacecraft. COBE 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 cold 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,
remains of a kind of blue-green algae called cyanobacteria, formed within a few
hundred million years. Life evolved, and 65 million years ago, the dinosaurs and
many other species were extinguished, probably from a catastrophic meteor
impact. The earliest humans evolved only a few hundred thousand years ago, a
blink of an eye on the cosmic timescale.
Back to top
Contributed By: Dr. Akash Garg
