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  1. Introduction and History
  2. Amateur astronomers
  3. Working as Astronomer
  4. The Sky
  5. The Solar System
  6. Stars
  7. Galaxies
  8. The Universe

1. Introduction and History

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 .

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2. Amateur Astronomers

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.

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3. Working as Astronomer

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.

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4. The Sky

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.


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5. The Solar System

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.


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6. Stars

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.

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7. Galaxies

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.


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8. The Universe

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.

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Contributed By: Dr. Akash Garg


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Copyright 2003 Dr. Akash Garg
Last modified: 07/26/03