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A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas dust, and an important but poorly understood component tentatively dubbed dark matter.[1][2] The name is from the Greek word galaxias [????????], literally meaning "milky", a reference to the Milky Way galaxy. Sony VAIO VPC CW2MFX/PU Battery

Typical galaxies range from dwarfs with as few as ten million (107) stars,[3] up to giants with a hundred trillion (1014) stars,[4] all orbiting the galaxy's center of mass. Galaxies may contain many star systems, star clusters, and various interstellar clouds.Sony VAIO VPC S11V9E/B Battery

The Sun is one of the stars in the Milky Way galaxy; the Solar System includes the Earth and all the other objects that orbit the Sun.

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology).Sony VAIO VPCB119GJ/B Battery

A common form is the elliptical galaxy,[5] which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with dusty, curving arms. Galaxies with irregular or unusual shapes are known as irregular galaxies, and typically result from disruption by the gravitational pull of neighboring galaxies. Sony VAIO VPCB11AGJ Battery

Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.[6]Sony VAIO VPCB11AVJ Battery

There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe.[7][8] Most galaxies are 1,000 to 100,000[9] parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).Sony VAIO VPCB11V9E Battery

Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters.Sony VAIO VPCB11X9E Battery

These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.[11]

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. Sony VAIO VPCCW18FJ/P Battery

They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy appears to harbor at least one such object within its nucleus.[12]


The word galaxy derives from the Greek term for our own galaxy, galaxias (????????), or kyklos galaktikos, meaning "milky circle" for its appearance in the sky. Sony VAIO VPCCW18FJ/R Battery

In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and then realizes she is nursing an unknown baby: Sony VAIO VPCCW18FJ/W Battery

she pushes the baby away and a jet of her milk sprays the night sky, producing the faint band of light known as the Milky Way.

In the astronomical literature, the capitalized word 'Galaxy' is used to refer to our galaxy, the Milky Way, to distinguish it from the billions of other galaxies. Sony VAIO VPCCW19FJ/W Battery

The term Milky Way first appeared in the English language in a story by Chaucer.

"See yonder, lo, the Galaxyë
Which men clepeth the Milky Wey,
For hit is whyt."

When William Herschel constructed his catalog of deep sky objects in 1786, he used the name spiral nebula for certain objects such as M31. Sony VAIO VPCCW1AFJ Battery

These would later be recognized as immense conglomerations of stars, when the true distance to these objects began to be appreciated, and they would be termed island universes. However, the word Universe was understood to mean the entirety of existence, so this expression fell into disuse and the objects instead became known as galaxies.Sony VAIO VPCCW1AHJ Battery

Observation history

The realization that we live in a galaxy, and that there were, in fact, many other galaxies, parallels discoveries that were made about the Milky Way and other nebulae in the night sky.Sony VAIO VPCCW1S1E Battery

The Milky Way

The Greek philosopher Democritus (450–370 BC) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.[17]Aristotle (384–322 BC), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large,Sony VAIO VPCCW1S1E/B Battery

numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions."[18] The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 AD) criticized this view, Sony VAIO VPCCW1S1E/L Battery

arguing that if the Milky Way were sublunary it should appear different at different times and places on the Earth, and that it should have parallax, which it does not. In his view, the Milky Way was celestial. This idea would be influential later in the Islamic world.Sony VAIO VPCCW1S1E/P Battery

The Arabian astronomer, Alhazen (965–1037), made the first attempt at observing and measuring the Milky Way's parallax,[20] and he thus "determined that because the Milky Way had no parallax, it was very remote from the Earth and did not belong to the atmosphere."Sony VAIO VPCCW1S1E/R Battery

The Persian astronomer Ab? Rayh?n al-B?r?n? (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars." The Andalusianastronomer Ibn Bajjah ("Avempace", d. 1138) proposed that the Milky Way was made up of many stars that almost touch one another and appear to be a continuous image due to the effect of refraction from sublunary material,Sony VAIO VPCCW1S1E/W Battery

citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near.[18] The Syrian-born Ibn Qayyim Al-Jawziyya (1292–1350) proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars".Sony VAIO VPCCW21FX/B Battery

Actual proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars.[26] In 1750 Thomas Wright, in his An original theory or new hypothesis of the Universe, Sony VAIO VPCCW21FX/L Battery

speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together bygravitational forces, akin to the solar system but on a much larger scale. The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk.Sony VAIO VPCCW21FX/R Battery

In a treatise in 1755,Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way.

The first attempt to describe the shape of the Milky Way and the position of the Sun in it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky.Sony VAIO VPCCW21FX/W Battery

He produced a diagram of the shape of the galaxy with the solar system close to the center.Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. Sony VAIO VPCCW26EC Battery

A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, Sony VAIO VPCCW26FX/B Battery

but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.[29]

Distinction from other nebulae

In the 10th century, the Persian astronomer, Abd al-Rahman al-Sufi (known in the West as Azophi), made the earliest recorded observation of the Andromeda Galaxy, describing it as a "small cloud".Sony VAIO VPCCW28EC Battery

Al-Sufi also identified the Large Magellanic Cloud, which is visible from Yemen, though not from Isfahan; it was not seen by Europeans until Magellan's voyage in the 16th century. These were the first galaxies other than the Milky Way to be observed from Earth. Al-Sufi published his findings in his Book of Fixed Stars in 964.Sony VAIO VPCCW28FJ/P Battery

In 1750 Thomas Wright, in his An original theory or new hypothesis of the Universe, speculated (correctly) that Milky Way was a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. In 1755 Immanuel Kant introduced the term "island universe" for these distant nebulae.Sony VAIO VPCCW28FJ/R Battery

Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae (celestial objects with a nebulous appearance), later followed by a larger catalog of 5,000 nebulae assembled by William Herschel.[27] In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae.Sony VAIO VPCCW28FJ/W Battery

He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.[34]

In 1912, Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine if they were made from chemicals that would be expected in a planetary system.Sony VAIO VPCCW29FJ/W Battery

However, Slipher discovered that the spiral nebulae had high red shifts, indicating that they were moving away at rate higher than the Milky Way's escape velocity. Thus they were not gravitationally bound to the Milky Way, and were unlikely to be a part of the galaxy.Sony VAIO VPCCW2AFJ Battery

In 1917, Heber Curtis had observed a nova S Andromedae within the "Great Andromeda Nebula" (Messier object M31). Searching the photographic record, he found 11 more novae. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred within our galaxy. Sony VAIO VPCCW2AHJ Battery

As a result he was able to come up with a distance estimate of 150,000 parsecs. He became a proponent of the so-called "island universes" hypothesis, which holds that spiral nebulae are actually independent galaxies. Sony VAIO VPCCW2S1E Battery

In 1920 the so-called Great Debate took place between Harlow Shapley and Heber Curtis, concerning the nature of the Milky Way, spiral nebulae, and the dimensions of the Universe. To support his claim that the Great Andromeda Nebula was an external galaxy,Sony VAIO VPCCW2S1E/B Battery

Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.[38]

The matter was conclusively settled in the early 1920s. In 1922, astronomer Ernst Öpik gave a distance determination which supported the theory that the Andromeda Nebula is indeed a distant extra-galactic object.Sony VAIO VPCCW2S1E/L Battery

Using the new 100 inch Mt. Wilson telescope, Edwin Hubble was able to resolve the outer parts of some spiral nebulae as collections of individual stars and identified some Cepheid variables, thus allowing him to estimate the distance to the nebulae: they were far too distant to be part of the Milky Way.Sony VAIO VPCCW2S1E/P Battery

In 1936 Hubble produced a classification system for galaxies that is used to this day, theHubble sequence.[41]

Modern research

In 1944, Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cm resulting from interstellar atomic hydrogen gas;this radiation was observed in 1951.Sony VAIO VPCCW2S1E/R Battery

The radiation allowed for much improved study of the Milky Way Galaxy, since it is not affected by dust absorption and its Doppler shift can be used to map the motion of the gas in the Galaxy. These observations led to the postulation of a rotating bar structure in the center of the Galaxy. With improved radio telescopes, hydrogen gas could also be traced in other galaxies.Sony VAIO VPCCW2S1E/W Battery

In the 1970s it was discovered in Vera Rubin's study of the rotation speed of gas in galaxies that the total visible mass (from the stars and gas) does not properly account for the speed of the rotating gas. This galaxy rotation problem is thought to be explained by the presence of large quantities of unseen dark matter.Sony VAIO VPCCW2S5C CN1 Battery

Beginning in the 1990s, the Hubble Space Telescope yielded improved observations. Among other things, it established that the missing dark matter in our galaxy cannot solely consist of inherently faint and small stars. The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, Sony VAIO VPCEA20 Battery

provided evidence that there are about 125 billion (1.25×1011) galaxies in the universe. Improved technology in detecting the spectra invisible to humans (radio telescopes, infrared cameras, and x-ray telescopes) allow detection of other galaxies that are not detected by Hubble. Sony VAIO VPCEB10 Battery

Particularly, galaxy surveys in the Zone of Avoidance (the region of the sky blocked by the Milky Way) have revealed a number of new galaxies.[48]

Types and morphology

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Sony VAIO VPCEB11FM Battery

Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) and activity in the core (in active galaxies).


The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated.Sony VAIO VPCEB11FM/BI Battery

These galaxies have an ellipsoidal profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little interstellar matter. Consequently these galaxies also have a low portion of open clusters and a reduced rate of new star formation.Sony VAIO VPCEB11FM/T Battery

Instead they are dominated by generally older, more evolved stars that are orbiting the common center of gravity in random directions. In this sense they have some similarity to the much smaller globular clusters.

The largest galaxies are giant ellipticals. Many elliptical galaxies are believed to form due to the interaction of galaxies, resulting in a collision and merger. Sony VAIO VPCEB11FM/WI Battery

They can grow to enormous sizes (compared to spiral galaxies, for example), and giant elliptical galaxies are often found near the core of large galaxy clusters.[50] Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.Sony VAIO VPCEB11FX Battery


Spiral galaxies consist of a rotating disk of stars and interstellar medium, along with a central bulge of generally older stars. Extending outward from the bulgeare relatively bright arms. In the Hubble classification scheme, spiral galaxies are listed as type S, followed by a letter (a, b, or c) that indicates the degree of tightness of the spiral arms and the size of the central bulge.Sony VAIO VPCEB11FX/BI Battery

An Sa galaxy has tightly wound, poorly defined arms and possesses a relatively large core region. At the other extreme, an Sc galaxy has open, well-defined arms and a small core region. A galaxy with poorly defined arms is sometimes referred to as a flocculent spiral galaxy; in contrast to the grand design spiral galaxy that has prominent and well-defined spiral arms.Sony VAIO VPCEB11FX/T Battery

In spiral galaxies, the spiral arms do have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars. Like the stars, the spiral arms rotate around the center, but they do so with constant angular velocity. Sony VAIO VPCEB11FX/WI Battery

The spiral arms are thought to be areas of high density matter, or "density waves".[53] As stars move through an arm, the space velocity of each stellar system is modified by the gravitational force of the higher density. (The velocity returns to normal after the stars depart on the other side of the arm.)Sony VAIO VPCEB11GX Battery

This effect is akin to a "wave" of slowdowns moving along a highway full of moving cars. The arms are visible because the high density facilitates star formation, and therefore they harbor many bright and young stars.

A majority of spiral galaxies have a linear, bar-shaped band of stars that extends outward to either side of the core, then merges into the spiral arm structure.Sony VAIO VPCEB11GX/BI Battery

In the Hubble classification scheme, these are designated by an SB, followed by a lower-case letter (a, b or c) that indicates the form of the spiral arms (in the same manner as the categorization of normal spiral galaxies). Bars are thought to be temporary structures that can occur as a result of a density wave radiating outward from the core, Sony VAIO VPCEB11GX/T Battery

or else due to a tidal interaction with another galaxy.[56] Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.[57]

Our own galaxy is a large disk-shaped barred-spiral galaxy[58] about 30 kiloparsecs in diameter and a kiloparsec in thickness. Sony VAIO VPCEB11GX/WI Battery

It contains about two hundred billion (2×1011)[59] stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.[60]

Other morphologies

Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. Sony VAIO VPCEB12FX Battery

An example of this is the ring galaxy, which possesses a ring-like structure of stars and interstellar medium surrounding a bare core. A ring galaxy is thought to occur when a smaller galaxy passes through the core of a spiral galaxy.[61] Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.Sony VAIO VPCEB12FX/BI Battery

A lenticular galaxy is an intermediate form that has properties of both elliptical and spiral galaxies. These are categorized as Hubble type S0, and they possess ill-defined spiral arms with an elliptical halo of stars.[63] (Barred lenticular galaxies receive Hubble classification SB0.)Sony VAIO VPCEB12FX/BIC Battery

In addition to the classifications mentioned above, there are a number of galaxies that can not be readily classified into an elliptical or spiral morphology. These are categorized as irregular galaxies. An Irr-I galaxy has some structure but does not align cleanly with the Hubble classification scheme.Sony VAIO VPCEB12FX/T Battery

Irr-II galaxies do not possess any structure that resembles a Hubble classification, and may have been disrupted.[64] Nearby examples of (dwarf) irregular galaxies include the Magellanic Clouds.


Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. Sony VAIO VPCEB14FX Battery

These galaxies are relatively small when compared with other galactic formations, being about one hundredth the size of the Milky Way, containing only a few billion stars. Ultra-compact dwarf galaxies have recently been discovered that are only 100 parsecs across.Sony VAIO VPCEB14FX/BI Battery

Many dwarf galaxies may orbit a single larger galaxy; the Milky Way has at least a dozen such satellites, with an estimated 300–500 yet to be discovered.[66]Dwarf galaxies may also be classified as elliptical, spiral, or irregular. Since small dwarf ellipticals bear little resemblance to large ellipticals, they are often called dwarf spheroidal galaxies instead.Sony VAIO VPCEB14FX/T Battery

A study of 27 Milky Way neighbors found that in all dwarf galaxies, the central mass is approximately 10 million solar masses, regardless of whether the galaxy has thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, Sony VAIO VPCEB14FX/WI Battery

and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.[67]


The average separation between galaxies within a cluster is a little over an order of magnitude larger than their diameter. Sony VAIO VPCEB15FM Battery

Hence interactions between these galaxies are relatively frequent, and play an important role in their evolution. Near misses between galaxies result in warping distortions due to tidal interactions, and may cause some exchange of gas and dust.Sony VAIO VPCEB15FM/BI Battery

Collisions occur when two galaxies pass directly through each other and have sufficient relative momentum not to merge. The stars within these interacting galaxies will typically pass straight through without colliding. However, the gas and dust within the two forms will interact. Sony VAIO VPCEB15FM/T Battery

This can trigger bursts of star formation as the interstellar medium becomes disrupted and compressed. A collision can severely distort the shape of one or both galaxies, forming bars, rings or tail-like structures.

At the extreme of interactions are galactic mergers. In this case the relative momentum of the two galaxies is insufficient to allow the galaxies to pass through each other. Sony VAIO VPCEB15FM/WI Battery

Instead, they gradually merge together to form a single, larger galaxy. Mergers can result in significant changes to morphology, as compared to the original galaxies. In the case where one of the galaxies is much more massive, however, the result is known as cannibalism. Sony VAIO VPCEB15FX Battery

In this case the larger galaxy will remain relatively undisturbed by the merger, while the smaller galaxy is torn apart. The Milky Way galaxy is currently in the process of cannibalizing theSagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy. Sony VAIO VPCEB15FX/BI Battery


Stars are created within galaxies from a reserve of cold gas that forms into giant molecular clouds. Some galaxies have been observed to form stars at an exceptional rate, known as a starburst. Should they continue to do so, however, they would consume their reserve of gas in a time frame lower than the lifespan of the galaxy.Sony VAIO VPCEB15FX/T Battery

Hence starburst activity usually lasts for only about ten million years, a relatively brief period in the history of a galaxy. Starburst galaxies were more common during the early history of the universe,[71] and, at present, still contribute an estimated 15% to the total star production rate.[72]Sony VAIO VPCEB15FX/WI Battery

Starburst galaxies are characterized by dusty concentrations of gas and the appearance of newly formed stars, including massive stars that ionize the surrounding clouds to create H II regions. These massive stars produce supernova explosions, resulting in expanding remnants that interact powerfully with the surrounding gas.Sony VAIO VPCEB16FX Battery

These outbursts trigger a chain reaction of star building that spreads throughout the gaseous region. Only when the available gas is nearly consumed or dispersed does the starburst activity come to an end.

Starbursts are often associated with merging or interacting galaxies.Sony VAIO VPCEB16FX/B Battery

The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity.[74]

Active nucleus

A portion of the galaxies we can observe are classified as active. That is, a significant portion of the total energy output from the galaxy is emitted by a source other than the stars, dust and interstellar medium.Sony VAIO VPCEB16FX/G Battery

The standard model for an active galactic nucleus is based upon an accretion disc that forms around a supermassive black hole (SMBH) at the core region. The radiation from an active galactic nucleus results from the gravitational energy of matter as it falls toward the black hole from the disc.Sony VAIO VPCEB16FX/L Battery

In about 10% of these objects, a diametrically opposed pair of energetic jets ejects particles from the core at velocities close to the speed of light. The mechanism for producing these jets is still not well understood.Sony VAIO VPCEB16FX/P Battery

Active galaxies that emit high-energy radiation in the form of x-rays are classified as Seyfert galaxies or quasars, depending on the luminosity. Blazars are believed to be an active galaxy with a relativistic jet that is pointed in the direction of the Earth. A radio galaxy emits radio frequencies from relativistic jets.Sony VAIO VPCEB16FX/W Battery

A unified model of these types of active galaxies explains their differences based on the viewing angle of the observer.[76]

Possibly related to active galactic nuclei (as well as starburst regions) are low-ionization nuclear emission-line regions (LINERs). The emission from LINER-type galaxies is dominated by weakly ionized elements.Sony VAIO VPCEB17FX Battery

Approximately one-third of nearby galaxies are classified as containing LINER nuclei.

Formation and evolution

The study of galactic formation and evolution attempts to answer questions regarding how galaxies formed and their evolutionary path over the history of the universe. Some theories in this field have now become widely accepted, but it is still an active area in astrophysics.Sony VAIO VPCEB17FX/B Battery


Current cosmological models of the early Universe are based on the Big Bang theory. About 300,000 years after this event, atoms of hydrogen and helium began to form, in an event called recombination. Nearly all the hydrogen was neutral (non-ionized) and readily absorbed light, and no stars had yet formed. Sony VAIO VPCEB17FX/G Battery

As a result this period has been called the "Dark Ages". It was from density fluctuations (or anisotropic irregularities) in this primordial matter that larger structures began to appear. As a result, masses of baryonic matter started to condense within cold dark matter halos.Sony VAIO VPCEB17FX/L Battery

These primordial structures would eventually become the galaxies we see today.

Evidence for the early appearance of galaxies was found in 2006, when it was discovered that the galaxy IOK-1 has an unusually high redshift of 6.96, corresponding to just 750 million years after the Big Bang and making it the most distant and primordial galaxy yet seen.Sony VAIO VPCEB17FX/P Battery

While some scientists have claimed other objects (such as Abell 1835 IR1916) have higher redshifts (and therefore are seen in an earlier stage of the Universe's evolution), IOK-1's age and composition have been more reliably established. The existence of such early protogalaxies suggests that they must have grown in the so-called "Dark Ages".Sony VAIO VPCEB17FX/W Battery

The detailed process by which such early galaxy formation occurred is a major open question in astronomy. Theories could be divided into two categories: top-down and bottom-up. In top-down theories (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years.Sony VAIO VPCEB190X Battery

In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.

Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. Sony VAIO VPCEB19FX Battery

These were composed almost entirely of hydrogen and helium, and may have been massive. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium.Sony VAIO VPCEB19GX Battery

This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.


Within a billion years of a galaxy's formation, key structures begin to appear. Globular clusters, the central supermassive black hole, and a galactic bulge of metal-poor Population II stars form. Sony VAIO VPCEB1AFX Battery

The creation of a supermassive black hole appears to play a key role in actively regulating the growth of galaxies by limiting the total amount of additional matter added.[88] During this early epoch, galaxies undergo a major burst of star formation.[89]

During the following two billion years, the accumulated matter settles into a galactic disc.Sony VAIO VPCEB1AFX/B Battery

A galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life.[91] This matter is mostly hydrogen and helium. The cycle of stellar birth and death slowly increases the abundance of heavy elements, eventually allowing the formation of planets.Sony VAIO VPCEB1AGX Battery

The evolution of galaxies can be significantly affected by interactions and collisions. Mergers of galaxies were common during the early epoch, and the majority of galaxies were peculiar in morphology.[93] Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. Sony VAIO VPCEB1AGX/BI Battery

However, gravitational stripping of the interstellar gas and dust that makes up the spiral arms produces a long train of stars known as tidal tails. Examples of these formations can be seen in NGC 4676[94] or the Antennae Galaxies.Sony VAIO VPCEB1BGX Battery

As an example of such an interaction, the Milky Way galaxy and the nearby Andromeda Galaxy are moving toward each other at about 130 km/s, and—depending upon the lateral movements—the two may collide in about five to six billion years. Sony VAIO VPCEB1BGX/BI Battery

Although the Milky Way has never collided with a galaxy as large as Andromeda before, evidence of past collisions of the Milky Way with smaller dwarf galaxies is increasing.

Such large-scale interactions are rare. As time passes, mergers of two systems of equal size become less common.Sony VAIO VPCEB1CGX Battery

Most bright galaxies have remained fundamentally unchanged for the last few billion years, and the net rate of star formation probably also peaked approximately ten billion years ago.

Future trends

At present, most star formation occurs in smaller galaxies where cool gas is not so depleted.Sony VAIO VPCEB1CGX/BI Battery

Spiral galaxies, like the Milky Way, only produce new generations of stars as long as they have densemolecular clouds of interstellar hydrogen in their spiral arms.Elliptical galaxies are already largely devoid of this gas, and so form no new stars. The supply of star-forming material is finite;Sony VAIO VPCEB1DGX Battery

once stars have converted the available supply of hydrogen into heavier elements, new star formation will come to an end.

The current era of star formation is expected to continue for up to one hundred billion years, and then the "stellar age" will wind down after about ten trillion to one hundred trillion years (1013–1014 years), Sony VAIO VPCEB1DGX/BI Battery

as the smallest, longest-lived stars in our astrosphere, tiny red dwarfs, begin to fade. At the end of the stellar age, galaxies will be composed of compact objects: brown dwarfs, white dwarfs that are cooling or cold ("black dwarfs"), neutron stars, and black holes.Sony VAIO VPCEB1EGX Battery

Eventually, as a result of gravitational relaxation, all stars will either fall into central supermassive black holes or be flung into intergalactic space as a result of collisions.

Larger-scale structures

Deep sky surveys show that galaxies are often found in relatively close association with other galaxies. Sony VAIO VPCEB1EGX/BI Battery

Solitary galaxies that have not significantly interacted with another galaxy of comparable mass during the past billion years are relatively scarce. Only about 5% of the galaxies surveyed have been found to be truly isolated; however, these isolated formations may have interacted and even merged with other galaxies in the past, and may still be orbited by smaller, satellite galaxies. Sony VAIO VPCEB1FGX Battery

Isolated galaxies can produce stars at a higher rate than normal, as their gas is not being stripped by other nearby galaxies.

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see Hubble's law). Sony VAIO VPCEB1FGX/BI Battery

Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early in the universe, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. Sony VAIO VPCEB1GGX Battery

This on-going merger process (as well as an influx of infalling gas) heats the inter-galactic gas within a cluster to very high temperatures, reaching 30–100 megakelvins.[103] About 70–80% of the mass in a cluster is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent of the matter in the form of galaxies.Sony VAIO VPCEB1GGX/BI Battery

Most galaxies in the universe are gravitationally bound to a number of other galaxies. These form a fractal-like hierarchy of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster, and these formations contain a majority of the galaxies (as well as most of the baryonic mass) in the universe.Sony VAIO VPCEB1HGX Battery

To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see Virial theorem). If there is insufficient kinetic energy, however, the group may evolve into a smaller number of galaxies through mergers.Sony VAIO VPCEB1HGX/BI Battery

Larger structures containing many thousands of galaxies packed into an area a few megaparsecs across are called clusters. Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the brightest cluster galaxy, which, over time, tidally destroys its satellite galaxies and adds their mass to its own.Sony VAIO VPCEB1JFX Battery

Superclusters contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the supercluster scale, galaxies are arranged into sheets and filaments surrounding vast empty voids.[109] Above this scale, the universe appears to be isotropic and homogeneous.Sony VAIO VPCEB1JFX/B Battery

The Milky Way galaxy is a member of an association named the Local Group, a relatively small group of galaxies that has a diameter of approximately one megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two galaxies.Sony VAIO VPCEB1JFX/G Battery

The Local Group itself is a part of a cloud-like structure within the Virgo Supercluster, a large, extended structure of groups and clusters of galaxies centered around the Virgo Cluster.

Multi-wavelength observation

After galaxies external to the Milky Way were found to exist, initial observations were made mostly using visible light. Sony VAIO VPCEB1JFX/L Battery

The peak radiation of most stars lies here, so the observation of the stars that form galaxies has been a major component of optical astronomy. It is also a favorable portion of the spectrum for observing ionized H II regions, and for examining the distribution of dusty arms.Sony VAIO VPCEB1JFX/P Battery

The dust present in the interstellar medium is opaque to visual light. It is more transparent to far-infrared, which can be used to observe the interior regions of giant molecular clouds and galactic cores in great detail. Infrared is also used to observe distant, red-shifted galaxies that were formed much earlier in the history of the universe. Sony VAIO VPCEB1JFX/W Battery

Water vapor and carbon dioxide absorb a number of useful portions of the infrared spectrum, so high-altitude or space-based telescopes are used for infrared astronomy.

The first non-visual study of galaxies, particularly active galaxies, was made using radio frequencies.Sony VAIO VPCEB1KGX Battery

The atmosphere is nearly transparent to radio between 5 MHz and 30 GHz. (The ionosphere blocks signals below this range.) Large radio interferometers have been used to map the active jets emitted from active nuclei. Radio telescopes can also be used to observe neutral hydrogen (via21 centimetre radiation), Sony VAIO VPCEB1KGX/B Battery

including, potentially, the non-ionized matter in the early universe that later collapsed to form galaxies.

Ultraviolet and X-ray telescopes can observe highly energetic galactic phenomena. An ultraviolet flare was observed when a star in a distant galaxy was torn apart from the tidal forces of a black hole.Sony VAIO VPCEB1KGX/W Battery

The distribution of hot gas in galactic clusters can be mapped by X-rays. The existence of super-massive black holes at the cores of galaxies was confirmed through X-ray astronomy.

Galactic clusters are gravitationally bound large-scale structures of multiple galaxies. Sony VAIO VPCEB1LFX Battery

The evolution of these aggregates is determined by time and manner of formation and the process of how their structures and constituents have been changing with time. Gamow (1952) and Weizscker (1951) showed that the observed rotations of galaxies are important for cosmology. Sony VAIO VPCEB1LFX/BI Battery

They postulated that the rotation of galaxies might be a clue of physical conditions under which these systems formed. Thus, understanding the distribution of spatial orientations of the spin vectors of galaxies is critical to understanding the origin of the angular momenta of galaxies.Sony VAIO VPCEB1LFX/WI Battery

There are mainly three scenarios for the origin of galaxy clusters and superclusters. These models are based on different assumptions of the primordial conditions, so they predict different spin vector alignments of the galaxies. The three hypotheses are the pancake model, the hierarchy model, and the primordial vorticity theory. Sony VAIO VPCEB1MFX Battery

The three are mutually exclusive as they produce contradictory predictions. However, the predictions made by all three theories are based on the precepts of cosmology. Thus, these models can be tested using a database with appropriate methods of analysis.Sony VAIO VPCEB1MFX/BI Battery


A galaxy is a large gravitational aggregation of stars, dust, gas, and an unknown component termed dark matter. The Milky Way Galaxy is only one of billions of galaxies in the known universe. Galaxies are classified into spirals, ellipticals, irregular, and peculiar.Sony VAIO VPCEB1NFX Battery

Sizes can range from only a few thousand stars (dwarf irregulars) to 1013 stars in giant ellipticals. Elliptical galaxies are spherical or elliptical in appearance. Spiral galaxies range from S0, the lenticular galaxies, to Sb, which have a bar across the nucleus, to Sc galaxies which have strong spiral arms. Sony VAIO VPCEB1NFX/B Battery

In total count, ellipticals amount to 13%, S0 to 22%, Sa, b, c galaxies to 61%, irregulars to 3.5% and peculiars to 0.9%.

At the center of the most galaxies is a high concentration of older stars. This portion of a galaxy is called the nuclear bulge. Beyond the nuclear bulge lies a large disc containing young, hot stars, called the disk of the galaxy. Sony VAIO VPCEB1NFX/L Battery

There is a morphological separation: Ellipticals are most common in clusters of galaxies, and typically the center of a cluster is occupied by a giant elliptical. Spirals are most common in the field, i.e., not in clusters.

Primordial Vorticity Model

The primordial vorticity theory predicts that the spin vectors of galaxies are distributed primarily perpendicular to the cluster plane. Sony VA


The primordial vorticity is called top-down scenario. Sometimes it is also called turbulence model. In the turbulence scenario, first flattened rotating proto-clusters formed due to cosmic vorticity in the early universe. Subsequent density and pressure fluctuations caused galaxies to form.Sony VAIO VPCEB1NFX/W Battery

The idea that galaxy formation is initiated by primordial turbulence has a long history. Ozernoy (1971, 1978) proposes that galaxies form from high-density regions behind the shocks produced by turbulence. According to the primordial vorticity theory, the presence of large chaotic velocities generates turbulence, which, in turn, produces density and pressure fluctuations.Sony VAIO VPCEB1PFX Battery

Density fluctuations on the scale of clusters of galaxies could be gravitationally bound, but galactic mass fluctuations are always unbound. Galaxies form when unbound galactic mass eddies, expanding faster than their bound cluster background. So forming galaxies collide with each other as clusters start to recollapse. Sony VAIO VPCEB1PFX/B Battery

These collisions produce shocks and high-density proto-galaxies at the eddy interfaces. As clusters recollapse, the system of galaxies undergoes a violent collective relaxation.

Pancake Model

The pancake model was first proposed in the 1970s by Yakob B. Zel'dovich at the Institute of Applied Mathematics in Moscow.Sony VAIO VPCEB1QGX Battery

The pancake model predicts that the spin vectors of galaxies tend to lie within the cluster plane. In the pancake scenario, formation of clusters took place first and it was followed by their fragmentation into galaxies due to adiabatic fluctuations. According to the non-linear gravitational instability theory, Sony VAIO VPCEB1QGX/BI Battery

a growth of small inhomogeneities leads to the formation of thin, dense, and gaseous condensations that are called `pancakes'. These condensations are compressed and heated to high temperatures by shock waves causing them to quickly fragment into gas clouds. The later clumping of these clouds results in the formation of galaxies and their clusters.Sony VAIO VPCEB1RGX Battery

Thermal, hydrodynamic, and gravitational instabilities arise during the course of evolution. It leads to the fragmentation of gaseous proto-clusters and, subsequently, clustering of galaxies takes place. The pancake scheme follows three simultaneous processes: first, gas cools and new clouds of cold gas form;Sony VAIO VPCEB1RGX/BI Battery

secondly, these clouds cluster to form galaxies; and thirdly, the forming galaxies and, to an extent, single clouds cluster together to form a cluster of galaxies.

Hierarchy Model

According to the hierarchy model, the directions of the spin vectors should be distributed randomly.Sony VAIO VPCEB20 Battery

In hierarchy model, galaxies were first formed and then they obtained their angular momenta by tidal force while they were gathering gravitationally to form a cluster. Those galaxies grow by subsequent merging of proto-galactic condensations or even by merging of already fully formed galaxies. Sony VAIO VPCEC20 Battery

In this scheme, one could imagine that large irregularities like galaxies grew under the influence of gravities from small imperfections in the early universe.

The angular momentum transferred to a developing proto-galaxy by the gravitational interaction of the quadrupole moment of the system with the tidal field of the matter.Sony VAIO VPCEE20 Battery

Observational astronomy is a division of the astronomical science that is concerned with getting data, in contrast with theoretical astrophysics which is mainly concerned with finding out the measurable implications of physical models. It is the practice of observing celestial objects by using telescopesand other astronomical apparatus.Sony VAIO VPCEF20 Battery

As a science, astronomy is somewhat hindered in that direct experiments with the properties of the distant universe are not possible. However, this is partly compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined. Sony VAIO VPCF112FX/B Battery

This allows for observational data to be plotted on graphs, and general trends recorded. Nearby examples of specific phenomena, such as variable stars, can then be used to infer the behavior of more distant representatives. Those distant yardsticks can then be employed to measure other phenomena in that neighborhood, including the distance to a galaxy.Sony VAIO VPCF115FG/B Battery


Galileo Galilei was the first person known to have turned a telescope to the heavens and to record what he saw. Since that time, observational astronomy has made steady advances with each improvement in telescope technology.Sony VAIO VPCF116FGBI Battery

A traditional division of observational astronomy is given by the region of the electromagnetic spectrum observed:

  • Optical astronomy is the part of astronomy that uses optical components (mirrors, lenses and solid-state detectors) to observe light from near infrared to near ultraviolet wavelengths. Sony VAIO VPCF117FJ/W Battery
  • Visible-light astronomy (using wavelengths that can be detected with the eyes, about 400 - 700 nm) falls in the middle of this range.
  • Infrared astronomy deals with the detection and analysis of infrared radiation (this typically refers to wavelengths longer than the detection limit of silicon solid-state detectors, about 1 ?m wavelength). Sony VAIO VPCF117HG/BI Battery
  • The most common tool is the reflecting telescope but with a detector sensitive to infrared wavelengths. Space telescopes are used at certain wavelengths where the atmosphere is opaque, or to eliminate noise (thermal radiation from the atmosphere).
  • Radio astronomy detects radiation of millimetre to dekametre wavelength.Sony VAIO VPCF118FJ/W Battery
  • The receivers are similar to those used in radio broadcast transmission but much more sensitive. See also Radio telescopes.
  • High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, and extreme UV astronomy, as well as studies of neutrinos and cosmic rays.Sony VAIO VPCF119FC Battery

Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is relatively transparent at the wavelengths being detected. Observatories are usually located at high altitudes so as to minimise the absorption and distortion caused by the Earth's atmosphere.Sony VAIO VPCF119FC/BI Battery

Some wavelengths of infrared light are heavily absorbed by water vapor, so many infrared observatories are located in dry places at high altitude, or in space.

The atmosphere is opaque at the wavelengths used by X-ray astronomy, gamma-ray astronomy,Sony VAIO VPCF119FJ/BI Battery

UV astronomy and (except for a few wavelength "windows") far infrared astronomy, so observations must be carried out mostly from balloons or space observatories. Powerful gamma rays can, however be detected by the large air showers they produce, and the study of cosmic rays is a rapidly expanding branch of astronomy.Sony VAIO VPCF11AFJ Battery

Optical telescopes

For much of the history of observational astronomy, almost all observation was performed in the visual spectrum with optical telescopes. Sony VAIO VPCF11AGJ Battery

While the Earth's atmosphere is relatively transparent in this portion of the electromagnetic spectrum, most telescope work is still dependent on seeing conditions and air transparency, and is generally restricted to the night time. The seeing conditions depend on the turbulence and thermal variations in the air. Sony VAIO VPCF11AHJ Battery

Locations that are frequently cloudy or suffer from atmospheric turbulence limit the resolution of observations. Likewise the presence of the full Moon can brighten up the sky with scattered light, hindering observation of faint objects.Sony VAIO VPCF11JFX/B Battery

For observation purposes, the optimal location for an optical telescope is undoubtedly in outer space. There the telescope can make observations without being affected by the atmosphere. However, at present it remains costly to lift telescopes into orbit. Sony VAIO VPCF11M1E Battery

Thus the next best locations are certain mountain peaks that have a high number of cloudless days and generally possess good atmospheric conditions (with good seeing conditions). The peaks of the islands of Mauna Kea, Hawaii and La Palma possess these properties, Sony VAIO VPCF11M1E/H Battery

as to a lesser extent do inland sites such as Llano de Chajnantor,Paranal, Cerro Tololo and La Silla in Chile. These observatory locations have attracted an assemblage of powerful telescopes, totalling many billion US dollars of investment.

The darkness of the night sky is an important factor in optical astronomy. Sony VAIO VPCF11MFX/B Battery

With the size of cities and human populated areas ever expanding, the amount of artificial light at night has also increased. These artificial lights produce a diffuse background illumination that makes observation of faint astronomical features very difficult without special filters. Sony VAIO VPCF11S1E Battery

In a few locations such as the state of Arizonaand in the United Kingdom, this has led to campaigns for the reduction of light pollution. The use of hoods around street lights not only improves the amount of light directed toward the ground, but also helps reduce the light directed toward the sky.Sony VAIO VPCF11S1E/B Battery

Atmospheric effects (astronomical seeing) can severely hinder the resolution of a telescope. Without some means of correcting for the blurring effect of the shifting atmosphere, telescopes larger than about 15-20 cm in aperture can not achieve their theoretical resolution at visible wavelengths. Sony VAIO VPCF11Z1E Battery

As a result, the primary benefit of using very large telescopes has been the improved light-gathering capability, allowing very faint magnitudes to be observed. However the resolution handicap has begun to be overcome by adaptive optics, speckle imaging and interferometric imaging, as well as the use of space telescopes.Sony VAIO VPCF11Z1E/BI Battery

Astronomers have a number of observational tools that they can use to make measurements of the heavens. For objects that are relatively close to the Sun and Earth, direct and very precise position measurements can be made against a more distant (and thereby nearly stationary) background. Sony VAIO VPCF11ZHJ Battery

Early observations of this nature were used to develop very precise orbital models of the various planets, and to determine their respective masses and gravitational perturbations. Such measurements led to the discovery of the planets Uranus, Neptune, and (indirectly) Pluto.Sony VAIO VPCF127HGBI Battery

They also resulted in an erroneous assumption of a fictional planet Vulcan within the orbit of Mercury (but the explanation of the precession of Mercury's orbit by Einstein is considered one of the triumphs of his general relativity theory).Sony VAIO VPCF137HG/BI Battery

Other instruments

In addition to examination of the universe in the optical spectrum, astronomers have increasingly been able to acquire information in other portions of the electromagnetic spectrum. The earliest such non-optical measurements were made of the thermal properties of the Sun. Sony VAIO VPC-P111KX/B Battery

Instruments employed during a solar eclipse could be used to measure the radiation from the corona.

With the discovery of radio waves, radio astronomy began to emerge as a new discipline in astronomy. Sony VAIO VPC-P111KX/D Battery

The long wavelengths of radio waves required much larger collecting dishes in order to make images with good resolution, and later led to the development of the multi-dish interferometer for making high-resolution aperture synthesis radio images (or "radio maps"). Sony VAIO VPC-P111KX/G Battery

The development of the microwave horn receiver led to the discovery of the microwave background radiation associated with the big bang.

Radio astronomy has continued to expand its capabilities, even using radio astronomy satellites to produce interferometers with baselines much larger than the size of the Earth. Sony VAIO VPC-P111KX/P Battery

However, the ever-expanding use of the radio spectrum for other uses is gradually drowning out the faint radio signals from the stars. For this reason, in the future radio astronomy might be performed from shielded locations, such as the far side of the Moon.Sony VAIO VPC-P111KX/W Battery

The last part of the twentieth century saw rapid technological advances in astronomical instrumentation. Optical telescopes were growing ever larger, and employing adaptive optics to partly negate atmospheric blurring. New telescopes were launched into space, and began observing the universe in the infrared, Sony VAIO VPC-P112KX/B Battery

ultraviolet, x-ray, and gamma ray parts of the electromagnetic spectrum, as well as observing cosmic rays. Interferometer arrays produced the first extremely high-resolution images using aperture synthesis at radio, infrared and optical wavelengths. Sony VAIO VPC-P112KX/D Battery

Orbiting instruments such as theHubble Space Telescope produced rapid advances in astronomical knowledge, acting as the workhorse for visible-light observations of faint objects. New space instruments under development are expected to directly observe planets around other stars, perhaps even some Earth-like worlds.Sony VAIO VPC-P112KX/G Battery

In addition to telescopes, astronomers have begun using other instruments to make observations. Huge underground tanks have been built to detect neutrino emissions from the Sun and supernovae.Gravity wave detectors are being designed that may capture events such as collisions of massive objects such as neutron stars.Sony VAIO VPC-P112KX/P Battery

Robotic spacecraft are also being increasingly used to make highly detailed observations of planets within the solar system, so that the field of planetary science now has significant cross-over with the disciplines of geology and meteorology.Sony VAIO VPC-P112KX/W Battery

Observation tools

The key instrument of nearly all modern observational astronomy is the telescope. This serves the dual purposes of gathering more light so that very faint objects can be observed, and magnifying the image so that small and distant objects can be observed. Sony VAIO VPCP113KX/B Battery

For optical astronomy, the optical components used in a telescope have very exacting requirements which require great precision in their construction. Typical requirements for grinding and polishing a curved mirror, for example, require the surface to be within a fraction of a wavelength of light of a particular conic shape. Sony VAIO VPC-P113KX/B Battery

Many modern "telescopes" actually consist of arrays of telescopes working together to provide higher resolution through aperture synthesis.

Large telescopes are housed in domes, both to protect them from the weather and to stabilize the environmental conditions. Sony VAIO VPCP113KX/D Battery

For example, if the temperature is different from one side of the telescope to the other, the shape of the structure will change, due to thermal expansion, pushing optical elements out of position, and affecting the image. For this reason, the domes are usually bright white (titanium dioxide) or unpainted metal. Sony VAIO VPC-P113KX/D Battery

Domes are often opened around sunset, long before observing can begin, so that air can circulate and bring the entire telescope to the same temperature as the surroundings. In order to prevent wind-buffet or other vibrations affecting observations, Sony VAIO VPCP113KX/G Battery

it is standard practice to mount the telescope on a concrete pier whose foundations are entirely separate from those of the surrounding dome/building.

In order to do almost any scientific work, telescopes must keep track of objects as they wheel across the visible sky. Sony VAIO VPC-P113KX/G Battery

In other words, they must smoothly compensate for the rotation of the Earth. Until the advent of computer controlled drive mechanisms, the standard solution was some form of equatorial mount, and for small telescopes this is still the norm. Sony VAIO VPCP113KX/P Battery

However, this is a structurally poor design and becomes more and more cumbersome as the diameter and weight of the telescope increases. The world's largest equatorial mounted telescope is the 200 inch (5.1 m) Hale Telescope, whereas recent 8-10 m telescopes use the structurally better Altazimuth mount, and are actually physically smaller than the Hale, Sony VAIO VPC-P113KX/P Battery

despite the larger mirrors. As of 2006, there are design projects underway for gigantic alt-az telescopes: the Thirty Metre Telescope [1], and the 100 m diameter Overwhelmingly Large Telescope

Amateur astronomers use such instruments as the Newtonian reflector, the Refractor and the increasingly popular Maksutov telescope.Sony VAIO VPCP113KX/W Battery

The photograph has served a critical role in observational astronomy for over a century, but in the last 30 years it has been largely replaced for imaging applications by digital sensors such as CCDs andCMOS chips. Specialist areas of astronomy such as photometry and interferometry have utilised electronic detectors for a much longer period of time. Sony VAIO VPC-P113KX/W Battery

Astrophotography uses specialised photographic film (or usually a glass plate coated with photographic emulsion), but there are a number of drawbacks, particularly a low quantum efficiency, of the order of 3%, whereas CCDs can be tuned for a QE >90% in a narrow band. Sony VAIO VPC-P114KX/B Battery

Almost all modern telescope instruments are electronic arrays, and older telescopes have been either been retrofitted with these instruments or closed down. Glass plates are still used in some applications, such as surveying, because the resolution possible with a chemical film is much higher than any electronic detector yet constructed.Sony VAIO VPC-P114KX/D Battery

Prior to the invention of photography, all astronomy was done with the naked eye. However, even before films became sensitive enough, scientific astronomy moved entirely to film, because of the overwhelming advantages:Sony VAIO VPC-P114KX/G Battery

  • The human eye discards what it sees from split-second to split-second, but photographic film gathers more and more light for as long as the shutter is open.
  • The resulting image is permanent, so many astronomers can use the same data.Sony VAIO VPC-P114KX/P Battery
  • It is possible to see objects as they change over time (SN 1987A is a spectacular example).

The blink comparator is an instrument that is used to compare two nearly identical photographs made of the same section of sky at different points in time. The comparator alternates illumination of the two plates, and any changes are revealed by blinking points or streaks. This instrument has been used to find asteroids, comets, and variable stars.Sony VAIO VPC-P114KX/W Battery

The position or cross-wire micrometer is an implement that has been used to measure double stars. This consists of a pair of fine, movable lines that can be moved together or apart. The telescope lens is lined up on the pair and oriented using position wires that lie at right angles to the star separation. Sony VAIO VPCP115JC Battery

The movable wires are then adjusted to match the two star positions. The separation of the stars is then read off the instrument, and their true separation determined based on the magnification of the instrument.

A vital instrument of observational astronomy is the spectrograph. Sony VAIO VPCP115JC/B Battery

The absorption of specific wavelengths of light by elements allows specific properties of distant bodies to be observed. This capability has resulted in the discovery of the element of helium in the Sun's emission spectrum, and has allowed astronomers to determine a great deal of information concerning distant stars, galaxies, and other celestial bodies. Sony VAIO VPCP115JC/D Battery

Doppler shift (particularly "redshift") of spectra can also be used to determine the radial motion or distance with respect to the Earth.

Early spectrographs employed banks of prisms that would split the light into a broad spectrum. Later the grating spectrograph was developed, which reduced the amount of light loss compared to prisms and provided higher spectral resolution. Sony VAIO VPCP115JC/G Battery

The spectrum can be photographed in a long exposure, allowing the spectrum of faint objects (such as distant galaxies) to be measured.

Stellar photometry came into use in 1861 as a means of measuring stellar colors. Sony VAIO VPCP115JC/P Battery

This technique measured the magnitude of a star at specific frequency ranges, allowing a determination of the overall color, and therefore temperature of a star. By 1951 an internationally standardized system of UBV-magnitudes (Ultraviolet-Blue-Visual) was adopted.Sony VAIO VPCP115JC/W Battery

Photoelectric photometry using the CCD is now frequently used to make observations through a telescope. These sensitive instruments can record the image nearly down to the level of individualphotons, and can be designed to view in parts of the spectrum that are invisible to the eye. Sony VAIO VPCP115KG Battery

The ability to record the arrival of small numbers of photons over a period of time can allow a degree of computer correction for atmospheric effects, sharpening up the image. Multiple digital images can also be combined to further enhance the image.Sony VAIO VPCP116KG Battery

When combined with the adaptive optics technology, image quality can approach the theoretical resolution capability of the telescope.

Filters are used to view an object at particular frequencies or frequency ranges. Multilayer film filters can provide very precise control of the frequencies transmitted and blocked, so that, for example, Sony VAIO VPC-P116KX/B Battery

objects can be viewed at a particular frequency emitted only by excited hydrogen atoms. Filters can also be used to partially compensate for the effects of light pollution by blocking out unwanted light.Polarization filters can also be used to determine if a source is emitting polarized light, and the orientation of the polarization.Sony VAIO VPC-P116KX/D Battery


Astronomers observe a wide range of astronomical sources, including high-redshift galaxies, AGNs, the afterglow from the Big Bang and many different types of stars and protostars.

A variety of data can be observed for each object. Sony VAIO VPC-P116KX/G Battery

The position coordinates locate the object on the sky using the techniques of spherical astronomy, and the magnitude determines its brightness as seen from the Earth. The relative brightness in different parts of the spectrum yields information about the temperature and physics of the object. Sony VAIO VPC-P116KX/P Battery

Photographs of the spectra allow the chemistry of the object to be examined.

Parallax shifts of a star against the background can be used to determine the distance, out to a limit imposed by the resolution of the instrument. Sony VAIO VPC-P116KX/W Battery

The radial velocity of the star and changes in its position over time (proper motion) can be used to measure its velocity relative to the Sun. Variations in the brightness of the star give evidence of instabilities in the star's atmosphere, or else the presence of an occulting companion. Sony VAIO VPCP118JC Battery

The orbits of binary stars can be used to measure the relative masses of each companion, or the total mass of the system. Spectroscopic binaries can be found by observing doppler shifts in the spectrum of the star and its close companion.Sony VAIO VPCP118JC/B Battery

Stars of identical masses that are formed at the same time and under the similar conditions will typically have nearly identical observed properties. Observing a mass of closely associated stars, such as in a globular cluster, allows data to be assembled about the distribution of stellar types. These tables can then be used to infer the age of the association.Sony VAIO VPCP118JC/P Battery

For distant galaxies and AGNs observations are made of the overall shape and properties of the galaxy, as well as the groupings in which they are found. Observations of certain types of variable starsand supernovae of known luminosity, called standard candles, in other galaxies allows the inference of the distance to the host galaxy. Sony VAIO VPCP118JC/W Battery

The expansion of space causes the spectra of these galaxies to be shifted, depending on the distance, and modified by the doppler effect of the galaxy's radial velocity. Both the size of the galaxy and its redshift can be used to infer something about the distance of the galaxy. Sony VAIO VPC-P118KX/B Battery

Observations of large numbers of galaxies are referred to as redshift surveys, and are used to model the evolution of galaxy forms.

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning,Sony VAIO VPC-P118KX/D Battery

the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. It is one of the most active research areas in astrophysics.Sony VAIO VPC-P118KX/G Battery

Galaxy formation is hypothesized to occur, from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model for this that is in general agreement with observed phenomena is the ? Cold Dark Matter cosmology; Sony VAIO VPC-P118KX/P Battery

that is to say that clustering and merging is how galaxies gain in mass, and can also determine their shape and structure.

Formation of the first galaxies

After the Big Bang, the universe, for a time, was remarkably homogeneous, as can be observed in the Cosmic Microwave Background or CMB (the fluctuationsof which are less than one part in one hundred thousand). Sony VAIO VPC-P118KX/W Battery

There was little-to-no structure in the universe, and thus no galaxies. Therefore we must ask how the smoothly distributed universe of the CMB became the clumpy universe we see today.

The most accepted theory of how these structures came to be is that all the structure we observe today was formed as a consequence of the growth of theprimordial fluctuations, Sony VAIO VPCP119JC Battery

which are small changes in the density of the universe in a confined region. As the universe cooled clumps of dark matter began to condense, and within them gas began to condense. The primordial fluctuations gravitationally attracted gas and dark matter to the denser areas, and thus the seeds that would later become galaxies were formed.Sony VAIO VPCP119JC/BI Battery

These structures constituted the first galaxies. At this point the universe was almost exclusively composed of hydrogen, helium, and dark matter. Soon after the first proto-galaxies formed the hydrogen and helium gas within them began to condense and make the first stars. Sony VAIO VPCP11S1E/B Battery

Thus the first galaxies were then formed. In 2007 the Keck telescope, a team from California Institute of Technology found six star forming galaxies about 13.2 billion light years (light travel distance) away and therefore created when the universe was only 500 million years old.Sony VAIO VPC-P11S1E/B Battery

The discovery of a galaxy more than 13 billion years old, was reported in January 2011 which existed only 480 million years after the Big Bang.

The universe was very violent in its early epochs, and galaxies grew quickly, evolving by accretion of smaller mass galaxies. Sony VAIO VPCP11S1E/D Battery

The result of this process is left imprinted on the distribution of galaxies in the nearby universe (see image of 2dF Galaxy Redshift Survey). Galaxies are not isolated objects in space, but rather galaxies in the universe are distributed in a great cosmic web of filaments. Sony VAIO VPC-P11S1E/D Battery

The locations where the filaments meet are dense clusters of galaxies, that began as the small fluctuations to the universe. Hence the distribution of galaxies is closely related to the physics of the early universe.

Despite its many successes, this picture is not sufficient to explain the variety of structure we see in galaxies.Sony VAIO VPCP11S1E/G Battery

Galaxies come in a variety of shapes, from round featureless elliptical galaxies to the pancake-flat spiral galaxies.

Commonly observed properties of galaxies

Some notable observed features of galaxy structure (including our own Milky Way) that astronomers wish to explain with galactic formation theories include (but are certainly not limited to) the following:Sony VAIO VPC-P11S1E/G Battery

  • Spiral galaxies and the Galactic disk are quite thin, dense, and rotate very fast. The Milky Way disk is 100 times longer than it is thick.
  • The majority of mass in galaxies is made up of dark matter, a substance which is not directly observable, and does not interact through any means except gravity.Sony VAIO VPCP11S1E/P Battery
  • Halo stars are typically much older and have much lower metallicities (that is to say they are almost exclusively composed of hydrogen and helium) than disk stars.
  • Many disk galaxies have a puffed up outer disk (often called the "thick disk") that is composed of old stars.Sony VAIO VPCP11S1E/W Battery
  • Globular clusters are typically old and metal-poor as well, but there are a few which are not nearly as metal-poor as most, and/or have some younger stars. Some stars in globular clusters appear to be as old as the universe itself (by entirely different measurement and analysis methods).Sony VAIO VPC-P11S1E/W Battery
  • High Velocity Clouds, clouds of neutral hydrogen are "raining" down on the galaxy, and presumably have been from the beginning (these would be the necessary source of a gas disk from which the disk stars formed).
  • Galaxies come in a great variety of shapes and sizes (see the Hubble Sequence) from giant featureless blobs of old stars (called elliptical galaxies) to thin disks with gas and stars arranged in highly ordered spirals.Sony VAIO VPCP11Z9E/B Battery
  • The majority of giant galaxies contain a supermassive black hole in their centers, ranging in mass from millions to billions of times the mass of our sun. The black hole mass is tied to properties of the galaxy that hosts it.Sony VAIO VPCS111FM/S Battery
  • Many of the properties of galaxies (including the galaxy color-magnitude diagram) indicate that there are fundamentally two types of galaxies. These groups divide into blue-star forming galaxies that are more like spiral types, and red-nonstar forming galaxies that are more like elliptical galaxies.Sony VAIO VPCS115EC Battery

The formation of disk galaxies

The key properties of disk galaxies, which are also commonly called spiral galaxies, is that they are very thin, rotate rapidly, and often show spiral structure. One of the main challenges to galaxy formation is the great number of thin disk galaxies in the local universe. Sony VAIO VPCS115FG Battery

The problem is that disks are very fragile, and mergers with other galaxies can quickly destroy thin disks.

Olin Eggen, Donald Lynden-Bell, and Allan Sandage[2] in 1962, proposed a theory that disk galaxies form through a monolithic collapse of a large gas cloud. Sony VAIO VPCS117GG Battery

As the cloud collapses the gas settles into a rapidly rotating disk. Known as a top-down formation scenario, this theory is quite simple yet no longer widely accepted because observations of the early universe strongly suggest that objects grow from bottom-up (i.e. smaller objects merging to form larger ones). Sony VAIO VPCS117GGB Battery

It was first proposed by Leonard Searle and Robert Zinn[3] that galaxies form by the coalescence of smaller progenitors.

More recent theories include the clustering of dark matter halos in the bottom-up process. Essentially early on in the universe galaxies were composed mostly of gas and dark matter, and thus, there were fewer stars. Sony VAIO VPCS118EC Battery

As a galaxy gained mass (by accreting smaller galaxies) the dark matter stays mostly on the outer parts of the galaxy. This is because the dark matter can only interact gravitationally, and thus will not dissipate. The gas, however, can quickly contract, and as it does so it rotates faster, until the final result is a very thin, very rapidly rotating disk.Sony VAIO VPCS119FJ/B Battery

Astronomers do not currently know what process stops the contraction. In fact, theories of disk galaxy formation are not successful at producing the rotation speed and size of disk galaxies. It has been suggested that the radiation from bright newly formed stars, or from an active galactic nuclei can slow the contraction of a forming disk.Sony VAIO VPCS119GC Battery

It has also been suggested that the dark matter halo can pull the galaxy, thus stopping disk contraction.

In recent years, a great deal of focus has been put on understanding merger events in the evolution of galaxies.Sony VAIO VPCS11AFJ Battery

Our own galaxy (the Milky Way) has a tinysatellite galaxy (the Sagittarius Dwarf Elliptical Galaxy) which is currently gradually being ripped up and "eaten" by the Milky Way. It is thought these kinds of events may be quite common in the evolution of large galaxies. The Sagittarius dwarf galaxy is orbiting our galaxy at almost a right angle to the disk.Sony VAIO VPCS11AGJ Battery

It is currently passing through the disk; stars are being stripped off of it with each pass and joining the halo of our galaxy. There are other examples of these minor accretion events, and it is likely a continual process for many galaxies. Such mergers provide "new" gas, stars, and dark matter to galaxies.Sony VAIO VPCS11AHJ Battery

Evidence for this process is often observable as warps or streams coming out of galaxies.

The Lambda-CDM model of galaxy formation underestimates the number of thin disk galaxies in the universe.[4] The reason is that these galaxy formation models predict a large number of mergers.Sony VAIO VPCS11AVJ Battery

If disk galaxies merge with another galaxy of comparable mass (at least 15 percent of its mass) the merger will likely destroy, or at a minimum greatly disrupt the disk, yet the resulting galaxy is not expected to be a disk galaxy. While this remains an unsolved problem for astronomers, Sony VAIO VPCS11J7E/B Battery

it does not necessarily mean that the Lambda-CDM model is completely wrong, but rather that it requires further refinement to accurately reproduce the population of galaxies in the universe.

Galaxy mergers and the formation of elliptical galaxies

The most massive galaxies in the sky are giant elliptical galaxies. Sony VAIO VPCS11M1E/W Battery

Their stars are on orbits that are randomly oriented within the galaxy (i.e. they are not rotating like disk galaxies). They are composed of old stars and have little to no dust. All elliptical galaxies probed so far have supermassive black holes in their center, and the mass of these black holes is correlated with the mass of the elliptical galaxy. Sony VAIO VPCS11V9E Battery

They are also correlated to a property called sigma which is the speed of the stars at the far edge of the elliptical galaxies. Elliptical galaxies do not have disks around them, although some bulges of disk galaxies look similar to elliptical galaxies. One is more likely to find elliptical galaxies in more crowded regions of the universe (such as galaxy clusters).Sony VAIO VPCS11V9E/B Battery

Astronomers now see elliptical galaxies as some of the most evolved systems in the universe. It is widely accepted that the main driving force for the evolution of elliptical galaxies is mergers of smaller galaxies. These mergers can be extremely violent; galaxies often collide at speeds of 500 kilometers per second.Sony VAIO VPCS11X9E/B Battery

Many galaxies in the universe are gravitationally bound to other galaxies, that is to say they will never escape the pull of the other galaxy. If the galaxies are of similar size, the resultant galaxy will appear similar to neither of the two galaxies merging.[5] An image of an ongoing merger of equal sized disk galaxies is shown left.Sony VAIO VPCS123FGB Battery

During the merger, stars and dark matter in each galaxy become affected by the approaching galaxy. Toward the late stages of the merger, thegravitational potential, the shape of galaxy, begins changing so quickly that star orbits are greatly affected, and lose any memory of their previous orbit. This process is called violent relaxation.Sony VAIO VPCS125EC Battery

Thus if two disk galaxies collide, they begin with their stars in an orderly rotation in the plane of the disk. During the merger, the ordered motion is transformed into random energy. And the resultant galaxy is dominated by stars that orbit the galaxy in a complex, and random, web of orbits. Sony VAIO VPCS128EC Battery

And this is what we see in elliptical galaxies, stars on random unordered orbits.

Mergers are also locations of extreme amounts of star formation.[7] During a merger some galaxies can make thousands of solar masses of new stars each year , which is large compared to our galaxy which makes about 10 new stars each year.Sony VAIO VPCS129GC Battery

Though stars almost never get close enough to actually collide in galaxy mergers, giant molecular clouds rapidly fall to the center of the galaxy where they collide with other molecular clouds. These collisions then induce condensations of these clouds into new stars. We can see this phenomenon in merging galaxies in the nearby universe.Sony VAIO VPCS12C7E/B Battery

Yet, this process was more pronounced during the mergers that formed most elliptical galaxies we see today, which likely occurred 1-10 billion years ago, when there was much more gas (and thus more molecular clouds) in galaxies. Also, away from the center of the galaxy gas clouds will run into each other producing shocks which stimulate the formation of new stars in gas clouds.Sony VAIO VPCS12L9E/B Battery

The result of all this violence is that galaxies tend to have little gas available to form new stars after they merge. Thus if a galaxy is involved in a major merger, and then a few billion years pass, the galaxy will have very few young stars (see Stellar evolution) left.Sony VAIO VPCS12V9E/B Battery

This is what we see in today's elliptical galaxies, very little molecular gas and very few young stars. It is thought that this is because elliptical galaxies are the end products of major mergers which use up the majority of gas during the merger, and thus further star formation after the merger is quenched.Sony VAIO VPCW111XX/P Battery

In the Local Group, the Milky Way and M31 (the Andromeda Galaxy) are gravitationally bound, and currently approaching each other at high speed. If the two galaxies do meet they will pass through each other, with gravity distorting both galaxies severely and ejecting some gas, dust and stars into intergalactic space. Sony VAIO VPCW111XX/PC Battery

They will travel apart, slow down, and then again be drawn towards each other, and again collide. Eventually both galaxies will have merged completely, streams of gas and dust will be flying through the space near the newly formed giant elliptical galaxy. M31 is actually already distorted: the edges are warped. Sony VAIO VPCW111XX/T Battery

This is probably because of interactions with its own galactic companions, as well as possible mergers with dwarf spheroidal galaxies in the recent past - the remnants of which are still visible in the disk populations.

In our epoch, large concentrations of galaxies (clusters and superclusters) are still assembling.Sony VAIO VPCW111XX/W Battery

While scientists have learned a great deal about ours and other galaxies, the most fundamental questions about formation and evolution remain only tentatively answered.Sony VAIO VPCW111XXP Battery,Sony VAIO VPCW111XXT Battery

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