STARS

A star is a massive, luminous ball of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible from Earth during the night, most commonly appearing as a multitude of fixed luminous points, when they are not outshone by the Sun or blocked by atmospheric phenomena. Historically, the most prominent stars on the celestial sphere were grouped together into constellations and asterisms, and the brightest stars gained proper names. Extensive catalogues of stars have been assembled by astronomers, which provide standardized star designations.

For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen in its core releasing energy that traverses the star's interior and then radiates into outer space. Almost all naturally occurring elements heavier than helium were created by stars, either via stellar nucleosynthesis during their lifetimes or by supernova nucleosynthesis when stars explode. Astronomers can determine the mass, age, chemical composition and many other properties of a star by observing its spectrum, luminosity and motion through space. The total mass of a star is the principal determinant in its evolution and eventual fate. Other characteristics of a star are determined by its evolutionary history, including diameter, rotation, movement and temperature. A plot of the temperature of many stars against their luminosities, known as a Hertzsprung-Russell diagram (H–R diagram), allows the age and evolutionary state of a star to be determined.

A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective processes. The star's internal pressure prevents it from collapsing further under its own gravity. Once the hydrogen fuel at the core is exhausted, those stars having at least 0.4 times the mass of the Sun expand to become a red giant, in some cases fusing heavier elements at the core or in shells around the core. The star then evolves into a degenerate form, recycling a portion of the matter into the interstellar environment, where it will form a new generation of stars with a higher proportion of heavy elements.

Binary and multi-star systems consist of two or more stars that are gravitationally bound, and generally move around each other in stable orbits. When two such stars have a relatively close orbit, their gravitational interaction can have a significant impact on their evolution.Stars can form part of a much larger gravitationally bound structure, such as a cluster or a galaxy.

A white dwarf star in orbit around Sirius(artist's impression)
NASA image

Massive Stars:

During their helium-burning phase, very high mass stars with more than nine solar masses expand to form red supergiants. Once this fuel is exhausted at the core, they can continue to fuse elements heavier than helium.
The core contracts until the temperature and pressure are sufficient to fuse carbon (see carbon burning process). This process continues, with the successive stages being fueled by neon (see neon burning process), oxygen (see oxygen burning process), and silicon (see silicon burning process). Near the end of the star's life, fusion can occur along a series of onion-layer shells within the star. Each shell fuses a different element, with the outermost shell fusing hydrogen; the next shell fusing helium, and so forth.
The final stage is reached when the star begins producing iron. Since iron nuclei are more tightly bound than any heavier nuclei, if they are fused they do not release energy—the process would, on the contrary, consume energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission.[68] In relatively old, very massive stars, a large core of inert iron will accumulate in the center of the star. The heavier elements in these stars can work their way up to the surface, forming evolved objects known as Wolf-Rayet stars that have a dense stellar wind which sheds the outer atmosphere.
Collapse
An evolved, average-size star will now shed its outer layers as a planetary nebula. If what remains after the outer atmosphere has been shed is less than 1.4 solar masses, it shrinks to a relatively tiny object (about the size of Earth) that is not massive enough for further compression to take place, known as a white dwarf. The electron-degenerate matter inside a white dwarf is no longer a plasma, even though stars are generally referred to as being spheres of plasma. White dwarfs will eventually fade into black dwarfs over a very long stretch of time.

Watch:Video Star Size Comparison

In larger stars, fusion continues until the iron core has grown so large (more than 1.4 solar masses) that it can no longer support its own mass. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay, or electron capture. The shockwave formed by this sudden collapse causes the rest of the star to explode in a supernova. Supernovae are so bright that they may briefly outshine the star's entire home galaxy. When they occur within the Milky Way, supernovae have historically been observed by naked-eye observers as "new stars" where none existed before.
Most of the matter in the star is blown away by the supernova explosion (forming nebulae such as the Crab Nebula)and what remains will be a neutron star (which sometimes manifests itself as a pulsar or X-ray burster) or, in the case of the largest stars (large enough to leave a stellar remnant greater than roughly 4 solar masses), a black hole.In a neutron star the matter is in a state known as neutron-degenerate matter, with a more exotic form of degenerate matter, QCD matter, possibly present in the core. Within a black hole the matter is in a state that is not currently understood.
The blown-off outer layers of dying stars include heavy elements which may be recycled during new star formation. These heavy elements allow the formation of rocky planets. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.

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GALAXIES

NGC 4414, a typical spiral galaxy in the constellationComa Berenices,
 is about 55,000 light-years in diameter and approximately
 60 million light-years away from Earth.

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.The name is from the Greek root galaxias [γαλαξίας], literally meaning "milky", a reference to the Milky Way galaxy. Typical galaxies range from dwarfs with as few as ten million stars,up to giants with a hundred trillion stars,all orbiting the galaxy's center of mass. Galaxies may contain many star systems, star clusters, and various interstellar clouds. 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). A common form is the elliptical galaxy,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. 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.
There are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe. Most galaxies are 1,000 to 100,000parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs).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. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.
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. 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.
ETYMOLOGY:
he word galaxy derives from the Greek term for our own galaxy, galaxias (γαλαξίας), or kyklos galaktikos, meaning "milky circle" for its appearance in the sky. 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: 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. 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."
—Geoffrey Chaucer. The House of Fame, c. 1380.
When William Herschel constructed his catalog of deep sky objects, he used the name spiral nebula for certain objects such as M31. 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.
The Milky Way:

The Greek philosopher Democritus (450–370 B.C.) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.Aristotle (384–322 B.C.), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, 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."The Neoplatonist philosopher Olympiodorus the Younger (c. 495–570 A.D.) criticized this view, 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.
The Arabian astronomer, Alhazen (965–1037), made the first attempt at observing and measuring the Milky Way's parallax,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."The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars."The Andalusian astronomer Ibn Bajjah 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,citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near.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".
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.In 1750 Thomas Wright, in his An original theory or new hypothesis of the Universe, speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational 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.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. 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. 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, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our galaxy, the Milky Way, emerged.

Sketch of the Whirlpool Galaxy by Lord Rosse in 1845

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".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.
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.
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.In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.

Photograph of the "Great Andromeda Nebula" from 1899, later identified as the
Andromeda Galaxy

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. 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.
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. 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.
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, Curtis noted the appearance of dark lanes resembling the dust clouds in the Milky Way, as well as the significant Doppler shift.
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.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.In 1936 Hubble produced a classification system for galaxies that is used to this day, the Hubble sequence.
Modern research

The most distant galaxy: UDFy-38135539

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. 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.
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.
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, 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. 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.
The most distant galaxy as seen in the Hubble Ultra Deep Field is UDFy-38135539 approximately 13 billion ly from Earth.
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. 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).

Types of galaxies according to the Hubble classification scheme. An Eindicates a type of elliptical galaxy; an S is a spiral; and SB is a barred-spiral galaxy

Ellipticals
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. 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. 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. 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.Starburst galaxies are the result of such a galactic collision that can result in the formation of an elliptical galaxy.
Spirals

NGC 1300, an example of a barred spiral galaxy.

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 bulge are 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. 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.
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. The spiral arms are thought to be areas of high density matter, or "density waves". 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.) 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.

The Whirlpool Galaxy , an example of an unbarred spiral galaxy.

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.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, or else due to a tidal interaction with another galaxy.Many barred spiral galaxies are active, possibly as a result of gas being channeled into the core along the arms.
Our own galaxy is a large disk-shaped barred-spiral galaxy about 30 kiloparsecs in diameter and a kiloparsec in thickness. It contains about two hundred billion (2×1011)stars and has a total mass of about six hundred billion (6×1011) times the mass of the Sun.
Other morphologies

Hoag's Object, an example of a ring galaxy.

Peculiar galaxies are galactic formations that develop unusual properties due to tidal interactions with other galaxies. 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.Such an event may have affected the Andromeda Galaxy, as it displays a multi-ring-like structure when viewed in infrared radiation.
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.(Barred lenticular galaxies receive Hubble classification SB0.)

NGC 5866, an example of a
lenticular galaxy. Credit: NASA/ESA.

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

Dwarf galaxy
Despite the prominence of large elliptical and spiral galaxies, most galaxies in the universe appear to be dwarf galaxies. 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.
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.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.
A study of 27 Milky Way neighbors found that dwarf galaxies were all approximately 10 million solar masses, regardless of whether they have thousands or millions of stars. This has led to the suggestion that galaxies are largely formed by dark matter, and that the minimum size may indicate a form of warm dark matter incapable of gravitational coalescence on a smaller scale.
Unusual dynamics and activities:
Interacting galaxy

The Antennae Galaxies are undergoing a collision that
will result in their eventual merg

The average separation between galaxies within a cluster is a little over an order of magnitude larger than their diameter. 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.
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. 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. 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. 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 the Sagittarius Dwarf Elliptical Galaxy and the Canis Major Dwarf Galaxy.

Starburst

M82, the archetype starburst galaxy, has experienced a
10-fold increase in star formation rate as
compared to a "normal" galaxy.

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. 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, and, at present, still contribute an estimated 15% to the total star production rate.

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. 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. 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.

Active galactic nucleus

A jet of particles is being emitted from the

 core of the elliptical radio galaxy M87.

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

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. 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.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.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".
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.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.Modern theories must be modified to account for the probable presence of large dark matter halos.
Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. 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.This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.
Evolution

I Zwicky 18 (lower left) resembles a newly formed galaxy

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. 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. During this early epoch, galaxies undergo a major burst of star formation.
During the following two billion years, the accumulated matter settles into a galactic disc.A galaxy will continue to absorb infalling material from high velocity clouds and dwarf galaxies throughout its life.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.
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.Given the distances between the stars, the great majority of stellar systems in colliding galaxies will be unaffected. 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 4676or the Antennae Galaxies.
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. 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. 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.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; 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), 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. 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

Seyfert's Sextet is an example of a compact galaxy group.

Deep sky surveys show that galaxies are often found in relatively close association with other galaxies. 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. 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). 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. 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. 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.

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. 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.
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.
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.Above this scale, the universe appears to be isotropic and homogeneous.
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.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. 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.
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. 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. 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 (via 21 centimetre radiation), 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.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.

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THE BIG BANG

INTRODUCTION:We certainly know that our universe exists, however, this knowledge alone has not satisfied mankind's quest for further understanding. Our curiosity has led us to question our place in this universe and furthermore, the place of the universe itself. Throughout time we have asked ourselves these questions: How did our universe begin? How old is our universe? How did matter come to exist? Obviously, these are not simple questions and throughout our brief history on this planet much time and effort has been spent looking for some clue. Yet, after all this energy has been expended, much of what we know is still only speculation.

We have, however, come a long way from the mystical beginnings of the study of cosmology and the origins of the universe. Through the understandings of modern science we have been able to provide firm theories for some of the answers we once called hypotheses. True to the nature of science, a majority of these answers have only led to more intriguing and complex questions. It seems to be inherent in our search for knowledge that questions will always continue to exist.

Although in this short chapter it will be impossible to tackle all of the questions concerning the creation of everything we know as reality, an attempt will be made to address certain fundamental questions of our being. It will be important to keep in mind that all of this information is constantly being questioned and reevaluated in order to understand the universe more clearly. For our purposes, through an examination of what is known about the Big Bang itself, the age of the universe, and the synthesis of the first atoms, we believe that we can begin to answer several of these key questions.

THE BIG BANG:
One of the most persistently asked questions has been: How was the universe created? Many once believed that the universe had no beginning or end and was truly infinite. Through the inception of the Big Bang theory, however,no longer could the universe be considered infinite. The universe was forced to take on the properties of a finite phenomenon, possessing a history and a beginning.
About 15 billion years ago a tremendous explosion started the expansion of the universe. This explosion is known as the Big Bang. At the point of this event all of the matter and energy of space was contained at one point. What exisisted prior to this event is completely unknown and is a matter of pure speculation. This occurance was not a conventional explosion but rather an event filling all of space with all of the particles of the embryonic universe rushing away from each other. The Big Bang actually consisted of an explosion of space within itself unlike an explosion of a bomb were fragments are thrown outward. The galaxies were not all clumped together, but rather the Big Bang lay the foundations for the universe.
The origin of the Big Bang theory can be credited to Edwin Hubble. Hubble made the observation that the universe is continuously expanding. He discovered that a galaxys velocity is proportional to its distance. Galaxies that are twice as far from us move twice as fast. Another consequence is that the universe is expanding in every direction. This observation means that it has taken every galaxy the same amount of time to move from a common starting position to its current position. Just as the Big Bang provided for the foundation of the universe, Hubbles observations provided for the foundation of the Big Bang theory.
Since the Big Bang, the universe has been continuously expanding and, thus, there has been more and more distance between clusters of galaxies. This phenomenon of galaxies moving farther away from each other is known as the red shift. As light from distant galaxies approach earth there is an increase of space between earth and the galaxy, which leads to wavelengths being stretched.
In addition to the understanding of the velocity of galaxies emanating from a single point, there is further evidence for the Big Bang. In 1964, two astronomers, Arno Penzias and Robert Wilson, in an attempt to detect microwaves from outer space, inadvertently discovered a noise of extraterrestrial origin. The noise did not seem to emanate from one location but instead, it came from all directions at once. It became obvious that what they heard was radiation from the farthest reaches of the universe which had been left over from the Big Bang. This discovery of the radioactive aftermath of the initial explosion lent much credence to the Big Bang theory.

Even more recently, NASAs COBE satellite was able to detect cosmic microwaves eminating from the outer reaches of the universe. These microwaves were remarkably uniform which illustrated the homogenity of the early stages of the universe. However, the satillite also discovered that as the universe began to cool and was still expanding, small fluctuations began to exist due to temperature differences. These flucuatuations verified prior calculations of the possible cooling and development of the universe just fractions of a second after its creation. These fluctuations in the universe provided a more detailed description of the first moments after the Big Bang. They also helped to tell the story of the formation of galaxies which will be discussed in the next chapter.
The Big Bang theory provides a viable solution to one of the most pressing questions of all time. It is important to understand, however, that the theory itself is constantly being revised. As more observations are made and more research conducted, the Big Bang theory becomes more complete and our knowledge of the origins of the universe more substantial.

THE FIRST ATOMS:
Now that an attempt has been made to grapple with the theory of the Big Bang, the next logical question to ask would be what happened afterward? In the minuscule fractions of the first second after creation what was once a complete vacuum began to evolve into what we now know as the universe. In the very beginning there was nothing except for a plasma soup. What is known of these brief moments in time, at the start of our study of cosmology, is largely conjectural. However, science has devised some sketch of what probably happened, based on what is known about the universe today.

Immediately after the Big Bang, as one might imagine, the universe was tremendously hot as a result of particles of both matter and antimatter rushing apart in all directions. As it began to cool, at around 10^-43 seconds after creation, there existed an almost equal yet asymmetrical amount of matter and antimatter. As these two materials are created together, they collide and destroy one another creating pure energy. Fortunately for us, there was an asymmetry in favor of matter. As a direct result of an excess of about one part per billion, the universe was able to mature in a way favorable for matter to persist. As the universe first began to expand, this discrepancy grew larger. The particles which began to dominate were those of matter. They were created and they decayed without the accompaniment of an equal creation or decay of an antiparticle.

As the universe expanded further, and thus cooled, common particles began to form. These particles are called baryons and include photons, neutrinos, electrons and quarks would become the building blocks of matter and life as we know it. During the baryon genesis period there were no recognizable heavy particles such as protons or neutrons because of the still intense heat. At this moment, there was only a quark soup. As the universe began to cool and expand even more, we begin to understand more clearly what exactly happened.

After the universe had cooled to about 3000 billion degrees Kelvin, a radical transition began which has been likened to the phase transition of water turning to ice. Composite particles such as protons and neutrons, called hadrons, became the common state of matter after this transition. Still, no matter more complex could form at these temperatures. Although lighter particles, called leptons, also existed, they were prohibited from reacting with the hadrons to form more complex states of matter. These leptons, which include electrons, neutrinos and photons, would soon be able to join their hadron kin in a union that would define present-day common matter.

After about one to three minutes had passed since the creation of the universe, protons and neutrons began to react with each other to form deuterium, an isotope of hydrogen. Deuterium, or heavy hydrogen, soon collected another neutron to form tritium. Rapidly following this reaction was the addition of another proton which produced a helium nucleus. Scientists believe that there was one helium nucleus for every ten protons within the first three minutes of the universe. After further cooling, these excess protons would be able to capture an electron to create common hydrogen. Consequently, the universe today is observed to contain one helium atom for every ten or eleven atoms of hydrogen.

While it is true that much of this information is speculative, as the universe ages we are able to become increasingly confident in our knowledge of its history. By studying the way in which the universe exists today it is possible to learn a great deal about its past. Much effort has gone into understanding the formation and number of baryons present today. Through finding answers to these modern questions, it is possible to trace their role in the universe back to the Big Bang. Subsequently, by studying the formation of simple atoms in the laboratory we can make some educated guesses as to how they formed originally. Only through further research and discovery will it be possible to completely understand the creation of the universe and its first atomic structures, however, maybe we will never know for sure.
AGE OF THE UNIVERSE:
We now have something of a handle on two of the most important quandaries concerning the universe; however, one major question remains. If the universe is indeed finite, how long has it been in existence? Again, science has been able to expand upon what it knows about the universe today and extrapolate a theory as to its age. By applying the common physical equation of distance over velocity equaling time, which again uses Hubbles observations, a fairly accurate approximation can be made.

The two primary measurements needed are the distance of a galaxy moving away from us and that galaxys red shift. An unsuccessful first attempt was made to find these distances through trigonometry. Scientists were able to calculate the diameter of the Earths orbit around the sun which was augmented through the calculation of the Suns motion through our own galaxy. Unfortunately, this calculation could not be used alone to determine the enormous distance between our galaxy and those which would enable us to estimate the age of the universe because of the significant errors involved.

The next step was an understanding of the pulsation of stars. It had been observed that stars of the same luminosity blinked at the same rate, much like a lighthouse could work where all lighthouses with 150,000 watt light bulbs would rotate every thirty seconds and those with 250,000 watt light bulbs would rotate every minute. With this knowledge, scientists assumed that stars in our galaxy that blinked at the same rate as stars in distant galaxies must have the same intensity. Using trigonometry, they were able to calculate the distance to the star in our galaxy. Therefore, the distance of the distant star could be calculated by studying the difference in their intensities much like determining the distance of two cars in the night. Assuming the two cars headights had the same intensity, it would be possible to infer that the car whose headlights appeared dimmer was farther away from the observer than the other car whose headlights would seem brighter. Again, this theory could not be used alone to calculate distance of the most far-away galaxies. After a certain distance it becomes impossible to distinguish individual stars from the galaxies in which they exist. Because of the large red shifts in these galaxies a method had to be devised to find distance using entire galaxy clusters rather than stars alone.

By studying the sizes of galaxy cluster that are near to us, scientists can gain an idea of what the sizes of other clusters might be. Consequently, a prediction can be made about their distance from the Milky Way much in the same way the distance of stars was learned. Though a calculation involving the supposed distance of the far-off cluster and its red shift, a final estimation can be made as to how long the galaxy has been moving away from us. In turn, this number can be used inversely to turn back the clock to a point when the two galaxies were in the same place at the same time, or, the moment of the Big Bang. The equation generally used to show the age of the universe is shown here:

(distance of a particular galaxy) / (that galaxys velocity) = (time)

or

4.6 x 10^26 cm / 1 x 10^9 cm/sec = 4.6 x 10^17 sec

This equation, equaling 4.6 x 10^17 seconds, comes out to be approximately fifteen billion years. This calculation is almost exactly the same for every galaxy that can be studied. However, because of the uncertainties of the measurements produced by these equations, only a rough estimate of the true age of the universe can be fashioned. While finding the age of the universe is a complicated process, the achievement of this knowledge represents a critical step in our understanding.

NOW WHAT?
In summary, we have made a first attempt at explaining the answers that science has revealed about our universe. Our understanding of the Big Bang, the first atoms and the age of the universe is obviously incomplete. As time wears on, more discoveries are made, leading to infinite questions which require yet more answers. Unsatisfied with our base of knowledge research is being conducted around the world at this very moment to further our minimal understanding of the unimaginably complex universe.

Since its conception, the theory of the Big Bang has been constantly challenged. These challenges have led those who believe in the theory to search for more concrete evidence which would prove them correct. From the point at which this chapter leaves off, many have tried to go further and several discoveries have been made that paint a more complete picture of the creation of the universe.

Recently, NASA has made some astounding discoveries which lend themselves to the proof of the Big Bang theory. Most importantly, astronomers using the Astro-2 observatory were able to confirm one of the requirements for the foundation of the universe through the Big Bang. In June, 1995, scientists were able to detect primordial helium, such as deuterium, in the far reaches of the universe. These findings are consistent with an important aspect of the Big Bang theory that a mixture of hydrogen and helium was created at the beginning of the universe.

In addition, the Hubble telescope, named after the father of Big Bang theory, has provided certain clues as to what elements were present following creation. Astronomers using Hubble have found the element boron in extremely ancient stars. They postulate that its presence could be either a remnant of energetic events at the birth of galaxies or it could indicate that boron is even older, dating back to the Big Bang itself. If the latter is true, scientists will be forced once again to modify their theory for the birth of the universe and events immediately afterward because, according to the present theory, such a heavy and complex atom could not have existed.

In this manner we can see that the research will never be truly complete. Our hunger for knowledge will never be satiated. So to answer the question, what now, is an impossibility. The path we take from here will only be determined by our own discoveries and questions. We are engaged in a never-ending cycle of questions and answers where one will inevitably lead to the other.

COBE continues to search the outer reaches of the universe

DEEP THOUGHTS:
It is extremely difficult to separate this subject of science from daily existential pondering. Everyone at some point in time has grappled with the question of why we are here? Some have found refuge in the sheer philosophic nature of this question while others have taken a more scientific approach. These particular wanderers have taken the question to a higher level, concentrating not only on human existence but the existence of everything we know as real.

If you sit and try to imagine the whole of the entire universe it would be mind-boggling. However, science has now told us that the universe is, in fact, finite, with a beginning, a middle, and a future. It is easy to get caught up in the large scale of the issue in discussing years by the billions, yet, this time still passes. As we travel through our own lives here on Earth, we also travel through the life of our universe.

In this chapter, we have made some attempts to explain this journey. It is odd that we will never truly know how it began. We can only speculate and give our best guess. Through our own devices we have been able to produce evidence that these guesses are close to the truth. But centuries from now, will the human race compare us to those who once thought of the Earth as the center of the universe?


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GLOSSARY
Baryons-- common particles including photons and neutrinos created at approximately 10^-33 seconds after the Big Bang

Deuterium-- a heavy isotope of hyrogen containing on proton and one neutron

Hadrons-- composite particles such as protons and neutrons forming after the temperature drops to 300 MeV

Leptons-- light particles existing with hadros including electrons, neutrinos and photons

Red Shift-- shift toward the red in the spectra of light reaching us from the stars in distant galaxies

Tritium-- transitional element between deuterium and the formation of a helium nucleus


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REFERENCES
Literature
Kaufmann, William J., III. Galaxies and Quasars. San Fransisco: W.H. Freeman and Company, 1979.

Silk, Joseph. A Short History of the Universe. New York: Scientific American Library, 1994.

Taylor, John. When the Clock Struck Zero. New York: St. Martins Press, 1993.

Trinh, Xuan Thuan. The Birth of the Universe: The Big Bang and After. New York: Harry N. Abrams, Inc., 1993.

World Wide Web
THE BIG BANG DOCUMENTARY MOVIE:

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THE COSMOS

In its most general sense, a cosmos is an orderly or harmonious system. It originates from the Greek term κόσμος (kosmos), meaning amongst others "order" but also "ornament"and is the antithetical concept of chaos. Today the word is generally used as a synonym of the word Universe (considered in its orderly aspect). The words cosmetics and cosmetology originate from the same root. In many Slavic languages such as Russian and Bulgarian, the word Космос cosmos means also the "outer space". In Mandarin Chinese, cosmos is translated as 宇宙 yuzhou, which literally translated means space-time (宇 yu = space + 宙 zhou = time).
Philosophy:
Pythagoras is said to have been the first philosopher to apply the term cosmos to the Universe, perhaps from application to the starry firmament.

Russian cosmism is a cosmocentric philosophical and cultural movement that emerged in Russia in the early 20th century.

Cosmicism is a philosophical position that mankind is an insignificant aspect of a universe at best indifferent and perhaps hostile. This philosophy, explored by writers such as H.P. Lovecraft (who some say is the original proponent of the philosophy) and later writers who actually represented the beliefs in books such as Hitchhiker's Guide to the Galaxy.

Theology:

In theology, the term can be used to denote the created Universe, not including the creator. The Septuagint uses both kosmos and oikumene for the inhabited world. In Christian theology, the word was also used synonymously with aion to refer to "worldly life" or "this world" as opposed to the afterlife.

The cosmos as originated by Pythagoras is parallel to the Zoroastrian term aša, the concept of a divine order, or divinely ordered creation.

Olaf Stapledon, in his science fiction novel Star Maker (1937), describes how God (the Star Maker) evolves by creating ever more complex cosmoses across multicosmic hypertime.

Cosmology:
Cosmology is the study of the cosmos in several of the above meanings, depending on context. All cosmologies have in common an attempt to understand the implicit order within the whole of being. In this way, most religions and philosophical systems have a cosmology.
In physical cosmology, the term cosmos is often used in a technical way, referring to a particular space-time continuum within the (postulated) multiverse. Our particular cosmos is generally capitalized as the Cosmos.

Integral Philosophy:
The philosopher Ken Wilber uses the term kosmos to refer to all of manifest existence, including various realms of consciousness. The term kosmos so used distinguishes a nondual Universe (which, in his view, includes both noetic and physical aspects) from the strictly physical Universe that is the concern of the traditional sciences.

Ancient Greek conception of the cosmos:
The Ancient Greek natural philosopher Archimedes in his essay The Sand Reckoner, estimated the diameter of the cosmos to be the equivalent in stadia of what we call two light years.

Age and size of the cosmos:
According to current scientific theory, the cosmos began 13.7 billion years ago short scale in the Big Bang. The current diameter of the observable cosmos is thought to be about 93 billion light years. The diameter of the entire cosmos is unknown and possibly infinite.

In some meanings of "cosmos" the speaker is speaking of the infinite of space, even that which reaches beyond the edges of the universe. "Beyond the edges of the universe" meaning the points in space where the universe has yet to expand into.

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