GALAXIES IN THE UNIVERSE
The group of three
galaxies seen in Hubble’s space telescope
Location of Solar system in Milky Galaxy (our Galaxy)
Introduction
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is about 55,000 light-years in diameter and approximately 60
million light-years away from Earth.
Modern Hubble’space telescope
of NASA
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants,
an interstellar medium of
gas and dust, and an
important but poorly understood component tentatively dubbed dark matter.
Galaxies contain varying amounts of star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of
gas, dust, and cosmic rays. 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 thought to be the
primary driver of active galactic nuclei found at the core of some galaxies. The
Milky Way galaxy appears to harbor at least one such object.
Galaxies have been historically
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 with dusty, curving
arms. Those with irregular or unusual shapes are known as irregular galaxies and typically originate from
disruption by the gravitational pull of neighboring galaxies. Such interactions
between nearby galaxies, which may ultimately result in a merging, sometimes
induce significantly increased incidents of star formation leading to starburst galaxies.
Smaller galaxies lacking a coherent structure are referred to as irregular galaxies.
There are probably more than
170 billion (1.7x1011) galaxies in
the observable universe. Most are 1,000 to 100,000 parsecs in diameter and 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 known as groups and clusters, which, in turn usually
form larger superclusters.
At the largest scale, these associations are generally
arranged into sheets and filaments,
which are surrounded by immense voids.
Emtymology
The word galaxy is derived from the Greek “galaxias” ,
literally "milky", a reference to the Milky Way galaxy, the Greek term for our own
galaxy with its appearance in the sky.
Examples of galaxies range from dwarfs with as few as ten million (107)
stars to giants with a hundred trillion (1014)
stars, each orbiting their galaxy's own center of mass.
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 English term Milky Way can be traced back to a story by Geoffrey Chaucer (1380):
"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. 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.
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.
Milky Way: The galaxy containing the
solar system
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. 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, 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 AD) was scientifically
critical of 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.
According to Mohani Mohamed, 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 ("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, citing his
observation of the conjunction of Jupiter and Mars as evidence of
this occurring when two objects are near.
In the 14th century, the Syrian-born Ibn Qayyim Al-Jawziyya 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 shape of
the Milky Way as deduced from star counts by William Herschel in 1785;
the solar
system was assumed to be near the center.
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.
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". The Andromeda Galaxy was
independently rediscovered by Simon Marius in 1612. Al-Sufi also identified the Large Magellanic Cloud, which is visible fromYemen, 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.
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"
(as the Andromeda Galaxy, Messier object M31, was known).
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.
Photograph of the
"Great Andromeda Nebula" from 1899,
later identified as theAndromeda Galaxy.
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 Öpikgave 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
In 1944, Hendrik van de Hulst predicted microwave radiation at a wavelength of 21 cmresulting
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 thespectra 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.
Types and morphology
Types of galaxies
according to the Hubble classification scheme.
An E indicates a type of elliptical
galaxy; an S is a spiral;
and SB is a barred-spiral galaxy.
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).
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 anellipsoidal 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. The stars contain low abundances of heavy
elements because star formation ceases after the initial burst. 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
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. 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.
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.
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NGC 1300,
an example of a barred 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.[55] 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
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.)
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.
Dwarfs 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
Unusual dynamics and activities
Interacting
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 ascannibalism. 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 expandingremnants 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 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.
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 asSeyfert
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.
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
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-poorPopulation 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 anddwarf 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.[93] 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 4676 or 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 dense molecular 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
.jpg)
The image spans about
400 million light years across.
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 30–100megakelvins. 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 thesupercluster 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
the emission of
ordinary stars and the light reflected by dust.
This ultraviolet
image of Andromeda shows
blue regions containing young, massive stars.
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.[113] Infrared is also used to observe
distant, red-shiftedgalaxies
that were formed much earlier in the history of the universe. Water vapor andcarbon 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 cm 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.
References
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