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Galaxy

Galaxy

:This article is about celestial bodies. For alternate meanings, see galaxy (disambiguation). galaxy (disambiguation), is about 56,000 light years in diameter and approximately 60 million light years distant.]] A galaxy is a vast gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain 10 million to one trillion (10⁷ to 10¹²) stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years. Although so-called dark matter and dark energy appear to account for well over 90% of the mass of most galaxies, the nature of these unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies. Intergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than 10¹¹ galaxies in the visible universe.

Types of galaxies

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. While the Hubble sequence does encompass all galaxies, it is entirely based upon visual morphological type. Hence, it may miss certain important characteristics of galaxies such as star formation rate. Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large disk-shaped barred spiral galaxy about 30 kiloparsecs or 100,000 light years in diameter and 3,000 light years in thickness. It contains about 3×10¹¹ stars and has a total mass of about 6×10¹¹ times the mass of the Sun. In spiral galaxies, the spiral arms 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 also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms. The spiral arms are thought to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this 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 they therefore harbor many bright and young stars. A new set of galaxies, classified as Ultra Compact Dwarf Galaxies, were discovered in 2003 by Michael Drinkwater of the University of Queensland.

Larger scale structures

Only a few galaxies exist by themselves; these are known as field galaxies. Most galaxies are gravitationally bound to a number of other galaxies. Structures containing up to about 50 galaxies are called groups of galaxies, and 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, which over time tidally destroys its satellite galaxies and adds their mass to its own. Superclusters are giant collections containing tens of thousands of galaxies, 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. Our galaxy is a member of the Local Group, which it dominates together with the Andromeda Galaxy; overall the Local Group contains about 30 galaxies in a space about one megaparsec across. The Local Group is part of the Virgo Supercluster, which is dominated by the Virgo Cluster (of which our Galaxy is not a member).

History

This account of the history of the investigation of our own and other galaxies is largely taken from [1]. In 1610, Galileo Galilei used a telescope to study the bright band on the night sky known as the Milky Way and discovered that it was composed of a huge number of faint stars. In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright, 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 much larger scales. The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Kant also conjectured that some of the nebulae visible in the night sky might be separate galaxies. Towards the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest nebulae, later followed by a catalog of 5000 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. However, the nebulae were not universally accepted as distant separate galaxies until the matter was settled by Edwin Hubble in the early 1920s using a new telescope. He 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. The first attempt to describe the shape of the Milky Way and the position of the Sun within it was carried out by William Herschel in 1785 by carefully counting the number of stars in different regions of the sky. Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter ~15 kiloparsecs) ellipsoid galaxy with the Sun close to the center. A different method by Harlow Shapley based on the cataloging of globular clusters lead to a radically different picture: a flat disk with diameter ~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; once Robert Julius Trumpler had quantified this effect in 1930 by studying open clusters, the present picture of our galaxy as described above emerged. 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. This radiation allowed for much improved study of the 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 realized that the total visible mass of galaxies (from stars and gas) does not properly account for the speed of the rotating gas, thus leading to the postulation of dark matter. dark matterBeginning 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. It photographed the Hubble Deep Field, providing evidence for hundreds of billions of galaxies in existence in the visible universe alone. Many scientists have tried to obtain a good estimate for the number of galaxies in the universe formally. The methods used to achieve such number varies, and therefore, the results are varying too. Also, as new and improved technology becomes available, astronomers can detect fainter objects that were not seen before. These objects that have come into view will in turn change the estimated number of galaxies. In 1999 the Hubble Space Telescope estimated that there were 125 billion galaxies in the universe, and recently with the new camera HST has observed 3000 visible galaxies, which is twice as much as they observed before with the old camera. The term "visible" is emphasized because observations with radio telescopes, infrared cameras, x-ray cameras, etc. would detect other galaxies that are not detected by Hubble. As observations keep on going and astronomers explore more of our universe, the number of galaxies detected will increase. In 2004, the galaxy Abell 1835 IR1916 became the most distant galaxy ever seen by humans.

Etymology

The word galaxy was derived from the Greek term for our own galaxy, kyklos galaktikos meaning "milky circle" for the system’s appearance in the sky. When astronomers speculated that certain objects previously classified as spiral nebulae were actually vast congeries of stars, this was called the "island universe theory"; but this was an obvious misnomer, since universe means everything there is. Consequently, this term fell into disuse, replaced by applying the term galaxy generically to all such bodies.

References

- James Binney: Galactic Astronomy, Princeton University Press, 1998
- Terence Dickinson: The Universe and Beyond (Fourth Edition), Firefly Books Ltd. 2004, 2004

External links

- [http://www.seds.org/messier/galaxy.html Galaxies, SEDS Messier pages]
- [http://www.anzwers.org/free/universe/ An Atlas of The Universe]
- [http://www.nightskyinfo.com/galaxies Galaxies - Information and amateur observations] Category:Astronomical objects Category:Large-scale structure of the cosmos ko:은하 ms:Galaksi ja:銀河 simple:Galaxy th:กาแล็กซี

Galaxy (disambiguation)

Galaxy (Greek γαλαξίας, galaxías [male noun] - the milky [nebula], Milky Way) can refer to:
- galaxy, a celestial body
- Galaxy, a candy bar made by the Mars company
- Galaxy Channel, a television channel operated by British Satellite Broadcasting
- Galaxy, a former cable television and satellite television company in Australia
- Galaxy, a science fiction magazine
- Star Trek's Galaxy class starship
- Star Wars's galaxy
- Galaxy, a cruise ship
- The Galaxy Network of British radio stations
- The Los Angeles Galaxy, a soccer team in the MLS
- The C-5 Galaxy, a large cargo airplane.
- The brand-name of Israel Aircraft Industries-manufactured IAI-1126 Galaxy, an executive jet airplane, now produced for Gulfstream Aerospace as the G200.
- Galactic, a band from New Orleans.
- Galactic, a rare computer game for the Amiga.
- The Frankfurt Galaxy, an NFL Europe team
- Galaxy is a web directory, the first web directory.

Gravitation

:This article is about the physical concept of gravitation. For information on the manga/anime series, see Gravitation (manga) Gravitation is the tendency of objects with mass to accelerate towards each other. There have been numerous theories of gravitation starting with the work of Aristotle in Ancient Greece. After the Renaissance, gravitational theory was dominated by Newton's theory, in which gravitation is ascribed to the force of gravity. During the 20th Century, the theory of gravity was replaced by Einstein's general relativity theory, which ascribes gravitation to the effects of spacetime curvature. It is important to note that gravitation is not gravity. Gravity is a force that was postulated to be responsible for gravitation. So the term gravitation describes an effect independent of any cause. It is possible for gravitation to exist without gravity, and in general relativity theory, that is indeed the case.

Star

:This article is about celestial bodies. A star is a massive body of plasma in outer space that is currently producing or has produced energy through nuclear fusion. Unlike a planet, from which most light is reflected, a star emits light because of its intense heat. Scientifically, stars are defined as self-gravitating spheres of plasma in hydrostatic equilibrium, which generate their own energy through the process of nuclear fusion. Small (dwarf) stars such as the Sun generally have essentially featureless disks with only small starspots. Larger (giant) stars have much bigger, much more obvious starspots, and also exhibit strong stellar limb-darkening (the brightness decreases towards the edge of the stellar disk). Stellar astronomy is the study of stars.

Star formation and evolution

Star formation occurs in molecular clouds, large regions of high density in the interstellar medium (though still less dense than the inside of an earthly vacuum chamber). Star formation begins with gravitational instability inside those clouds, often triggered by shockwaves from supernovae or collision of two galaxies (as in a starburst galaxy). High mass stars powerfully illuminate the clouds from which they formed. One example of such a nebula is the Orion Nebula. Stars spend about 90% of their lifetime fusing hydrogen to produce helium in high-temperature and high-pressure reactions near the core. Such stars are said to be on the main sequence. Small stars (called red dwarfs) burn their fuel very slowly and last tens to hundreds of billions of years. At the end of their lives, they simply become dimmer and dimmer, fading into black dwarfs. However, since the lifespan of such stars is greater than the current age of the universe (13.6 billion years), no black dwarfs exist yet. As most stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. In about 5 billion years, when the Sun is a red giant, it will be so large that it will consume both Mercury and Venus. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron, which is the end point of the process. Since iron nuclei are more tightly bound than any heavier nuclei, they cannot be fused to release energy. Likewise, since they are more tightly bound than all lighter nuclei, energy cannot be released by fission. In old, very massive stars, a large core of inert iron will accumulate in the center of the star. An average-size star will then shed its outer layers as a planetary nebula. The core that remains will be a tiny ball of degenerate matter not massive enough for further fusion to take place, supported only by degeneracy pressure, called a white dwarf. These too will fade into black dwarfs over very long stretches of time. white dwarf In larger stars, fusion continues until an iron core accumulates that is too large to be supported by electron degeneracy pressure. This core will suddenly collapse as its electrons are driven into its protons, forming neutrons and neutrinos in a burst of inverse beta decay. 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. Eventually, most of the matter in a star is blown away by the explosion (forming nebulae such as the Crab Nebula) and what remains will be a neutron star (sometimes a pulsar or X-ray burster) or, in the case of the largest stars, a black hole. 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.

Appearance and distribution of stars

All stars except the Sun appear to the human eye as shining points in the nighttime sky that twinkle because of the effect of the Earth's atmosphere. Interferometer telescopes are required in order to produce images of these objects. The Sun is also a star, but it is close enough to Earth to appear as a disk instead, and to provide daylight. Stars are not spread uniformly across the universe, but are typically grouped into galaxies. A typical galaxy contains hundreds of billions of stars. The majority of stars are gravitationally bound to other stars, forming binary stars. Larger groups called star clusters also exist. Astronomers estimate that there are at least 70 sextillion (7×10²²) stars in the known universe [http://news.bbc.co.uk/2/hi/science/nature/3085885.stm]. That is 70 000 000 000 000 000 000 000, or 230 billion times as many as the 300 billion in our own Milky Way. The nearest star to the Earth, apart from the Sun, is Proxima Centauri, which is 39.9 trillion kilometers, or 4.2 light years away (light from Proxima Centauri takes 4.2 years to reach Earth). Travelling at the orbit speed of the Space Shuttle (5 miles per second -- almost 30,000 kilometers per hour), it would take about 150,000 years to get there. Distances like this are typical inside galactic discs, where the Sun and Earth are located. Stars can be much closer to each other in the centres of galaxies and globular clusters, or much further apart in galactic halos.

Age and size of stars

galactic halo Many stars are between 1 billion and 10 billion years old. Some stars may even be close to 13.7 billion years old, which is the observed age of the universe. (See Big Bang theory and stellar evolution.) They range in size from the tiny neutron stars (which are actually dead stars) no bigger than a city, to supergiants like the North Star (Polaris) and Betelgeuse, in the Orion constellation, which have a diameter about 1,000 times larger than the Sun—about 1.6 billion kilometers. However, these have a much lower density than the Sun. One of the most massive stars known is η Carinae, with 100–150 times as much mass as the Sun. Recent work by Donald Figer, an astronomer at the Space Telescope Science Institute in Baltimore, Maryland, suggests that 150 solar masses is the upper limit of stars in the current era of the universe. He used the Hubble Space Telescope to observe about a thousand stars in the Arches cluster, a massive young star cluster near the core of the Milky Way, and found no stars over that limit despite a statistical expectation that there should be several. The reason for this limit is not precisely known, but the Eddington limit is part of the answer. The very first stars to form after the Big Bang may have been larger, up to 300 solar masses or more, due to the complete absence of elements heavier than lithium in their composition. This generation of supermassive star is long extinct, however, and currently only theoretical. With a mass only 93 times that of Jupiter, AB Doradus C, a companion to AB Doradus A, is the smallest known star undergoing nuclear fusion in its core. Smaller bodies are brown dwarfs, which occupy a poorly-defined grey area between stars and gas giants. The minimum mass a star can have is estimated to be in the vicinity of 75 Jupiters.

Star classification

There are different classifications of stars ranging from type W, which are very large and bright, to M, which is often just large enough to start ignition of the hydrogen. Some of the more common classifications are O, B, A, F, G, K, M, and can perhaps be more easily remembered using the mnemonic "Oh, Be A Fine Girl, Kiss Me" (variant: change "girl" to "guy"), invented by Annie Jump Cannon (1863-1941). There are many other mnemonics for star classification; the most frequent addition tacks "Right Now, Sweetheart" for the red dwarf sub-types R, N and S. The new types L and T have also been recently appended to the end of the OBAFGKM sequence to classify the coldest low-mass stars and brown dwarfs, prompting such additions as "Lovingly Tonight" to the mnemonic. Each letter has 10 subclassifications. Our Sun is a G2, which is very near the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type. The Sun is taken as the prototypical star (not because it is special in any way, but because it is the closest and most studied star we have), and most characteristics of other stars are usually given in solar units.
For example, the mass of the Sun is :M_Sun = 1.9891×10³⁰ kg The masses of other stars can be given in terms of M_Sun.

Naming of stars

Most stars are identified only by catalogue numbers; only a few have names as such. The names are either traditional names (mostly from Arabic), Flamsteed designations, or Bayer designations. The only body which has been recognized by the scientific community as having competence to name stars or other celestial bodies is the International Astronomical Union (IAU). A number of private companies (e.g. the "International Star Registry") purport to sell names to stars; however, these names are not recognized by the scientific community, nor used by them, and many in the astronomy community view these organizations as frauds preying on people ignorant of how stars are in fact named. See star designations for more information on how stars are named. For a list of traditional names, see the list of stars by constellation.

Energy production

The energy produced by stars radiates into space as electromagnetic radiation, as a stream of neutrinos from the star's core, and as a stream of particles from the star's outer layers (its stellar wind). The peak frequency of the light depends on the temperature of the outer layers of the star. Besides the emitted visible light, the ultraviolet and infrared components are typically significant. The apparent brightness of a star is measured by its apparent magnitude.

Nuclear fusion reaction pathways

A variety of different nuclear fusion reactions take place inside the cores of stars, depending upon their mass and composition (see Stellar nucleosynthesis). Stars begin as a cloud of mostly hydrogen with about 25% helium and heavier elements in smaller quantities. In the Sun, with a 10⁷ K core, hydrogen fuses to form helium in the proton-proton chain: :4¹H → 2²H + 2e⁺ + 2ν_e (4.0 MeV + 1.0 MeV) :2¹H + 2²H → 2³He + 2γ (5.5 MeV) :2³He → ⁴He + 2¹H (12.9 MeV) These reactions result in the overall reaction: :4¹H → ⁴He + 2e⁺ + 2γ + 2ν_e (26.7 MeV) In more massive stars, helium is produced in a cycle of reactions catalyzed by carbon, the carbon-nitrogen-oxygen cycle. In stars with cores at 10⁸ K and masses between 0.5 and 10 solar masses, helium can be transformed into carbon in the triple-alpha process: :⁴He + ⁴He + 92 keV → ^{8
-}Be :⁴He + ^{8
-}Be + 67 keV → ^{12
-}C :^{12
-}C → ¹²C + γ + 7.4 MeV For an overall reaction of: :3⁴He → ¹²C + γ + 7.2 MeV

Star mythology

As well as certain constellations and the Sun itself, stars as a whole have their own mythology. They were thought to be the souls of the dead, or gods/goddesses.

References

- Cliff Pickover (2001) "The Stars of Heaven", Oxford University Press
- John Gribbin, Mary Gribbin (2001) "Stardust: Supernovae and Life — The Cosmic Connection", Yale University Press.

External links

- [http://www.mrao.cam.ac.uk/telescopes/coast/betel.html Images of starspots on the surface of Betelgeuse]
- [http://simbad.u-strasbg.fr/sim-fid.pl Find out what is known about any given star by entering its name or position]
- [http://www.enchantedlearning.com/subjects/astronomy/stars/startypes.shtml Star Classification] Category:Astronomical objects ko:항성 ms:Bintang ja:恒星 simple:Star th:ดาวฤกษ์

Plasma

:This article is about plasma in the sense of an ionized gas. For other uses of the term, such as blood plasma, see plasma (disambiguation). plasma (disambiguation) In physics and chemistry, a plasma is an ionized gas, and is usually considered to be a distinct phase of matter. "Ionized" in this case means that at least one electron has been removed from a significant fraction of the molecules. The free electric charges make the plasma electrically conductive so that it couples strongly to electromagnetic fields. This fourth state of matter was first identified by Sir William Crookes in 1879 and dubbed "plasma" by Irving Langmuir in 1928, because it reminded him of a blood plasma [http://www.plasmacoalition.org/what.htm].

Common plasmas

blood plasma Plasmas are the most common phase of matter. The entire visible universe outside the Solar System is plasma, since all we can see are stars. Since the space between the stars is filled with a plasma, although a very sparse one (see interstellar- and intergalactic medium), essentially the entire volume of the universe is plasma. In the Solar System, the planet Jupiter accounts for most of the non-plasma, only about 0.1% of the mass and 10⁻¹⁵ of the volume within the orbit of Pluto. Alfvén also noted that due to their electric charge, very small grains also behave as ions and form part of a plasma (see dusty plasmas). Commonly encountered forms of plasma include:
- Artificially produced
- Inside fluorescent lamps (low energy lighting), neon signs
- Rocket exhaust
- The area in front of a spacecraft's heat shield during reentry into the atmosphere
- Fusion energy research
- The electric arc in an arc lamp or an arc welder
- Plasma ball (sometimes called a plasma sphere or plasma globe)
- Earth plasmas
- Flames (ie. fire)
- Lightning
- The ionosphere
- The polar aurorae
- Space and astrophysical
- The Sun and other stars (which are plasmas heated by nuclear fusion)
- The solar wind
- The Interplanetary medium (the space between the planets)
- The Interstellar medium (the space between star systems)
- The Intergalactic medium (the space between galaxies)
- The Io-Jupiter flux-tube
- Accretion disks
- Interstellar nebulae

Characteristics

The term plasma is generally reserved for a system of charged particles large enough to behave as one. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e. respond to magnetic fields and be highly electrically conductive). In technical terms, the typical characteristics of a plasma are: # Debye screening lengths that are short compared to the physical size of the plasma. # Large number of particles within a sphere with a radius of the Debye length. # Mean time between collisions usually is long when compared to the period of plasma oscillations.

Plasma scaling

Plasmas and their characteristics exist over a wide range of scales (ie. they are scaleable over many orders of magnitude). The following chart deals only with conventional atomic plasmas and not other exotic phenomena, such as, quark gluon plasmas:

	Typical plasma scaling ranges: orders of magnitude (OOM)
Characteristic	Terrestrial plasmas	Cosmic plasmas
Size in metres (m)	10⁻⁶ m (lab plasmas) to: 10² m (lightning) (~8 OOM)	10⁻⁶ m (spacecraft sheath) to 10²⁵ m (intergalactic nebula) (~31 OOM)
Lifetime in seconds (s)	10⁻¹² s (laser-produced plasma) to: 10⁷ s (fluorescent lights) (~19 OOM)	10¹ s (solar flares) to: 10¹⁷ s (intergalactic plasma) (~17 OOM)
Density in particles per cubic metre	10⁷ to: 10²¹ (inertial confinement plasma)	10³⁰ (stellar core) to: 10⁰ (i.e., 1) (intergalactic medium)
Temperature in kelvins (K)	~0 K (Crystalline non-neutral plasma[http://sdphca.ucsd.edu/]) to: 10⁸ K (magnetic fusion plasma)	10² K (aurora) to: 10⁷ K (Solar core)
Magnetic fields in teslas (T)	10⁻⁴ T (Lab plasma) to: 10³ T (pulsed-power plasma)	10⁻¹² T (intergalactic medium) to: 10⁷ T (Solar core)

Temperatures

plasma scaling characteristic of the gas being excited.]] The defining characteristic of a plasma is ionization. Although ionization can be caused by UV radiation, energetic particles, or strong electric fields, (processes that tend to result in a non-Maxwellian electron distribution function), it is more commonly caused by heating the electrons in such a way that they are close to thermal equilibrium so the electron temperature is relatively well-defined. Because the large mass of the ions relative to the electrons hinders energy transfer, it is possible for the ion temperature to be very different from (usually lower than) the electron temperature. The degree of ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density) in accordance with the Saha equation. If only a small fraction of the gas molecules are ionized (for example 1%), then the plasma is said to be a cold plasma, even though the electron temperature is typically several thousand degrees. The ion temperature in a cold plasma is often near the ambient temperature. Because the plasmas utilized in plasma technology are typically cold, they are sometimes called technological plasmas. They are often created by using a very high electric field to accelerate electrons, which then ionize the atoms. The electric field is either capacitively or inductively coupled into the gas by means of a plasma source, e.g. microwaves. Common applications of cold plasmas include plasma-enhanced chemical vapor deposition, plasma ion doping, and reactive ion etching. A hot plasma, on the other hand, is nearly fully ionized. This is what would commonly be known as the "fourth-state of matter". The Sun is an example of a hot plasma. The electrons and ions are more likely to have equal temperatures in a hot plasma, but there can still be significant differences.

Densities

Next to the temperature, which is of fundamental importance for the very existence of a plasma, the most important property is the density. The word "plasma density" by itself usually refers to the electron density, that is, the number of free electrons per unit volume. The ion density is related to this by the average charge state

\langle Z\rangle

of the ions through

n_e=\langle Z\rangle n_i

. (See quasineutrality below.) The third important quantity is the density of neutrals

n_0

. In a hot plasma this is small, but may still determine important physics. The degree of ionization is

n_i/(n_0+n_i)

Potentials

reactive ion etching Since plasmas are very good conductors, electric potentials play an important role. The potential as it exists on average in the space between charged particles, independent of the question of how it can be measured, is called the plasma potential or the space potential. If an electrode is inserted into a plasma, its potential will generally lie considerably below the plasma potential due to the development of a Debye sheath. Due to the good electrical conductivity, the electric fields in plasmas tend to be very small, although where double layers are formed, the potential drop can be large enough to accelerate ions to relativistic velocities and produce synchrotron radiation such as x-rays and gamma rays. This results in the important concept of quasineutrality, which says that, on the one hand, it is a very good approximation to assume that the density of negative charges is equal to the density of positive charges (

n_e=\langle Z\rangle n_i

), but that, on the other hand, electric fields can be assumed to exist as needed for the physics at hand. The magnitude of the potentials and electric fields must be determined by means other than simply finding the net charge density. A common example is to assume that the electrons satisfy the Boltzmann relation,

n_e \propto e^

. Differentiating this relation provides a means to calculate the electric field from the density:

\vec = (k_BT_e/e)(\nabla n_e/n_e)

. It is, of course, possible to produce a plasma that is not quasineutral. An electron beam, for example, has only negative charges. The density of a non-neutral plasma must generally be very low, or it must be very small, otherwise it will be dissipated by the repulsive electrostatic force. In astrophysical plasmas, Debye screening prevents electric fields from directly affecting the plasma over large distances (ie. greater than the Debye length). But the existence of charged particles causes the plasma to generate and be affected by magnetic fields. This can and does cause extremely complex behavior, such as the generation of plasma double layers, an object that separates charge over a few tens of Debye lengths. The dynamics of plasmas interacting with external and self-generated magnetic fields are studied in the academic discipline of magnetohydrodynamics.

In contrast to the gas phase

Plasma is often called the fourth state of matter. It is distinct from the three lower-energy phases of matter; solid, liquid, and gas, although it is closely related to the gas phase in that it also has no definite form or volume. There is still some disagreement as to whether a plasma is a distinct state of matter or simply a type of gas. Most physicists consider a plasma to be more than a gas because of a number of distinct properties including the following:

Property	Gas	Plasma
Electrical Conductivity	Very low	Very high For many purposes the electric field in a plasma may be treated as zero, although when current flows the voltage drop, though small, is finite, and density gradients are usually associated with an electric field according to the Boltzmann relation. The possibility of currents couples the plasma strongly to magnetic fields, which are responsible for a large variety of structures such as filaments, sheets, and jets. Collective phenomena are common because the electric and magnetic forces are both long-range and potentially many orders of magnitude stronger than gravitational forces.
Independently acting species	One	Two or three Electrons, ions, and neutrals can be distinguished by the sign of their charge so that they behave independently in many circumstances, having different velocities or even different temperatures, leading to new types of waves and instabilities, among other things
Velocity distribution	Maxwellian	May be non-Maxwellian Whereas collisional interactions always lead to a Maxwellian velocity distribution, electric fields influence the particle velocities differently. The velocity dependence of the Coulomb collision cross section can amplify these differences, resulting in phenomena like two-temperature distributions and run-away electrons.
Interactions	Binary Two-particle collisions are the rule, three-body collisions extremely rare.	Collective Each particle interacts simultaneously with many others. These collective interactions are about ten times more important than binary collisions.

Complex plasma phenomena

Boltzmann relation. Langmuir coined the name plasma because of its similarity to blood plasma, and Hannes Alfvén noted its cellular nature. Note also the filamentary blue outer shell of X-ray emitting high-speed electrons]] Plasma may exhibit complex behaviour. And just as plasma properties scale over many orders of magnitude (see table above), so do these complex features. Many of these features were first studied in the laboratory, and in more recent years, have been applied to, and recognised throughout the universe. Some of these features include:
- Filamentation, the striations or "stringy things" seen in a "plasma ball", the aurora, lightning, electric arcs, and nebulae. They are caused by larger current densities, and are also called magnetic ropes or plasma cables.
- Double layers, localised charge separation regions that have a large potential difference across the layer, and a vanishing electric field on either side. Double layers are found between adjacent plasmas regions with different physical characteristics, and can accelerate ions and produce synchrotron radiation (such as x-rays and gamma rays).
- Birkeland currents, a magnetic-field-aligned electric current, first observed in the Earth's aurora, and also found in plasma filaments.
- Circuits. Birkeland currents imply electric circuits, that follow Kirchhoff's circuit laws. Circuits have a resistance and inductance, and the behaviour of the plasma depends on the entire circuit. Such circuits also store inductive energy, and should the circuit be disrupted, for example, by a plasma instability, the inductive energy will be released in the plasma.
- Cellular structure. Plasma double layers may separate regions with different properties such as magnetization, density, and temperature, resulting in cell-like regions. Examples include the magnetosphere, heliosphere, and heliospheric current sheet.
- Critical ionization velocity in which the relative velocity between an ionized plasma and a neutral gas, may cause further ionization of the gas, resulting in a greater influence of electomagnetic forces.

Ultracold plasmas

It is also possible to create ultracold plasmas, by using lasers to trap and cool neutral atoms to temperatures of 1 mK or lower. Another laser then ionizes the atoms by giving each of the outermost electrons just enough energy to escape the electrical attraction of its parent ion. The key point about ultracold plasmas is that by manipulating the atoms with lasers, the kinetic energy of the liberated electrons can be controlled. Using standard pulsed lasers, the electron energy can be made to correspond to a temperature of as low as 0.1 K a limit set by the frequency bandwidth of the laser pulse. The ions, however, retain the millikelvin temperatures of the neutral atoms. This type of non-equilibrium ultracold plasma evolves rapidly, and many fundamental questions about its behaviour remain unanswered. Experiments conducted so far have revealed surprising dynamics and recombination behaviour that are pushing the limits of our knowledge of plasma physics.

Mathematical descriptions

Plasmas may be usefully described with various levels of detail. However the plasma itself is described, if electric or magnetic fields are present, then Maxwell's equations will be needed to describe them. The coupling of the description of a conductive fluid to electromagnetic fields is known generally as magnetohydrodynamics, or simply MHD.

Fluid

The simplest possibility is to treat the plasma as a single fluid governed by the Navier Stokes Equations. A more general description is the two-fluid picture, where the ions and electrons are considered to be distinct.

Kinetic

For some cases the fluid description is not sufficient. Kinetic models include information on distortions of the velocity distribution functions with respect to a Maxwell-Boltzmann distribution. This may be important when currents flow, when waves are involved, or when gradients are very steep.

Particle-in-cell

Particle-in-cell (PIC) models include kinetic information by following the trajectories of a large number of individual particles. Charge and current densities are determined by summing the particles in cells which are small compared to the problem at hand but still contain many particles. The electric and magnetic fields are found from the charge and current densities with appropriate boundary conditions. PIC codes for plasma applications were developed at Los Alamos National Laboratory in the 1950's. Although often more calculationally intensive than alternative models, they are relatively easy to understand and program and can be very general.

Fundamental plasma parameters

Los Alamos National Laboratory All quantities are in Gaussian cgs units except temperature expressed in eV and ion mass expressed in units of the proton mass

\mu = m_i/m_p

; Z is charge state; k is Boltzmann's constant; K is wavelength; γ is the adiabatic index; ln Λ is the Coulomb logarithm.

Frequencies

- electron gyrofrequency, the angular frequency of the circular motion of an electron in the plane perpendicular to the magnetic field: :

\omega_ = eB/m_ec = 1.76 \times 10^7 B \mbox

- ion gyrofrequency, the angular frequency of the circular motion of an ion in the plane perpendicular to the magnetic field: :

\omega_ = eB/m_ic = 9.58 \times 10^3 Z \mu^ B \mbox

- electron plasma frequency, the frequency with which electrons oscillate when their charge density is not equal to the ion charge density (plasma oscillation): :

\omega_ = (4\pi n_ee^2/m_e)^ = 5.64 \times 10^4 n_e^ \mbox

- ion plasma frequency: :

\omega_ = (4\pi n_iZ^2e^2/m_i)^ = 1.32 \times 10^3 Z \mu^ n_i^ \mbox

- electron trapping rate :

\nu_ = (eKE/m_e)^ = 7.26 \times 10^8 K^ E^ \mbox^

- ion trapping rate :

\nu_ = (ZeKE/m_i)^ = 1.69 \times 10^7 Z^ K^ E^ \mu^ \mbox^

- electron collision rate :

\nu_e = 2.91 \times 10^ n_e\,\ln\Lambda\,T_e^ \mbox^

- ion collision rate :

\nu_i = 4.80 \times 10^ Z^4 \mu^ n_i\,\ln\Lambda\,T_i^ \mbox^

Lengths

plasma oscillation http://history.nasa.gov/SP-345/ch15.htm#250 Ref]]
- Electron thermal de Broglie wavelength, approximate average de Broglie wavelength of electrons in a plasma: :

\Lambda_e= \sqrt= 6.919\times 10^\,T_e^\,\mbox

- classical distance of closest approach, the closest that two particles with the elementary charge come to each other if they approach head-on and each have a velocity typical of the temperature, ignoring quantum-mechanical effects: :

e^2/kT=1.44\times10^\,T^\,\mbox

- electron gyroradius, the radius of the circular motion of an electron in the plane perpendicular to the magnetic field: :

r_e = v_/\omega_ = 2.38\,T_e^B^\,\mbox

- ion gyroradius, the radius of the circular motion of an ion in the plane perpendicular to the magnetic field: :

r_i = v_/\omega_ = 1.02\times10^2\,\mu^Z^T_i^B^\,\mbox

- plasma skin depth, the depth in a plasma to which electromagnetic radiation can penetrate: :

c/\omega_ = 5.31\times10^5\,n_e^\,\mbox

- Debye length, the scale over which electric fields are screened out by a redistribution of the electrons: :

\lambda_D = (kT/4\pi ne^2)^ = 7.43\times10^2\,T^n^\,\mbox

Velocities

- electron thermal velocity, typical velocity of an electron in a Maxwell-Boltzmann distribution: :

v_ = (kT_e/m_e)^ = 4.19\times10^7\,T_e^\,\mbox

- ion thermal velocity, typical velocity of an ion in a Maxwell-Boltzmann distribution: :

v_ = (kT_i/m_i)^ = 9.79\times10^5\,\mu^T_i^\,\mbox

- ion sound velocity, the speed of the longitudinal waves resulting from the mass of the ions and the pressure of the electrons: :

c_s = (\gamma ZkT_e/m_i)^ = 9.79\times10^5\,(\gamma ZT_e/\mu)^\,\mbox

- Alfven velocity, the speed of the waves resulting from the mass of the ions and the restoring force of the magnetic field: :

v_A = B/(4\pi n_im_i)^ = 2.18\times10^\,\mu^n_i^B\,\mbox

Dimensionless

waves meeting the heliopause]]
- square root of electron/proton mass ratio :

(m_e/m_p)^ = 2.33\times10^ = 1/42.9

- number of particles in a Debye sphere :

(4\pi/3)n\lambda_D^3 = 1.72\times10^9\,T^n^

- Alven velocity/speed of light :

v_A/c = 7.28\,\mu^n_i^B

- electron plasma/gyrofrequency ratio :

\omega_/\omega_ = 3.21\times10^\,n_e^B^

- ion plasma/gyrofrequency ratio :

\omega_/\omega_ = 0.137\,\mu^n_i^B^

- thermal/magnetic energy ratio :

\beta = 8\pi nkT/B^2 = 4.03\times10^\,nTB^

- magnetic/ion rest energy ratio :

B^2/8\pi n_im_ic^2 = 26.5\,\mu^n_i^B^2

Miscellaneous

- Bohm diffusion coefficient :

D_B = (ckT/16eB) = 6.25\times10^6\,TB^\,\mbox^2/\mbox

- transverse Spitzer resistivity :

\eta_\perp = 1.15\times10^\,Z\,\ln\Lambda\,T^\,\mbox = 1.03\times10^\,Z\,\ln\Lambda\,T^\,\Omega\,\mbox

Fields of active research

Bohm diffusion is so effective at accelerating ions, that electric fields are used in ion drives]] This is just a partial list of topics. A more complete and organised list can be found on the Web site for Plasma science and technology [http://www.plasmas.com/topics.htm].
- Plasma theory
- Plasma equilibria and stability
- Plasma interactions with waves and beams
- Guiding center
- adiabatic invariant
- Debye sheath
- Coulomb collision
- Plasmas in nature
- The Earth's ionosphere
- Space plasmas, e.g. Earth's plasmasphere (an inner portion of the magnetosphere dense with plasma)
- plasma cosmology
- Plasma sources
- Dusty Plasmas
- Plasma diagnostics
- Thomson scattering
- Langmuir probe
- Spectroscopy
- Interferometry
- Ionospheric heating
- Incoherent scatter radar
- Plasma applications
- Fusion power
- Magnetic fusion energy (MFE) -- tokamak, stellarator, reversed field pinch, magnetic mirror, dense plasma focus
- Inertial fusion energy (IFE) (also Inertial confinement fusion — ICF)
- Plasma-based weaponry
- Industrial plasmas
- plasma chemistry
- plasma processing
- plasma display

External links

- [http://fusedweb.pppl.gov/CPEP/Chart_Pages/5.Plasma4StateMatter.html Plasmas: the Fourth State of Matter]
- [http://www.plasmas.org/ Plasma Science and Technology]
- [http://plasma-gate.weizmann.ac.il/PlasmaI.html Plasma on the Internet] comprehensive list of plasma related links.
- [http://farside.ph.utexas.edu/teaching/plasma/lectures/lectures.html Introduction to Plasma Physics: a graduate level lecture course given by Richard Fitzpatrick]
- [http://plasmas.org/ An overview of plasma links and applications]
- [http://wwwppd.nrl.navy.mil/nrlformulary/index.html NRL Plasma Formulary online] (or an [http://w3.pppl.gov/~dcoster/nrl/ html version])
- [http://www.plasmacoalition.org/ Plasma Coalition page]
- [http://starfire.ne.uiuc.edu/ Plasma Material Interaction]
- [http://jnaudin.free.fr/html/oa_plasmoid.htm How to build a Stable Plasmoid at One Atmosphere] (requires pre-ignition)
- [http://jnaudin.free.fr/html/oa_plsm4.htm How to build a Stable Plasmoid with this Enhanced Generator] (self-igniting)
- [http://c3po.barnesos.net/homepage/lpl/grapeplasma/ How to make a glowing ball of plasma in your microwave with a grape] Category:Astrophysics ko:플라즈마 ja:プラズマ

Dark matter

In cosmology, dark matter refers to hypothetical matter particles, of unknown composition, that do not emit or reflect enough electromagnetic radiation to be detected directly, but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. The dark matter hypothesis aims to explain several anomalous astronomical observations, such as anomalies in the rotational speed of galaxies (the galaxy rotation problem). Estimates of the amount of matter present in galaxies, based on gravitational effects, consistently suggest that there is far more matter than is directly observable. The existence of dark matter would also resolve a number of inconsistencies in the Big Bang theory, and is crucial for structure formation. If dark matter does exist, it vastly outmasses the "visible" part of the universe [http://map.gsfc.nasa.gov/m_mm/mr_limits.html]. Only about 4% of the total mass in the universe (as inferred from gravitational effects) can be seen directly. About 23% is thought to be composed of dark matter. The remaining 73% is thought to consist of dark energy, an even stranger component, distributed diffusely in space, that probably cannot be thought of as ordinary particles. Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. The first to hypothesize dark matter was Fritz Zwicky, of the California Institute of Technology (Caltech) in 1933. He applied the virial theorem to the Coma cluster of galaxies and obtained evidence of unseen mass.

Evidence for dark matter

In 1913, Norwegian explorer and physicist Kristian Birkeland may have been the first to predict that space is not only a plasma, but also contains "dark matter". He wrote: "It seems to be a natural consequence of our points of view to assume that the whole of space is filled with electrons and flying electric ions of all kinds. We have assumed that each stellar system in evolutions throws off electric corpuscles into space. It does not seem unreasonable therefore to think that the greater part of the material masses in the universe is found, not in the solar systems or nebulae, but in "empty" space. (Ref. See notes) Dark matter was first hypothesized to exist by the Swiss astrophysicist Fritz Zwicky. In 1933 Zwicky estimated the total amount of mass in a cluster of galaxies, the Coma Cluster, based on the motions of the galaxies near the edge of the cluster. When he compared this mass estimate to one based on the number of galaxies and total brightness of the cluster, he found that there was about 400 times more mass than expected. The gravity of the visible galaxies in the cluster would be far too small for such fast orbits, so something extra was required. This is known as the "missing mass problem". Based on these conclusions, Zwicky inferred that there must be some other form of matter existent in the cluster which we have not detected, which provides enough of the mass and gravity to hold the cluster together. At present, the density of ordinary baryons and radiation in the universe is estimated to be about one hydrogen atom per cubic meter of space. However, dark matter and dark energy are together said to account for 96% of all matter in the universe. This means that only about 4% of all matter can be directly observed. Things like gas clouds, small lumps of iron, and brown dwarfs are ordinary baryonic matter and are all accounted for in the 4% that is observed [http://arxiv.org/abs/astro-ph/0007444] [http://arxiv.org/abs/astro-ph/0002058]. The term dark matter specifically refers to stuff other than what makes up ordinary matter (baryons) that we are familiar with. Since it cannot be directly detected via optical means, many aspects of dark matter remain speculative. The DAMA/NaI experiment has claimed to directly detect dark matter passing through the Earth, though most scientists remain sceptical since negative results of other experiments are (almost) incompatible with the DAMA results if dark matter consists of neutralinos.

Galactic rotation

Much of the evidence for dark matter comes from the study of the motions of galaxies. Many of these appear to be fairly uniform, so by the virial theorem the total kinetic energy should be half the total gravitational binding energy of the galaxies. Experimentally, however, it is found to be much greater: in particular, stars far from the center of galaxies have much higher velocities than predicted by the virial theorem. Galactic rotation curves, which illustrate the velocity of rotation versus the distance from the galactic center, cannot be explained by only the visible matter. Assuming that the visible material makes up only a small part of the cluster is the most straightforward way of accounting for this. Galaxies show signs of being composed largely of a roughly spherical halo of dark matter with the visible matter concentrated in a disc at the center. Low surface brightness dwarf galaxies are important sources of information for studying dark matter, as they have an uncommonly low ratio of visible matter to dark matter, and have few bright stars at the center which impair observations of the rotation curve of outlying stars. Recently, astronomers from Cardiff University claim to have discovered a galaxy made almost entirely of dark matter, 50 million light years away in the Virgo Cluster, which was named VIRGOHI21 (Wikinews, [http://www.newscientist.com/article.ns?id=dn7056 New Scientist]). Unusually, VIRGOHI21 does not appear to contain any visible stars: it was seen with radio frequency observations of hydrogen. Based on rotation profiles, the scientists estimate that this object contains approximately 1000 times as much dark matter as hydrogen and has a total mass of about 1/10th of that of the Milky Way Galaxy we live in. For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter. Models of the Big Bang and structure formation have suggested that such dark galaxies should be very common in the universe, but none have previously been detected. If the existence of this dark galaxy is confirmed, it provides strong evidence for the theory of galaxy formation and poses problems for alternative explanations of dark matter. Dark matter is believed to affect galaxy clusters as well. The galaxy cluster Abell 2029 is composed of thousands of galaxies enveloped in a cloud of hot gas, and an amount of dark matter equivalent to more than a hundred trillion Suns. At the center of this cluster is an enormous, elliptically shaped galaxy that is thought to have been formed from the mergers of many smaller galaxies. More info is available here: [http://chandra.harvard.edu/photo/2003/abell2029/ http://chandra.harvard.edu/photo/2003/abell2029/].

Structure formation

A significant amount of non-baryonic, cold matter is necessary to explain the large-scale structure of the universe. Observations suggest that structure formation in the universe proceeds hierarchically, with the smallest structures, such as stars, forming first, and followed by galaxies and then clusters of galaxies. In the universe, it is thought that the first structures that form are quasars, which are supermassive black holes. This, bottom up model of structure formation requires something like cold dark matter to succeed. Ordinary baryonic matter had too high a temperature, and too much pressure left over from the big bang to collapse and form smaller structures, such as stars, via the Jeans instability. Large computer simulations of billions of dark matter particles have been used to confirm that the cold dark matter model of structure formation is consistent with the structures observed in the universe through galaxy surveys, such as the Sloan Digital Sky Survey and 2dF Galaxy Redshift Survey, as well as observations of the Lyman-alpha forest. These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter. Another important tool for future dark matter observations is gravitational lensing, in particular a technique called weak lensing that allows astrophysicists to characterize the distribution of dark matter by statistical means.

Composition

Data from galaxy rotation curves indicate that nearly 90% of the mass of a galaxy cannot be seen. It can only be detected by its gravitational effect. Several categories of dark matter have been postulated.
- Hot dark matter
- Warm dark matter
- Cold dark matter
- Baryonic dark matter Hot dark matter consists of particles that travel with relativistic velocities. One kind of hot dark matter is known, the neutrino. Neutrinos have a very small mass, do not interact via either the electromagnetic or the strong nuclear force and so are incredibly difficult to detect. This is what makes them appealing as dark matter. However, bounds on neutrinos indicate that ordinary neutrinos make only a small contribution to the density of dark matter. Hot dark matter cannot explain how individual galaxies formed from the Big Bang. The microwave background radiation as measured by the COBE and WMAP satellites, while incredibly smooth, indicates that matter has clumped on very small scales. Fast moving particles, however, cannot clump together on such small scales and, in fact, suppress the clumping of other matter. Hot dark matter, while it certainly exists in our universe in the form of neutrinos, is therefore only part of the story. To explain structure in the universe it is necessary to invoke cold (non-relativistic) dark matter. Large masses, like galaxy-sized black holes can be ruled out on the basis of gravitational lensing data. Possibilities involving normal baryonic matter include brown dwarfs or perhaps small, dense chunks of heavy elements; such objects are known as massive compact halo objects, or "MACHOs". However, studies of big bang nucleosynthesis have convinced most scientists that baryonic matter such as MACHOs cannot be more than a small fraction of the total dark matter. At present, the most common view is that most dark matter is made of one or more elementary particles other than the usual electrons, protons, neutrons, and ordinary neutrinos. Currently, the most commonly considered particles are axions, sterile neutrinos, SIMPs (Strongly Interacting Massive Particles), and WIMPs (Weakly Interacting Massive Particles) (which include neutralinos). None of these are part of the standard model of particle physics. Instead, particles in this last category are frequently suggested by theorists proposing supersymmetric extensions of the standard model of particle physics. In such theories, the WIMP involved is usually the neutralino. Another candidate is so-called sterile neutrinos. Sterile neutrinos can be added to the standard model to explain the small neutrino mass. These sterile neutrinos are expected to be heavier than the ordinary neutrinos, and are a candidate for dark matter.

Alternative explanations

An alternative to dark matter is to suppose that the inconsistencies are due to an incomplete understanding of gravitation. One task could be given through the need of conciling gravitation with quantum mechanics and to explain mass and its creation (Higgs) within gravitation, as in some scalar-tensor theories, which couple scalar fields like the Higgs one to the curvature given through the Riemann tensor or its traces. In many of such theories, the scalar field equals the inflaton field, which is needed in some theories for explaining the inflation of the universe after the Big Bang, as the dominating factor of the quintessence or Dark Energy. To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distances or in weak fields. For instance, this can be done by assuming a negative value of the cosmological constant (the value of which is believed to be positive based on recent observations) or by assuming Modified Newtonian Dynamics (MOND), which corrects Newton's laws at small acceleration. However, constructing a relativistic MOND theory has been troublesome, and it is not clear how the theory can be reconciled with gravitational lensing measurements of the deflection of light around galaxies. The leading relativistic MOND theory, proposed by Milgrom's colleague Professor Bekenstein in 2004 is called "TeVeS" for Tensor-Vector-Scalar and solves many of the problems of earlier attempts. Another approach, proposed by Finzi (1963) and again by Sanders (1984), is to replace the gravitational potential energy with the expression :

U=\frac

where B and ρ are adjustable parameters. However, such approaches run into difficulties explaining the different behavior of different galaxies and clusters, whereas one can easily describe such differences by assuming different quantities of dark matter. For a deeper discussion of this subject, see Modified Newtonian dynamics. Another proposed explanation of the mystery is Nonsymmetric Gravitational Theory. Two other theories which propose modifications to general relativity have recently been proposed. M. Reuter and H. Weyer have proposed that Newton's constant grows at large scales due to quantum effects [http://arxiv.org/abs/hep-th/0410117]. Another proposal by Cooperstock and Tieu suggested that the galaxy rotation problem could be explained with the results of general relativity, amplified by non-linear effects so that the behavior of the galaxy as a whole becomes non-Newtonian [http://arxiv.org/abs/astro-ph/0507619]. A problem in this model was found when it was shown that this model gives rise to a "thin, singular disk" of 2-dimensional matter in the galactic plane [http://arxiv.org/abs/astro-ph/0508377/]. In a [http://arxiv.org/abs/astro-ph/0510750 recent article] it is shown that Cooperstock's and Tieu's model implies that the thin disk must be made out of "exotic matter, either cosmic strings or struts with negative energy density". Cooperstock and Tieu have since [http://arxiv.org/abs/astro-ph/0512048 responded to the potential flaw in their model.] Their amended model has no singularity at the plane of symmetry, yet still it can explain galactic rotation without assuming dark matter exists.

Dark matter in popular culture

Mentions of dark matter occur in some video games and other works of fiction. In such cases, it is usually attributed extraordinary physical or magical properties. Such descriptions are often inconsistent with the properties of dark matter proposed in physics and cosmology.

References

- Polar Magnetic Phenomena and Terrella Experiments, in The Norwegian Aurora Polaris Expedition 1902-1903 (publ. 1913, p.720 on 'dark matter')

External links

- [http://astron.berkeley.edu/~mwhite/darkmatter/hdm.html Hot dark matter]
- [http://lpsc.in2p3.fr/tep/fred/dm.html Dark matter Portal]
- [http://arxiv.org/abs/hep-ph/0404175 G. Bertone, D. Hooper, and J. Silk, "Particle Dark Matter: Evidence, Candidates and Constraints"]
- [http://www.livingreviews.org/lrr-2002-4 Timothy J. Sumner, "Experimental Searches for Dark Matter"]
- [http://www.economist.com/science/displaystory.cfm?story_id=3556105 The Economist: Young solar systems are like cosmic snooker games, and the universe is flat]
- [http://www.wired.com/news/space/0,2697,66487,00.html Scientists Find Missing Matter (Wired.com Feb 3rd 2005) ]
- [http://news.bbc.co.uk/1/hi/wales/south_east/4288633.stm Astronomers find star-less galaxy (BBC News Feb 23rd 2005) ]
- [http://www.physorg.com/news6850.html Elliptical galaxies have dark matter halo as well]
- [http://www.isracast.com/tech_news/061005_tech.htm Dark matter History and More on Elliptical galaxies and the Mystery of dark matter]
- [http://www.ebicom.net/~rsf1/missmass.htm Cosmology's Missing Mass Problems]
- [http://xxx.lanl.gov/find/grp_physics/1/abs:+AND+Dark+Matter/0/1/0/past,2005/0/1 recent papers on dark matter on arXiv.org] Category:Celestial mechanics Category:Cosmology
- Category:Large-scale structure of the cosmos ko:암흑물질 ja:暗黒物質

Center of gravity

In physics, the center of gravity (CoG) of an object is a point at which the object's mass can be assumed, for many purposes, to be concentrated. For example, if you hang an object from a string, the object's center of gravity will be directly below the string. The path of an object in orbit depends only on its center of gravity. Most astronomical objects are radially symmetric, causing both the center of gravity and the center of mass to coincide at the center of the sphere. The center of gravity of an object is the average location of its weight. In a uniform gravitational field, it coincides with the object's center of mass. (In modern Britain the spelling centre is standard. Both spellings originated in England; center is now standard in America.) Note that the center of gravity of a body is not a point such that the gravitational field due to that body is equal to the gravitational field if all mass were concentrated there. Such a point usually does not exist. For example, in the case of two equal bodies the center of gravity of the system would have to be (and is) midway, but gravity due to the system is not very large near that point.

Locating Center of Gravity

A simple experiment to locate the center of gravity of a "2-D" object:
You will need:
1) A roughly cut piece of cardboard [lamina](for best results just tear a piece of carboard off)
2) Some string
3) A weight of some sort (not very heavy, but heavy enough to hold string down)
4) A punch (or some sort of device which can be used to puncture a hole in the carboard)
5) 2 pencils
6) Scissors with which to cut the string
7) A tailor's pin or a very thin nail
Conducting the experiment:
1) After tearing your cardboard to create your lamina, use the punch to punch two holes on it : one on the left hand side and one on the top (it doesn't really matter where you punch the holes, as long as they aren't too close to each other).
2) Cut a piece of string and put it through one of the holes. Knot it at the ends and put one of the pencils through the loop. The loop must be loose so that the lamina is not restricted.
3) Cut a significantly longer piece of string (around thrice the size of the height of your cardboard). Tie one end around the weight, making sure that when you hold the string up, the weight is reasonably centered.This 'device' is known as a plumbline. Make a loose loop at the other end and put the SAME pencil through that as well.
4) After making sure that the two loops on the pencil are very close together, place the pencil between two high tables (you can put equal number of books on the tables if the height is not enough) so that the string, the weight and the lamina hang without touching the floor, or any other object.
5) Wait for the strings to steady and draw a line where the string with the weight overlaps the lamina with the second pencil.
6) Repeat the same process with the other hole on the lamina.
7) Once you have done this, there will be a point where the two lines on the lamina intersect. If you have done the job accurately, this should be the center of gravity (CoG) of this "2-D" object(as the cardboard is very thin, the width is almost negligable, hence it serves the purpose of being a 2-D object). If you balance your lamina on the tip of the pin at this point, it should stay forever without tilting/turning (besides effects of external forces such as the air). If this does not work properly, then try placing the tip of the pin a litle bit to the left/right/top/bottom of the pont you have located.
8) Once you locate your final, accurate center of gravity, calculate the percentage of your error to see how accurate you carried out your measurement. The percentage of error should not be more than 5%, otherwise you have not been accurate enough while conducting your experiment.

Similarities between center of mass and center of inertia

In a uniform gravitational field (in other words, when the tidal force is insignificant), the center of mass and the center of gravity are at the same location. In a radially uniform gravitational field (such as the one formed by a typical star), the center of mass and the center of gravity of a radially symmetric object (such as planets and stars) are both at the center of the sphere.

Differences between center of mass and center of inertia

- the "center of buoyancy" of an object depends only on its geometric shape, independent of its density.
- the "center of mass" of an object depends on its shape and its density
- the center of gravity of an object depends on its shape, density, and the external gravitational field. The center of mass of a long, uniform beam (of rectangular or circular cross section) is always at the center of the beam. In locations where Earth's gravity dominates, the center of gravity of a vertical, long, uniform beam is closer to Earth than the center of the beam (although it is still inside the beam). In locations where Earth's gravity dominates, the center of gravity of a horizontal, long, uniform beam is further away from Earth than the center of the beam. (The CoG may be outside the beam, if the beam is long enough and narrow enough). When we say :

\mathbf = m\mathbf

(Newton's second law of motion), the "a" (acceleration) referred to is the acceleration of the center of mass. The "F" (force), however, when caused by gravitational forces, depends on the center of gravity.

References

-
-

Star cluster

.]] Star clusters are groups of stars which are gravitationally bound. Two distinct types of star cluster can be distinguished: globular clusters are tight groups of hundreds of thousands of very old stars, while open clusters generally contain less than a few hundred members, and are often very young. Open clusters become disrupted over time by the gravitational influence of giant molecular clouds as they move through the galaxy, but cluster members will continue to move in broadly the same direction through space even though they are no longer gravitationally bound; they are then known as a stellar association, sometimes also referred to as a moving group.

Globular clusters

Globular clusters are roughly spherical groups of anything between 10,000 and several million stars in a region about 10 to 30 light years across. They generally consist of very old Population II stars, just a few million years younger than the universe itself. The constituent stars tend to be yellow and red, and weigh less than about two solar masses. This is because the hotter, more massive stars have either exploded as supernovae or passed through a planetary nebula phase to become white dwarfs. However, some anomalous blue stars are found in globulars, and are believed to have been formed by stellar mergers in the dense inner regions of the cluster. These stars are known as blue stragglers. In our galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the galactic centre, orbiting the centre in highly elliptical orbits. In 1917, the astronomer Harlow Shapley was able to estimate the Sun's distance from the galactic centre based on the distribution of globular clusters; previously the Sun's location within the Milky Way was by no means well established. Until recently, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the Hipparcos satellite and increasingly accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less. Our galaxy has about 150 globular clusters, some of which may have been captured from small galaxies disrupted by the Milky Way, as seems to be the case for the globular cluster M79. Some galaxies are much richer in globulars: the giant elliptical galaxy M87 contains over a thousand. A few of the brightest globular clusters are visible to the naked eye, with the brightest, Omega Centauri, having been known since antiquity and catalogued as a star before the telescopic age. The best known globular cluster in the northern hemisphere is M13 (modestly called the Great Globular Cluster in Hercules).

Open clusters

Great Globular Cluster in Hercules Open clusters are very different to globular clusters. Unlike the spherically-distributed globulars, they are confined to the galactic plane, and are almost always found within spiral arms. They are generally young objects, up to a few tens of millions of years old. They form from H II regions such as the Orion Nebula. Open clusters usually contain up to a few hundred members, within a region up to about 30 light years across. Being much less densely populated than globular clusters, they are much less tightly gravitationally bound, and over time, are disrupted by the gravity of giant molecular clouds and other clusters. Close encounters between cluster members can also result in the ejection of stars, a process known as 'evaporation'. The most prominent open clusters are the Pleiades and Hyades in Taurus. The Double Cluster of h+Chi Persei can also be prominent under dark skies. Open clusters are often dominated by hot young blue stars, because although such stars are short-lived in stellar terms, only lasting a few tens of millions of years, open clusters tend to have dispersed before these stars die.

Stellar associations

Once an open cluster has become gravitationally unbound, the constituent stars will continue to move on similar paths through space. The group is then known as a stellar association, or a moving group. Several of the brightest stars in Ursa Major are members of a former open cluster, and have similar proper motions. Other bright stars across the sky, including Sirius and Alpha Ophiuchi, seem to also be related to this group. Our Sun lies within this stream of stars at the moment, but isn't a true member as shown by its different galactic orbit. Another stellar association is that surrounding Mirfak (α Persei), which is very prominent in binoculars. Distant moving clusters can't readily be detected since the proper motions of the stars need to be known.

Astronomical significance of clusters

The study of star clusters is very important in many areas of astronomy. Because the stars were all born at roughly the same time, the different properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters. Clusters are also a crucial step in determining the distance scale of the universe. A few of the nearest clusters are close enough for their distances to be measured using parallax. A Hertzsprung-Russell Diagram can be plotted for these clusters which has absolute values known on the luminosity axis. Then, when similar diagrams are plotted for clusters whose distance is not known, the position of the main sequence can be compared to that of the first cluster and the distance estimated.

External links

- [http://www.seds.org/messier/cluster.html Star Clusters], SEDS Messier pages
- ja:星団

Light year

:There was also a 1988 animated science fiction film named "Light Years". A light year (or light-year), abbreviated ly, is the distance light travels in one year: about 9.461 × 10¹⁵ metres (9.461 petametres), or about 5.879 × 10¹² (nearly six trillion) miles. More specifically, a light year is defined as the distance that a photon would travel, in free space and infinitely distant from any gravitational or magnetic fields, in one Julian year (365.25 days of 86,400 seconds each). Since the speed of light in a vacuum is exactly 299,792,458 m/s by the definition of metre, one light year is exactly equal to 9,460,730,472,580,800 m. The light year is often used to measure distances to stars: a light year is not a unit of time. In astronomy, the preferred unit of measurement for such distances is the parsec which is defined as the distance at which an object will generate one arcsecond of parallax when the observing object moved one astronomical unit perpendicular to the line of sight to the observer. This is equal to approximately 3.26 light years. The parsec is preferred because it can be more easily derived from, and inter-compared with, observational data. However, outside scientific circles, the term light year is more widely used by the general public. A light year is also equal to about 63,241.077 astronomical units (AU). For a list of lengths on the order of one light year, see the article 1 E15 m. Units related to the light year are the light minute and light second, the distance light travels in a vacuum in one minute and one second, respectively. A light minute is equal to 17,987,547,480 m. Since light travels 299,792,458 m in one second, a light second is 299,792,458 m in length.

Miscellaneous facts

- It takes 8.3 minutes for light to travel from the Sun to the Earth (a distance of light years).
- The most distant space probe, Voyager 1, was 13 light hours (only light years) away from Earth in September 2004. It took Voyager 27 years to cover that distance.
- The nearest known star, Proxima Centauri is 4.22 light years away.
- The center of our galaxy, the Milky Way, is about 28,000 light years away. The Galaxy is about 100,000 light years across.
- The nearest large galaxy cluster, the Virgo Cluster, is about 60 million light years away.
- The particle horizon (observable part) of the universe has a radius of about 46 billion light years, but light from the edge of the observable universe was emitted only 13.7 billion years ago (the age of the universe). The figures differ because distant objects have continued to recede from us due to cosmological expansion (see Hubble's law).
- One gigaparsec is equal to approximately 3.2 billion light years.

External link

- [http://www.ex.ac.uk/trol/scol/ccleng.htm Conversion Calculator for Units of LENGTH] Category:Astronomical units of length ko:광년 ms:Tahun cahaya ja:光年 simple:Light year th:ปีแสง

Supermassive black hole

] A supermassive black hole is a black hole with a mass in the range of millions or billions of solar masses. It is currently thought that most if not all galaxies, including the Milky Way, contain a supermassive black hole at their galactic centers. Supermassive black holes have some interesting properties which distinguish them from relatively low-mass cousins:
- The average density of a supermassive black hole can be very low, and may actually be lower than the density of water. This is because the Schwarzschild radius is directly proportional to mass, such that density is inversely proportional to the square of the mass.
- The tidal forces in the vicinity of the event horizon are significantly weaker. Since the central singularity is so far away from the horizon, a hypothetical astronaut travelling towards the black hole center would not experience significant tidal force until very deep into the black hole. Black holes of this size can only form in two ways: by slow accretion of matter (starting from a stellar size), or directly from external pressure in the first instances of the Big Bang. The first method requires a long time and large amounts of matter available for the black hole growth. Direct Doppler measures of the matter surrounding the nucleus of nearby galaxies have revealed a very fast motion, only possible with a high concentration of matter in the center. Currently, the only known object that can pack enough matter in such a small space is a black hole. For active galaxies farther away, the width of br

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