Category:Classification Of Galaxies
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Classification of Galaxies
Hubble classification of galaxies Method devised by Edwin Powell Hubble (1889-1953) for classifying galaxies by shape.
Irregular galaxy (Hubble Ir) No obvious symmetry, i.e. not obviously a spiral galaxy or an elliptical galaxy.
Elliptical Galaxies
Elliptical galaxy (Hubble E) Structureless agglomerations of stars, that make up c. 80% of normal galaxies whose visible shape forms an ellipse.
- Oblate Elliptical galaxy like a pancake.
- Prolate Elliptical galaxy like a cylindrical sausage shape.
- Triaxial Elliptical galaxy like a slightly squashed sausage, unstable shape.
BL Lac object (BL Lacertae object; Lacertid) Type of elliptical galaxy with a bright, highly variable, compact nucleus, discovered 1929.
D galaxy Type of large elliptical galaxy with a bright nucleus surrounded by an extensive envelope, of radio galaxies, designation introduced by W. Morgan, late-1950s.
- cD galaxy Member of class of giant elliptical galaxies with an extended halo of stars, designation introduced by W. Morgan, late-1950s.
Lenticular galaxy Elliptical galaxy in elongated form with nearly pointed ends.
Spiral Galaxies
Spiral galaxy (Hubble S) Galaxy with a bright ellipsoidal central nucleus from which spiral arms emerge tangentially at two diametrically opposite points to form a disk, and die out about one complete turn.
- Barred spiral (Hubble SB) Characterized by a bright nucleus with a bar running through it, from the ends of which the two spiral arms extend part of the way around the galactic nucleus.
- Transitional galaxy (Hubble SO) Transitional type suggested by Hubble in 1923 as the ‘missing link’ in an evolutionary chain from EO through open spirals Sc and SBc. This concept of progression is no longer accepted.
Disc galaxy Spiral galaxy that has been stripped of most of its interstellar gas through interaction with the intergalactic medium in a cluster of galaxies.
- Galactic disc Plate-shaped component of a spiral galaxy, in which the spiral arms are found.
- Galactic cluster Group of stars located in the galactic disc. The stars typically number in the hundreds.
- Disc Flattened portion of a spiral galaxy without the arms.
- Whirlpool galaxy Interacting spiral galaxy connected to its companion galaxy by a bridge of stars.
Hypergalaxy Group of galaxies consisting of a dominating spiral galaxy surrounded by a mass of dwarf galaxies.
Liner (= Low-ionization narrow emission line region) Regions that are found in a high proportion of spiral galaxies.
Tully-Fisher relation Correlation between the width of the twenty-one-centimeter line radiation from spiral galaxies and their absolute photographic magnitude, used for estimating distances of spiral galaxies, developed by B. Tully and R. Fisher 1977.
Other Types of Galaxies
(alphabetical listing)
Active galaxy New-born galaxy with a giant black hole at its core. An accretion disk of matter swirls around it, which eventually falls inwards.
- Starburst galaxy Galaxy undergoing a rapid wave of stellar birth.
- Infrared galaxy Galaxy that emits most of its energy, typically more than 90%, in the infrared region of the spectrum. They are usually starburst galaxies.
Dwarf galaxy Small elliptical or spheroidal galaxy containing between a few hundred thousand and a few million stars.
- Satellite galaxy Dwarf galaxy in orbit around a larger one.
Emission line galaxy Any galaxy that displays emission lines in its spectrum.
Faint blue blobs Faint bluish galaxies that have been showing up on astronomical plates since the 1970s. They have turned out to be the most common galaxies in the universe, the blueness being the result of many new hot starts being formed. The belief is these galaxies were born from dust and gas left over from the formation of larger elliptical and spiral galaxies. Shortly after the giant galaxies formed, quasars erupted inside their cores, spewing out radiation that heated intergalactic gas, making it so energetic that the gas could not organize into galaxies for 7 or 8 billion years. Only after the quasars died did the small blue galaxies form. Observations of faint blue blobs are the result of the Medium Deep Survey, using the Hubble Space Telescope, which examines galaxies up to 10 billion light-years from Earth.
Haro galaxy Member of a class of galaxies characterized by their blue color and narrow emission line spectra.
Markarian galaxy Any of the galaxies in the list drawn up by the Soviet astronomer B. E. Markarian in the 1970s, characterized by strong continuum emission of ultraviolet light.
N stars Galaxies which resemble quasars, whose light is dominated by a point-like stellar nucleus.
Normal galaxy Any spiral or elliptical galaxy that does not have an unusual structure.
Peculiar galaxy Any galaxy that does not readily fit into the Hubble classification.
Radio galaxy Galaxy that emits an intense source of radio radiation. First definite optical identification of a radio source beyond our galaxy, Cygnus A, made by German-American astronomers Rudolph Leo Minkowski (1895-1976) and Wilhelm Heinrich Walter Baade (1893-1960) 1954.
- Head-tail galaxy Radio galaxy with radio emission streaming away to one side of the corresponding optical galaxy, giving a shape resembling a tadpole.
Ring galaxy Shaped like a ring, thought to be the result from a collision in which one galaxy passes through another.
Seyfert-1 galaxy Type of galaxy with a brilliant point-like nucleus and inconspicuous spiral arms, first described by Carl Seyfert 1943.
- Seyfert-2 galaxy Classified as type-2 quasars.
Starburst galaxy Exceptionally high rate of star formation within the galaxy, characterized by excessive emission of infrared radiation.
Ultraluminous infrared galaxies (ULIRGs) Exotic class of galaxies that emit most of their total energy output in the far-infrared, a telltale sign that they are saturated with dust. Nearly all ULIRGs show signs of having undergone a collision with another galaxy.
- NGC 6240 Ultraluminous infrared galaxy that churns out new stars that also has one of the most voracious black holes.
Galaxy Clusters
Cluster of galaxies (Cluster; Galaxy cluster; Supercluster of galaxies) Huge assemblages of galaxies linked by their mutual gravitational attraction, the most massive structures in the universe, that include huge amounts of gas threading the space between the galaxies.
- Galaxy group Large number of galaxies linked by their mutual gravitational attraction.
- Coma cluster (Berenice's cluster) Very large group of galaxies that emit x-rays. Fre Zwicky shows that most of the mass in the Coma cluster is invisible, 1937.
Crossing time Ratio between the diameter of a rich cluster of galaxies and the average random velocity for galaxies within the cluster, a measure of the time for a galaxy to drift through the cluster,
Absorption of a galaxy group Process that allows a galaxy cluster to grow to a colossal size. Pulled in by gravity, the galaxy group slams into the cluster, pushing gas out the sides. The galaxies themselves pass through the cluster, their progress unimpeded by the tenuous gas. Eventually the galaxies and the gas mix together, forming a unified cluster that continues to draw in other galaxy groups until no more are found.
- Cooling flows Gas within a cluster that is losing temperature, a process remaining a mystery.
Supercluster Long and thin strands of clusters and galaxies, intercluster gases and, presumably, dark matter that stretch across tens of millions of light-years, typically about one hundred million (108) light-years in diameter and contain tens of thousands of galaxies. The strands are interspersed by large voids nearly empty of galaxies. The reality of superclusters did not sink in until the 1980s and only the beginnings of a clear picture of them had been developed by the late 1990s.
- Virgo Supercluster Aggregation of galaxies, roughly 10,000 of them, to which the Virgo Cluster and our own galaxy belong.
- Virgo cluster Nearby cluster of hundreds of galaxies seen near the constellation Virgo, at 60 million light-years from Earth.
Superwinds Flow of matter across a supercluster that are the right direction and at the right speed to feed matter, including stars, galaxies and gas, into growing galactic clusters.
Great Wall Largest gigantic cosmic conglomeration of supergalaxies stretching for 500 million light-years which was claimed to have grown out of irregularities in the distribution of matter that existed in the era of decoupling, mapped by Margaret J. Geller and John P. Huchra 1989.
Note : Astronomers were at a loss to explain how something so enormous could have formed during the lifetime of the universe. The Great Wall was declared a great cosmic optical illusion, 1996.
Metagalaxy Entire system of galaxies including the Milky Way. That is the entire contents of the universe together with the region of space it occupies.
Notable Galaxies
0902+34 Most remote galaxy ever observed, discovered by Simon Liffly in 1988.
Centaurus Giant elliptical galaxy, located between the Local Group and the center of the Virgo Supercluster.
M31 (Andromeda galaxy) (NGC 224) Major spiral galaxy, 2.2. million light-years from Earth. Gravitationally bound to the Milky Way galaxy, with which it shares membership with the Local Group.
M33 (Triangulum galaxy) (NGC 598) Spiral galaxy in the constellation Triangulum, part of the Local Group.
M51 (white nebula.)
M74 (Phantom Galaxy) Galaxy in Pisces, about 32 million light years away, and is a spiral galaxy like our Milky Way, but a bit smaller. A French astronomer, Pierre Mechain, found the galaxy in 1780.
M77 (NGC 1068) First Seyfert galaxy, whose characteristics were first observed in M77 in 1908. Seyferts were defined as a class in 1943.
M81 Giant galaxy in the constellation Virgo with a black hole in the center that shoots out high-speed jets of charged subatomic particles which expand until they hit infalling material from outside the galaxy.
M82 Galaxy M82 makes up to 50 times more stars than other galaxies. This is the prototype starburst galaxy.
- Magnetic bubble 3,000 light-years across discovered in galaxy M82. Nothing like it has been seen before, observed on the James Clerk Maxwell telescope, photographed by Scuba camera, by Joint Astronomy Center, Hawaii, 2000.
M87 Central galaxy of the Virgo Cluster, the central cluster of the Local Supercluster.
M101 (Messier 101) Giant spiral galaxy of stars, dust, and gas 170,000 light-years across or nearly twice the diameter of the Milky Way. M101 is estimated to contain at least one trillion stars. Approximately 100 billion of these stars could be like our Sun in terms of temperature and lifetime. The galaxy's spiral arms are sprinkled with large regions of star-forming nebulae.
M102 Galaxy cannot be definitively identified, with the most likely candidate being NGC 5866, and a good chance of it being a misidentification of M101. Other candidates have also been suggested.
M104 (Sombrero Galaxy) 28 million light years from Earth. It has 800 billion suns and is 50,000 light years across
