Many spirals have a halo of stars and star clusters arrayed above and below the disk. A large majority of galaxies have these bars, and astronomers study them to understand what function they play within the galaxy. In addition to bars, many spirals may also contain supermassive black holes in their cores.
Subgroups of spirals are defined by the characteristics of their bulges, spiral arms, and how tightly wound those arms are. Elliptical galaxies are roughly egg-shaped ellipsoidal or ovoid found largely in galaxy clusters and smaller compact groups. Another common type of galaxy is an ellipsoid with radially decreasing brightness. Additionally, ellipticals are subclassified according to the shape of their lines of constant brightness on images, so-called isophotes.
Very luminous E galaxies show isophotes with boxlike shapes boxy E while less luminous E galaxies show isophotes with disk-like shapes disky E. Ellipticals may vary in size from thousands to several hundred thousand light-years. They are most commonly found in clusters of galaxies and often contain a hot gas halo and in about half of all cases appreciable amounts of dust.
The brightest galaxies are usually ellipticals. They are dominated by Population II stars. Other, rarer forms of galaxies include a transition class called S0 that has a disk superimposed on an otherwise elliptical type of light distribution, and an irregular Irr class composed of galaxies with chaotic forms Fig. Some irregular galaxies appear to be small spiral galaxies that had their ordered shapes gravitationally scrambled by interactions with other galaxies.
The Small Magellanic Cloud, classified as a dwarf irregular galaxy, likely lost its formerly barred, spiral shape from perturbations by its much larger neighbor, the Milky Way. Some galaxies lie outside the normal range of morphologies. Most of these galaxies have suffered some disturbing event, such as a gravitational encounter, a merger, or violent nuclear activity.
Other such galaxies seem to have started out with anomalous characteristics, such as the low-surface-brightness galaxies. Those with activity in their nuclei are described below. One of the more spectacular examples of exotic galaxies is the starbursters, galaxies that are presently manufacturing stars at an unusually vigorous rate.
It is now known that some gravitational impulse has triggered the unusual star formation activity in most cases. The burst is a temporary condition and the galaxies now bursting must have spent most of their lives in a quieter state.
See also: Starburst galaxy. Another type of exotic galaxy is the low-surface-brightness galaxy, which has such a low spatial density of stars that it is almost invisible. The Sculptor dwarfs have many characteristics similar to the globular star clusters very old stars, cluster-type variable stars, and smooth stellar distribution , but are millions of times less dense and dominated by dark matter. Other low-surface-brightness galaxies have a composite population of stars.
For example, at least two star-forming events occurred in the Carina dwarf, one about 13 billion years ago and one about 7 billion years ago. See also: Star clusters. Another type of low-surface-brightness galaxy includes extreme irregular and spiral galaxies, which have some of the structural properties of the normal examples of these types but are so faint that it is difficult to detect them against the sky's ambient brightness.
A significant fraction of the mass of the universe may be in the form of these nearly invisible galaxies. The stars and interstellar matter in a galaxy revolve around the center of mass of the galaxy, which is often a bright nucleus. In a spiral galaxy such as the Milky Way, there are generally two types of stellar orbits. The stars in the flat plane tend to have nearly circular orbits, while the stars in the bulge and halo have more highly elliptical orbits.
All of the motions are the result of the stars responding to the gravitational field of the galaxy. The galaxies are supported against gravitational collapse by these motions in the same sense in which the planets of the solar system are kept from falling into the Sun by their orbital motions.
Usually, the inner regions in the plane of a spiral galaxy undergo nearly solid-body rotation velocity proportional to radius , while the outer regions rotate differentially velocity constant. The spread in velocity of the material within galaxies varies from one to several hundred miles per second.
Typical orbital periods for stars are several hundred million years. See also: Orbital motion ; Solar system. The distribution of kinetic energy into randomly oriented and circular rotational motions varies with the galaxy type. See also: Energy. The number is exponentially cut off for higher luminosities. The brightest observed galaxies are fainter than billion solar luminosities; the faintest, brighter than about 1 million.
The Milky Way Galaxy's total luminosity is roughly 10 billion times greater than the Sun's. The distribution of luminosities is such that while there are very many faint galaxies, they do not contribute a large fraction of the total light given off by galaxies. Only the brighter galaxies, visible for great distances through space, can be observed easily and in great numbers.
It is generally found that the masses of galaxies are roughly proportional to their luminosities. See also: Mass-luminosity relation. At each luminosity, the fraction of different galaxy types strongly varies with luminosity. The most luminous galaxies are mainly ellipticals and the less luminous mainly spirals.
Additionally the number of elliptical galaxies at all luminosities increases in environments where the density of galaxies is high. Although galaxies are scattered through space in all directions for as far as they can be observed, their distribution is not uniform or random. Most galaxies are found in associations Fig. The E and S0 galaxies tend to be concentrated in large clusters more strongly than spirals. See also: Andromeda Galaxy ; Local group.
Galaxy mergers with ellipticals could even produce weird hybrids, such as the Sombrero Galaxy. A examination by NASA's Spitzer Space Telescope found that Sombrero is made up of one galaxy inside of another; the study reclassified Sombrero to an elliptical galaxy with a flat disk inside of it.
Such examinations sometimes also reclassify elliptical galaxies into other types, such as with UGC in Scientists determined that the galaxy was not an elliptical, but a sort of strange combination of cosmic pieces that came about through various mergers.
Scientists are now interested in how many other similar galaxies reside in our universe. Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community space. Elizabeth Howell is a contributing writer for Space. She is the author or co-author of several books on space exploration.
Elizabeth holds a Ph. She also holds a bachelor of journalism degree from Carleton University in Canada, where she began her space-writing career in As already remarked, different astronomers have invented slightly differing versions of the above basic classification, in order to fit in with the particular aspects of galaxies that they have been researching. Hubble originally arranged the different shapes of galaxy in the form of a tuning fork, because he noticed a gradual variation in visual appearance between the different galaxies that he observed.
When Hubble devised his classification scheme, he thought that spiral galaxies evolved from elliptical ones. This idea has long since been recognised to be false, but confusingly the terms 'early type' and 'late type' are still often used. Thus astronomers often refer to an Sa galaxy as an 'early type spiral' and to an Sc galaxy as a 'late type' spiral. We now know that the tuning fork diagram is an arrangement of galaxies according to their rotation.
Spiral galaxies rotate rapidly, while elliptical ones have little or no rotation. We also know that elliptical galaxies probably form as a result of spiral galaxies colliding.
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