![]() For this reason they are now observed to consist primarily of old, red, population II stars. Whatever the formation mechanism, all of the different types of elliptical galaxies contain significantly less dust and gas than spiral galaxies and irregular galaxies, and certainly not enough to support much star formation at present times. Dwarf ellipticals may also form in this manner, but others have suggested that they form out of the leftover material of a larger galaxy or in the tidal tails of interacting galaxies. In this case, a rapid burst of star formation would convert almost all the gas into stars simultaneously, leaving nothing to form a disk. Ordinary elliptical galaxies may also form in this manner, or may have formed from the gravitational collapse of an interstellar gas cloud. Stellar population properties as a function of the central v. Giant elliptical galaxies are generally thought to be the result of galaxy mergers. As we want to compare spiral and elliptical galaxies with quiescent stellar populations. ![]() This is partially reflected in our theories of how the different galaxies formed. The above comparisons encompass the different types of elliptical galaxy, however, it should be noted that astronomers are not sure if dwarf ellipticals, ellipticals and giant ellipticals form a continuous physical sequence. In some cases, the density of stars in a dwarf elliptical can be so low that we can see straight through the galaxy! At the other extreme, dwarf ellipticals are faint (10 5 times as bright as the Sun) and contain as little as 10 7 solar masses of stars. Giant ellipticals can be 10 13 times as bright as the Sun and contain up to 10 13 solar masses of stars. Not surprisingly, the luminosities and masses of elliptical galaxies also range enormously. Measurements reveal that elliptical galaxies come in a large range of sizes, from the rare giant ellipticals found in the centres of galaxy clusters and stretching over hundreds of kiloparsecs, to the very common dwarf ellipticals which may have diameters as small as 0.3 kiloparsecs. The size of an elliptical galaxy is measured as an effective radius which corresponds to the size of a circle encompassing half of the light coming from the galaxy. Faster moving stars can travel further before they are turned back by gravity, resulting in the creation of the long axis of the elliptical galaxy in the direction these stars are moving. ![]() The orbits of the constituent stars are random and often very elongated, leading to a shape for the galaxy determined by the speed of the stars in each direction. Unlike spiral galaxies, elliptical galaxies are not supported by rotation. In the Hubble Classification scheme, elliptical galaxies are allocated a number from 0 to 7 indicating their ellipticity. ![]()
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