Types of Galaxies Help
Types Of Galaxies
When telescopes became powerful enough to resolve nebulae into definite shapes, one type of nebula presented a conundrum. Many of the spiral nebulae , which looked like whirlpools of glowing gas, had spectral lines whose wavelengths were much longer than they ought to be. This phenomenon, called red shift , suggests that an object is receding. Red shifts were seen commonly for spiral nebulae, but blue shifts—foreshortening of the waves when an object is approaching—were almost never seen, and when they were observed, they were minimal. Some astronomers thought that the spiral nebulae actually were huge congregations of stars at immense distances and that our own Milky Way was just one such congregation. Until individual stars could be resolved within the spiral nebulae, however, this idea remained an unproved hypothesis.
Today we know that the spiral nebulae do consist of stars, and we call them spiral galaxies . Spirals are not the only type of galaxy. Other objects, previously thought to be emission nebulae or globular star clusters within our Milky Way, turned out to be irregular galaxies or elliptical galaxies . Some of these are billions (units of 1 billion or 10 9 ) of light-years away from us. Many contain hundreds of billions of individual stars.
Some of the brightest galaxies, containing the greatest numbers of stars, are ellipsoidal or spherical in shape. These galaxies are classified according to their eccentricity, the extent to which they are elongated from a perfectly spherical shape. (Actually, the sphere or ellipsoid represents the median boundary, the two-dimensional region such that half the stars are inside and half are outside.) Eccentricity zero (E0) represents a perfect sphere; E1 and E2 are egg-shaped. The E3 and E4 elliptical galaxies resemble elongated eggs. When we get to E5, the median boundary is football-shaped. Elliptical galaxies of E6 and E7 classification are even more elongated. Figure 15-1 shows approximate representations for E0 through E7 elliptical galaxies. This scheme was devised by astronomer Edwin Hubble in the 1930s.
Elliptical galaxies contain comparatively little gas and dust. It is believed that this is so because most of the interstellar material has developed into stars. This suggests that elliptical galaxies are old. There are numerous red giants in these galaxies, and this is consistent with the theory that they are old. Some of these must be supergiants, with diameters hundreds of times that of our Sun, and thousands of times brighter. Despite the vast distances separating other galaxies from ours, some of the stars in elliptical galaxies resolve into points of light in large telescopes. Were it not for the red shift, these giant galaxies could be mistaken for globular star clusters within our own galaxy.
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