Hubble classification of galaxies. Ellipticals range from E0 (round) to E7 (the most oblate).The regular spirals are divided according to the relative size of the nucleus and the disk, and the tightness of the spiral arms.The Sa have the largest nuclei and the most open arms.The barred spirals, SB, follow the same classification as the normal spirals. S0 galaxies have nuclei and small disks but no spiral arms.
metal abundances are not low. Giant ellipticals have metal abundances that are quite high, about twice the solar value.
Some ellipticals are rotating very slowly. They have a higher ratio of random velocities to rotational velocities than do spirals. We think that their slow rotation means that they could collapse without much flattening. Remember, when we discussed collapsing interstellar clouds, so if spirals are merely evolved ellipticals, we have no way of understanding why spirals have more gas and dust than ellipticals.
Ellipticals generally contain an evolved stellar population, with no O or B stars. However, their
Elliptical galaxies. (a) M87, in Virgo, which is a giant elliptical, type E0.The fuzzy patches visible near the edge of the galaxy are globular star clusters.The inset shows a blow-up of the center. (b) M49, in Virgo, type E1. It is about 15 Mpc away, and about 15 kpc across. (c) M32 in Andromeda, which is a dwarf elliptical companion to the Andromeda Galaxy (M31, Fig. 17.3b) and is type E2. It is only 800 pc across. [NOAO/AURA/NSF]
we found that rapid rotation retards collapse perpendicular to the axis of rotation, resulting in the formation of a disk.
We can use photometry to study the brightness distribution in ellipticals. Since we see the galaxy projected as a two-dimensional object on the sky, it is convenient to speak of the luminosity per unit surface area L(r), where r is the projected distance from the center of the elliptical. Studies show that the light from most ellipticals can be described well by a simple relationship (known as de Vaucouleur's law):
to galaxies that are so far away that we cannot distinguish individual stars of HII regions. As suggested by Sydney Van den Bergh, astronomers have taken to adding a luminosity class to the spiral
In this expression L(0) and r0 are constants. The values of L(0) are found not to vary very much, with a typical value of about 2 X 105 I0/pc2. The values of r0, however, show a very large spread.
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