 |
Spiral Galaxies
Spiral galaxies usually consist of two major
components: A flat, large disk which often contains a lot of interstellar
matter and an ellipsoidally formed bulge component, consisting of an old
stellar population without interstellar matter.Our sun is one of several
100 billion stars in a spiral galaxy, the Milky Way.
|
 |
Lenticular Galaxies
These are, in short, "spiral galaxies without
spiral structure", i.e. smooth disk galaxies, where stellar formation
has stopped long ago, because the interstellar matter was used up.
|
 |
Elliptical Galaxies
Elliptical galaxies are actually of ellipsoidal
shape, and it is now quite safe from observation that they are usually
triaxial (cosmic footballs, as Paul Murdin, David Allen, and David Malin
put it). They have little or no global angular momentum, i.e. do not rotate
as a whole (of course, the stars still orbit the centers of these galaxies,
but the orbits are statistically oriented so that only little net orbital
angular momentum sums up). Normally, elliptical galaxies contain very little
or no interstellar matter. They appear like luminous bulges of spirals,
without a disk component.
|
 |
Irregular Galaxies
Often due to distortion by the gravitation of
their intergalactic neighbors, these galaxies do not fit well into the
scheme of disks and ellipsoids, but exhibit peculiar shapes. A subclass
of distorted disks is however frequently occuring. Galaxies of all types,
though of a wide variety of shapes and appearances, have many basic common
features. They are huge agglomerations of stars like our Sun, counting
several millions to several trillions. Most of the stars are not lonely
in space like our Sun, but occur in pairs (binaries) or multiple systems.
|