When powerful telescopes are used, in most galaxies only the mixed light of all stars is detected; however, the nearest show individual stars.
The galaxies present a great variety of forms. In 1930 Edwin hubble classified galaxies in elliptical, spiral and irregular. The first two classes are more frequent.
Some galaxies have a complete globular profile with a bright nucleus.
These galaxies, called ellipticals, contain a large population of old stars, usually little gas and dust, and some newly formed stars. Elliptical galaxies have a variety of sizes, from giants to dwarfs. In the photo, the elliptical Hat Galaxy, M104.
Hubble symbolized elliptical galaxies with the letter E and subdivided them into eight classes, from E0, almost spherical, to E7, husiform. In elliptical galaxies the concentration of stars is decreasing from the nucleus, which is small and very bright, towards its edges.
Spiral galaxies are flattened discs that contain not only some old stars but also a large population of young stars, quite a lot of gas and dust, and molecular clouds that are the birthplace of the stars.
Generally, a halo of weak old stars surrounds the disk, and there is usually a smaller nuclear protuberance that emits two jets of energy in opposite directions. One of them is the Bode Galaxy, M81.
Spiral galaxies are designated with the letter S (spiral). Depending on the minor or major development of each arm, a letter a, b or c is assigned (Sa, Sb, Sc, SBa, SBb, SBc).
There are other intermediate galaxies between ellipticals and spirals. Are the lenticular calls or normal lenticular, identified as SO and classified in groups SO1, SO2 and SO3. In turn, we distinguish the barred lenticular (SBO) that are classified into three groups, as they present the bar more or less defined and bright.
Irregular galaxies are symbolized with the letter I or IR, although they are usually dwarf or uncommon. Those galaxies that do not have well defined structure and symmetry are included in this group.
They are classified as irregular type 1 or Magellanic, which contain a large number of young stars and interstellar matter, and irregular galaxies type 2, less frequent and whose content is difficult to identify.
Irregular galaxies are generally close to larger galaxies, and usually contain large amounts of young stars, gas and cosmic dust. An example is our neighbor the Barnard Galaxy, NGC 6822.
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