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"Types of Galaxies"

How many types of galaxies are there in the Universe? 


Edwin Hubble divided the galaxies into three basic groups: ellipticals, spirals, and irregulars. The ellipticals are smooth and round or elliptical, the spirals are flat with a spiral pattern in their disk, and the irregulars have stars and gas in random patches.
 Most galaxies are small and faint so only the luminous galaxies are seen at great distances. These spectacular galaxies tend to be either the elliptical or spiral type, so they are the ones often displayed in astronomy textbooks.


Sprial Galaxy
15 percent of galaxies are spirals, flat galaxies with a central light concentration that show spiral arms in an outer disk.
The central regions of spiral galaxies appear reddish and are composed of older Population II stars, such as those in the halo of the Milky Way Galaxy.
Spiral galaxies range from intermediate to large galaxies, with masses in the range of 10 9 to 10 12 solar masses, diameters 6 kpc to 100 kpc, and luminosities 10 8 to 10 11 solar luminosities. The observed appearance of a spiral depends on the observer's point of view: Seen from above or below,
a spiral looks basically round, but if viewed from the side, a spiral appears very flat, typically with an axial ratio b/a ≈ 0.1. Making allowance for this, spirals still exhibit a far greater range of intrinsic shapes than do the ellipticals.


Ellipticals
Elliptical galaxies are smooth and elliptical in appearance. There are four distinguishing characteristics of the ellipticals:
(a) they have much more random star motion than orderly rotational motion (star orbits are aligned in a wide range of angles and have a wide range of eccentricities); (b) they have very little dust and gas left between the stars; (c) this means
that they have no new star formation occuring now and no hot, bright, massive stars in them (those stars are too short-lived); and (d) they have no spiral structure.





Irregular
Irregular galaxies have no definite structure. The stars are bunched up but the patches are randomly distributed throughout the galaxy. Some irregulars have a lot of dust and gas so star formation is possible. Some are undergoing a burst of star formation now, so many H II regions are seen in them.
Others have very little star formation going on in them (even some of those with a lot of gas and dust still in them).

Most irregulars are small and faint. The dwarf irregulars may be the most common type of galaxy in the universe (or maybe the dwarf ellipticals are). The estimates of the number of dwarf irregulars and dwarf ellipticals are based on the proportions of these types of galaxies in nearby groups. The dwarf galaxies far away are too faint to be seen and are, therefore, overlooked in surveys of the sky. Perhaps if the dwarf galaxies were brighter,
Hubble would have arranged the galaxies in a different sequence instead of the two-pronged sequence. Examples of irregular galaxies are the Large and Small Magellanic Clouds (two small irregulars that orbit the Milky Way).
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