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The galaxy is a collection of dust, gas and stare measuring thousands of light years across. If we observe a clear night sky through a telescope, we observe tiny points of light on slightly larger, hazy splotches.

The points of light are the stars while the spots are nebulae, stellar clusters or other galaxies. Our solar system is in the Milky Way galaxy.

There Are Two Broad Classes Of Galaxies:

Elliptical (probably the most common) and spiral. Spiral galaxies are farther divided into normal, which constitute the majority of spirals, and barred spirals. Barred spirals have their center in the form of a bar or spindle shaped. About 3 percent of galaxies do not, however, fit into any of these categories and are called irregular galaxies.

Various classes of galaxies are classified by the Hubble system, according to their degree of flatness. The elliptical galaxies range from EO to E7 — from an almost spherical shape to flattened disc. The normal spirals are classified as SO (with large nucleus and little or no arm structure), Sa and Sb (with progressively less tightly coiled arms and progressively smaller nuclei), and Sc (which have virtually no nucleus and wide open arms) The shape of a galaxy is partly a measure of the intensity of rotational motion of the system.

In spiral galaxies, stars in the nucleus revolve about the center in the same direction as the arms, which trail behind. Galaxies vary in size from dwarfs to giants. Andromeda Nebula (in the local group) is one of the largest spiral galaxies known and has a diameter of 60,000 parsecs. Elliptical galaxies are usually much smaller, and a dwarf elliptical galaxy may only be 2000 parsecs in diameter.

The mass of a galaxy is calculated in terms of solar masses (mass of sun). Andromeda Nebula has an estimated mass equal to 3 x 1011 solar masses. In other words it has about 300 billion stars. Spiral galaxies are estimated to have masses between about 109 to more than 1011 solar masses.

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