Galaxies - Introduction

A galaxy is an organized system of hundreds of millions to thousands of billions of stars, sometimes mixed with interstellar gas and dust.

Our sun and solar system are part of the Milky Way galaxy.

Galaxies can be seen in every direction in space, each with billions of stars. Galaxies often appear to be distinct but fuzzy patches of light.

Charles Messier (1730-1817) cataloged more than 100 fuzzy celestial objects, sometimes called Messier objects, and named M1 to M110.

Dreyer compiled the New General Catalog of nearly 8000 objects around 1900. Most of these fuzzy objects are planetary nebulae and star clusters that are part of our galaxy, but extragalactic objects (or galaxies) were also included.

Nearest neighboring large galaxy = Andromeda Galaxy (M31). The relatively "nearby" Andromeda Galaxy (M31) is about 2.2 million light years away.

The Local Group is a group of our nearest galaxy neighbors, held together by their mutual gravitational attraction. About 20 galaxies are in this area.

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Classification of Galaxies

In the 1920's, Hubble devised a classification of galaxies:

1. Spiral galaxies (30%)
2. Elliptical galaxies (most common - 60%)
3. Lenticular galaxies (transitional orms between sprial and elliptical galaxies)
4. Irregular galaxies (10%)

Spiral galaxies are flat disks with a nuclear bulge, a halo of old stars, and spiral arms with young stars. Some have a bar-shaped concentration of stars in the center (barred spirals). Arms emerge fromt he ends of the bar. Dust is readily visible as dark streaks. The Milky Way galaxy is a spiral galaxy.

Globular clusters encircle spiral galaxies. Elliptical galaxies are spheroidal in shape (elliptical in two dimensions). Old stars are dominant. There is no prominent internal structure. They are circled by a halo of globular clusters. Little or no gas and dust are present. Almost all has been converted into stars.

Irregular galaxies are ot disk-like or spheroidal and have no nucleus. They have a chaotic, irregular appearance. Some have bars, but no arms. Sites of active star formation with young stars and luminous gas clouds. Some very old stars are present in globular clusters.

Examples of irregular galaxies are the Small Magellanic Cloud and the Large Magellanic Cloud.