Galaxies

Galaxies

At the beginning of this century people still believed that the Universe exists wholly in our own galaxy, the Milky Way, and that all nebulae visible in the night sky were nearby gas clouds.

The true nature of the spiral nebulae was discovered by Edwin Hubble. He surveyed Cepheid variables in the Andromeda Galaxy (M31) and showed that the distances to these objects were much too great to be part of our galaxy. Today we know that there are billions of galaxies with distances of millions of lightyears between them.

Hubble also developed the Hubble Sequence, a classification scheme for galaxies based on their shape and structure. Galaxies can be divided into 3 categories: Spiral, Elliptical and Irregular. Generally galaxies consist of a bulge, a disc and a surrounding Halo.

The Hubble Sequence

In Spiral Galaxies the disc shows spiral arms containing mostly young stars (appearing blue) and the greatest part of the galaxy's dust and gas. The disc can have a size of 50,000 to 150,000 lightyears in diameter. The bulge on the other hand consists mostly of older stars that appear red in color. Examples of Spiral Galaxies are Andromeda, the Milky Way, M63 (left) or the Sombrero Galaxy (right).
Spiral galaxies are subdivided among three classes: Sa, Sb, Sc.

The first one describes spirals with a large bulge, tightly wound spiral arms and only little HII regions, while those with a small bulge, open arms and many HII regions are Sc.

Barred Spiral Galaxies are classified in the same way: SBa, SBb and SBc (notice the bar visible in Galaxy NGC1365).

M86 The intermediate classification between Spiral and Elliptical is S0 and SB0 describing galaxies with a disc but no spiral arms (left).

At the left end of the Hubble Sequence are the Elliptical Galaxies, subdivided among 7 classes describing their ellipticity, from E0 (circular) to E7 (cigar shaped). Ellipticals are usually red in color because they only contain old stars and virtually no gas or dust, much like the bulge of a Spiral Galaxy (left picture, M87).

Large_Magellanic_Cloud Irregular Galaxies don't fit into this classification scheme, because they are not symmetrical, have no bulge and contain much gas. An example is the Large Magellanic Cloud, a satellite galaxy of the Milky Way (right picture).

Click here for a Table of Galaxy Characteristics

Nearly all galaxies are arranged in groups called Galaxy Clusters that can have only a few or up to 10,000 members. The Virgo Galaxy Cluster for example contains 2,500 galaxies (such as M86 and M87, see above) in a span of 12 million lightyears.

Our own galaxy is part of the Local Group, a cluster with more than 30 members. Other members of the Local Group are Andromeda, the Large and the Small Magellanic Cloud and the Triangulum Galaxy M33 (right). On the picture below is the Hercules Galaxy Cluster, Abell 2151.

Despite the fact that the expansion of the universe is driving galaxies away from each other, inside of galaxy clusters the gravitational forces can bring them close together... too close sometimes, as on the right picture. The influence of the bigger galaxy NGC2207 is pulling stars and gas out of the smaller one, IC2163. (right)

For more pictures and a complete list of all Messier Objects visit this site:
http://www.tietjen-hamburg.de/jdt/astro/astro1.html

At the beginning of this century people still believed that the Universe exists wholly in our own galaxy, the Milky Way, and that all nebulae visible in the night sky were nearby gas clouds.
The true nature of the spiral nebulae was discovered by Edwin Hubble. He surveyed Cepheid variables in the Andromeda Galaxy (M31) and showed that the distances to these objects were much too great to be part of our galaxy. Today we know that there are billions of galaxies with distances of millions of lightyears between them.
Hubble also developed the Hubble Sequence, a classification scheme for galaxies based on their shape and structure. Galaxies can be divided into 3 categories: Spiral, Elliptical and Irregular. Generally galaxies consist of a bulge, a disc and a surrounding Halo.
The Hubble Sequence
In Spiral Galaxies the disc shows spiral arms containing mostly young stars (appearing blue) and the greatest part of the galaxy's dust and gas. The disc can have a size of 50,000 to 150,000 lightyears in diameter. The bulge on the other hand consists mostly of older stars that appear red in color. Examples of Spiral Galaxies are Andromeda, the Milky Way, M63 (left) or the Sombrero Galaxy (right). Spiral galaxies are subdivided among three classes: Sa, Sb, Sc.
	The first one describes spirals with a large bulge, tightly wound spiral arms and only little HII regions, while those with a small bulge, open arms and many HII regions are Sc.
Barred Spiral Galaxies are classified in the same way: SBa, SBb and SBc (notice the bar visible in Galaxy NGC1365).
	The intermediate classification between Spiral and Elliptical is S0 and SB0 describing galaxies with a disc but no spiral arms (left).
At the left end of the Hubble Sequence are the Elliptical Galaxies, subdivided among 7 classes describing their ellipticity, from E0 (circular) to E7 (cigar shaped). Ellipticals are usually red in color because they only contain old stars and virtually no gas or dust, much like the bulge of a Spiral Galaxy (left picture, M87).
		Irregular Galaxies don't fit into this classification scheme, because they are not symmetrical, have no bulge and contain much gas. An example is the Large Magellanic Cloud, a satellite galaxy of the Milky Way (right picture).
Click here for a Table of Galaxy Characteristics
Nearly all galaxies are arranged in groups called Galaxy Clusters that can have only a few or up to 10,000 members. The Virgo Galaxy Cluster for example contains 2,500 galaxies (such as M86 and M87, see above) in a span of 12 million lightyears.
Our own galaxy is part of the Local Group, a cluster with more than 30 members. Other members of the Local Group are Andromeda, the Large and the Small Magellanic Cloud and the Triangulum Galaxy M33 (right). On the picture below is the Hercules Galaxy Cluster, Abell 2151.
Despite the fact that the expansion of the universe is driving galaxies away from each other, inside of galaxy clusters the gravitational forces can bring them close together... too close sometimes, as on the right picture. The influence of the bigger galaxy NGC2207 is pulling stars and gas out of the smaller one, IC2163. (right)

For more pictures and a complete list of all Messier Objects visit this site: http://www.tietjen-hamburg.de/jdt/astro/astro1.html