Nearby Galaxies

Outside the Milky Way galaxy, our own, are two small irregular galaxies known as the Magellanic Clouds. The Large Magellanic Cloud is at about 160,000 light years and the Small Magellanic Cloud is at about 200,000 light years. This compares to a size of about 100,000 light years for our galactic diameter, so they would be considered to be relatively close by. The Andromeda galaxy is a spiral galaxy similar to our own and is at about 900 kiloparsecs away or 2.9 million light years , the nearest major galaxy. The closest galaxy is believed to be the Sagittarius Dwarf, a small elliptical galaxy some 50,000 light years away from the Milky Way's center. It is moving toward us.

These nearby galaxies can be seen with the naked eye, and everything else that can be seen with the naked eye is within our own galaxy. This makes more impressive the deep space view of the Hubble Telescope that shows so many galaxies.

Clusters of Galaxies

Our Milky Way galaxy has about 20 neighboring galaxies that make up what is called the "Local Group" of galaxies. This group extends to about 1 Mpc from our galaxy. Besides the Milky Way, only two others are spiral galaxies: Andromeda and M33. The remainder are dwarf irregular and elliptical galaxies.

At about 18 Mpcs from the Milky Way is the large Virgo Cluster, containing approximately 2500 galaxies spanning a region of space about 3 Mpc across. A galaxy contains typically 100 billion stars.

The Coma cluster is a large cluster of mostly elliptical galaxies.

H-II Regions

Hydrogen makes up about 74% of the visible universe, so it is everywhere. One of its visible forms is that of a glowing cloud of gas called an ionization nebula. If the hydrogen gas in in the vicinity of hot stars which emit large amounts of ultraviolet light, the hydrogen atoms can be excited (electrons bumped to higher quantum levels) or ionized. Astronomers call ionized hydrogen "H-II", so these glowing nebulae are often called H-II regions.

One of the most famous of these H-II regions is the Orion Nebula, about 1,500 light years away in the "sword" of the constellation Orion. The characteristic colors of the hydrogen specturm are seen in such nebulae, and they often have a noticeably red color from the hydrogen n=3->2 electron transition in hydrogen.