The Milky Way

The Milky Way's Center Credit: NASA, COsmic Background Explorer (COBE) Project.

Here is a picture of the Milky Way using infrared wavelength - this picture is taken by the COBE satellite that orbits around the Earth. Thus, this is a picture of our own galaxy, taken obviously from within, and looking towards the center. COBE measured the infrared light instead of the visible light because gas and dust in the galaxy absorb much of the visible light.

This is an image of the Milky Way (using visible light)

Galaxies, each of them containing hundreds of billions of stars along with gas and dust, form the basic structural units of the universe, and if we find life in the universe, we shall almost certainly find it within a galaxy. Our own Milky Way (see picture above), a typical giant spiral galaxy , includes at least 100 billion stars in its diameter of 30,000 parsecs (1 parsec=3.34 light years). The solar system is located toward the edge of our galaxy, at about 8,000 parsecs from the center. The sun travels around the center of the Milky Way; it takes the sun about 240 million years to complete a close orbit.

Within a few billion parsecs of our Milky Way, the region we may call the "known universe", billions of individual galaxies exist, usually within clusters that each include thousands of members. According to recent tentative estimates following the data gathered by the Hubble space telescope, there are probably 50 billion galaxies, each containing of the order of 100 billion stars. These galaxies formed from what was once a featureless and nearly uniform medium, and they have done so during the 15 billion years since the Big Bang . The study of other galaxies provides some of the most exciting research in modern astronomy, because we still know so little about the ways that they formed, and how some of them produced such exotic objects as quasars, the most energetic of all objects, and giant "halos" of dark matter.