Issue 4.06

The Astrophysics Spectator

April 18, 2007

With this issue of the web site I begin a series on the Milky Way Galaxy. The long-overdue first page of this series gives a short description of our Galaxy's structure.

To anyone living in the desert regions of the world, the Milky Way is a familiar feature on the sky. This hazy, wandering ribbon that follows a great circle around the sky is the collected light from the numerous nearby stars in the Galactic plane. Dust in the Galactic plane limits our sight along the Galactic plane to about 1 kpc, a very short distance compared to the 7.6 kpc to the Galactic Center. The Milky Way we see in the sky does not bring to mind the pictures of our neighboring galaxies, but to an observer in one of those galaxies, the Milky Way Galaxy would appear as a big, tightly-coiled spiral of gas and stars wrapping around a short central bar of stars, an impressive example of a barred spiral galaxy.

Sitting near the plane of the Galactic disk, in the space between two spiral arms, we are in the wrong place to see much of the structure of our galaxy at visible wavelengths. The central black hole candidate at the galactic center can be seen with radio, infrared, and x-ray telescopes. The spiral arms can be mapped by radio telescopes, although the positions of these arms are imprecise. Nearby stars tell us about the rotation of the Galactic disk and the properties of the galactic halo. But large regions of our Galaxy remain blocked from view or are invisible because of the absence of bright stars.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for May 2.

Stars

Structure of the Milky Way Galaxy. The galaxy in which we live, the Milky Way Galaxy, appears to be a barred spiral galaxy. The Galaxy is often thought of as three components: a central bulge, a thin disk, and a spherical halo. A black hole candidate of almost 4 million solar masses sits at the center of the central bulge. The spiral structure of the Galaxy is confined to the Galactic disk. The disk contains all of the Galaxy's bright, young stars, which is why it is visible to the eye as a hazy band ringing the sky. The Galactic halo is mostly dark, making its presence known principally through its gravitational field. Many physicists believe, or hope, that the dark mass of the galactic halo is in an exotic form that has not been seen in the laboratory. The Sun sits inside the Galactic disk, 7.6 kpc from the Galactic center, between two of the Galaxy's spiral arms. (continue)