Introductory Astronomy: The Halo

The halo is a spherical cloud of thinly scattered stars and globular clusters. It is the largest component of the Galaxy, extending to radii of about 40,000 - 50,000 parsecs. The disk and bulge components are located in the center of the halo.

The halo contains very little dust or gas. No star formation currently takes place there. This means that the halo contains very few young stars. Most of the halo stars are, in fact, 10 - 14 billion years, which is very close to the age of the galaxy itself. Halo stars are, in general, old, cool, lower-main-sequence stars and giants. Halo stars are extreme Population II stars. This means they have randomly tipped, elliptical orbits and very low metal content.

Globular clusters are dense collections of stars which are gravitationally bound together. They contain roughly 100,000 - 1,000,000 stars in a region abut 25 parsecs in diameter. Globular Clusters are generally spherical or elliptical in shape. The clusters are stable and have survived for billions of years (basically as long as the oldest stars in them). They also tend to have randomly tipped, elliptical orbits which distributes them spherically about the center of the Galaxy. (Almost half of all known globular clusters are located in or near the constellation of Sagittarius, which is in the direction to the Galactic center.) Like all Halo object, they contain stars which are very metal poor.

The most massive globular cluster in the MW is Omega Centauri, which contains about 10 million stars. It is shown left.