Introductory Astronomy: The Bulge

The nuclear bulge is a dense swarm of stars with a radius of about 2,000 parsecs at the center of the Galaxy. The bulge is slightly flattened, but is considered part of the spherical component of the Galaxy. Bulge stars are generally Population II stars. Although the bulge does not contain much gas or dust, it is obscured by the dust of the disk so we cannot observe it optically. However, radio, infrared, and X ray observations indicate that star formation does take place deep inside the bulge.

The bulge includes the nucleus of the Galaxy. Not much is known about the nucleus. Non-optical observations suggest that it is a region very dense in stars and that there is a disk of neutral hydrogen gas at the center. Radio observations indicate a ring of molecular clouds only about 250 parsecs from the center of the galaxy which are expanding outward. They also suggest that Sagittarius A, a very powerful source of radio emission, is at exactly the Galactic core.

Infrared observations tell us that the stars at the core are only about 1000 A.U. apart. (Near the sun, stars are 330,000 A.U. apart!) They also suggest that the gas at the center is rotating very rapidly, with speeds of about 200 km/sec. These speeds indicate that there must be a central mass at the core of about 3 million solar masses, leading many astronomers to speculate that there is a supermassive black hole at the center of our galaxy!

There is also evidence that the bulge of our galaxy contains a central bar. If this is true, our Galaxy's spiral arms would begin at the ends of the bar, spiraling out from this starting point.