THE MILKY WAY GALAXY: THE CENTER

Lesson #1. Don't wear a redshirt on an "away team".

When viewed in different regions of the electromagnetic spectrum (gamma-ray, X-ray, ultraviolet, visible, infrared, microwave, or radio), we see different aspects of the galaxy. Gamma and X-rays show us the hottest most energetic astronomical objects. The galactic center is bright in gamma and X-rays. Something is hot and energetic in the galactic center.

The center of the Milky Way cannot be seen in visible light because so much gas and dust blocks the light. However, infrared light passes right through the gas clouds and we can see the center clearly in infrared light. This infrared light is emitted from the dust in the gas.

What we see: The center of the galaxy had lots of exploded stars (which we see as supernovae remnants) and a very bright spot called Sagittarius A. In the center of Sagittarius A, there lurks a black hole with a mass 3 million times that of the sun. We know this because of how fast material is orbiting the black hole.

How do we know?: To determine the mass of the black hole,, we measure the velocity of the disk of material spiraling down into the event horizon. This material is confined to a small area and the math shows that the mass in the center is gigantic. Basically, knowing the radius of the gas disk and the velocity of rotation of the gas disk, we can compute the central mass around which it is spinning (orbiting). Answer: 3 million times the mass of the sun. It is confined to a tiny region such the density of the massive object is greater than 10¹⁴ g/cm³. Recall that this is the maximum density that anything can have before it must turn into a black hole. Thus, the object must be a black hole.