Inventory of the Galactic Center 

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• 28 magnitudes of optical extinction -- can't do it in visible light
• infrared emission (2.2 microns - K band)
• much less obscured
• old Population I red giants (T ~ 4000 K) are brightest in the infrared.
• radio emission
• unobscured
• not emission from stars
• non-thermal emission (synchrotron emission, supernovae, etc)

What do we see?

A rising density of stars towards the center: n ~ r^-2 from 1 parsec to 0.1 parsec. Stars are very close together, so close interactions are common.

IRS 16: A bright infrared source located very close to the density peak of stars. IRS actually consists of 15 or more individual sources, most likely very luminous O and B stars. A recent starburst?

Lots of non-thermal synchrotron radio emission:

The brightest radio source is called Sagittarius A (Sgr A). Sgr A is not a single object, but a complex region at the center of the Galaxy.

• Sgr A East is most-likely a recent supernovae remnant (again, perhaps suggesting a recent starburst?).
• Sgr A West is a bizarre HII region which shows a mini-spiral pattern:

• Sgr A* is a very bright, compact radio source located in the center of Sgr A West.

• The inner edge of the ring is sharp, too sharp to be a long-lived feature (random motions of the gas would smear out any sharp edge in < 10⁵ years).
• Inside the ring, the gas is ionized; the ring itself is mostly molecular.
• This all suggests that a recent explosion of some sort carved out the central part of the ring. How much energy is needed? About 10⁵¹ ergs, which is sort of what a supernova releases...