It is understood that a very large black hole resides at the heart of every galaxy - our own galaxy has one. This object is often called a supermassive black hole - meaning they are very large, at least 100 solar masses or more.

Material that is near this supermassive black hole will be accelerated and release very high energy radiation. Galaxies that are undergoing this event is called an active galaxy. More information on this is found in the Galaxy section.

A normal galaxy is one that does not have material for the supermassive black hole to "feed" on. But even if these normal galaxies are quite in regards to core activity, they still emit in other wavelengths. There are three sources of emission outside the optical band in normal galaxies:

• Synchrotron Radiation
• Thermal Emission
• Bremsstrahlung

Thermal emission are the most dominant. This emission comes from the billions of stars that reside in a galaxy. Synchrotron emission comes from the supernova remnants of the large stars that have ended their lives in a supernova. Bremsstrahlung (braking radiation) comes from the emission from HII regions (ionized hydrogen gas).

An example of a normal galaxy emitting in multiple wavelengths is the spiral galaxy M81:

X-Ray Image	Ultraviolet Image
Visible Image	Near Infrared
Mid Infrared	Far Infrared
Radio Image	Image credits

The X-Ray and Ultraviolet images show areas of intense star birth and locations of very bright, hot stars. The infrared images show the more cooler red stars and dust areas surrounding bright stars. The Radio image shows the distribution of hydrogen through the galaxy. In addition, radio emission can also result from residual supernova remnants.

In radio and the far infrared, bursts of star formation are optically thin - the brightness in these regions is a good indicator of star formation rate. Bremsstrahlung from these regions can sometimes be competitive with synchrotron radiation but can be difficult to determine. Most easily recognized is synchrotron radiation at about 1 GHz, which arises from old relativistic electrons which have moved a long way from the supernova remnants in which they were created. The supernova remnant themselves may have long since merged with the interstellar medium and are no longer visible (C. Flynn, 2005).

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