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A spectrum of a starburst galaxy, M82 (also shown in Fig. 17.9). [ESA/ISO, SWS, R. Gerzel and D. Lutz]

nova remnants are detected by their radio emission. The centers of starburst galaxies also have radio emission that is characteristic of supernova remnants. This tells us that the star formation must have included high mass stars.

When we look at the amount of molecular material out of which the stars could have formed, and we look at the stars that have formed, we come to another interesting conclusion. The star formation must have been very efficient. That is, a large fraction of the available molecular cloud mass was converted into stars in a short period of time. This is in contrast to what we found in the nearby star forming regions, like Orion. In those cases, a very small fraction of the available cloud mass has been converted into stars. We discussed the fact that, in general, in our galaxy, star formation has a low efficiency. That is, something appears to be supporting the clouds until the conditions are just right for star formation. In starburst galaxies, the clouds are not as well supported, and more easily form massive stars (as discussed in Chapter 15). There is some evidence that the density of molecular material is much higher than in normal GMCs near us. They are more like the GMCs in our galactic center, which also have high densities.

What creates the conditions that favor a star-burst? This is a topic of current research. Some

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