An important opportunity provided by other galaxies is the opportunity to study star formation in a variety of environments. We can see how various factors, such as the type of galaxy, its metallicity, and the interstellar radiation field within the galaxy, affect star formation. We discussed the basic idea of star formation in the interstellar medium in Chapters 14-16. We would like to apply the ideas we developed when studying star formation in our galaxy to help us understand other galaxies. In turn, our understanding of other galaxies will help our analysis of our galaxy. We can ask a number of questions about star formation in galaxies.
(1) Does star formation take place in molecular clouds, as it does in the Milky Way? If so, are
the properties of those clouds similar to the ones we find in our galaxy?
(2) What is the large-scale distribution of star-forming material within the galaxy?
(3) What is the distribution of newly formed stars?
(4) Does the mix of stellar masses (the initial mass function) appear to vary from galaxy to galaxy, or within galaxies?
(5) How does the star formation rate vary from galaxy to galaxy and within galaxies?
To study molecular material, we still need to observe trace constituents, such as CO, using millimeter telescopes. For more distant galaxies, angular resolution with single dishes is a limiting factor, but millimeter interferometers are becoming more powerful, and give much more useful resolutions. To study hot cores and protostars, near infrared observations are useful, and angular resolution is not a serious problem. To study recent sites of massive star formation, we look for HII regions, either by their Ha emission (through Ha filters), or by observing their radio continuum emission, at centimeter wavelengths. The latter requires interferometers for good angular resolution.
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