Galaxy, star, and planet formation

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Professor Murray pursues quantitative studies of galaxy, star, and planet formation. Galaxies form stars at rates that vary as the universe ages. Currently, spiral galaxies form between one and ten Suns per year. It is surprising is that spiral galaxies, which contain somewhere between a billion and ten billion solar masses of gas, form stars so slowly. Professor Murray is exploring the idea that the light from young stars is what limits the rate of star formation. In contrast, five billion years ago, when the universe was roughly half its present age, most galaxies formed between ten and one hundred Suns per year. When first discovered, this increased rate of star formation seemed surprising, but in recent years it has been found that galaxies contained roughly ten times as much gas at that time. In the end, the fraction of gas turned into stars per orbital period is the same in these younger galaxies as it is in present day galaxies. Again, Murray is trying to determine if the higher luminosity associated with the higher star formation rate explains the rather slow rate at which gas is made into stars. Closer to home, we have discovered more than five hundred planets orbiting stars other than the sun. Many of these extra-solar planets are on extremely small orbits. Professor Murray and his coworkers have suggested that a substantial fraction of the very short period planets ``migrated'' from their original (much larger) orbits via the influence of a second stellar companion. He is attempting to answer the question, What fraction of the known short period planets were produced as a result of this mechanism? Professor Murray has also suggested that migration should result when a star is orbited by a massive planetesimal disk, consisting of objects tens to hundreds of kilometers across. Recent observations of 3-10 Earth mass planets in short period orbits indicate that such ``super Earths'' occur around 10 percent of solar type stars. Such low mass objects are not expected to be found in models that rely on migration in a gas disk. Professor Murray is revising the planetesimal migration models to see what they say about planet formation in such massive rocky disks.