Galaxy, star, and planet formation

默里教授从事星系、星星和行星形成的定量研究。银河系形成恒星的速度随着宇宙年龄的增长而变化。目前，螺旋星系每年形成1到10个太阳。令人惊讶的是，包含10亿到100亿太阳质量气体的螺旋星系形成恒星的速度如此之慢。默里教授正在探索年轻恒星发出的光限制了星星形成速度的观点。相比之下，50亿年前，当宇宙大约是现在年龄的一半时，大多数星系每年形成10到100个太阳。当第一次被发现时，这种星星形成速度的增加似乎令人惊讶，但近年来人们发现星系中含有的气体大约是当时的十倍。最后，在这些年轻星系中，每个轨道周期转化为恒星的气体比例与现在的星系相同。默里再次试图确定，与较高的星星形成速率相关的较高光度是否可以解释气体形成恒星的速率相当缓慢。 在地球附近，我们已经发现了500多颗围绕太阳以外的恒星运行的行星。这些太阳系外行星中的许多都在非常小的轨道上。默里教授和他的同事们认为，很大一部分非常短周期的行星是通过第二颗恒星伴星的影响从它们原来的（大得多的）轨道上“迁移”过来的。他试图回答这样一个问题：在已知的短周期行星中，有多少是由这种机制产生的？默里教授还提出，当一颗星星被一个巨大的小行星盘环绕时，应该会导致迁移，小行星盘由数十到数百公里宽的物体组成。最近对3-10颗地球质量的行星在短周期轨道上的观测表明，这样的“超级地球”出现在10%的太阳型恒星周围。这种低质量的物体预计不会在依赖于气体盘中迁移的模型中发现。默里教授正在修改微行星迁移模型，看看他们对行星在如此巨大的岩石盘中形成的看法。

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.