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ScienceWeek ASTROPHYSICS: STAR FORMATION HISTORY OF THE UNIVERSE
The following points are made by Alan Heavens (American Scientist 2005 93:36):
1) More than 9000 billion billion (9 Ã 10^(21)) stars have been formed in the observable Universe since it began 13.7 billion years ago. Despite the apparent wealth of stars in the sky, current cosmological models suggest that the Universe was quite dark for much of its first billion years. During these dark ages, the Universe contained clouds of gas and dark matter, but little else the first stars did not form until several hundred million years had passed. Once the cosmic star-making machinery got going it seems to have churned out stars at a prodigious rate.
2) Differing rates of star formation provide clues about the physical circumstances in which star birth takes place. These "physical circumstances" are, of course, the galaxies, and the rate at which stars were made is intimately related to how the galaxies were formed.
4) Despite the admittedly mysterious circumstances, we know quite well in broad outline how the Universe formed the structures we see. Observations of the cosmic microwave background, which dates to when the Universe was only 300,000 years old, show that the Universe was not quite uniform in its early phases. These observations have been made with a number of ground-, balloon-and satellite-based experiments, most famously with the Cosmic Background Explorer in the early 1990s, and now at higher resolution with the Wilkinson Microwave Anisotropy Probe. These experiments revealed small irregularities in the density of the early Universe. The irregularities also happened to be unstable: Denser-than-average regions had slightly stronger gravity, and so pulled matter in to form "clumps." In this way dense objects formed over time through a combination of gentle accretion and the merger of smaller units. | |
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