Population III stars

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Population III stars are a hypothetical population of extremely massive stars that are believed to have been formed in the early universe. They have not been observed directly, but are thought to be components of faint blue galaxies. Their existence is necessary to account for the fact that heavy elements, which could not have been created in the Big Bang, are observed in quasar emission spectra as well as the existence of faint blue galaxies. It is believed that these stars triggered a period of reionization.

Current theory is divided on whether the first stars were very massive or not. One theory, which seems to be borne out by computer models of star formation, is that with no heavy elements from the Big Bang, it was easy to form stars much more massive than the ones visible today. Typical masses for population III stars would be expected to be about several hundred solar masses, which is much larger than current stars. Analysis of data on low-metallicity Population II stars, which are thought to contain the metals produced by Population III stars, suggests that these metal-free stars had masses of 10 to 100 solar masses instead. This also explains why there have been no low-mass stars with zero metallicity observed. Confirmation of these theories awaits the launch of NASA's James Webb Space Telescope.

The highest-mass star which may form today is about 110 solar masses. Any attempt to form a star greater than this results in the protostar blowing itself apart during the initial ignition of nuclear reactions. Without enough carbon, oxygen and nitrogen in the core, however, the CNO cycle could not begin and the star would not go nuclear with such enthusiasm. Direct fusion through the proton-proton chain does not proceed quickly enough to produce the copious amounts of energy such a star would need to support its immense bulk. The end result would be the star collapsing into a black hole without ever actually shining properly. This is why astronomers consider population III to be somewhat of a mystery--by all rights they should not exist, yet they're needed to explain the quasar observations.

If these stars were able to form properly, their lifespan would be extremely short, certainly less than one million years. As they can no longer form today, viewing one would require us to look to the very edges of the observable universe. (Since the time it takes light to reach Earth from great distances is extremely long, it is possible to see "back in time" by looking farther away.) Seeing this distance while still being able to resolve a star could prove difficult even for the James Webb Space Telescope.

Population III stars

See also