Stellar populations

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Studies of the spectra of stars show that stars vary in the amount of metal[1] that each contain. Stars who's spectra show prominent lines of metals are called Population I stars. Stars who's spectra only contain weak metal lines are called Population II stars.


Population I

Population I stars are said to be rich in metals. In this case rich is a relative term since even Population I stars have only a few percent of their mass being elements other than hydrogen and helium. As heavier elements are only formed in the interior of stars when they are undergoing nuclear fusion (main sequence stars) or during supernova explosions, Population I stars must be amongst the youngest in the galaxy, again youngest being a relative term. Population I stars tend to lie in the plane of our galaxy, primarily in the spiral arms. Our Sun is a Population I star[2].

Population II

Population II stars are said to be metal poor. This suggests that that the nebulae Population II stars formed from contained little metal and hence must have been from an early time in the universe when few stars had formed. This means that the Population II stars that we see must be older than the Population I stars that we see. In order to survive this long, Population II stars that are around must be somewhat cooler and relatively small as to have survived so long. Population II stars tend to lie in the core of our galaxy as well as in the globular clusters[3].

Population III

Hypothetically, shortly after the Big Bang, there would have been virtually no heavy elements around as stars had yet to form to create these heavy elements. This would suggest that there should have been, at one time, stars with no metal. These initial stars would have created the first heavy elements that would have then formed the Population II stars[3]. These stars are a theoretical construct as none have ever been observed. It may be that these initial stars were so massive that they were incredibly short lived and that they exploded as supernovae, creating the low metal nebulae that the Population II stars formed from[4].

Notes and References

  1. Note that in astronomy, a metal is any element heavier than helium.
  2. Freedman, Roger A and Kaufmann III, William J., Universe, 8th ed.
  3. 3.0 3.1 Abell, George O., Exploration of the Universe, 4th Ed
  4. Bromm, V, Coppi, P, & Larson, R, The Formation of the First Stars, The Astrophysical Journal 564:23-51, 2002
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