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Stars - Stellar Populations

Stars can be grouped by various populations. The most obvious being star cluster: globular clusters and open clusters.

More specifically stars can be divided by main population based on Metallicity.

  • Population I stars - new stars that contain numerous heavy metals in their atmosphere
  • Population II stars - old stars that contain little heavy metals in their atmosphere

When astronomers think about metals, they are not referring to iron and nickel (while they certainly are metals). To an astronomer, any element heavier than helium is considered a metal. The main reason for this is that the only elements that existed in the early Universe was hydrogen and helium. Other "heavier" elements were formed in the process of stellar evolution.

Population II stars were believed to have formed first. These stars occupy the globular clusters that reside in the halo of the galaxy. However, it should be noted that the search is on for Population III stars. Astronomers suggest that the very first stars to ever form in the universe were Population III - that only burned hydrogen and helium only. The suggested comes from the determination that most Population II stars do have some heavy elements.

Some characteristics between a Population II star versus a Population I star is:

  • Population II stars burn hotter
  • Population II stars burn faster

Astronomers believe this has to do with the opacity of the stellar atmosphere. More metals mean a more opaque atmosphere in a Population I means less energy escapes (when compared to Population II stars anyway).

So how does a Population I star contain metal when a Population II star does not?

We will cover this in stellar evolution, but much of the heavy elements in our Universe today is created when a giant star end its life in a supernova. The intense heat for this creates in the upper atmosphere of stars much of the elements we know - like iron, gold, even fluoride (yes, the same stuff in your toothpaste).

These elements disperse themselves to nearby molecular clouds. When that cloud undergoes contraction and give birth to a new star, the end result is a Population I star - one that is now metal rich.

One consequence of a metal rich star is that they are likely to contain a system of planets!

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