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Mar 22 2013

Populations of Stars

How old are stars?

Stars are categorised into three groups:-

  • Population III – The oldest
  • Population II
  • Population I – The youngest (Our Sun)

Population III are the oldest stars that are thought to have burned close to the big bang at around 400 Million years afterwards. In the time since recombination (~380,000 years after the Big Bang) until ~400 Million years after the Big Bang there were no stars but lots of gas. This gas was made up of mostly Hydrogen and Helium with a little (teeny tiny) bit of lithium and beryllium. Astronomers call this “metal poor.” After recombination this gas started to collapse and form clumps which eventually grew big enough to start nuclear fusion. These stars would have been absolutely massive with calculations estimating that they would have been 30 – 1000 times the mass of our Sun.

Stars this size would have been very bright, with a 1000 solar mass star being millions of times brighter than the Sun. They would also have burned all their fuel very quickly resulting in a huge explosion that would scatter the stellar material back into the galactic void.

What about population II stars?

Population II stars coalesced from the remains of population III stars and also contain more elements, or “metals”, which results in them being smaller, less luminous and longer lived. Because they are smaller they will also produce more elements through nuclear fusion such as carbon, oxygen etc.

Can we see population II stars today?

Yes, but only the smallest of them. They are found in Globular Clusters and Elliptical galaxies. The larger population II stars will have exhausted their fuel through nuclear fusion far quicker than smaller population II stars and exploded in huge supernovae, sending the stellar material out into the cosmic void.

Okay, so what about population I stars?

Population I stars are the youngest stars to form and will have formed from the planetary nebula left over from larger population II stars which have burn out and exploded. They are generally smaller again than population II stars but contain higher concentrations of “metals”, elements heavier than helium. Our Sun is a population I star and as it contains higher concentrations of metals it formed from the planetary nebula left over from a population II star. This also tells us that earth, the planets and almost everything else in the solar system formed from the same nebula.

Wow, we came from the stars?

Yes, we are all born from star dust. All the elements that make up everything in and around us came from the stars. See this explanation.

Awesome.

It is isn’t it? There is a twist though which will have to wait as I don’t have the information to hand right now.

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