Why Were Early Universe Stars More Massive Than Today's Stars?

[ian2012]

Apparently, in the early universe, stars were massive - ranging up to about 500 solar masses. Furthermore the metallicity (elements other than H, He) in the early universe was effectively zero with only hydrogen, helium and trace amounts of lithium. Apparently, stars could reach large masses because much larger quantities of these H, He had to accumulate before they could come together and form a star. I was wondering if anyone knows an intuitive explanation for this? Or knows any equations which could explain this?

 

[bapowell]

Apparently, in the early universe, stars were massive - ranging up to about 500 solar masses. Furthermore the metallicity (elements other than H, He) in the early universe was effectively zero with only hydrogen, helium and trace amounts of lithium. Apparently, stars could reach large masses because much larger quantities of these H, He had to accumulate before they could come together and form a star. I was wondering if anyone knows an intuitive explanation for this? Or knows any equations which could explain this?

As you mention, the early star forming regions were devoid of metals. Metals play an important role in cooling -- 'modern' star forming regions are cooled efficiently by metals to temperatures as low as 10K. Without metals in the early days, the star forming regions were much hotter. Now, there's a special amount of mass that a system needs to acquire before it undergoes gravitational collapse -- the Jeans mass. The Jeans mass depends on temperature -- the hotter the gas the more of it you need to form an instability. So, early stars were consequently more massive than those of today.