# How can a very old star be formed among Population III stars?

After looking at some articles about old stars, (this, that, and this Wikipedia article), I thought about how such an old and small star could have formed. As the early universe had very massive, early-type stars and (no? almost none?) smaller, late-type stars. So how could a ~$$1M_\odot$$ star have formed in the early universe, where massive Population III stars dominated?

Here are my theories, if you may find them helpful.

## Mass loss via a long-dead companion

Star Foo and Star Bar are both $$300 M_\odot$$ stars closely orbiting each other. Star Bar is slightly more massive than Star Foo, causing Star Foo to lose mass to Star Bar. Now, Star Bar is $$580 M_\odot$$ and Star Foo is now drained to $$20 M_\odot$$. As Star Bar ages and becomes a supergiant, Star Foo gets engulfed by Star Bar, reducing Foo's mass to $$\text{1-2} M_\odot$$. Star Bar then goes supernova, ejecting Star Foo. 13 billion years later, Star Foo ages and becomes a subgiant, what we see today.

## Formed in a massive star's disk

When the universe was young, there were plenty of materials to create stars. One massive star, star Foo, formed. It had a vast disk of gaseous material, weighing over $$2 M_\odot$$. In the disk, perturbations caused a massive clump of gas to form together, causing the ignition of hydrogen and creating a small, yellow dwarf of mass $$0.8 M_\odot$$, called star Bar. When Foo went supernova, Bar was ejected. 13 billion years later, we see an ultra-metal poor, very old, late-G type borderline-subgiant.

## Split off from a massive star's envelope

A massive star forming region creates a star, Foo. Foo is very massive and large, and begins losing mass through the stellar wind. In a massive eruption, over $$1 M_\odot$$ is ejected from the star's envelope. This gas clump collapses under its own weight to form a small, Sun-like star. As the universe ages, we see a metal-poor, very old red giant.

• Where do you get your information that no small stars were formed in the early universe? Mar 3 '21 at 23:27
• Maybe you suggest a tag-definition for stellar-population, please? Mar 26 '21 at 9:57