In the modern-day universe, stars face a weight limit – about 150 times the mass of the Sun. Anything heavier would be so hot and bright that it would blow away any more gas that tried to fall on its surface, keeping it from getting bigger.

In the early universe, though, the weight limit might have been much higher – hundreds of times the mass of the Sun. Such stars would have blazed millions of times brighter than the Sun.

The difference is caused by slight changes in the recipe for making stars. The early universe contained only hydrogen, helium, and a tiny smattering of a couple of other elements – all forged in the Big Bang. As the universe expanded and cooled, these elements clumped together to make stars.

The cores of these newborn stars were hot enough to ignite nuclear fusion, which forces lighter elements together to make heavier ones. More-massive stars plowed through their hydrogen and helium quickly, forging carbon, oxygen, and many other elements. And when they could no longer sustain the fusion reactions, they exploded. The energy of these blasts created many more elements.

The explosions filled the universe with many of these heavier elements, which could be incorporated into later generations of stars.

Models of star formation say these extra ingredients inhibit the growth of stars that are born today. The stars can still grow to monstrous proportions – just not as monstrous as when the universe was young.

Script by Damond Benningfield