What is the magic "X factor" that determines which stars become the biggest and brightest in the universe? The answer, new calculations suggest, is how dense their parent gas clouds are. Denser clouds heat up more evenly, preventing the clouds from fragmenting into lots of tiny stars and allowing one or two big stars to form instead.

The research suggests astronomers may be underestimating the number of small- to medium-sized stars in the universe.

Astronomers believe stars condense from cold clouds of gas in space. But it has been unclear why the vast majority of stars grow no more massive than the Sun, while some balloon to more than 100 solar masses.

"Why would a massive cloud collapse monolithically to form one big star instead of fragmenting to make lots of little ones?" asks Mark Krumholz of Princeton University in New Jersey, US.