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|Title:||Universe's Biggest Stars Form In The Densest Gas Clouds|
Universe's biggest stars form in the densest gas clouds
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.
He and colleague Christopher McKee of the University of California in Berkeley, US, say the answer is the density of the stars' natal gas clouds, which span less than a light year across.
As the gas in such a cloud begins to contract under its own gravity, it forms warm knots, or proto-stars, that radiate heat. If the cloud's density is low, that heat does not have much influence on its surroundings, and the proto-stars can keep collapsing until they become full-fledged stars.
But if the density is above a certain point, more and more proto-stars get packed together, and their heat prevents gas from collapsing onto them. This stunts their growth, allowing the gas cloud to condense into one or two high-mass stars instead.
This threshold density is 1 gram per square centimetre, according to the researchers' calculations. "It turns out to be an amusingly terrestrial number," Krumholz told New Scientist. "If you take a pool of water 1 cm deep and look straight down into it, you're looking through 1 g/cm2 of water."
"I think the paper makes a very interesting point," says Volker Bromm of the University of Texas in Austin, US. "Low-mass stars form first and are then enabling the formation of massive stars later on – a bottom-up scenario, as it were."
"The idea is very intriguing but needs to be studied further with sophisticated numerical simulations," Bromm told New Scientist.
The density dependence could mean estimates of the total star formation rate in the universe are incorrect. Massive stars are brighter and are therefore easier to see than normal stars. So astronomers use them to estimate how many normal stars exist in a given region.
"We measure the tip of the iceberg and use it to estimate the weight of the iceberg," explains Krumholz.
But he says many cosmic gas clouds – perhaps those in low-density dwarf galaxies – may not be dense enough to form massive stars, even though they are dense enough to form smaller stars. "Our measurement of the tip is just fine, but our measurement of the weight of the iceberg may be wrong," he says.
Eric Keto of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US, says the new paper adds to an ongoing debate about how gas clouds fragment before they form stars. But he is not convinced that it casts doubt on astronomers' estimates of the distribution of different-sized stars in the universe.
Previous research suggests this distribution is "in some sense universal, despite the fact that galaxies are not all the same and despite the fact that within galaxies there are regions of very high gas density and regions of very low density", Keto told New Scientist.