Unlike Hollywood, the universe is full of big stars
Research finds massive star numbers have been underestimated – affecting calculations for black holes, neutron stars and gravitational waves. Richard A Lovett reports.
Giant stars hundreds of times more massive than the sun may have been much more common in the early universe than previously believed, astronomers say.
The find, published in the journal Science, used the European Southern Observatory’s Very Large Telescope in Chile to examine about 800 stars in a “starburst” region called 30 Doradus (also known as the Tarantula Nebula) in the Large Magellanic Cloud, a galaxy about 160,000 light years away from the Milky Way.
Using a spectrometer so sensitive it could pick out individual stars only 1.2 arcseconds apart (about 1/1500 the width of the full moon), the researchers counted substantially more high-mass stars – ranging from 30 to 200 times the mass of the sun – than predicted by long-standing models of star formation. Furthermore, the discrepancy was particularly large for the largest stars.
Historically, astronomers have believed that the vast majority of stellar matter is in the form of myriad small stars, with only a fraction of it in giants of the type observed in 30 Doradus. (In fact, it was only recently that astronomers realized that the largest of these gigantic stars even existed.)
But the new research appears to have stood the traditional notion on its head. “Our results suggest that a significant fraction [of the mass] is in high-mass stars,” says one of the authors, Chris Evans of the UK’s Astronomy Technology Centre, in Edinburgh, Scotland.
That’s important, adds the study’s lead author, Fabian Schneider, an astrophysicist from the University of Oxford, because a star 100 times the mass of our sun isn’t equivalent to 100 suns.
“These are extremely bright,” he says. “A 100 solar-mass star would be a million times brighter than our sun. You need only a handful of these to outshine all the others.”
Such bright stars, he adds, are “cosmic engines” that blast out not only light but ionising radiation and strong stellar winds. They burn bright, but also die young in massive explosions that not only create black holes and neutron stars, but disperse the elements of planets — and life — into space: carbon, oxygen, silicon, iron, and many others.
In the earliest universe, after it had cooled down from the initial fury of the Big Bang, there was nothing but hydrogen and helium, cold and dark, Schneider says. But about 150 million years later, astrophysicists believe that the infant universe’s “dark age” ended with the coalescing of these materials into the first stars and galaxies.
The resulting burst of radiation not only brought light back to the universe, but produced a series of other important effects, including the production of ionising radiation, stellar winds, and supernovae. In combination, these shaped galaxies and slowed the rate of star formation enough to keep the first generation of stars from gobbling up all of the available matter.
The result, Schneider says, was to “regulate” the star forming process “so that you [still] see stars forming today. Otherwise, it would have stopped early on.”
In today’s universe, giant star-forming regions such as 30 Doradus are relatively rare. Ancient regions can still be studied by peering at distant galaxies, whose light has been traveling for billions of years, but these are far away and difficult to observe in detail.
Having one nearby, where we can study it closely, is therefore a perfect opportunity, especially because 30 Doradus is so close and large that it is easily visible in a small telescope.
And it is so bright that if it were in our own galaxy at the distance of the Orion Nebula’s star-forming cluster (easily visible to the naked eye) it would span an area 60 times larger than the full moon and cast visible shadows on cloudless nights, Schneider says.
And while it doesn’t constitute a perfect laboratory – it has too many heavier elements, for example, to be a perfect analogy to star-forming regions in the earliest galaxies – the fact it contains so many super-massive stars has major ramifications for our understanding of our universe’s history.
“There might [have been] 70% more supernovae, a tripling of the chemical yields and towards four times the ionising radiation from massive star populations,” says Schneider.
“Also, the formation rate of black holes might be increased by 180%, directly translating into a corresponding increase of binary black hole mergers that have recently been detected via their gravitational wave signals.”
Brad Tucker, an astrophysicist and cosmologist at Australian National University, calls the new study “a very good paper” with “wide-reaching impact.”
Its authors, he adds comprise a “who’s who” of experts in the field.
“[It] suggests we should expect more core-collapse supernovae, and thus more metals, in the early Universe,” he says. There should also be more black hole mergers to be detected in the future by the gravitational waves they produced.
“Simply put,” he says, “more larger stars equals a more exciting universe.”