Stars between 18 and 30 times the mass of the Sun may quietly collapse to form black holes, Hubble Space Telescope observations suggest. Previously, astronomers thought the stars would go out in a blaze of glory, producing brilliant explosions known as supernovae.
What happens to a star when it runs out of fuel at the end of its life depends on its mass. Stars with a few to about 30 times the mass of the Sun are thought to collapse to form neutron stars, producing a supernova in the process.
These explosions are so bright they can be seen at distances of millions or even billions of light years from Earth. But gaining more information about the stars that produce the explosions has been tough because the stars themselves are much fainter and harder to see than the explosions.
Now, a team led by Stephen Smartt of Queen's University in Belfast, UK, has found four of these 'progenitor' stars in old Hubble Space Telescope images fortuitously taken before the stars exploded. All four of the stars were between 7 and 18 times the mass of the Sun.
Originally, the team set out to find the progenitor stars for 19 supernovae, but they could not find 15 of the stars. Those 15 must have weighed less than 18 times the Sun's mass, the team argues, or the stars would have been too bright to miss.
Although higher mass stars are rarer than lower mass ones, astronomers had expected to find three or four of them with masses between 18 and 30 times the mass of the Sun.
The fact that they found none heavier than 18 times the Sun's mass suggests these heavier stars may not produce supernovae, or that they only produce very faint ones that are too dim to detect, the team says.
"These very massive stars should be easy to detect and it's strange that we haven't found any of them," Smartt told New Scientist.
He speculates that these more massive stars are collapsing to form black holes instead of neutron stars. Excess material that normally explodes outwards to create a supernova when a neutron star is born "ends up being sucked in by the black hole instead of being ejected," he suggests. In that case, no supernova occurs.
Other studies indicate that stars born even heavier, with at least 30 times the Sun's mass, do produce supernovae. The puzzle has been over the fate of the middleweight stars with between 18 and 30 times the mass of the Sun.
Another search using archival images taken from the ground led by Weidong Li of the University of California in Berkeley, US, found a similar lack of relatively heavy stars among those responsible for supernovae.
Li's team suggested that these stars might simply be producing different, rarer kinds of core-collapse supernovae, such as type II-L or type II-n. These were not included in either Li's or Smartt's study, both of which focused on type II-P supernovae, the most common type of core-collapse supernova. The different supernova types are distinguished by their light spectra.
Neither study searched for the stars responsible for so-called type Ia supernovae, which are explosions of white dwarf stars that have grown overweight by feasting on material from a companion star.
By Polemos
Tue Apr 01 18:15:24 BST 2008
The concept with a temporary neutron star looks more plausible, since the collapse is accompanied by release of "crystallisation heat," whose entropy needs to be emitted into the outer space before the collapse can proceed to the phase of a black hole.All comments should respect the New Scientist House Rules. If you think a particular comment breaks these rules then please use the "Report" link in that comment to report it to us.
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