How stars end their lives depends on how massive they are.
Large stars are thought to die in explosive fits and
collapse into the densest objects in the universe — black holes and neutron
stars. Small
stars languish as dim objects called white dwarfs. But what happens to
stars right on the border is not certain. Now astronomers have observed two peculiar
white
dwarfs that may represent the end point for these objects.
White dwarfs can be about the size of Earth, but contain
roughly the mass of the sun. They have already burned up most of their fuel and
shine weakly by releasing heat. Most white dwarfs are made of compacted carbon
and oxygen, with small amounts of a few other elements.
The new observations found, for the first time, two white
dwarfs that contained much more oxygen than carbon, perhaps representing the
end stage for stars that are between seven and 10 times the mass of the sun — just
under the mass threshold for neutron stars and black
holes.
"These are literally the first two white dwarfs which
have this kind of chemical composition," said Boris Gänsicke of the University
of Warwick, lead author of a paper describing the finding in the Nov. 13 issue
of the journal Science. "These stars may define the upper boundary of
stars that can make white dwarfs."
The stars, called SDSS 0922+2928 and SDSS 1102+2054, are 400
and 220 light-years from Earth, respectively.
While stars are still young they power themselves by burning
hydrogen and helium in nuclear fusion reactions in their cores. Our sun is
still doing this, but at some point in a star's life, it will run out of this
fuel and its gaseous outer layers will puff up and float out into space. Then
the inner parts will condense; just how compacted they get depends on the
star's mass.
The hugest stars will crumple all the way down to neutron
stars or even black holes, while the majority of stars will stop at the white
dwarf stage. This is the predicted fate of
our sun.
But the most massive of white dwarfs will undergo another
period of nuclear fusion where they burn up carbon to form the element neon.
That's what researchers think the newly-discovered stars are.
"Models predict that if you go to the top end of the mass
range of white dwarfs, they manage to burn most of the carbon layer,"
Gänsicke told SPACE.com. "These two stars definitely have lower abundances
of carbon."
The researchers discovered the stars using spectroscopy data
from the Sloan Digital Sky Survey. They hope to take more detailed follow-up
measurements that could confirm whether there is neon present in the stars,
which would result from the carbon burning.
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