http://news.bbc.co.uk/2/hi/science/nature/5260008.stm
Last Updated: Thursday, 17 August 2006, 18:53 GMT 19:53 UK
Hubble glimpses faintest stars
By Elli Leadbeater
Researchers peering at the Universe's first-born stars have uncovered
the key to predicting a star's destiny.
Stars that don't have enough mass never shine, dying billions of years
before their bigger counterparts.
But astronomers have never been able to measure the exact mass limit,
because the lightest stars that do shine can be simply too faint to
detect.
Now, new images show for the first time how big a star must be to avoid
impending doom.
Reporting in the journal Science, astronomers have viewed high quality
pictures of some of the faintest stars in our galaxy for the first time.
The images come from the dimmest members of the NGC 6397 cluster -
ancient stars that orbit the Milky Way's centre in a close-knit group.
"The light from these faint stars is so dim that it is equivalent to
that produced by a birthday candle on the Moon, as seen from Earth,"
said lead author Harvey Richer, from University of British Columbia,
Vancouver, Canada.
The Hubble Space telescope's advanced camera was focused on the stars
for five days to detect the tiny pinpricks of light.
Although the telescope would have been able to detect fainter stars,
none could be found- so it appears that they simply don't exist. "We
checked the instruments over and over again," said Professor Richer,
"but we don't see any stars fainter than this".
The new pictures finally answer one of astronomy's burning questions. By
calculating the mass of the faintest ancient stars, researchers can work
out the minimum mass needed for a star to survive. Potential stars that
fall just short of the limit die before they're even born.
Survival of the fattest
Almost everything about a star's fate is determined by the mass of the
gas cloud from which it is formed.
Gravity pulls the gas into a giant ball, or protostar. As the ball gets
bigger and more solid, its centre becomes extremely hot.
For large protostars, the heat becomes so intense that the star begins
to burn hydrogen by fusion, and so starts to shine. These stars can
sustain themselves for billions of years, because their heat is
self-replenishing. Some could live longer than the estimated lifespan of
our Universe - in effect, forever.
Small protostars never make it this far. Their cores are just not hot
enough for hydrogen fusion, so they never light up. They quickly stop
shrinking and fade into brown dwarfs, or giant planets.
A small difference in mass can therefore mean the difference between
effective immortality and an untimely death.
But how big is big enough? The problem has been that those stars that do
start to shine - but have only just enough mass - burn very faintly and
are nearly impossible to see.
The long-awaited new images finally lay this question to rest, say the
authors. The dimmest stars were measured as being 8.3% of the Sun's
mass. All protostars that are smaller than this are headed for life as a
brown dwarf.
Star death
The pictures also provide a spectacular new record of the end of a
star's life.
Large stars, which burn out more quickly, can become white dwarfs-
glowing cinders which slowly fade with age. Astronomers had predicted
that these should turn blue as they move towards death.
The new findings provide the first images of this signature colour
change, confirming expectations.
These ancient white dwarfs, which have never been seen before, are
amongst the Universe's oldest stars. Now that astronomers can work out
how long they have lived, they can refine estimates of the age of the
Universe.
/end
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Fundies and trolls are cordially invited to
shove a wooden cross up their arses and rotate
at a high rate of speed. I trust you'll
be 'blessed' with a plethora of splinters.
.
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