 The
dying star puffs off its outer layers
By BBC News Online science editor Dr David Whitehouse
Astronomers
have seen a remarkable, almost perfect circle in space.
It is actually
a shell of gas expanding into space from the surface of a cool,
red-giant star called TT Cygni, about 1,500 light-years away in
the constellation Cygnus.
This false-colour
picture of TT Cyg, showing its remarkable symmetry, was made recently
using a co-ordinated array of radio telescopes.
It shows radio
emission from carbon monoxide molecules in the gas around the
star. The radiation from the centre is from material blown off
the red giant over a few hundred years.
Next generation
The thin ring,
which has a radius of about 1/4 light-year (2.3 million million
kilometres or 1.4 million million miles), represents a shell of
gas that has been expanding outward for 6,000 years.
Carbon stars
like TT Cyg are so named because of the abundance of carbon containing
molecules found in their atmospheres.
The carbon
is likely the dredged-up ashes of nuclear reactions involving
helium burning deep in the stellar interior.
Astronomers
observe that carbon stars lose a significant fraction of their
total mass in the form of a stellar wind, which enriches the interstellar
gas providing source material for future generations of stars.
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