http://www.skyandtelescope.com.au 61ESA / HERSCHEL / PACS / L. DECIN ET AL.
than 700 years to reach us.)
Before it goes supernova, the
star might even loop back across
the Hertzsprung-Russell diagram, the
standard plot of star types, and become
a blue supergiant like Rigel or even
a superhot Wolf-Rayet star. It might
burst as one of those, or re-loop back
to erupt as a red supergiant. While
astronomers hope beyond hope that
they’ll get to witness Betelgeuse go
supernova in their lifetime, most think
that’s wishful thinking. “Most likely
it’s in helium core-burning and we still
have, I’m sorry to say, 100,000 years to
go,” says Wheeler.
Would we see any hints just before
it explodes? “This is a really active and
interesting question for people studying
red supergiants right now,” Levesque
says. Some kinds of stars give off what
you might consider death throes,
she says. Luminous blue variables, for
instance, can produce a visible outburst
before they die. “We haven’t seen red
supergiants do this, but that doesn’t
mean they don’t,” she says. “It just
means we have a limited sample.”Aftermath
Whenever it does blow itself to pieces,
Betelgeuse will pose no threat to Earth.
Even at the closest distance estimates,
it’s too far away for its X-rays, UV
radiation or debris to affect us here. “It
will be really intensely bright — and just
a point, of course — but not dangerous,”
Wheeler says. “A supernova would have
to be like 30 light-years away for it to be
really dangerous.”
But it will be a tremendous learning
opportunity. “We haven’t had a visible
galactic supernova in hundreds of years,
so it’ll be exciting and answer a lot of
questions when that does eventually
happen,” says Levesque. One of those
questions concerns the flux of neutrinos
released. After Supernova 1987A in
the Large Magellanic Cloud, which
lies about 165,000 light-years away,
detectors on Earth picked up just a
handful of neutrinos. But Betelgeuse
is over 200 times closer than the
LMC, so the dose of neutrinos that weWHY AN IRON
CORE MEANS
THE END IS NEAR
Throughout its life, a star
keeps an extraordinary
balance between energy
pushing out from its core
and gravity pulling in. As
long as the star burns
elements lighter than iron at
its centre, this hydrostatic
equilibrium will hold steady.
But once only iron remains
in the core, its demise is
imminent.
Until this moment, the
star could generate the
energy needed to hold
gravity at bay by fusing
elements lighter than
iron. Energy can also be
produced by fissioning, or
splitting, elements heavier
than iron. But to transform
iron into something else,
you need an input of energy.
Without that, a star that has
wound up with an iron heart
keeps it together for only a
fraction of a second.
Suddenly, the long-
restrained gravity becomes
unleashed and the star
implodes, collapsing at near
the speed of light into an
ultra-dense neutron-star
core. Iron nuclei break up,
and neutrons form, but only
for an instant. When the
density of nuclear matter is
reached, the core cannot
stand more pressure, and
the imploding material
rebounds. Bursting back out
at thousands of kilometres
per second, it creates a
blindingly bright supernova,
spews heavy elements into
the interstellar medium, and
leaves the core exposed
as a neutron star — or, if
the star is massive enough,
collapsed as a black hole.receive, while still not enough to harm
us, would be much greater, providing
unprecedented insight into Alpha
Ori’s apocalypse, Wheeler says. “We’d
learn an immense amount about the
explosion through the neutrino flux
that we didn’t learn from ’87A because
it was so far away.”
In the meantime, Alpha Ori
continues to serve as a fantastic
laboratory for studying red supergiants.
“Because it’s so close, we can ask
questions about Betelgeuse that we can’t
ask about other stars,” Levesque says.
These include questions about how its
surface is behaving, what its core is
doing, and how it’s losing mass. “If we
can figure out exactly what stage this
star is in, we can study those stages in
just amazing detail and then generalise
to other red supergiants.Ӣ PETER TYSON hopes Betelgeuse
goes supernova right after this issue hits
the newsstand.PLOUGHING AHEAD Seen here in a
far-infrared image taken by the European
Space Agency’s Herschel Space Telescope,
Betelgeuse appears to be pushing crescent-
shaped waves of gas and dust ahead of it in
the direction it’s heading, like a ship pushes
waves before its bow. Astronomers think this
‘bow shock’ comprises material that the star
has released. The origin and nature of the
straight bar at left is unknown.