http://www.skyandtelescope.com.au 57ROGELIO BERNAL ANDREO / WIKIMEDIA COMMONS / CC BY-SA 3.0
BETELGEUSE, THAT ORANGE BEACON shining from the
shoulder of Orion, the Hunter, is one of the best-studied stars
out there. Yet despite all we’ve learned, virtually everything we
know — or think we know — about this aged, highly volatile
red supergiant remains uncertain. This includes everything
from its distance and mass to the question that astronomers
can’t reliably answer without getting a better handle on those
and other basic stellar parameters: When will this amber-
glowing behemoth go supernova?
The suspicion is, it could explode anytime. Increasingly
frustrated by the uncertainties, astronomers are trying every
tool in their toolkits to improve their understanding of this
nearby supernova progenitor.What we know
Betelgeuse — Alpha (α) Orionis — was born roughly 10
million years ago, condensing out of a molecular cloud
probably in the area of Orion’s Belt. It’s thought to be a
runaway star from that region, possibly forced out by a
supernova. Today Alpha Ori is hurtling away from the Belt at
a speed of 30 kilometres per second (108,000 kph).
Betelgeuse is a star of superlatives. It’s the ninth-brightest
star in the night sky, but if we could perceive all the
wavelengths of radiation it emits, it would be the brightest
star of all. With an average apparent angular diameter of
about 44 milliarcseconds — equivalent to a lunar crater 80
metres wide as seen from Earth — Alpha Ori is also one of
the largest stars in our sky. It’s so near to us that we can
actually resolve its surface like we can that of our own Sun.
With sophisticated enough scopes, the red supergiant appears
as a disk rather than a point, and like a planet it doesn’t
twinkle. It’s not just close but big. If Betelgeuse were placed
at the centre of our Solar System, it would extend out beyond
the asteroid belt, engulfing Mercury, Venus, Earth, Mars and
possibly even Jupiter.
The star is most likely burning helium in its core, with a
surrounding shell that’s consuming hydrogen. Beyond that
shell and all the way to its distant surface lies an envelope
of churning convective cells constituting a hugely extended
atmosphere. At some point, Betelgeuse will have burned up all
the helium in its core and will begin fusing heavier elements:
carbon, oxygen, silicon and so on. When only iron remains in
its core, the star’s explosive demise will be imminent.
When Alpha Ori eventually does go supernova, it will be
as bright as the quarter or even full Moon, perhaps for weeks
on end. We’d be able to read outdoors at night by its light.
Not all astronomers would likely welcome that. “It will ruin
night-time astronomy for a few months when it happens,”
says Edward Guinan (Villanova University).Distance
When studying a star, almost the first thing astronomers
want to know is: How far away is it? Distance provides the
key to unlocking luminosity and, beyond that, other criticalBLAZING BEACON Betelgeuse (upper right)
stands out boldly from its blue-white Orion
neighbours, including the three Belt stars
(middle) and Rigel (lower left). Blazing its own
path, the pulsating red supergiant seems to
taunt us about when it will go supernova.