Sky & Telescope - May 2019 - 35

Astronomers are itching to know when this
red supergiant will go supernova, but a clear
answer eludes them.

B

etelgeuse, 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
kilometers per second (67,000 mph).

"There's a real scientific issue
here: We really are ignorant of
when it's going to blow up. What
can we possibly do about that?"

LIKE A SORE THUMB Betelgeuse
(upper right) stands out boldly from its
blue-white Orion neighbors, 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.

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 angular diameter of about 44 milliarcseconds - equivalent to a lunar crater 80 meters 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 center 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
s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 9

35



http://www.skyandtelescope.com

Sky & Telescope - May 2019

Table of Contents for the Digital Edition of Sky & Telescope - May 2019

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