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When Betelgeuse Goes Supernova!

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Wade Hampton III

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When Betelgeuse Goes Supernova!

PostThu Mar 14, 2019 11:39 pm

Betelgeuse has inspired a lot of astronomical scare-stories because it is a
nearby red giant star that is expected to explode soon as a powerful
supernova. What these stories often gloss over is that “nearby” and
“soon” are relative terms. The way astronomers use them is quite
different from the way we use those words in everyday conversation.
First, let’s look at “soon.” Astronomers estimate that Betelgeuse is
approximately 10 million years old, and it began expanding into a red
giant 40,000 years ago. That means it has begun nuclear fusion of helium
in its core, creating oxygen and carbon and starting down the pathway to
core collapse and eventual supernova detonation. Exactly how long it will
take for that to happen is unknown; astronomers can only make estimates
using models of stellar evolution. Those models, in turn, depend on Betelgeuse’s
mass and rotation period, both of which are imprecisely known. If Betelgeuse
is almost 20 times as massive as the Sun, as most studies indicate, then it
will explode sometime within the next 100,000 years, leaving a celestial
splatter similar to Cassiopeia A. It’s more likely to blow up later in that
time-frame, but it’s not impossible that it could explode tomorrow. Still,
even if you assume that an explosion could happen randomly any time within
that period, the odds of Betelgeuse exploding in your lifetime are less
than 0.1%.
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60374.JPG
Future Betelgeuse
60374.JPG (54.38 KiB) Viewed 175 times

Then again, if Betelgeuse is closer to 15 times the mass of the Sun, as implied
by a few other studies, and if it is rotating slowly, then it could take a million
years or more to go supernova. In that case, the likelihood that you will live
to see Betelgeuse go boom is a good, solid zero. Now, let’s look at “close.” It’s
not so easy to measure the distance to a bright red giant star like Betelgeuse.
Different methods give answers ranging from 520 light years to nearly 700 light
years, about 150 times as far away as Alpha Centauri. (Betelgeuse looks bright
in our sky because it is so intrinsically large and luminous, as shown in this
illustration.) Even at the low end of the distance estimates, Betelgeuse is too
far away to do significant damage to Earth. As Charlie Kilpatrick explains, the
material ejected directly by the Betelgeuse supernova will have expanded and
cooled to insignificance long before it reaches Earth.

Radiation from the Betelgeuse supernova will certainly have some measurable
effects on Earth’s environment, but probably only a minor impact on life.
Betelgeuse is too far away to significantly ionize Earth’s atmosphere, for
instance. One way to evaluate the risk is to look at the consequences of
past nearby supernovas. It’s not easy to find evidence of them, which is
one strong indication that only the very closest supernovas present much
of a risk. A recent study claims to find chemical evidence of two supernova
explosions between 1.7 million and 3.2 million years ago. These explosions
allegedly happened on the order of 300 light years from Earth, meaning they
hit us with radiation 4 times as strong (give or take) as what we’d expect
from Betelgeuse. There’s no clear sign that they had any effect on life,
however. It’s possible they caused a period of climate cooling, but it’s
also possible that the changing climate was completely unrelated. At any
rate, there was no mass extinction during that era. Statistically speaking,
supernova explosions should occur within 100 parsecs (300ish light years)
every 2 million–4 million years. Whatever effect they’ve had on ancient life
is too subtle to recognize in the fossil record. So it’s safe too say that
even if Betelgeuse were to explode really soon, in your lifetime, it still
isn’t close enough to pose much of a risk. One less thing to worry about!

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