From October 19, 2020.
Hosted by: Dr. Pamela L. Gay
(@CosmoQuest)
Today’s top story brings back our favorite red giant star,
Betelgeuse. A new paper published in The Astrophysical
Journal (ApJ) with lead author Dr. Meridith
Joyce looks at Betelgeuse as a pulsating variable star
and uses atmospheric physics to bring us new insights on what the
red giant is up to.
This paper combines new observations with new computer models to
derive the star’s mass and size from its outward behavior in much
the same way that one might derive the size of a jug based on
what notes can be created by blowing across its opening.
Different sized objects naturally resonate in specific ways. By
understanding how Betelgeuse pulsates in the fundamental mode and
first overtone mode – those first two pitches you can make with
your bottle – they were able to work backward to figure out
Betelgeuse’s physical size. This method let them then figure out
more accurately the star’s distance because we know how big it
appears on the sky, and there is a simple mathematical
relationship between how big something is and how big it
appears.
Our current estimates had been based
on Hipparcos measurements of how Betelgeuse appears to
move relative to background objects when the Earth moves around
the Sun. This technique, called parallax, can be super useful,
but, trying to measure how much an object
shifts, when that object is a blobby star like Betelgeuse, can be
a challenge!
Betelgeuse appears to be about 42 microarcseconds (mas) across
and seems to shift just 5.95+0.58 or -0.85 mas. This is a really
hard measurement, and Betelgeuse hasn’t been looked at yet
with Gaia.
By working from the theoretical size they got from its pulsation
modes, and calculating a new distance, this team found that the
lower edge of that error in the distance appears to be accurate,
putting Betelgeuse just 530 lightyears away. This is 25% of the
distance from the prior estimate, but as I said, it’s within
error. This closer distance makes Betelgeuse a bit smaller; it
appears to be 750 times the radius of the Sun. Nevertheless, it
still probably started with 20+ solar masses of material, and it
will still explode in about 100,000 years. At this point, they
believe, it is still burning helium in its core and is nowhere
near going boom.
It’s always cool when completely different techniques can narrow
in on more precise values. This shows us once again that there is
no one way to do science.
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