Betelgeuse - Orion's Red Supergiant

Betelgeuse - Orion's Red Supergiant
Betelgeuse is the star that represents the right shoulder of Orion the hunter. It's also one apex of the Winter Triangle, and it marks the center of the Winter Hexagon. Being bright red-orange and part of the hunter's commanding presence, it's not hard to find in the winter sky.

Ptolemy's Almagest – a second century astronomical work – listed the constellation Orion. Betelgeuse was unnamed. It was just described as “the bright, reddish star on the right shoulder”. Later on, Arabic astronomers named the star, and over the centuries we've ended up with something challenging to pronounce.

I usually hear the name pronounced BET.uhl.jooz. Yet I've found seven different pronunciations, not counting Beetlejuice which became popular after the movie of that name. But properly speaking, the name should be variations on “Yetelgeuse”. Apparently medieval scholars translating the Arabic misread a 'y' as a 'b'.

The best strategy is probably just to adopt the pronunciation of people around you.

Bright but inconstant
Although tricky to pronounce, Betelgeuse is easy to see. Not only is it the tenth brightest star in the night sky, but it's also one of the seven bright stars of Orion. As a red supergiant it's the odd one out since the other six are blue. This bright star group is so prominent that it's part of the skylore of civilizations around the globe.

Betelgeuse is a variable star. English astronomer John Herschel first described its variation in 1836, and continued to observe it over a number of years. Rigel is usually the brightest star in Orion, but on occasion Herschel saw Betelgeuse outshine it.

Herschel was the first western astronomer to describe Betelgeuse's variability, but not the first person to notice it. The aboriginal people of the Australian Great Victoria Desert knew it and incorporated it into their sky traditions. The hunter Nyeeruna [Orion] pursues the sisters of the Yugarilya [Pleiades] with lecherous intent. However, the eldest sister Kambugudha [Hyades] wades in and foils him. Betelgeuse is Nyeeruna's club, and with it he commands fire magic. But – HA! - it has no effect on Kambugudha who has fire magic of her own to dim the fire in Betelgeuse.

In November 2019 Betelgeuse began very noticeably to dim. There was widespread speculation about the cause of the dimming, though Kambugudha was not implicated. Many thought it meant that the star was about to go supernova, but astronomers tended to disagree with this.

A research team investigated, and concluded that Betelgeuse is burning helium in the core and therefore unlikely to become a supernova for another 100,000 years or so. They concluded that the dimming was caused by a massive dust cloud. However they couldn't determine whether the cloud was somehow generated by the star or came from elsewhere.

Not so big, not so massive, not so far away
As a bonus to their study of the dimming, the researchers found out more about the star. It had been common to describe Betelgeuse as being so big that if it were where our Sun was, it could fill the space out to about the orbit of Jupiter. However, it seems that it's only about 2/3 of that size and has a slightly lower mass than what was usually cited. It's also about 25% closer to us than had previously been thought.

Big star, big boom
Although Betelgeuse is smaller than we thought, it's still much bigger than the Sun – and thousands of times more luminous. Interestingly, its surface temperature is noticeably lower than that of the Sun. Its tremendous energy output is just due to its size.

Betelgeuse is a young star – just ten million years old compared to the Sun's 4.6 billion. It has already completed over 90% of its lifetime. A massive star has more fuel, but it doesn't live longer. It burns more quickly, fusing ever heavier elements until its core is iron.

Fusing iron doesn't release energy, it uses it, so fusion stops abruptly. The core collapses suddenly. A rebound produces a mammoth explosion. When that happens, Betelgeuse's stardom will end in a supernova as bright as a whole galaxy. What will be left is a neutron star at the center of an expanding gas cloud.

When Betelgeuse finally goes, it will be a spectacular event in the night sky that will probably also be bright enough to see during the day for a time. This won't happen in our lifetimes. And if there is anyone around to see it in the distant future, they will be quite safe. Betelgeuse is closer than we thought, but it's still way too far away to be a danger to Earth.

(1) Leaman & Hamacher, Aboriginal Astronomical Traditions from Ooldea, South Australia, arXiv:1403.7849 [physics.hist-ph]
(2) Meridith Joyce et al 2020 ApJ 902 63

You Should Also Read:
Orion the Hunter
Death of a Massive Star
AstroFest 2021 – An Evening with the Stars

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