The Red Supergiant Star Betelgeuse

Betelgeuse position in Orion

Betelgeuse position in Orion

Red supergiant Betelgeuse sits at Orion’s left shoulder in the northern winter sky. Everyone has probably looked at it, in this striking constellation. But you will never see it again as just a boring point of light. It is a turbulent, volatile monster of a star on a one way trip to destruction in a supernova.

Maybe just a few million years old, Betelgeuse has evolved at a rapid rate due to its large mass, it is a live fast die young star. In fact it is so large that if it was placed at the centre of our solar system, Jupiter would disappear inside it’s swollen atmosphere. At 520 light years from Earth, up to 1,000 times the diameter of our Sun and an unimaginable 10,000 times brighter, Betelgeuse doesn’t do things by halves. Betelgeuse is a big star, really big, but a mere tiddler compared to some stars out there that are so frighteningly gargantuan they would would make your hair stand on end.

This red super giant is the second brightest star in the constellation of Orion after Rigel, and the ninth brightest star in the sky. If you look at Betelgeuse in the night sky, and then down at the bright star Rigel at Orion’s foot at the bottom right, and switch between the two noticing their colours, you will see just how redder Betelgeuse is, and how much bluer Rigel is.

Betelgeuse hot spots

Betelgeuse in infrared showing hot spots

Betelguese has used up its supply of hydrogen, the core has compressed down into a hot dense ball, and the outer layers of the star have expanded into the overblown red sun we see today. Red super giants such as this are a rare find as their short lives mean that they do not stay around for long before they go supernova. We know of only about 200 red supergiants in our Galaxy. It is hard to believe that the twinkling red point of light that you see in the constellation Orion is one of the largest and most luminous stars known. This behemoth actually appears to the Hubble Space Telescope as a disk rather than a point of starlight like most stars out there. In fact it’s the only star other than the Sun where hotspots and other features have been seen on its surface.

But it doesn’t just sit there in space doing nothing, this famous star spews out large amounts of material into space. A plume of gas was spotted erupting from its surface, by astronomers using the European Southern Observatory’s VLT Telescope in 2009. This plume was enormous, and would cover the distance from our Sun to Neptune as this artist’s impression shows.


Astronomers also discovered a huge bubble boiling on its surface and came to realise that these bubbles exist beneath the star, can be nearly as big as Betelgeuse itself, and that they cause the vast outflows of material that shoot out into space at such colossal distances. These bubbles are large convection cells rising up from beneath the surface. A similar analogy is to think about how water moves in a boiling kettle. Betelgeuse has managed to throw out material into space equal to the mass of our Sun in just 10,000 years, and remember just how big the Sun is…1.3 million Earths would fit in it’s volume.

Betelgeuse plume

Betelgeuse from ESO’s VLT showing plumes of gas

So this star is chucking out serious amounts of stuff. In fact the star is surrounded by shell type structures of gas and dust resulting from its previous outbursts. Betelgeuse is an enormous seething restless cauldron of belching plasma, that makes our Sun look like a pussycat by comparison.

Betelgeuse also known as Alpha Orionis, changes its brightness because it’s atmosphere actually swells and contracts in size. The mammoth star pulsates, and is known as a semi regular variable star, its brightness goes from about magnitude +0.2 to +1.2 . Betelgeuse varies in diameter from about 550 to 920 times the Sun. A star that changes it’s diameter, with a churning and bubbling surface, surrounded by shells of gas and dust that are the results of it’s previous flare ups. No wonder astronomers can’t agree on it’s true size.

Betelgeuse contains no more than 20 solar masses, meaning that amazingly it actually has a density of less than air. But this 20 solar mass has turned it into the bloated red sun we are witnessing today, as any star over 10 solar masses will usually results in a red supergiant after all the fuel has gone. Our Sun will turn into the smaller version, a red giant, in around 5 billion years or so.

Its fate is sealed, Betelgeuse is losing mass and has been seen to have shrunk over the last ten years, and by as much as 15% since 1993. This is the death knell of a dying giant, one that has lived out its short existence in a matter of just a few million years, and probably heading for destruction in a blaze of violence. Betelgeuse will likely rip itself apart in a supernova event, one of the universe’s most violent explosions, so bright that to a extraterrestrial observer in another galaxy it would outshine our entire Milky Way. This explosion could happen any time, but before you rush out to cancel your home insurance, we are quite safe here on Earth as it is far enough away from us to pose any danger. Betelgeuse meeting it’s death within the time frame of the next few thousand years is thought possible, although today is as good as any. When this star eventually does go supernova it will be a spectacular event, rivalling or exceeding the view Chinese and Arab astronomers got of the supernova that produced the Crab Nebula in 1054. It will be nearly as bright as the Moon in the night sky, cast a strong shadow, be clearly visible in daylight, and may hang around in the sky for weeks. The once mighty red supergiant Betelgeuse will be reduced to a neutron star the size of a small town, surrounded by the stellar wreckage that is the aftermath of the supernova explosion. This gas and dust material may over hundreds of millions of years, go on to slowly collapse under gravity to produce a next generation star, and eventually maybe even a new solar system.

Of course Betelgeuse may have even already exploded, but as it’s 520 light years away the light hasn’t had time to reach us yet, and to us it’s still the glowing red star it always was. But  if you catch out of the corner of your eye what you think are two Moons shining in the night sky, you may be witnessing a once in a millennia event.


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