Betelgeuse's Mysterious Dimming Drops It Off The Top 20 Brightest Stars

This orange blob shows the star Betelgeuse, as seen by the Atacama Large Millimeter/submillimeter Array (ALMA). This is the first time that ALMA has ever observed the surface of a star and this first attempt has resulted in the highest-resolution image of Betelgeuse available. ALMA (ESO/NAOJ/NRAO)/E. O’Gorman/P. Kervella

On the shoulder of Orion, there is a red star at the end of its life. Its name is Betelgeuse, and it has many astronomers wondering what on earth is going on. For the last month or so it has experienced extreme dimming, defying expectations, and leading many to wonder if the star going supernova will happen sooner than we thought possible.

Betelgeuse is a supergiant, and a semi-regular variable star; its brightness naturally oscillates over time. It has a main period of about 400 days, a longer one of 2,100 days, and a shorter one of about half a year. The latest update on the state of Betelgeuse shows that its dimming continues, although the rate at which it's dimming appears to have slowed down.

The changes in brightness due to these periods are big enough to be visible to the naked eye. The star can go from brighter than Vega, the fifth brightest star in the sky, to dimmer than Deneb, the 19th brightest star in the night sky. The latest measurement, announced in an Astronomer Telegram recently, puts Betelgeuse out of the Top 20 brightest stars for the first time since records began a century ago.

It's important to note that Betelgeuse, and red supergiants in general, are not well-defined spheres of hot plasma such as you might expect with a star like our Sun. It is 11 times the mass of our own star, but it is much much bigger in size. It has a radius 900 times bigger than the Sun (626 million kilometers/ 389 million miles), which means it would comfortably fit 729 million Sun in its volume.

So don’t picture a sphere, think of it more as an ever-changing blob: Its outer layers move, it shrinks and expands, heats up and cools down, and gets brighter and dimmer. Since last September it became 100°C (180°F) cooler and its luminosity dropped by a quarter.

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If these changes are due to internal processes, then those processes have led to the radius of Betelgeuse expanding about 9 percent. However, this is not the only explanation. The star might have expelled gas and dust that is now cooling off and that's blocking some of the light we receive from the star.

If the dimming is part of an extra-strong episode of the 420-day period, the minimum is almost upon us, expected between late January and early February. If it continues to dim after that, then something else might be going on.

Betelgeuse will eventually die in a dramatic supernova explosion, which will be very clearly visible from Earth. Any day could be a good day for it to go boom, but don't get your hopes up just yet. Despite its recent peculiar behavior, it's more likely that it still has several thousand years left.

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