12 January 2020
So that's a funny term... fainting. But it's actually the astronomical term for what is happening to what is normally the brightest star in the constellation Orion (The Hunter). It means the star is dimming - and rather dramatically! Because Betelgeuse is a red super giant star, it will likely explode at some point in the future in an event called a supernova.
With a diameter of about 700 times that of our Sun, Betelgeuse would extend out past the orbit of Mars if it was plunked into the middle of our Solar System. It's surface temperature is lower than our Sun's (3500K vs. 5778K) so it glows with a distinctive red colour. It is a semi-regular variable, which means it does dim and brighten over time - so in some respects this fainting (dimming) is normal.
However, the range of variability is normally between +0.0 and +1.3 magnitudes (the lower the number, the brighter the object's appearance in our sky), so this current fainting spell (pardon the pun!) is unusual as the star has dipped to +1.5 which doesn't seem like much, but it means that Betelgeuse went from being the 10th brightest star in our night sky to being the 23rd brightest.
Because it is a pulsating semi-regular variable, the brightness changes in the star are due to changes in its size and surface temperature. It is expected that the range of this pulsation (and thus it's apparent brightness) will become more erratic as it approaches the end of its life and explodes. The good news for us is that even if it does explode, at a distance of around 550 light years, it will merely be the most spectacular light show we've ever seen rather than being something more dangerous (ie: if Betelgeuse was much closer and exploded, it could bathe the entire Solar System in gamma radiation that could end life as we know it).
But scientists don't think we're about to witness a supernova. Betelgeuse has had these episodes before and nothing has come of it. Likely, it won't explode until about 100 million years from now, so all we can do is enjoy the change in brightness and ponder this magnificent star.
One thing you can do is visually compare the apparent brightness of Betelgeuse against nearby Aldebaran in the constellation Taurus which is also a red giant star of magnitudes between +0.75 and +0.95 (it too is a variable star).