When Beetlejuice goes off, it's going to be the show of a lifetime. But it’s not going to hurt us.
Betelgeuse is the shoulder of Orion, a red giant sitting about 500 light years away. It’s huge, weighing somewhere between 15-20 solar masses, but so extended and bloated that if you plopped it down in our own solar system, it would stretch to roughly the orbit of Mars.
And it's not doing so great. Massive stars do not live very long lives, with their precise lifetimes depending on a host of factors like their mass, their metallicity, and spin rate. On the low side, we're talking only a few hundred thousand years. On the high side, we've got a few million.
But either way, as stars go, that's not a lot. Our own Sun will outlive multiple generations of such giants, and red dwarfs, the smallest stars in the universe, can stretch for trillions of years at a time. In fact, just a fun side note, red dwarfs live for so long that the entire universe isn't even old enough for them to start dying yet.
No matter how you slice it, Betelgeuse is on its last legs.
It's in what's called the red giant phase, and it's pretty obvious to see why astronomers picked that name for this phase in a stellar life cycle. It's red, and it's gigantic. And it is so close to being dead that it is in an incredibly unstable phase. In fact, we saw some very dramatic dimming episodes a few years ago where it dimmed by around 15% out of nowhere over the course of a few weeks. And then just after a few months, it popped back up to full brightness.
When a star is near the end of its life, it’s all chaos. Sometimes it's fusing hydrogen, sometimes it's fusing helium, sometimes it will shut down for a while, sometimes it'll start back up. The outer edges of the atmosphere are so far away from the central core that they start getting a mind of their own. It just gets complicated.
Estimates based on the mass of Betelgeuse, its rotation rate, the group of stars it was born with, and the amount of metals we can measure in the upper layers of its atmosphere, all suggest that it's somewhere in the neighborhood of a few hundred thousand years from now, it's going to go supernova. But honestly it could be tomorrow. In fact, because it's 500 light years away. It could have gone off a hundred years ago, and we won't find out for a while. It may already be dead.
When Betelgeuse goes off as a supernova, it will be a sight to behold. Keep in mind that typical supernova can outshine entire galaxies of over a hundred billion stars. And at a distance of a few hundred light years, Betelgeuse is going to put on an impressive show.
It will be visible during the day. It will be brighter than any planet. It will be almost as bright as the full moon. You'll be able to read a book by the light of the Betelgeuse supernova at midnight.
But it will actually be painful to look at because unlike the full moon that is this gorgeous disc in the sky, Betelgeuse is still going to be a tiny pinprick of light. So it won't be comfortable to look at, and it will last a few months before fading away as all supernovae do. But as impressive as it is, it won't be dangerous.
What saves us from most supernova dangers is that as bright as they are, as much radiation as they pour into the universe, stars are really stinking far apart. What helps here is something called the inverse square law. There's a fixed amount of light that a star or a supernova or any radiating object in the universe gives off.
And so that light moves away from the star, that same amount of light has to cover more and more area. If you double the distance, the radiation in any one spot gets cut to 25%. If you go to ten times the distance, then you get a factor of a 100 drop off. It goes as the square of the distance.
If you're trying to stay warm by a fire, you will notice if you stand really close to the fire that it’s maybe a little bit too hot, but then you take one step back, and all of a sudden, you’re cold again. That's because of that inverse square law of the infrared radiation being emitted by the fire. But in the case of a supernova, we're going to be grateful for the inverse square law. Because we're talking about a giant star turning itself into an uncontrolled nuclear bomb and detonating with enough energy to overwhelm an entire galaxy's worth of starlight.
From our perspective, Betelgeuse will go from a dot of light in the night sky to a brighter dot of light in the night sky. It's simply not going to be a threat.