What will the Betelgeuse supernova be like—and will it hurt us?

 

Credit: Royal Astronomical Society/ ESO/ L. Calçada.

It will be the show of a lifetime when Beetlejuice opens.  However, it won't harm us.

 About 500 light years away, Orion, a crimson giant, has a shoulder called Betelgeuse.  It is enormous, weighing between 15 and 20 solar masses, yet it is so bloated and stretched that it would reach around the orbit of Mars if it were placed in our own solar system.

 Additionally, it's not doing well.  The exact lifetime of a massive star depends on a number of variables, including its mass, metallicity, and spin rate. Massive stars do not have very lengthy lives.  We're simply talking about a few hundred thousand years on the low side.  We have a few million on the upper side.

In any case, that's not a lot of stars.  Red dwarfs, the tiniest stars in the cosmos, can last for trillions of years at a time, and our own sun will survive several generations of these giants.  As an entertaining aside, red dwarfs are so long-lived that the cosmos isn't even old enough for them to begin dying yet.

 Betelgeuse is dying, regardless of how you look at it.

The reason astronomers used the term "red giant phase" to describe this stage of a star's life cycle is rather clear. It is enormous and crimson. It is also in a very unstable phase because it is so near death. Indeed, a few years ago, we witnessed some quite dramatic dimming incidents in which it abruptly decreased by about 15% over a few weeks. After a few months, it suddenly returned to its full brightness.

Chaos reigns when a star is nearing the end of its existence. Depending on the situation, it may fuse hydrogen or helium, shut down for a period of time, and then restart. The atmosphere's outer edges begin to take on a life of their own since they are so distant from the core. It simply becomes more difficult.

Betelgeuse is expected to go supernova in a few hundred thousand years, according to estimates based on its mass, rotation rate, the group of stars it was born with, and the quantity of metals we can measure in its upper layers of atmosphere.  To be honest, though, it might be tomorrow.  Because it's 500 light years distant, actually.  We won't know for a while if it went off a century ago or not.  It might be dead already.

The explosion of Betelgeuse as a supernova will be spectacular.  Remember, a typical supernova can outshine galaxies with more than 100 billion stars.  And Betelgeuse will be putting on a spectacular performance from a few hundred light years away.

 You will be able to see it during the day.  No planet will be as bright as it.  It will have nearly the full moon's brightness.  At midnight, the light from the Betelgeuse supernova will be enough to read a book.

However, it will be uncomfortable to look at since Betelgeuse will still be a small pinprick of light, unlike the full moon, which is this beautiful disk in the sky. It will therefore be uncomfortable to observe, and like all supernovae, it will fade away after a few months. Despite its impressiveness, it won't pose a threat.

The fact that stars are so far away, despite their brightness and the amount of radiation they emit into the cosmos, protects us from the majority of supernova threats. The inverse square law is helpful in this situation. A star, a supernova, or any other radiating object in the universe emits a certain amount of light.

The same amount of light must therefore traverse an increasing amount of space as it goes away from the star.  The radiation in a single location is reduced by 25% when the distance is doubled.  You get a factor of 100 drop off if you travel ten times as far.  The square of the distance is what it goes as.

Standing very close to a fire while attempting to stay warm will make you realize that it may be a touch too hot, but as soon as you take a step back, you'll be cold once more.  The reason for this is the inverse square law of the infrared radiation that the fire is emitting.  However, we will be thankful for the inverse square law in the event of a supernova.  We are discussing the possibility of a massive star transforming into an uncontrollable nuclear bomb and exploding with enough energy to outweigh the starlight of an entire galaxy.

From our vantage point, Betelgeuse will increase in brightness from a dot of light in the night sky.  Simply put, it won't pose a threat.

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