Why will Betelgeuse go supernova?

[tovisonnenberg]

Betelgeuse is a red supergiant, yet I have heard that Betelgeuse will go supernova in the next million years. How is this possible? Wouldn't Betelgeuse be too cool to go supernova? Normally red supergiants produce white dwarfs and planetary nebulae.

 

[mathman]

Size! When it runs out of fuel it will collapse under its own weight.

 

[Ken G]

Red giants are the ones that make white dwarfs, red supergiants go supernova. The issue is whether or not the core ever gets enough mass to release enough gravitational energy to accelerate the electrons to near the speed of light. Relativistic electrons have a different relation between momentum and energy, such that they actually have only 4/5 as much momentum flux per energy density when they go relativistic. That might seem like a small difference, but it is enough to make the core undergo catastrophic collapse when gravity squeezes it. You need about 1.4 solar masses in the iron core to get the electrons to go relativistic, which requires stars of at least the mass of Betelgeuse.

 

[stefan r]

...You need about 1.4 solar masses in the iron core to get the electrons to go relativistic, which requires stars of at least the mass of Betelgeuse.

You get supernovas in white dwarfs when they reach 1.4 solar mass. The equation for the Chandrasekhar limit has a term in units of nucleons per electron. Most white dwarfs are carbon/oxygen/neon so there is 2 nucleons per electron. Iron has 56 nucleons and 26 electrons so 2.15. Squared is 4.64 instead of 4.
1.4/4.64*4 = 1.2 solar mass.

...Wouldn't Betelgeuse be too cool to go supernova? ...

Red giants are cool on the outermost layers. The core is extremely hot and dense. The outer layer is blown out by the radiation from the hot core.
In the solar system our planet has ice on planets 1 astronomical unit from the sun. Around Betelgeuse the star's photo sphere includes the area 1 astronomical unit radius.

 

[CygnusX-1]

Stars born with more than 8 solar masses go supernova. The mass of Betelgeuse is between 15 and 25 solar masses (see this), so Betelgeuse will explode as a supernova.

 

[Ken G]

That paper seems more like a study of how asteroseismology data might be useful for understanding Betelgeuse, than a paper that comes to definitive conclusions about Betelgeuse's mass. For example, the Wiki on Betelgeuse (https://en.wikipedia.org/wiki/Betelgeuse) still lists its mass as 12 solar masses! Perhaps that number is a little out of date, but it remains a bit unclear given the uncertainties about Betelgeuse. It's still a bit tricky to get the mass of a single star just by modeling it. But in any event, whether it is in the range 15-25, or just 12 solar masses, it will go boom.

 

[snorkack]

How precisely do you derive the 8 solar mass limit?

 

[Ken G]

Not precisely at all, the exact mass needed to go supernova is not known. You can't get it from simulations because simulations usually don't even produce a supernova at all, and the observations are limited because we don't know the masses that well and there aren't that many supernovae where we know the star that produced it. I don't know the confidence level for what the minimum mass is, but I'm sure it's at least a 1 solar mass uncertainty and I wouldn't be shocked if it was even 2. So we have uncertainty at both ends-- we usually don't know the precise mass of the star, nor the precise mass to go supernova.

 

[snorkack]

That paper seems more like a study of how asteroseismology data might be useful for understanding Betelgeuse, than a paper that comes to definitive conclusions about Betelgeuse's mass. For example, the Wiki on Betelgeuse (https://en.wikipedia.org/wiki/Betelgeuse) still lists its mass as 12 solar masses! Perhaps that number is a little out of date, but it remains a bit unclear given the uncertainties about Betelgeuse. It's still a bit tricky to get the mass of a single star just by modeling it. But in any event, whether it is in the range 15-25, or just 12 solar masses, it will go boom.

The number for Betelgeuse is from a link:
https://arxiv.org/abs/1109.4562
which tries to estimate the mass via limb darkening.
The uncertainty is from 8 to 17 solar.
And that's just present mass. Not original mass (how much mass has Betelgeuse already shed), nor presupernova mass (how much mass Betelgeuse will yet shed quietly as a red giant before exploding).

 

[Ken G]

Yes, the uncertainty in the mass of a single star is generally pretty large, especially when the distance is also uncertain. It seems likely that Betelgeuse would go supernova, but it's hard to say for sure, because one can take the smallest observed mass for Betelgeuse, and the largest theoretical limit, and get that it won't. It would be a good bet for a gambler, but hard to collect! By and large, astronomical conclusions are statistical in nature, more so than intended to apply to given objects. Sometimes predictions for specific objects can be made and tested, such as the prediction that KIC 9832227 will undergo a red nova outburst in 2022 (https://arxiv.org/abs/1704.05502), but that's pretty rare.