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Comment by raxxorrax | Hacker News Reader

> that the old star was about to go supernova

I think the estimate is that Betelgeuse is about 10 million years old, some suggest it is closer to 8 million. Our sun is in its prime with 4.5 billion years.

Glad that stars can have a vastly different life expectancy.

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Red dwarfs have to counter less gravity pressure and can be fully convective - they can burn for trillions of years: https://en.wikipedia.org/wiki/Red_dwarf

The absolute magnitude scale is hilarious: logarithmic and inverted. A value lower by 5 means it's 100 times brighter.

The magnitude (absolute or apparent) actually started as an ordinal number: in Italian we still say sometimes first magnitude instead of magnitude 1. Of course that doesn't work too well for the Sun's magnitude of -26.

So, that graph on the GP's link has a total scale of 10^14 times between the smallest and largest point. What is roughly the difference between the energy of electrical and nuclear phenomena.

Every star has the force of gravity that is pressing all the gas to collapse, and heat from fusion that is pushing all the gas apart. These forces are balanced in a stable star: fusion reaction provides the energy that prevents the gas from collapsing. In comparison to our sun, a larger star like Betelgeuse needs to sustain a higher level of fusion to fight off the effects of the higher gravitational force. Larger stars have shorter lifespans because they burn through their fuel faster.

For hydrogen-burning stars, the life expectancy is reciprocal to their mass. As Betelgeuse is much more massive than the Sun, its lifetime is considerably shorter. As it is already a red giant, it is in its last phase before going supernova.

One far day in the future, the Sun is also going to become a red giant, extending out till the Mars orbit. But that is still much smaller than Betelgeuse is right now. Actually it is so large, that its diameter can be directly measured by astronomers.

It's more than reciprocal. Fusion rate and luminosity go up enormously with stellar mass, like m^4 or even higher, so the lifetime goes more like 1/m^4.

Sirius for example is 2x the mass of the sun, but 25x as luminous, therefore 2/25 the lifetime until it runs out of hydrogen.

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