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chriswhoppers t1_j220ekj wrote

Reply to comment by [deleted] in What is the timeline of star death? by jfgallay

So is fusion not limitless? If a star uses fusion in its process and lasts only 10 billion years, then what chance does a man-made device have for lasting even close to that long with time dilation in play?

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Lyrian_Rastler t1_j22bct6 wrote

It's not limitless because there is a finite amount of hydrogen in its storage. But even more so because it uses a stupendous amount of hydrogen every single second.

So 1) Man made devices just need to be fueled with hydrogen, and hydrogen is very very common. Not limitless, sure, but very common still

  1. We would require the tiniest fraction of what the sun uses to generate all the the energy we use, which means even with a smaller amount of hydrogen available they might last longer

Of course, all this is moot if humans don't manage to survive long enough to really do anything with fusion because of climate change

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galqbar t1_j26he5o wrote

The other commenter said it well, but just to add an analogy: if you burn a chunk of coal slowly at low temperatures it will last many hours, but if you crush it up a bit and heat it more you can burn it faster. If you blast it at super high temperatures you can get all of your energy in a single big WHOOSH. The total amount of energy released is the same.

The sun burns really really slowly, and only in a relatively small region at the center where it is hottest. The reactors that people are trying to build are trying to burn up the fuel quickly for a number of reasons, one of which is because it’s hard to keep it burning slowly without it sputtering out (to use the same analogy).

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EBtwopoint3 t1_j22bpj2 wrote

Can you better explain what the question is? If you mean a fusion reactor, the fusion reaction continues until you either run out of fuel or can no longer maintain the containment and allow the materials to spread/cool too much to continue fusing. I don’t see how time dilation comes into play, or what a 10 billion year time scale has to do with anything.

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chriswhoppers t1_j2381wa wrote

You answered the question wonderfully. Thank you. Time dilation comes into play because of size of the massive object. Since the sun is 100x bigger than earth, time is perceived to go much slower. The passage of time happens based on gravitational potential and velocity, so when the sun says 10 billion years, its more like 100 million years of duration on earths time scale. But like you said, as long as there is a power source it won't end, so how does a star run out of hydrogen in an endless sea of it?

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EBtwopoint3 t1_j23pqjb wrote

That is not how time dilation works. It is not a 1 to 1 rate or people who are twice as heavy would experience them moving at super speed. At the surface of the Sun, time dilation is at a rate of ~67 seconds per year. Or, for every year on Earth only 364 days, 23 hours, 58 minutes, and 53 seconds will have passed.

As for how the star runs out, the end result of fusion is that two hydrogen atoms turn into 1 helium atom. Eventually, all the hydrogen atoms in the star have been fused. Of course there will be some hydrogen left in the star, but it is too diffuse to continue fusing at a rate to sustain the star. Remember that stars are a delicate balance between the pressure exerted by all the heat and fusion, and the gravity trying to contract it.

Last, the sun is 1.3 million times the size of Earth by volume and 333,000 times the mass of Earth.

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