Why Does Gravity Remain when Star Mass Dissipates?

[ahandyman59]

TL;DR

Stars contain mass. Gravity has a proportionate effect on that mass. When the star runs out of fuel, why doesn't the gravity reduce as the burnable material reduces?

Stars have mass. This mass has a gravitational pull. The nuclear fission of the star pushes against the effects of gravity. Why doesn't the gravity reduce as the star burns more and more of the hydrogen/helium/carbon that it's made up of? In other words, how can a black hole be created when the majority of the star has been burned away? Shouldn't gravity reduce based on the amount of the star's mass that remains after all the fuel is burned?

Stars emit huge amounts of energy into space. We are alive because of some of that very energy from our own star (the Sun). As this energy is dispersed, wouldn't the mass of the star be lowering as the process of fusion continues? How can there be enough mass left after a star burns out, to cause a black hole?

 

[PeroK]

Not all of the mass of a giant star is radiated away. It collapses before than happens and enough mass may remain to form a neutron star or black hole. See, for example:

https://en.m.wikipedia.org/wiki/Stellar_evolution

 

[ahandyman59]

But when the final collapse occurs, the net resulting black hole is theoretically more mass than the planet was in its entirety. I realize that the hydrogen gets converted to other elements including helium, carbon, sulfur and eventually iron, but each of these transitions can't be free - energy wise or mass wise. Something has to be lost during the creation of these other elements. Not all of that mass/energy gets recaptured. If the mass isn't constant, then the gravity should change accordingly, shouldn't it?

 

[gmax137]

Pick a star, say the sun. What is it's mass? What rate does it release energy, and what does this correspond to in terms of rate at which it's mass decreases? Divide the two to find out how many years before the mass has been released as energy.

Everything you need to do this simple Calc can be found with a few Google searches. The answer may surprise you.

 

[Ibix]

But when the final collapse occurs, the net resulting black hole is theoretically more mass than the planet was in its entirety.

No it isn't. It's the same mass, less the mass of all the matter that gets blasted into space in the final collapse. Where did you get the idea that its mass increases?

If the mass isn't constant, then the gravity should change accordingly, shouldn't it?

Yes. Leaving aside interactions where a star can gain matter from some other body, the mass of a star steadily decreases throughout its lifetime.

 

[Vanadium 50]

Fuel has mass, but so does ash.

 

[ahandyman59]

Pick a star, say the sun. What is it's mass? What rate does it release energy, and what does this correspond to in terms of rate at which it's mass decreases? Divide the two to find out how many years before the mass has been released as energy.

Everything you need to do this simple Calc can be found with a few Google searches. The answer may surprise you.

Looks like 4.67 billion years. Wow! Thank you for that.

 

[ahandyman59]

Thank you all.

 

[collinsmark]

Pick a star, say the sun. What is it's mass? What rate does it release energy, and what does this correspond to in terms of rate at which it's mass decreases? Divide the two to find out how many years before the mass has been released as energy.

Everything you need to do this simple Calc can be found with a few Google searches. The answer may surprise you.

Looks like 4.67 billion years. Wow! [...]

My calculations lead me to believe it's longer than that.

 

[gmax137]

Looks like 4.67 billion years. Wow! Thank you for that.

My calculations lead me to believe it's longer than that.

Me too, by quite a lot.
@ahandyman59 , show us your numbers!

 

[Ibix]

A youtube video that seems to have had their facts wrong...

It's a continuing issue, I'm afraid.

It is worth noting that because black holes and neutron stars are very dense you can get closer to them than you could to their progenitor stars, so the gravity near them can be a lot higher. It's the same at a distance, though.

So if you replaced the Sun with a one solar mass black hole it would make no difference to the Earth's orbit. And if you took a ship and flew to near where the Sun's surface was you could hover just the same as over the Sun. But if you flew inside the space where the Sun was (its radius is about 700,000km but the black hole's radius would be 3km) the thrust needed to hover would grow and grow, going to infinity as you approach the event horizon.

It's possible the video meant that.