Gravity and how it starts and stops

[Eric pelland]

They say gravity is because mass stretches space time and it makes matter fall to it and can prove it. And that makes sense when there is a lot of mass. But they say stars are made from gas clouds mostly hydrogen and helium very small atoms that start to clump together. So a star starts out a one atom not much mass. But is able to stretch space like make a dip that other atoms fall into and gather. They should all have the same mass say one atom of hydrogen so there should be many dips in space time . But one must have just a bit more mass to get it started. And once started you would think it would use the whole gas cloud. Yet we see nebulas still around giving birth to many stars. So at some time mass must quit falling in making the star bigger and not use all the gas. So does the dip in space get less dip. Less gravity. Something doesn’t make sense. It’s big enough to bend light. But not big enough to use all the gas in the cloud.

 

[Ibix]

This might be better in cosmology. I'll report and suggest it be moved.

For reasons we don't completely understand, the early universe wasn't completely homogeneous. There were slightly overdense regions and slightly underdense regions. The overdense regions collapse, not because any hydrogen atom was heavier than another, but because there were slightly more of them in one area than another. This leads to the formation of stars, galaxies, and galactic clusters.

But once fusion ignites in a star, the stellar wind it generates blows nearby gas away. So it doesn't all form into stars in the first place. Furthermore, stars eventually explode, blasting a mix of hydrogen and heavier elements into space. In other words, some of the nuclear energy gets released as kinetic energy, enough for at least some of the star to achieve escape velocity. This, I believe, is the source of the nebulae we see today. Frequently nebulae collapse into new stars (with different element mixes and different properties).

I'm not an expert on stellar formation, so there may be other processes and subtleties I'm not aware of. But the answer to your question is that it isn't to do with "less gravity". Rather, it's other processes making themselves felt.

 

[Drakkith]

As Ibix said, the main thing to take away here is that a collection of large numbers of atoms attract each other and pull each other together if they are denser than the surrounding space. The distribution of matter in the universe is very uneven, with matter clumped together into galaxies, nebulas, and gas clouds surrounded by low-density regions where there are only a few atoms per cubic meter or less.

So a star starts out a one atom not much mass.

No, stars start out as collections of something like 10⁵⁰-10⁶⁰ atoms. You cannot pick a single atom out of all of these and say that it is the starting point.

But is able to stretch space like make a dip that other atoms fall into and gather. They should all have the same mass say one atom of hydrogen so there should be many dips in space time

Yes, that is exactly what happens. The gravity from all of these atoms adds together, pulling material inwards and growing stronger as the density increases.

But one must have just a bit more mass to get it started.

Not true. All that is needed is an uneven distribution so that the density is greater in one region than another. Gravity will then take over and pull this denser region together.

And once started you would think it would use the whole gas cloud. Yet we see nebulas still around giving birth to many stars. So at some time mass must quit falling in making the star bigger and not use all the gas.

That's right. In addition to what Ibix said about stars blowing gas and dust away, slightly uneven densities within the gas cloud itself cause the cloud to break into smaller sections as it collapses. And then these commonly break into even smaller sections. So you end up with a gas cloud of thousands of solar masses breaking apart into dozens or hundreds of smaller clouds of tens of solar masses each, with each of these small sub-clouds forming a separate proto-star. Not all of the gas will end up in one of these proto-star clouds, as gravitational interactions between them tends to throw some material out into interstellar space.

Edit: One more thing. I highly recommend not even trying to think about gravity in terms of spacetime curvature. At these scales we can use Newtonian gravity and predict things perfectly well, and Newtonian gravity is much easier to understand. General Relativity is counter intuitive and contains many subtleties that you need to understand if you want to grasp it. Understanding 4-dimensional variable geometry is... difficult.

 

[Eric pelland]

This might be better in cosmology. I'll report and suggest it be moved.

For reasons we don't completely understand, the early universe wasn't completely homogeneous. There were slightly overdense regions and slightly underdense regions. The overdense regions collapse, not because any hydrogen atom was heavier than another, but because there were slightly more of them in one area than another. This leads to the formation of stars, galaxies, and galactic clusters.

But once fusion ignites in a star, the stellar wind it generates blows nearby gas away. So it doesn't all form into stars in the first place. Furthermore, stars eventually explode, blasting a mix of hydrogen and heavier elements into space. In other words, some of the nuclear energy gets released as kinetic energy, enough for at least some of the star to achieve escape velocity. This, I believe, is the source of the nebulae we see today. Frequently nebulae collapse into new stars (with different element mixes and different properties).

I'm not an expert on stellar formation, so there may be other processes and subtleties I'm not aware of. But the answer to your question is that it isn't to do with "less gravity". Rather, it's other processes making themselves felt.

Thanks for responding. How can stars be different sizes then? Yes at a certain point they ignite and stop growing. Once the ball of gasses gets so big it would have to ignite no way to stop that.

 

[Ibix]

Thanks for responding. How can stars be different sizes then? Yes at a certain point they ignite and stop growing. Once the ball of gasses gets so big it would have to ignite no way to stop that.

Like I say, I'm not an expert. A quick google turns up this thread: https://www.physicsforums.com/threads/why-stars-are-different-sizes.294234/

 

[Eric pelland]

As Ibix said, the main thing to take away here is that a collection of large numbers of atoms attract each other and pull each other together if they are denser than the surrounding space. The distribution of matter in the universe is very uneven, with matter clumped together into galaxies, nebulas, and gas clouds surrounded by low-density regions where there are only a few atoms per cubic meter or less.
No, stars start out as collections of something like 10⁵⁰-10⁶⁰ atoms. You cannot pick a single atom out of all of these and say that it is the starting point.
Yes, that is exactly what happens. The gravity from all of these atoms adds together, pulling material inwards and growing stronger as the density increases.
Not true. All that is needed is an uneven distribution so that the density is greater in one region than another. Gravity will then take over and pull this denser region together.
That's right. In addition to what Ibix said about stars blowing gas and dust away, slightly uneven densities within the gas cloud itself cause the cloud to break into smaller sections as it collapses. And then these commonly break into even smaller sections. So you end up with a gas cloud of thousands of solar masses breaking apart into dozens or hundreds of smaller clouds of tens of solar masses each, with each of these small sub-clouds forming a separate proto-star. Not all of the gas will end up in one of these proto-star clouds, as gravitational interactions between them tends to throw some material out into interstellar space.

Edit: One more thing. I highly recommend not even trying to think about gravity in terms of spacetime curvature. At these scales we can use Newtonian gravity and predict things perfectly well, and Newtonian gravity is much easier to understand. General Relativity is counter intuitive and contains many subtleties that you need to understand if you want to grasp it. Understanding 4-dimensional variable geometry is... difficult.

Thanks for responding. So you’re saying gravity doesn’t start till there are a great number of atoms collect in one place ? Nother question once the gas ball gets so big it will ignite no way to stop that. So how can there be super big stars? What keeped them from ignition. Are they made of something else lighter than hydrogen?

 

[Drakkith]

Thanks for responding. How can stars be different sizes then? Yes at a certain point they ignite and stop growing. Once the ball of gasses gets so big it would have to ignite no way to stop that.

As you will find in the thread that Ibix linked, stars don't start shining when fusion starts in their cores. In fact they begin shining well before that. The collapsing gas and dust begins to heat up well before it becomes a protostar, and further collapse and compression heats up this gas and dust even more. By the time that a protostar is formed, the gas and dust in the center of the cloud (the stuff that forms the actual protostar) has been heated up to hundreds of thousands or millions of degrees in the very center, with the temperature gradually falling as you get further out.

Once the protostar blows away the remaining gas and dust surrounding it it becomes a pre-main sequence star, a star which has yet to begin fusing hydrogen in its core. The star continues to collapse and heat up until hydrogen burning begins, which stabilizes the star and puts it on the main sequence track.

The entire stellar formation process is a balance between the inward force of gravity and the outward force from radiation pressure and heat. Starting with more or less mass alters the final end point such that there is no single cutoff value for the mass of a star. More starting mass in the cloud generally creates larger, more massive stars.

Thanks for responding. So you’re saying gravity doesn’t start till there are a great number of atoms collect in one place ? Nother question once the gas ball gets so big it will ignite no way to stop that. So how can there be super big stars? What keeped them from ignition. Are they made of something else lighter than hydrogen?

If by 'gravity starting' you mean that the collapse process starts, then yes, it doesn't happen until there is enough mass in a small enough area to overcome the pressure holding the cloud up against itself. A hotter cloud will need more mass since it has a greater outward pressure, a cooler cloud will need less mass.

The largest stars simply start with a lot more mass in their progenitor clouds than smaller, less massive stars. As I said above, 'ignition' doesn't even happen until well after the star has already blown away the remains of its progenitor cloud, so it has little effect on the size of a star as far as I understand.

 

[Eric pelland]

As you will find in the thread that Ibix linked, stars don't start shining when fusion starts in their cores. In fact they begin shining well before that. The collapsing gas and dust begins to heat up well before it becomes a protostar, and further collapse and compression heats up this gas and dust even more. By the time that a protostar is formed, the gas and dust in the center of the cloud (the stuff that forms the actual protostar) has been heated up to hundreds of thousands or millions of degrees in the very center, with the temperature gradually falling as you get further out.

Once the protostar blows away the remaining gas and dust surrounding it it becomes a pre-main sequence star, a star which has yet to begin fusing hydrogen in its core. The star continues to collapse and heat up until hydrogen burning begins, which stabilizes the star and puts it on the main sequence track.

The entire stellar formation process is a balance between the inward force of gravity and the outward force from radiation pressure and heat. Starting with more or less mass alters the final end point such that there is no single cutoff value for the mass of a star. More starting mass in the cloud generally creates larger, more massive stars.
If by 'gravity starting' you mean that the collapse process starts, then yes, it doesn't happen until there is enough mass in a small enough area to overcome the pressure holding the cloud up against itself. A hotter cloud will need more mass since it has a greater outward pressure, a cooler cloud will need less mass.

The largest stars simply start with a lot more mass in their progenitor clouds than smaller, less massive stars. As I said above, 'ignition' doesn't even happen until well after the star has already blown away the remains of its progenitor cloud, so it has little effect on the size of a star as far as I understand.

Yes, I agree more available mass the bigger the gas ball will become. And gravity should increase with more mass. And the center of gas ball should get more dense it has too. So somehow the very big gas balls Centers don't get dense for longer? Even though gravity keeps on increasing. The collapse has to be based on something else then

 

[Drakkith]

Yes, I agree more available mass the bigger the gas ball will become. And gravity should increase with more mass. And the center of gas ball should get more dense it has too. So somehow the very big gas balls Centers don't get dense for longer? Even though gravity keeps on increasing. The collapse has to be based on something else then

I'm not sure how the density of the core behaves as mass increases, so I can't help you there.

 

[stefan r]

I'm not sure how the density of the core behaves as mass increases, so I can't help you there.

All stars must have started at extremely low density and increased to high density. Fusion takes place at lower density in stars with more mass.

 

[Drakkith]

All stars must have started at extremely low density and increased to high density. Fusion takes place at lower density in stars with more mass.

Got any links? I'd love to read more.

 

[DaveC426913]

And the center of gas ball should get more dense it has too. So somehow the very big gas balls Centers don't get dense for longer? Even though gravity keeps on increasing.

Internal pressure from gravity rises to a point where the radiation pressure and kinetic energy from the hot atoms (plasma) balances out the crush from gravity.

That results in a stable volume.

Now, you can do interesting things if you add a lot more mass. (not all stars have access to so much mass). But if enough is captured, gravity overcomes even the internal pressures mentioned above, and the atoms pack even further together until they are (essentially) touching.

 

[Eric pelland]

there are some gas ball with a lot less mass yet some of them some how are able to compress the gas till it gets so hot it starts to fusion. They say the core collapses on it self. So it gets denser. Gravity must increase or the gas molecules must get smaller? There are very large gas balls that the core has not collapsed. So mass is not a guarantee for collapse? I'm sure we know how tightly you have to compress hydrogen to start fusion and the amount of mass needed for gravity to do that. Something doesn't make sense.thanks

 

[Drakkith]

there are some gas ball with a lot less mass yet some of them some how are able to compress the gas till it gets so hot it starts to fusion. They say the core collapses on it self. So it gets denser. Gravity must increase or the gas molecules must get smaller?

Gravity increases as density increases.

There are very large gas balls that the core has not collapsed. So mass is not a guarantee for collapse?

There are large gas balls that the core has not yet collapsed. It takes time.

I'm sure we know how tightly you have to compress hydrogen to start fusion and the amount of mass needed for gravity to do that.

It's not just the compression, it's also the temperature. Compression both increases density, which allows for a higher fusion rate since the ions are all much closer together, and also raises the core temperature.

 

[Eric pelland]

Gravity increases as density increases.
There are large gas balls that the core has not yet collapsed. It takes time.
It's not just the compression, it's also the temperature. Compression both increases density, which allows for a higher fusion rate since the ions are all much closer together, and also raises the core temperature.

Gravity is responsible for density. And as the density goes up the so does the temperature. I don't think the mass of a gas ball has anything to do with the collapse of the core to start fusion. Because there are different sizes stars. Yes bigger gas balls don't need as much extra gravity to collapse the core. Smaller ones would need a lot more collapse to start fusion. Yes the fusion would start small and slowly grow. If all the stars had to be a certain mass to start fusion then would be similar in size.

 

[Drakkith]

I don't think the mass of a gas ball has anything to do with the collapse of the core to start fusion.

Of course it does. The cloud of gas that collapsed to form Jupiter was never able to get to a high enough temperature to start fusion in its core because it did not have enough mass. The same is true for things like brown dwarfs, which only achieve deuterium fusion, but not protium fusion.

Yes bigger gas balls don't need as much extra gravity to collapse the core.

I don't know what 'extra gravity' is supposed to mean.

If all the stars had to be a certain mass to start fusion then would be similar in size.

Support your claim with a valid reference please.

 

[mfb]

If all the stars had to be a certain mass to start fusion then would be similar in size.

Objects need a minimal mass to start fusion, and indeed all stars have a mass above this minimum. The mass of an object depends on the mass of the initial cloud that collapses. If it is too small, you don't get a star. If it is just enough for protium fusion you get a red dwarf. If it is larger you get Sun-like stars, and so on.
There is also an upper limit for the mass of stable stars, but that is much larger and given by stellar winds at the surface, not by fusion.

 

[Eric pelland]

Of course it does. The cloud of gas that collapsed to form Jupiter was never able to get to a high enough temperature to start fusion in its core because it did not have enough mass. The same is true for things like brown dwarfs, which only achieve deuterium fusion, but not protium fusion.
I don't know what 'extra gravity' is supposed to mean.
Support your claim with a valid reference please.

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


[URL]https://www.universetoday.com/25348/what-is-the-smallest-star/[/URL]

 

[DaveC426913]

If all the stars had to be a certain mass to start fusion then would be similar in size.

Why do you think this is the case?
There is a minimum mass, as others have pointed out. But what makes you think that a given collapsing gas cloud can only form a star up to a certain size?
More starting material and/or more time equals bigger star.

Your logic doesn't make sense.

 

[Drakkith]

https://en.wikipedia.org/wiki/Stellar_evolution
https://www.universetoday.com/25348/what-is-the-smallest-star/

Those references do not support your claim.

 

[Eric pelland]

Why do you think this is the case?
There is a minimum mass, as others have pointed out. But what makes you think that a given collapsing gas cloud can only form a star up to a certain size?
More starting material and/or more time equals bigger star.

Your logic doesn't make sense.

Yes my post is wrong. Some continue to gain mass once they have started fusion of hydrogen. If there is mass available. But some gain so much mass that they should not stay together. Yet get even more mass. They have theories to explain what the how its possible. But really don't know.

 

[rootone]

Larger mass stars go beyond fusion of hydrogen, and can fuse elements up as far as iron/nickel while remaining stable.
Those stars end up as type 2 supernovae (which briefly produce elements even more heavy than iron), but this is unstable so the core collapses.
The remains of the collapsed core become a neutron star, or in extreme cases a black hole.

 

[mfb]

Some continue to gain mass once they have started fusion of hydrogen.

Rarely. Most stars simply start with more mass.

But some gain so much mass that they should not stay together. Yet get even more mass.

Do you have an example?

 

[Eric pelland]

Rarely. Most stars simply start with more mass.Do you have an example?

https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-lifecycles.html
https://www.spaceanswers.com/deep-space/how-do-some-stars-get-so-massive/
These are the articles. Do you mean that it gathers gas and dust till there is no more close by in the nebula? Then it starts fusion?

 

[Eric pelland]

https://imagine.gsfc.nasa.gov/educators/lessons/xray_spectra/background-lifecycles.html
https://www.spaceanswers.com/deep-space/how-do-some-stars-get-so-massive/
These are the articles. Do you mean that it gathers gas and dust till there is no more close by in the nebula? Then it starts fusion?

Are sun now doesn't gain mass from its gravity except when a piece of iron might crash into it.
But I believe when it was young it gathered mass. And it's core started fusion long before it had gathered all the mass available. It ran out of stuff to gather at some point or it would have been bigger.

 

[Drakkith]

These are the articles. Do you mean that it gathers gas and dust till there is no more close by in the nebula? Then it starts fusion?

He means that for most stars, all the mass the star will ever have is gathered well before fusion begins to occur in its core. The only exception might be high-mass stars with more than around 8 solar masses. These collapse so rapidly (relative to less massive stars) that they start fusing before they blow away their envelope of gas.

Are sun now doesn't gain mass from its gravity except when a piece of iron might crash into it.

A piece of iron? Do you mean something like an asteroid?

But I believe when it was young it gathered mass. And it's core started fusion long before it had gathered all the mass available.

No, that's incorrect. The Sun spent a few million years as a pre-main sequence star, where it had already blown away most of its parent nebula but hadn't yet started fusing hydrogen in its core.

See here: https://en.wikipedia.org/wiki/Pre-main-sequence_star
And here: https://en.wikipedia.org/wiki/T_Tauri_star

 

[DaveC426913]

It ran out of stuff to gather at some point or it would have been bigger.

Yes. What wasn't blown away formed into a protoplanetary disc, which coalesced into the current solar system bodies we see today - about 0.2% of the Sun's mass.

 

[Eric pelland]

https://www.independent.co.uk/news/...-new-how-do-universe-galaxy-a8329581.html?amp
Maybe things are a little different

 

[Eric pelland]

https://physicsworld.com/a/how-do-stars-form/

 

[Eric pelland]

After the protostar gains mass it blows away what it doesn't use. Okay then depending on how much mass it has after that and it varies. That's the problem I have. The energy to blow away comes from friction. That means to me the mass gets to a certain size. If it is too gain more mass and it must be able too. Then the friction energy would have to be controlled by something other than mass and gravity. I can't find were it says how this is done. Thanks