• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

Astrophysicists around here?

G

Gunnar

Senior member
I know academically what these stars are, but I was wondering why these stars maintain their density after they have expelled an outer layer in "death".

If these stars are that dense, I would assume they are still performing fusion, of massive elements like carbon, but if the star expelled matter, that would mean there is less gravitational force holding it together, meaning that the density should lower, since particles would be more free to repulse each other using the strong and weak forces.

Am I missing something here?

And does anyone know a good scientific forum? I have a lot of questions like this(medicine, astronomy, etc) I need answered, and this is the only forum I frequent (because someone actually responds here...).
 
white dwarfs and neutron stars do not maintain their density. Like every other object in the night sky they are finite as well.

I believe white dwarfs eventually cool down to black dwarfs, the carbon leftover exerts enough gravity to hold it together.

I'm not quite sure I understand your question. Shouldn't the mass of the these leftover cores exert enough gravity to keep them intact? After the "evacutation" phase of super red giant an equilibrium should be met where the mass of the core exerts enough gravity to maintain the leftover matter.
 
You could have posted this in the highly technical.

From what I have read a long time ago, highly dense stars such as neutron stars are created after super massive stars collapse, they become black holes, and then finally the black hole has an extremely strong gravitational pull that creates these super dense neutron stars.
 
Both white dwarf and neutron stars are stars that collapsed after the red giant phase. They didn't have enough mass to become either a supernova or collapse fully into a black hole. They were 'in-between'. A white dwarf (similar but slightly different from a red dwarf or brown dwarf) has blown off it's outer layers in the Red Giant phase and has collapsed into a small star that is still conducting fusion but on a smaller scale. It's very dense (relative to other stars but not enough to be a black hole).

Not much is known about neutron stars. A neutron star collapsed further than a dwarf but still not fully to a black hole. Theories suggest one teaspoon full of matter from a neutron star would weigh several tons if it were possible to weigh on Earth. The nuetron star (often also called a pulsar) is rotating at an extremely high rate, perhaps around 2 rotations per second. It is also giving off radiation. The name pulsar came from the 'pulses' as this star rotates and the area that is streaming radiation passes over our field of observation.

It's been a long time since I finished my astronomy minor so some of this may be incorrect as my memory is rusty.
 
Oh, and like JoeKing said, these are finite objects. They're all just phases in star life cycles and will eventually move to another phase.
 
Originally posted by: Gunnar
I know academically what these stars are, but I was wondering why these stars maintain their density after they have expelled an outer layer in "death".

If these stars are that dense, I would assume they are still performing fusion, of massive elements like carbon, but if the star expelled matter, that would mean there is less gravitational force holding it together, meaning that the density should lower, since particles would be more free to repulse each other using the strong and weak forces.

Am I missing something here?

And does anyone know a good scientific forum? I have a lot of questions like this(medicine, astronomy, etc) I need answered, and this is the only forum I frequent (because someone actually responds here...).
Click to expand...

Astrophysicist here 🙂

Neither white dwarfs nor neutron stars are undergoing fusion. This is the key.

White dwarfs are just radiating blackbody radiation (heat from just being hot) and will eventually dim and cool down to a superhuge diamond hurtling through space. The best way to explain a white dwarf (in nerd terms) is that it's the athlon t-bird you've oc/d for the last time. It locked, burned out, and now is just smoldering. The star burned through its hydrogen, then when the hydrogen in the core ran out, the star collapsed under gravity until the pressure was enough to ignite helium fusion. Once the helium in the core was burned up, lithium and boron were burned, and on to carbon. If the star does not have enough mass to burn the carbon, it expels its outer layers and you're left with the remaining carbon-rich, extremely dense core. There's no reaction going on inside the star though.

Neutron stars occur when carbon is burned, exhausted, etc etc, all the way up to iron. Once you hit iron in the core, the reaction stops cold because it takes more energy to fuse iron than you get out of the process. The star collapses onto its core as there's no reaction producing an outward pressure anymore, and the outer layers essentially bounce off. This is a supernova. The forces in this reaction actually push the electrons into the protons in the atomic nuclei and form neutrons. All you have is a huge neutron ball which doesn't do much other than crack under its own gravity (huge amounts of energy are released when hairline cracks form in the surface layers of neutron stars).
 
Oh, and more about the OP:

Mass does not determine density really. You can have a 1 solar mass star that is 10 times the sun's radius, or a 1 solar mass black hole the size of a beachball (I'm guessing at the exact scale there). The gravity is the same at large distances in either case. There would be less gravity from a dwarf than it's predecessing giant, but because the dwarf is much more compact, the force on any piece of the dwarf is much greater (closer to the centre = more force).

The strong force is attractive, and doesn't really come into play until you get into the neutron star formation bit. Actually the same is pretty much true for the weak force as this is what allows the p + e + nu --> n reaction.
 
Hmm, its making more sense.

I guess that in something that is still super hot, like a neutron star or white dwarf, the electrons are still free-flowing, and the forces between the atoms (or particles...) aren't repulsive (guess its apparent that my knowledge only extends as far as EM). I thought that the gravitational force of the mass of the star allowed it to overcome the energy necessary to fuse atoms.

I dont think I'm understanding this. I'm thinking that atoms cannot get too close to one another, meaning that they cannot exist in low energy states as closely as they are in a neutron star or white dwarf, which is why I thought the star should expand to lower densities as it cools and it turns from a soup of particles into regular atoms (like that huge diamond...).

I guess that latter part of this is, do atomic elements have natural densities? Is there a natural distance between two atoms? (doesnt diamond have to have carbon bonds a certain defined length?)....

Or do I have this completely wrong? How can matter exist in such a dense state?
 
Originally posted by: Gunnar
Hmm, its making more sense.

I guess that in something that is still super hot, like a neutron star or white dwarf, the electrons are still free-flowing, and the forces between the atoms (or particles...) aren't repulsive (guess its apparent that my knowledge only extends as far as EM). I thought that the gravitational force of the mass of the star allowed it to overcome the energy necessary to fuse atoms.

I dont think I'm understanding this. I'm thinking that atoms cannot get too close to one another, meaning that they cannot exist in low energy states as closely as they are in a neutron star or white dwarf, which is why I thought the star should expand to lower densities as it cools and it turns from a soup of particles into regular atoms (like that huge diamond...).

I guess that latter part of this is, do atomic elements have natural densities? Is there a natural distance between two atoms? (doesnt diamond have to have carbon bonds a certain defined length?)....

Or do I have this completely wrong? How can matter exist in such a dense state?
Click to expand...

1. In a neutron star there are no free flowing electrons as they have all been pushed inside the protons to form neutrons. It's called a neutron star because it is just a large ball of neutrons and that's it.

2. There is a balance in stars called hydrostatic equilibrium. Gravity pulls the star's material in and the fusion process pushes the material out. Gravity provides enough pressure to initiate enough fusion to keep the star from collapsing further. The forces at work are gravity in and EM out. When fusion stops, you still have EM out, but this is just the repulsion between electrons in the atoms, as well as the fermi pressure (can't have electrons in the same quantum state so they repel...)

If anything, the star should contract as it cools as there is less thermal energy (kinetic atomic/particular energy), but it shouldn't expand. Gravity is constant in this case and is of course attractive.

Atomic elements have natural densities at given temperatures and pressures yes, but there is no real inherant density for any element unless you want to define it to be the mass of one atom of element x in the volume that atom takes up... but even this is hard to do as the electrons aren't in perfectly well defined orbits, so there is no exact radius you can use.
 
You must log in or register to reply here.

TRENDING THREADS

Back
Top