# The Naked Scientists Forum

### Author Topic: What is the density of a Neutron Star?  (Read 1822 times)

#### thedoc

• Sr. Member
• Posts: 511
• Thanked: 12 times
##### What is the density of a Neutron Star?
« on: 01/05/2013 15:30:01 »

What is the density of a Neutron Star?

What do you think?
« Last Edit: 01/05/2013 15:30:01 by _system »

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: What is the density of a Neutron Star?
« Reply #1 on: 01/05/2013 18:48:37 »
Depends on its size and density, but it can't be greater that the size/density given for becoming a black hole. So first you have ordinary matter, then you have a further state due to compression called degenerate matter, from which we get a neutron star. But if the compression from that is being feed by some other mass you might get it to pass that state and instead become a black hole. I saw someone claim that about five percent of all neutron stars are binary. And adding that mass should be sufficient for it to pass the Schwarzschild radius from where you get a Black Hole.

NASA use this calculation approximately defining where you get a neutron star.

#### distimpson

• Sr. Member
• Posts: 118
##### Re: What is the density of a Neutron Star?
« Reply #2 on: 01/05/2013 18:54:32 »

"Neutron stars have overall densities predicted by the APR EOS of 3.7×1017 to 5.9×1017 kg/m3 ... which compares with the approximate density of an atomic nucleus of 3×1017 kg/m3. The neutron star's density varies from below 1×109 kg/m3 in the crust, increasing with depth to above 6×1017 or 8×1017 kg/m3 deeper inside (denser than an atomic nucleus). This density is approximately equivalent to the mass of a Boeing 747 compressed to the size of a small grain of sand." this information from http://en.wikipedia.org/wiki/Neutron_star

Several years ago I asked an astrophysicist (can not remember name) "if somehow a spoonful of this dense matter could be removed from the star, would it expand into  a large volume of ordinary matter?". Her answer was yes it would, any thoughts here?

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: What is the density of a Neutron Star?
« Reply #3 on: 01/05/2013 18:59:36 »
Never thought of that one, but she should be right :)
That would be something to see.

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: What is the density of a Neutron Star?
« Reply #4 on: 01/05/2013 19:42:33 »
If it had been defined the way a black hole is I wouldn't expect it to expand, as described outside a event horizon, assuming a 'infinite center' is all there is behind that. But a Neutron star, even if in a equilibrium? Then again, after a compression it will consist of neutrons, not electron-proton pairs any more?? so what would it 'expand' into? Maybe we both are wrong there?
=

No, she's right, it will decay into electron-proton pairs again I think. And that should happen in about fifteen minutes of your local time, after you lifted it up, through (radiation) beta decay. But I'm still not sure? What happens to the nucleus as it becomes a degenerate 'neutron gas'? Does that exist any more?
« Last Edit: 01/05/2013 19:55:31 by yor_on »

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: What is the density of a Neutron Star?
« Reply #5 on: 01/05/2013 20:02:01 »
You could imagine a Einstein Rosen bridge opening up inside a neutron star. If she is right then that should mean a explosion and a he of a lot of radiation coming from that 'portals' other end. But it builds on a idea of nucleus's, and, does that still exist inside a degenerate neutron gas?

That one was very tricky, to me at last:).
=

Then again, is there something leading to particles of a very compressed state, and also if treated as 'matter waves' being compressed into a very small 'volume', to expand into electron-proton pairs again? Is there some quantum mechanical rule that guarantees it? I don't think there is myself?

Pressure is something defined and restricted to a constricting volume, no matter if we discuss it as waves or rest mass. And so it belongs to dimensions, and distances? I don't know?
« Last Edit: 01/05/2013 20:15:16 by yor_on »

#### distimpson

• Sr. Member
• Posts: 118
##### Re: What is the density of a Neutron Star?
« Reply #6 on: 01/05/2013 22:08:28 »
Very far out of my area of understanding, I was OK with the idea of some sort of decompression of the dense matter, unless there is some other force to keep it together, some conditions where the strong force would hold the material together like a macroscopic radioactive super nucleus. That is what you are saying with beta decay to get some protons in the mix? Sorry if this is really out there, I don't even know what questions to ask.

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: What is the density of a Neutron Star?
« Reply #7 on: 02/05/2013 01:08:40 »

"Neutron stars have overall densities predicted by the APR EOS of 3.7×1017 to 5.9×1017 kg/m3 ... which compares with the approximate density of an atomic nucleus of 3×1017 kg/m3.
I didn't check you figures but I'm assuming their correct. You can think of a neutron star as a large nucleus of an atom where all the nucleons are neutrons. Think of it as if all the electrons have been absorbed by the protons to form neutrons.

#### yor_on

• Naked Science Forum GOD!
• Posts: 12001
• Thanked: 4 times
• (Ah, yes:) *a table is always good to hide under*
##### Re: What is the density of a Neutron Star?
« Reply #8 on: 02/05/2013 09:40:16 »
The weirdest thing I know about a neutron star, is that the more mass you add, the smaller it becomes. (As defined by you looking at it, from afar.)

#### The Naked Scientists Forum

##### Re: What is the density of a Neutron Star?
« Reply #8 on: 02/05/2013 09:40:16 »