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"Neutron stars are the city-size, extremely compact leftovers of supernova explosions. Beams of high-energy particles and radiation from their magnetic poles sw

by Zenst 4y ago

"Neutron stars are the city-size, extremely compact leftovers of supernova explosions. Beams of high-energy particles and radiation from their magnetic poles sweep through space as they rotate. " I wished they would define what they mean by City as globally that varies even per country. I was also wondering how much mass it is loosing as from my understanding the mass is converted to energy it emits and given the amount of energy - how much mass would that be. I did have a read of https://www.physicsforums.com/threads/do-neutron-stars-decay.613515/ https://www.physicsforums.com/threads/do-neutron-stars-decay... though not clear and is it that they are just emitting massless particals in the form of neutrons so the mass does not change?

ramraj07 4y ago
A sun mass sized neutron star is 20km across. So a medium sized city.
mrintegrity 4y ago
Neutrons are not massless. They are neutral in charge but have a mass greater than protons.
noneeeed 4y ago
Neutron stars vary in size, the comparison to a city is to give the rough order of magnitude, not the specific size. The rough order of magnitude in this case is “big, but not incomprehensible so” unlike most objects in cosmology which are incomprehensibly massive.
eis 4y ago
Regarding the city-size: you are right that "city" is a whole range of sizes but so is the size of a neutron star, they are not all of the same size. Don't read too much into it, they just want to give a very rough idea. You can easily calculate how much mass it is losing via E=mc^2. If something emits neutrons then it is losing energy and therefore mass. There are no energyless particles. Everything has a certain energy and by that mass. Some particles have no rest-mass but that is something different. Fun fact: you could create a black hole from beams of light.
- paulrouget 4y ago
  I don’t think you can just use E=mc2 in that case. The rotation of the star is so high that you have to consider relativistic angular momentum effect in the equation: https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...
  - eis 4y ago
    While you are correct that the neutron star has angular momentum and then you'd use the E^2 = (pc)^2 + m0^2c^4 for it, the question was if it is losing mass by emitting neutrons and how much. For that, the momentum energy of the Neutron star can be ignored. What does though play a role is the momentum of the particles that get shot into space and due to the escape velocity of these stars being so high it would still make sense to use this formula instead of E = mc^2. BTW these stars emit a lot more than neutrons just in case the GP thought due to their name that that's what they emit mostly. Most of the emission of neutron stars is electromagnetic radiation (EMR) aka photons. Actually neutron stars have two ways to lose energy: slowing down their rotational speed ("spin down") and losing mass. Which one of these contributes more to the EMR varies. For magnetars for example, only about 1% of the radiation is thought to be powered by rotation and for those we could actually use E = mc^2 to calculate how much rest mass the star is losing due to its radiation. That's my layman understanding at least :)
- pdonis 4y ago
  > If something emits neutrons Pulsars don't emit neutrons, they emit radio waves. They are neutron stars, so they're made of neutrons, but they don't emit any.
andrewflnr 4y ago
Not even neutrinos, which is probably what you meant, are quite massless. But my understanding is that the radio beams and such come out of the neutron star's magnetic field, and maybe its interaction with the surrounding gas. So its energy budget probably comes from the star's angular momentum, not converting its mass into energy.
deleted 4y ago
[deleted]
wglb 4y ago
Perhaps this will help: https://en.wikipedia.org/wiki/Cities_in_Flight https://en.wikipedia.org/wiki/Cities_in_Flight

by Zenst 4y ago

ramraj07 4y ago

A sun mass sized neutron star is 20km across. So a medium sized city.

mrintegrity 4y ago

Neutrons are not massless. They are neutral in charge but have a mass greater than protons.

noneeeed 4y ago

Neutron stars vary in size, the comparison to a city is to give the rough order of magnitude, not the specific size. The rough order of magnitude in this case is “big, but not incomprehensible so” unlike most objects in cosmology which are incomprehensibly massive.

eis 4y ago

Regarding the city-size: you are right that "city" is a whole range of sizes but so is the size of a neutron star, they are not all of the same size. Don't read too much into it, they just want to give a very rough idea. You can easily calculate how much mass it is losing via E=mc^2. If something emits neutrons then it is losing energy and therefore mass. There are no energyless particles. Everything has a certain energy and by that mass. Some particles have no rest-mass but that is something different. Fun fact: you could create a black hole from beams of light.

paulrouget 4y ago
I don’t think you can just use E=mc2 in that case. The rotation of the star is so high that you have to consider relativistic angular momentum effect in the equation: https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relation https://en.wikipedia.org/wiki/Energy%E2%80%93momentum_relati...
- eis 4y ago
  While you are correct that the neutron star has angular momentum and then you'd use the E^2 = (pc)^2 + m0^2c^4 for it, the question was if it is losing mass by emitting neutrons and how much. For that, the momentum energy of the Neutron star can be ignored. What does though play a role is the momentum of the particles that get shot into space and due to the escape velocity of these stars being so high it would still make sense to use this formula instead of E = mc^2. BTW these stars emit a lot more than neutrons just in case the GP thought due to their name that that's what they emit mostly. Most of the emission of neutron stars is electromagnetic radiation (EMR) aka photons. Actually neutron stars have two ways to lose energy: slowing down their rotational speed ("spin down") and losing mass. Which one of these contributes more to the EMR varies. For magnetars for example, only about 1% of the radiation is thought to be powered by rotation and for those we could actually use E = mc^2 to calculate how much rest mass the star is losing due to its radiation. That's my layman understanding at least :)
pdonis 4y ago
> If something emits neutrons Pulsars don't emit neutrons, they emit radio waves. They are neutron stars, so they're made of neutrons, but they don't emit any.

andrewflnr 4y ago

Not even neutrinos, which is probably what you meant, are quite massless. But my understanding is that the radio beams and such come out of the neutron star's magnetic field, and maybe its interaction with the surrounding gas. So its energy budget probably comes from the star's angular momentum, not converting its mass into energy.

deleted 4y ago

[deleted]

wglb 4y ago

Perhaps this will help: https://en.wikipedia.org/wiki/Cities_in_Flight https://en.wikipedia.org/wiki/Cities_in_Flight