9 ms·

Black Widow Pulsar Sets Mass Record

59 points by wglb 4y ago

egman_ekki 4y ago
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
zogomoox 4y ago
The article states the neutron star spins up as it accretes mass from its partner.. surprising as you'd expect something that gains mass to slow down.
- ISL 4y ago
  It spins up because that mass carries angular momentum.
  - pantulis 4y ago
    This begs the question: what _is_ angular momentum? How does it gets transferred from the swirling gas to whatever degenerate state of matter is inside the neutron star? What kind of interaction is that?
    - techdragon 4y ago
      Im not completely confident, given my knowledge isn’t by an understanding of momentum in “particle physics” … but at least as far as I understand it, momentum and angular momentum are the same “thing”, angular momentum is just momentum constrained by some other physical force (such as gravity for things moving on their obits, or a gravitationally bound object such as stars or rubble pile asteroids spinning on their axis of rotation)
    - CarVac 4y ago
      Neutron degenerate matter still interacts in ways other than gravity, it just interacts with non-electromagnetic forces like the Strong force.
    - philipswood 4y ago
      One nice perspective on what angular momentum _is_: It's a consequence of the laws of physics being the same for all directions. Noether's theorem can be paraphrased as: If a system has a continuous symmetry property, then there are corresponding quantities whose values are conserved in time. If the laws of physics are the same even if your coordinate system is rotated that means there is a continous symmetry for that. When The conserved property is written out is is angular momentum.
BlueTemplar 4y ago
jmyeet 4y ago
The article mentions why this is important but doesn't really explain the details. We don't have an equation of state [1] for neutron stars. Part of the reason why is that neutron stars are incredibly complex. You have fluid dynamics. Because matter is so dense you likely have neotronium and probably weird states and interactions with quarks. Contrast this with black holes. Black holes can be perfectly described with 3 properties, only 2 of which are really relevant. The two relevant ones are spin and mass. The less relevant one is electric charge. Why is it less relevant? Because if the charge is too high then it can't be a black hole. Because of this we can calculate how large black holes can be, figure out the fastest that they can grow (ie by sucking matter from the accretion disk; there's an uppper bound to that), etc. So we have a rough guess on the range of masses neutron stars can be but we observation is really our only tool here. This is why examples outside of our estimated range are important. [1]: https://en.wikipedia.org/wiki/Equation_of_state https://en.wikipedia.org/wiki/Equation_of_state
- dfee 4y ago
  I’d never heard that there’s an upper limit to the size of a black hole. What is supposed to happen when it approaches this limit?
  - Thiez 4y ago
    As far as I am aware there is no theoretical limit that they cannot surpass, but of course they are limited in practice by the available mass, and how fast they can consume that mass; a lot of mass ends up orbiting, instead of falling into the black hole. So black holes such as the one mentioned in the article are expected to be rare.
  - sebzim4500 4y ago
    Once they reach a certain size the stars in the closest stable orbit are no longer ripped apart by tidal forces, which means that they do not grow significantly beyond this.
mabbo 4y ago
Fun question: can spinning this fast allow for larger neutron stars? The neutron star keeps growing, and eventually may have too much mass, causing it to collapse into a black hole. The forces pulling inward are just too much. But if it's spinning fast enough, then centripetal forces would be pulling outward as well. This one is completing a full rotation in 1.41 ms- that's a lot of rotational speed. Or is there some intrinsic property of black hole formation that does not care about this?
- zaarn 4y ago
  Rotation does indeed allow for heavier neutron stars, the outward force helps counteracting the inward pull of gravity that would otherwise overcome the forces that keep neutrons not turning into black holes. edit: There is a relativistic limit, as faster rotation eventually is not possible without adding ludicrous amounts of energy that would make the neutron star too heavy. The non-rotating limit is about 2.1 solar masses (2.1 to 2.9), the rotating limit is about 20% higher (2.5 - 3.6 solar masses)
  - mabbo 4y ago
    > There is a relativistic limit, as faster rotation eventually is not possible without adding ludicrous amounts of energy that would make the neutron star too heavy. That is exactly the kind of thing I'm here for. Spinning so fast that the energy adds more mass than the spinning can counteract. Delightful.
- ben_w 4y ago
  There is an upper limit to how fast a black hole can spin. I'm not sure if that's the same limit you'd get if you tried spinning a material object as fast as possible while increasing its mass until it became a black hole.
  - zaarn 4y ago
    Black holes don't have a limit on their rotation rate as far as current physics go, since black holes do not have a traditional surface. If the spin exceeds the speed of light, that simply makes the event horizon go away and leaves you with a naked singularity.
JackFr 4y ago
As required with any neutron star story on HN, recommendation for “Dragon’s Egg” by Robert Forward. Science fiction novel which takes the surface of a neutron star as its setting.

59 points by wglb 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

zogomoox 4y ago

The article states the neutron star spins up as it accretes mass from its partner.. surprising as you'd expect something that gains mass to slow down.

ISL 4y ago
It spins up because that mass carries angular momentum.
- pantulis 4y ago
  This begs the question: what _is_ angular momentum? How does it gets transferred from the swirling gas to whatever degenerate state of matter is inside the neutron star? What kind of interaction is that?
  - techdragon 4y ago
    Im not completely confident, given my knowledge isn’t by an understanding of momentum in “particle physics” … but at least as far as I understand it, momentum and angular momentum are the same “thing”, angular momentum is just momentum constrained by some other physical force (such as gravity for things moving on their obits, or a gravitationally bound object such as stars or rubble pile asteroids spinning on their axis of rotation)
  - CarVac 4y ago
    Neutron degenerate matter still interacts in ways other than gravity, it just interacts with non-electromagnetic forces like the Strong force.
  - philipswood 4y ago
    One nice perspective on what angular momentum _is_: It's a consequence of the laws of physics being the same for all directions. Noether's theorem can be paraphrased as: If a system has a continuous symmetry property, then there are corresponding quantities whose values are conserved in time. If the laws of physics are the same even if your coordinate system is rotated that means there is a continous symmetry for that. When The conserved property is written out is is angular momentum.

BlueTemplar 4y ago

jmyeet 4y ago

The article mentions why this is important but doesn't really explain the details. We don't have an equation of state [1] for neutron stars. Part of the reason why is that neutron stars are incredibly complex. You have fluid dynamics. Because matter is so dense you likely have neotronium and probably weird states and interactions with quarks. Contrast this with black holes. Black holes can be perfectly described with 3 properties, only 2 of which are really relevant. The two relevant ones are spin and mass. The less relevant one is electric charge. Why is it less relevant? Because if the charge is too high then it can't be a black hole. Because of this we can calculate how large black holes can be, figure out the fastest that they can grow (ie by sucking matter from the accretion disk; there's an uppper bound to that), etc. So we have a rough guess on the range of masses neutron stars can be but we observation is really our only tool here. This is why examples outside of our estimated range are important. [1]: https://en.wikipedia.org/wiki/Equation_of_state https://en.wikipedia.org/wiki/Equation_of_state

dfee 4y ago
I’d never heard that there’s an upper limit to the size of a black hole. What is supposed to happen when it approaches this limit?
- Thiez 4y ago
  As far as I am aware there is no theoretical limit that they cannot surpass, but of course they are limited in practice by the available mass, and how fast they can consume that mass; a lot of mass ends up orbiting, instead of falling into the black hole. So black holes such as the one mentioned in the article are expected to be rare.
- sebzim4500 4y ago
  Once they reach a certain size the stars in the closest stable orbit are no longer ripped apart by tidal forces, which means that they do not grow significantly beyond this.

mabbo 4y ago

Fun question: can spinning this fast allow for larger neutron stars? The neutron star keeps growing, and eventually may have too much mass, causing it to collapse into a black hole. The forces pulling inward are just too much. But if it's spinning fast enough, then centripetal forces would be pulling outward as well. This one is completing a full rotation in 1.41 ms- that's a lot of rotational speed. Or is there some intrinsic property of black hole formation that does not care about this?

zaarn 4y ago
Rotation does indeed allow for heavier neutron stars, the outward force helps counteracting the inward pull of gravity that would otherwise overcome the forces that keep neutrons not turning into black holes. edit: There is a relativistic limit, as faster rotation eventually is not possible without adding ludicrous amounts of energy that would make the neutron star too heavy. The non-rotating limit is about 2.1 solar masses (2.1 to 2.9), the rotating limit is about 20% higher (2.5 - 3.6 solar masses)
- mabbo 4y ago
  > There is a relativistic limit, as faster rotation eventually is not possible without adding ludicrous amounts of energy that would make the neutron star too heavy. That is exactly the kind of thing I'm here for. Spinning so fast that the energy adds more mass than the spinning can counteract. Delightful.
ben_w 4y ago
There is an upper limit to how fast a black hole can spin. I'm not sure if that's the same limit you'd get if you tried spinning a material object as fast as possible while increasing its mass until it became a black hole.
- zaarn 4y ago
  Black holes don't have a limit on their rotation rate as far as current physics go, since black holes do not have a traditional surface. If the spin exceeds the speed of light, that simply makes the event horizon go away and leaves you with a naked singularity.

JackFr 4y ago

As required with any neutron star story on HN, recommendation for “Dragon’s Egg” by Robert Forward. Science fiction novel which takes the surface of a neutron star as its setting.