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I'm a layman here. This is a genuine question because I don't know as much about this as I'd like: Why do we expect neutron stars to spin rapidly? I understand

I'm a layman here. This is a genuine question because I don't know as much about this as I'd like: Why do we expect neutron stars to spin rapidly? I understand the "ice skater pulling their arms in" analogy, but why should the pre-collapse star have been appreciably spinning in the first place? To my lay lack-of-understanding, if the neutron star is spinning slowly, then that just implies the earlier version didn't have a whole lot of angular momentum in the first place. What's wrong with that?

Waterluvian 2y ago
Not an expert. How slow would it have to spin for it to be appreciably “slow” after it speeds up as a neutron star, and how many celestial bodies are spinning that slowly in general? So, why do bodies accrete in a non-uniform way as to inherit a significant spin far more often than not? Is that bias found in the matter being accreted? Similar to why things seem to rotate the same way: one direction eventually prevails?
- hatthew 2y ago
  Super basic estimate (I'm not an astronomer, this could be wrong): The core of a star has a radius of 100000 km, and after collapse the radius becomes 10 km. Angular momentum is proportional to fr^2, where f is the rotation frequency. Since the radius decreases by 4 orders of magnitude, to keep the same angular momentum the frequency must increase by 8 orders of magnitude. This means that if the new rotation frequency is 1/hour, the original must have been around 10000 years. Statistically some stars like that probably exist somewhere, but it's unlikely that one of the few thousand neutron stars we know of started that way.
  - Waterluvian 2y ago
    Plus the survivorship bias of those very few. We don’t observe the ones that aren’t spinning much.
pfdietz 2y ago
The pre-collapse star is spinning for the same reason: any nonzero angular momentum in the very extended gas/dust cloud from which it formed gets concentrated as it collapses into the star. In fact, there's typically so much angular momentum it can't go into the star, but instead either goes into more than one star, or into a pre-planetary disk around the star.
jdiff 2y ago
The skater is a small human sized object. They pull their arms in, don't honestly shrink much, but they still spin appreciably faster. When you go from something the size of a pre-collapse star and compare it to the size of a neutron star, the speed up factor is way higher. It doesn't matter how slowly you're spinning when big, you will be spinning fast when you get small as all that angular momentum must be conserved.
- kstrauser 2y ago
  I understand that bit, but it's proportional to the shrinkage. If star A is spinning at X turns per unit time, and identical star B is spinning at 4X turns per the same unit time, and they both collapse to the same size neutron star, tiny A' should be spinning at 4X the turns per unit time as little B'. If the observations in the article are correct, then this neutron star is spinning around 1/10,000th as fast as some others. Neutron star weirdness aside - and that's a whole awful lot of weirdness to set aside - I think that'd mean its earlier version was spinning about 1/10,000th as fast as others. What I don't know, again, as a layman, is whether that's especially unusual or unexpected. Could it be that instead we're looking at its pole, and the pole has an axial precession of an hour while the little fellow's equator is spinning around quickly?
  - sandworm101 2y ago
    Or that it has slowed down. Maybe two objects with opposite rotations merged, then collapsed into a neutron star?
  - pavel_lishin 2y ago
    I thought that the flashes we see from pulsars were entirely due to the axial precession.
  - ravetcofx 2y ago
    I think you're onto something as earth has precessions of 25,000 years...
sandworm101 2y ago
Everything in space is spinning. Put three or more objects in space, let them fall towards each other under gravity, and the result will be some amount of rotation. So every star forms from spinning material.
idiotsecant 2y ago
You're right, lots of things could happen to neutron stars to make them slow or even stop with the right exchanges. The commonly understood model, though, was that these slow spinning objects would not be pulsars. The radio pulsar mechanism was thought to 'shut off' below certain rotational speeds
- kstrauser 2y ago
  Ohhhh, I see! That was the part I wasn’t getting. Thanks!
- Hnrobert42 2y ago
  Was that based on theoretical or empirical analysis? If empirical, is the data biased? The article mentioned ASKAP reads a lot of the sky at the time. If there are objects emitting every 15 hours, would that even be detected or just look like noise?
throwaway211 2y ago
It's not a silly question. The USS Enterprise used to stop by pulsars and neutron stars all the time to observe them

Waterluvian 2y ago

Not an expert. How slow would it have to spin for it to be appreciably “slow” after it speeds up as a neutron star, and how many celestial bodies are spinning that slowly in general? So, why do bodies accrete in a non-uniform way as to inherit a significant spin far more often than not? Is that bias found in the matter being accreted? Similar to why things seem to rotate the same way: one direction eventually prevails?

hatthew 2y ago
Super basic estimate (I'm not an astronomer, this could be wrong): The core of a star has a radius of 100000 km, and after collapse the radius becomes 10 km. Angular momentum is proportional to fr^2, where f is the rotation frequency. Since the radius decreases by 4 orders of magnitude, to keep the same angular momentum the frequency must increase by 8 orders of magnitude. This means that if the new rotation frequency is 1/hour, the original must have been around 10000 years. Statistically some stars like that probably exist somewhere, but it's unlikely that one of the few thousand neutron stars we know of started that way.
- Waterluvian 2y ago
  Plus the survivorship bias of those very few. We don’t observe the ones that aren’t spinning much.

pfdietz 2y ago

The pre-collapse star is spinning for the same reason: any nonzero angular momentum in the very extended gas/dust cloud from which it formed gets concentrated as it collapses into the star. In fact, there's typically so much angular momentum it can't go into the star, but instead either goes into more than one star, or into a pre-planetary disk around the star.

jdiff 2y ago

The skater is a small human sized object. They pull their arms in, don't honestly shrink much, but they still spin appreciably faster. When you go from something the size of a pre-collapse star and compare it to the size of a neutron star, the speed up factor is way higher. It doesn't matter how slowly you're spinning when big, you will be spinning fast when you get small as all that angular momentum must be conserved.

kstrauser 2y ago
I understand that bit, but it's proportional to the shrinkage. If star A is spinning at X turns per unit time, and identical star B is spinning at 4X turns per the same unit time, and they both collapse to the same size neutron star, tiny A' should be spinning at 4X the turns per unit time as little B'. If the observations in the article are correct, then this neutron star is spinning around 1/10,000th as fast as some others. Neutron star weirdness aside - and that's a whole awful lot of weirdness to set aside - I think that'd mean its earlier version was spinning about 1/10,000th as fast as others. What I don't know, again, as a layman, is whether that's especially unusual or unexpected. Could it be that instead we're looking at its pole, and the pole has an axial precession of an hour while the little fellow's equator is spinning around quickly?
- sandworm101 2y ago
  Or that it has slowed down. Maybe two objects with opposite rotations merged, then collapsed into a neutron star?
- pavel_lishin 2y ago
  I thought that the flashes we see from pulsars were entirely due to the axial precession.
- ravetcofx 2y ago
  I think you're onto something as earth has precessions of 25,000 years...

sandworm101 2y ago

Everything in space is spinning. Put three or more objects in space, let them fall towards each other under gravity, and the result will be some amount of rotation. So every star forms from spinning material.

idiotsecant 2y ago

You're right, lots of things could happen to neutron stars to make them slow or even stop with the right exchanges. The commonly understood model, though, was that these slow spinning objects would not be pulsars. The radio pulsar mechanism was thought to 'shut off' below certain rotational speeds

kstrauser 2y ago
Ohhhh, I see! That was the part I wasn’t getting. Thanks!
Hnrobert42 2y ago
Was that based on theoretical or empirical analysis? If empirical, is the data biased? The article mentioned ASKAP reads a lot of the sky at the time. If there are objects emitting every 15 hours, would that even be detected or just look like noise?

throwaway211 2y ago

It's not a silly question. The USS Enterprise used to stop by pulsars and neutron stars all the time to observe them