[HN Gopher] Slow-spinning radio neutron star breaks all the rules
___________________________________________________________________
Slow-spinning radio neutron star breaks all the rules
Author : doener
Score : 86 points
Date : 2024-06-07 21:46 UTC (1 days ago)
(HTM) web link (www.sydney.edu.au)
(TXT) w3m dump (www.sydney.edu.au)
| kstrauser wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| Plus the survivorship bias of those very few. We don't
| observe the ones that aren't spinning much.
| pfdietz wrote:
| 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 wrote:
| 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 wrote:
| 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 wrote:
| Or that it has slowed down. Maybe two objects with opposite
| rotations merged, then collapsed into a neutron star?
| pavel_lishin wrote:
| I thought that the flashes we see from pulsars were
| entirely due to the axial precession.
| ravetcofx wrote:
| I think you're onto something as earth has precessions of
| 25,000 years...
| sandworm101 wrote:
| 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 wrote:
| 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 wrote:
| Ohhhh, I see! That was the part I wasn't getting. Thanks!
| Hnrobert42 wrote:
| 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 wrote:
| It's not a silly question. The USS Enterprise used to stop by
| pulsars and neutron stars all the time to observe them
| xqcgrek2 wrote:
| From all I've read for this, it should not be possible for such a
| slow star to emit anything, ie against known physics of natural
| objects.
|
| Why are they so quick to discard a technological origin?
| ceejayoz wrote:
| Because we've run into plenty of "huh we didn't think those
| existed" in astronomy so far. We'll try to understand more
| mundane possibilities first.
| ben_w wrote:
| > Why are they so quick to discard a technological origin?
|
| A sufficiently advanced society could cause any possible
| phenomena we ever observe, Occam's razor says you should look
| for other solutions first.
| xqcgrek2 wrote:
| Occam's razer also says we should not have different laws of
| physics in different places.
|
| Something very interesting is going on here.
| rimunroe wrote:
| > Occam's razer also says we should not have different laws
| of physics in different places.
|
| You're starting from an odd place if you're assuming that's
| the case. There's absolutely no reason to believe the laws
| of physics are different in this star's case compared to
| any other we've seen. It's an absurd leap to believe aliens
| are at work here rather than this being yet another case of
| our knowledge of physics being incomplete.
| tithe wrote:
| > Something very interesting is going on here.
|
| Agreed.
|
| If we interpret Occam's razor as to "choose the simplest
| solution with the greatest explaining power," then the
| "solution" may be to admit that we have an inadequate
| understanding of how these things physically work.
|
| This doesn't rule out the possibility of a technological
| origin, but I think Ben's point is that we should explore
| possiblities that generate more answers than questions
| first.
| Sharlin wrote:
| No, there's nothing except the fact that the "laws of
| physics", ie. our models of nature, are eternally
| incomplete simplifications. All that's going on is that our
| models have a prediction error and thus tell us we've
| failed to take something into account. Which is an utterly
| standard part of scientific progress. Papers will be
| written, hypotheses proposed, more evidence gathered, and
| after some number of years we'll have a new, more complete
| theory of neutron star evolution and/or emission processes.
| goatlover wrote:
| Because as of yet, there's no evidence of ETs, but there's
| plenty of evidence for natural phenomena that were puzzling for
| a while.
| kstrauser wrote:
| That path is riskily close to " _shrug_ God did it! _shrug_ ",
| which is kind of a conversation stopper in science.
| margalabargala wrote:
| > Why are they so quick to discard a technological origin?
|
| For the same reason that they haven't presented God as a
| serious option. It's not falsifiable, it being true would
| require a truly enormous amount of preconditions that are
| utterly unknown, and it's much more likely that something about
| our current understanding isn't correct.
|
| If you say "aliens!" every time you see something that doesn't
| make sense, you'll be saying it a lot, but you will find it
| will actually be normal physics hiding behind a curtain.
| nurettin wrote:
| Ok let's assume aliens at the first sight of unexplained
| phenomenon. Now what?
| bunabhucan wrote:
| Obviously, "now what?" is that they fly here and leave
| detection traces only certain people can see.
| spacecadet wrote:
| Because "space" can contain almost anything at this point...
| and I doubt we have discovered 1% of natural entities or
| combinations of natural entities.
| dave4420 wrote:
| My first thought was that maybe it's falling into a black hole
| and thus heavily time dilated, but the article doesn't mention
| that as a possibility.
|
| So... why isn't this a serious possibility?
| pavel_lishin wrote:
| I'm going to take a guess not backed up by any mathematics - to
| be that close to a black hole, it would have to be well within
| the Roche limit and get torn apart.
|
| Plus, I'd wager they'd be able to tell if it was eclipsed by
| the black hole at all.
| Zardoz84 wrote:
| Plus a very visible Doppler effect. We know many stars
| orbiting neutron stars or blackholes because the Doppler
| effect, indicates that it's moving fast around something that
| we can't see.
| pdonis wrote:
| _> maybe it's falling into a black hole and thus heavily time
| dilated_
|
| If it were doing that it would have redshifted into
| undetectability and disappeared from view on a time scale much
| shorter than 8 months, which is how long it has been observed.
| GlobalFrog wrote:
| You're making Sparks here!
| davidhyde wrote:
| It occurred to me that the neutron star may be wobble spinning
| and we only catch it flashing by now and again when it gets round
| to pointing in our direction. Does anyone here know if this even
| a possibility for such an object or would it's internal
| uniformity (in composition) prohibit such behaviour?
| jiggawatts wrote:
| It should be impossible.
|
| The rotational inertia of these things is absolutely insane.
| They're the mass of a star! Nothing short of a very close very
| massive black hole could alter their spin axis quickly.
|
| They're also extremely symmetrical objects because of the
| enormous surface gravity. A "mountain" on a neutron star is
| about a millimeter high.
| davidhyde wrote:
| It could be a periodic spin change caused by a large, unseen,
| object orbiting it. Alternatively, the angle of the cone of
| radiation coming from the poles might be tightening and
| enlarging on a cyclical basis and we may see the glancing
| blows as a large sector sweeps by our viewpoint.
|
| Periodic spin changes:
| https://sciencex.com/news/2021-09-pulse-neutron-star-
| decades...
| kazinator wrote:
| I would kill for a video of the radio star.
| kubanczyk wrote:
| Marconi plays the mamba.
| tromp wrote:
| A pun on
| https://en.wikipedia.org/wiki/Video_Killed_the_Radio_Star I
| guess...
| larsrc wrote:
| The rules are more like guidelines, you see.
|
| The universe has had a lot of opportunities to come up with wacky
| stuff.
___________________________________________________________________
(page generated 2024-06-08 23:01 UTC)