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    Default Brightness of neutron stars

    Stranngely, this seems to be a question that you can't easily search online.

    What's the common range of brightness for neutron stars? They are very extreme objects, but also extremely small. How much light do they put out?
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    Default Re: Brightness of neutron stars

    Because it's complicated. Neutron stars emit a great deal of radiation from their magnetic poles, can RIP matter apart realizing huge amounts of energy, and their intense magnetic fields can energize surrounding gas clouds and cause them to emit radiation on their own, but neutron stars aren't "bright" in the same way that other stars are. You might find The Electromagnetic Spectrum of Neutron Stars interesting.
    Last edited by georgie_leech; 2018-07-17 at 02:31 PM.
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    Default Re: Brightness of neutron stars

    I did find one place that calculates the luminosity of a neutron star at 1 million K and a diameter of 20 km (more or less average for neutron stars) to be 18% that of the sun. Red dwarves peak at only 10% the luminosity of the sun. That's a lot of energy from such a little ball.

    However, luminosity appplies to the entire em-radiation spectrum and red dwarves barely reach beyond visible light, while neutron stars have most of their radiation as x-rays, which means most of it would be invisible to the eye.
    So I would estimate that the visible light from a neutron star would be significantly lower than a red dwarf. Yet even when it seems you are standing in moonlight (coming from what looks like just a star), you would get pretty roasted by x-rays and UV radiation.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by Yora View Post
    I did find one place that calculates the luminosity of a neutron star at 1 million K and a diameter of 20 km (more or less average for neutron stars) to be 18% that of the sun. Red dwarves peak at only 10% the luminosity of the sun. That's a lot of energy from such a little ball.
    1 million K is a lot of temperature! Even the largest blue-white stars only have a surface temperature of 40,000 K. Since you can approximate the energy output of stars as black body radiation (it's a lot more complicated, of course, but it's a good first approximation) this explains why most of their output is in the X-ray spectrum, too.

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    Default Re: Brightness of neutron stars

    Originally, the only way that astronomers wqere able to prove the existense of neutron stars was finding the x-rays from pulsars, which came in pulses at very distantict periods. The visible light from these stars was way too low to be seen with conventional telescopes.

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    Default Re: Brightness of neutron stars

    I heard that pulsars pulsed because they are rotating, but that doesn't seem to really explain it if all of them pulse, because if they pulse in a beam, why does that beam usually find us, a beam ought to miss pretty often.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by halfeye View Post
    I heard that pulsars pulsed because they are rotating, but that doesn't seem to really explain it if all of them pulse, because if they pulse in a beam, why does that beam usually find us, a beam ought to miss pretty often.
    Presumably, the answer is that the beam doesn't usually find us. Remember, the reason we discovered pulsars to begin with is that we observed the periodic behavior caused by the beam hitting us. If there are a ton of pulsars that don't hit us (and I'm guessing that there are), it seems like it would be very challenging to identify them as pulsars and not some other celestial object.

    It's also possible that, angular momentum being what it is, that pulsars are biased towards rotating in the same plane as whatever galaxy it resides in, in which case you'd be more likely to hit a planet than you would given unbiased random chance.

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    Default Re: Brightness of neutron stars

    Quote Originally Posted by Xyril View Post
    If there are a ton of pulsars that don't hit us (and I'm guessing that there are), it seems like it would be very challenging to identify them as pulsars and not some other celestial object.
    If the beams are (say) one degree wide, that would mean 179 pulsars we don't see, for every one we do. That might be all the dark matter. Except they would fall into black holes and not come out again, and they would interact with each other.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by halfeye View Post
    I heard that pulsars pulsed because they are rotating, but that doesn't seem to really explain it if all of them pulse, because if they pulse in a beam, why does that beam usually find us, a beam ought to miss pretty often.
    A Pulsar is a rotating Neutron Star that happens to "flash" us with the EMR being emitted from the poles. Compare to throwing a laser pointer in the air. It has a constant beam of light (or in the pulsar's case, other EMR, usually X-rays), but we only detect it when it points at us.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by halfeye View Post
    If the beams are (say) one degree wide, that would mean 179 pulsars we don't see, for every one we do.
    As I said previously, it might not be as simple as an unweighted game of spin the bottle, but yes, that's pretty much the point.

    That might be all the dark matter.
    Well, as georgie just pointed out, a pulsar is still a neutral star, and it isn't emitting nothing outside of the beam angles. We could very well detect it even if the beam doesn't hit us--the issue is, we would have trouble differentiating it as a pulsar and not a regular neutron star.

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    Default Re: Brightness of neutron stars

    Quote Originally Posted by georgie_leech View Post
    A Pulsar is a rotating Neutron Star that happens to "flash" us with the EMR being emitted from the poles. Compare to throwing a laser pointer in the air. It has a constant beam of light (or in the pulsar's case, other EMR, usually X-rays), but we only detect it when it points at us.
    That's another puzzle, if the poles are what it rotates around, how are they moving that fast? or we don't mean those poles? in which case, which poles are we talking about?

    Quote Originally Posted by Xyril View Post
    As I said previously, it might not be as simple as an unweighted game of spin the bottle, but yes, that's pretty much the point.
    It's not the numbers, so much as the mass. These are solar mass objects, and we know of a lot of pulsars, I thought they were all in this galaxy, if they are all here that's a lot of mass.
    Last edited by halfeye; 2018-07-18 at 06:50 PM.
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    Default Re: Brightness of neutron stars

    Our best guess is that there's about four times as much dark matter as visible matter. It's unlikely that so many neutron stars exist, especially in certain regions.

    On the other hand, we might also be wrong about the amount of dark matter.
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    Default Re: Brightness of neutron stars

    As far as solar mass goes, that's not all that big where the galaxy is concerned. it would be a bit like theorizing "there's a lot of big rocks under the water here, I bet that accounts for a bunch of the earth's mass." The universe is massive on a scale that our intuitions don't readily map to.

    And while I'm at it, we've got a pretty good idea of how much dark matter there is.
    Last edited by georgie_leech; 2018-07-18 at 07:06 PM.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by halfeye View Post
    That's another puzzle, if the poles are what it rotates around, how are they moving that fast? or we don't mean those poles? in which case, which poles are we talking about?
    The magnetic poles of pulsars tend to be mis-aligned with the rotational poles of pulsars. Typical pulsars are spinning once every second (give or take) when they form, and slowly lose rotational momentum over the course of kilo- to mega- years. Because they are very tightly gravitationally bound, they can spin at very high rates without breaking apart.

    Then there are "millisecond pulsars" that spin every few milliseconds, but my understanding is that those are the result of the pulsar gaining angular momentum from a companion body at some point.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by halfeye View Post
    It's not the numbers, so much as the mass. These are solar mass objects, and we know of a lot of pulsars, I thought they were all in this galaxy, if they are all here that's a lot of mass.
    As I have stated previously, it's not that we don't know they exist, it's that we don't know that they're pulsars. We know about a lot of roughly neutron-star-massed objects. We even know that a lot of objects are probably neutron stars. However, as far as I know, unless we happen to be on the arc where we see the "flashes," we can't actually tell that one of these neutron-star-sized things is a pulsar, which implies to me that if we don't see flashes, we can't no for sure that it isn't a pulsar.

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    Default Re: Brightness of neutron stars

    Quote Originally Posted by Xyril View Post
    As I have stated previously, it's not that we don't know they exist, it's that we don't know that they're pulsars. We know about a lot of roughly neutron-star-massed objects. We even know that a lot of objects are probably neutron stars. However, as far as I know, unless we happen to be on the arc where we see the "flashes," we can't actually tell that one of these neutron-star-sized things is a pulsar, which implies to me that if we don't see flashes, we can't no for sure that it isn't a pulsar.
    I'm fairly certain that if can see its spectrum we can tell the difference between a .3 solar mass red dwarf, a .3 solar mass white dwarf, and a .3 solar mass neutron star. Since pulsars are neutron stars that spin and have strong magnetic fields, I think we cannot tell the difference between a .3 solar mass neutron star that doesn't spin and a .3 solar mass pulsar that just never points its beam in our direction.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by Rockphed View Post
    I think we cannot tell the difference between a .3 solar mass neutron star that doesn't spin and a .3 solar mass pulsar that just never points its beam in our direction.
    I'm not sure there's any such thing as a neutron star that doesn't spin, given conservation of angular momentum means they have to be spinning much faster than the star they collapsed from. Is there such a thing as a non-rotating star?

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    Default Re: Brightness of neutron stars

    Quote Originally Posted by factotum View Post
    I'm not sure there's any such thing as a neutron star that doesn't spin, given conservation of angular momentum means they have to be spinning much faster than the star they collapsed from. Is there such a thing as a non-rotating star?
    Probably not. I have a half remembered memory that pulsar timing caps out at some point. I'm not sure if that is because the oldest pulsars haven't had time to slow down past that point or if neutron stars that spin more slowly than that rate aren't pulsars.
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    Default Re: Brightness of neutron stars

    The slowest spinning neutron star has a period of 6.5 hours. It's also a magnetar, so super weird: https://news.psu.edu/story/425762/20...g-neutron-star

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    Quote Originally Posted by Excession View Post
    The slowest spinning neutron star has a period of 6.5 hours. It's also a magnetar, so super weird: https://news.psu.edu/story/425762/20...g-neutron-star
    Wow, cool! I wonder why that one is spinning so slowly?

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    Default Re: Brightness of neutron stars

    More on Magnetars, courtesy of Phil Plait. The one we detected in 2004 affected us from 50,000 light years away, half the diameter of the Milky Way.
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    Default Re: Brightness of neutron stars

    A non-rotating neutron star, or rather one with very slow rotation, is possible as a result of a collision but unlikely.
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    Default Re: Brightness of neutron stars

    The events leading to the formation of a pulsar begin when the core of a massive star is compressed during a supernova, which collapses into a neutron star. The neutron star retains most of its angular momentum, and since it has only a tiny fraction of its progenitor's radius (and therefore its moment of inertia is sharply reduced), it is formed with very high rotation speed. A beam of radiation is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every rotation of the neutron star, which leads to the "pulsed" nature of its appearance.

    In rotation-powered pulsars, the beam originates from the rotational energy of the neutron star, which generates an electrical field from the movement of the very strong magnetic field, resulting in the acceleration of protons and electrons on the star surface and the creation of an electromagnetic beam emanating from the poles of the magnetic field.[23][24] This rotation slows down over time as electromagnetic power is emitted. When a pulsar's spin period slows down sufficiently, the radio pulsar mechanism is believed to turn off (the so-called "death line"). This turn-off seems to take place after about 10–100 million years, which means of all the neutron stars born in the 13.6 billion year age of the universe, around 99% no longer pulsate.[25]
    https://en.wikipedia.org/wiki/Pulsar...nism,_turn_off

    There ought to be a lot of mass in neutron stars out there.

    Typically, neutron stars have a radius on the order of 10 kilometres (6.2 mi) and a mass between 1.4 and 2.16 solar masses
    https://en.wikipedia.org/wiki/Neutron_star

    <edit>
    Like other neutron stars, magnetars are around 20 kilometres (12 mi) in diameter and have a mass 2–3 times that of the Sun.
    https://en.wikipedia.org/wiki/Magnetar

    Those don't match
    Oops!

    The mass is of a neutron star is still greater than the mass of Sol. <edit>
    Last edited by halfeye; 2018-07-19 at 10:58 AM.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by halfeye View Post

    https://en.wikipedia.org/wiki/Neutron_star



    https://en.wikipedia.org/wiki/Magnetar

    Those don't match, but they both say the mass is > Sol.
    The Neutron Star page does raise the possibility that 3 solar masses is the maximum (as per the Magnetar page):

    A neutron star has a mass of at least 1.1 and perhaps up to 3 solar masses (M☉).[15][16] The maximum observed mass of neutron stars is about 2.01 M☉. But in general, compact stars of less than 1.39 M☉ (the Chandrasekhar limit) are white dwarfs, whereas compact stars with a mass between 1.4 M☉ and 3 M☉ (the Tolman–Oppenheimer–Volkoff limit) should be neutron stars (though there is an interval of a few tenths of a solar mass where the masses of low-mass neutron stars and high-mass white dwarfs can overlap).
    Also, radius 10 km is the same as Diameter 20 km - so that bit's OK.
    Last edited by hamishspence; 2018-07-19 at 10:50 AM.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by hamishspence View Post
    Also, radius 10 km is the same as Diameter 20 km - so that bit's OK.
    Yeah I was reading radius as diameter, or vice versa, sorry about that, will edit.
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    Default Re: Brightness of neutron stars

    I was in fact also first confused to see the numbers based on r=10km, as 20km is the number you usually hear when talking about neutron stars.
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    Default Re: Brightness of neutron stars

    Quote Originally Posted by factotum View Post
    Wow, cool! I wonder why that one is spinning so slowly?
    I would assume something with tidal breaking, but for a neutron star that's still incredibly slow,
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    Quote Originally Posted by halfeye View Post
    The mass is of a neutron star is still greater than the mass of Sol. <edit>
    But you'd still need there to be more neutron stars than actual visible ones for them to be an explanation for dark matter, and that would mean a *lot* more pulsars than we see. Consider: according to Wikipedia, there are only 11 pulsars within 300 parsecs of the solar system. It's harder to judge how many actual stars are within that distance, because many of the ones out there will be too faint for us to see at those sort of distances, so we have to estimate it. The Gliese star catalogue says there are 63 stars within 5 parsecs of us, which is a density of about 0.12 stars per cubic parsec. If that density holds out to the 300 parsec range, we're talking approximately 13.5 *million* stars in that volume. We need even more neutron stars than that, so for only 11 of them to be actual visible pulsars seems highly unlikely.

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    Quote Originally Posted by factotum View Post
    But you'd still need there to be more neutron stars than actual visible ones for them to be an explanation for dark matter, and that would mean a *lot* more pulsars than we see. Consider: according to Wikipedia, there are only 11 pulsars within 300 parsecs of the solar system. It's harder to judge how many actual stars are within that distance, because many of the ones out there will be too faint for us to see at those sort of distances, so we have to estimate it. The Gliese star catalogue says there are 63 stars within 5 parsecs of us, which is a density of about 0.12 stars per cubic parsec. If that density holds out to the 300 parsec range, we're talking approximately 13.5 *million* stars in that volume. We need even more neutron stars than that, so for only 11 of them to be actual visible pulsars seems highly unlikely.
    I like your working, but I still have questions. The stars that we don't see are typically small ones, with relatively low masses. For 11 pulsars that shine on us, there would be some number, I've suggested 179, we could perhaps argue about that if anyone has numbers on the widths of the beams, that don't. For every pulsar, whether we see them or not, there are from the wikipedia pages I mentioned above (was it in the bit I quoted?) there are estimated to be 99 dead ones that have stopped pulsing. 11 * 179 * 99 = 195, 000 according to my old calculator, and those are all heavier than Sol.

    I'm no longer sure there are enough to account for all dark matter, but they do add quite a bit to the account.
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    Quote Originally Posted by halfeye View Post
    I like your working, but I still have questions. The stars that we don't see are typically small ones, with relatively low masses. For 11 pulsars that shine on us, there would be some number, I've suggested 179, we could perhaps argue about that if anyone has numbers on the widths of the beams, that don't. For every pulsar, whether we see them or not, there are from the wikipedia pages I mentioned above (was it in the bit I quoted?) there are estimated to be 99 dead ones that have stopped pulsing. 11 * 179 * 99 = 195, 000 according to my old calculator, and those are all heavier than Sol.

    I'm no longer sure there are enough to account for all dark matter, but they do add quite a bit to the account.
    Based on a paper from the 70s, it looks like 10 - 15 degrees is the beam width of pulsars. Also, while a linear distribution model is probably wrong, I am fairly certain that we should be assuming 2 magnetic pole distributed around the circle. Thus we have 1 pole per 90 degrees. Assuming the same 99% of neutron stars are not pulsars you used above, we get 6500 - 9800 total pulsars.

    Looking a little deeper, the beam-width of pulsars looks like it decreases as the pulsar slows down.
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