8 Neutron Star “Facts”!

Neutron stars are amongst the most extreme objects in the known universe. And I don’t mean extreme as in frontside 360 stalefish varial, I mean the kind of extreme that would’ve given Einstein bad dreams. Born in supernovae, neutron stars are the ultracompact insanely dense cores of once massive stars. And, it has to be said, these stellar corpses are rather odd. So, for your enterntainment, here’s 8 random facts about neutron stars and their general oddness!

EDIT– Incidentally, on realising how much of this is based on theories and that we don’t actually know much for certain about these strange beasts, I’ve changed the post title to “facts”. It seems more appropriate.

1. They’re difficult to find

Neutron stars are genuinely tiny (and it’s quite rare in astronomy to be able to say that and mean it!). They’re only around 15-25km in diameter, which is comparable to the size of a city! They’re mostly found associated with supernova remnants (like Puppis A, here) or discovered as pulsars. Only one has ever been found on its own. Slightly worryingly, there must be quite a few neutron stars drifting silently through space, virtually impossible for us to detect…

Pulsars, by the way, are all neutron stars. The magnetic poles of neutron stars aren’t quite aligned with the rotational poles (mind you, the same is true of Earth), and a neutron star’s magnetic poles emit beams of radio waves. In effect, they act a lot like lighthouses — Radio telescopes can see “flashes” every time they point in Earth’s direction, as radio frequency pulses. Pulsing stars = pulsars. No, I know, it isn’t very imaginative, is it?

2. They’re incredibly dense

And I really do mean incredibly. Take twice the mass of the Sun and compact it into the size of Los Angeles, and that’s roughly how dense a neutron star is. Which is just ludicrous, when you think about it. A cubic metre of neutron star material would weigh just under 400 billion tonnes. Approximately the same weight as all the water in the Atlantic Ocean! In fact, on average, neutron stars are denser than atomic nuclei (at least twice as dense deep inside their cores).

3. They make gravity go crazy

All of that density makes their surface gravity truly immense. The escape velocity from the surface of a neutron star is around one third the speed of light. Any matter falling towards a neutron star would probably be torn asunder by brutal tidal forces long before it got near the surface. It might even be spaghettified (and yes, that means exactly what it sounds like). As this matter fell further towards the star, it would be accelerated to a speed of around 100 million kilometres per hour. Slamming into the surface of the star at that speed, any matter would simply be destroyed. Atoms would be smashed. Atomic nuclei would be fragmented, probably causing a brief flurry of nuclear fusion. The fate of whatever it was that fell into the star would be to end up, unrecognisably, as neutron star matter.

4. They can warp light

One interesting effect of such harsh gravitational fields is gravitational lensing. Specifically, because gravity can interact with photons, it can bend light around it. This also affects light leaving the surface of the neutron star. The bizarre effect is that if you were to look at a neutron star, you would be able to see more than half of it at any one time! Light leaving the star’s surface on the side facing away from you would be bent, giving you a view something like this image shows. If you could hypothetically get a view from the surface of the star, it would probably appear a lot bigger than it actually was!

5. They’re not actually very star-like

In fact, neutron stars have a structure closer to planets than stars. Under an atmosphere of electron degenerate gas about a metre thick, they can actually have a solid crust. Solid and extremely hard, neutron stars start to solidify when their surface temperature cools below about a million degrees. Recent simulations suggest that neutron star crust is around 10 billion times as strong as steel. This crust is estimated to be around a mile thick and is extremely flat, due to the overpoweringly strong gravitational field. It was previously thought that any “mountains” on a neutron star wouldn’t be more than about 5mm tall — making them the smoothest objects in the universe. However, if the recent study is correct, then their crust might actually be hard enough to support slightly bigger “mountains”. They’d still be small, by Earth standards, but they might just be big enough that the star’s rotation would cause gravitational waves — ripples in spacetime predicted by Einstein’s equations.

Because neutron stars have a crust, they also have periodic starquakes. Magnetic fields put stress on the star’s crust. Eventually this causes the crust to rupture. With a violent crack, the crust shifts and magnetic field lines reconnect, powering a flare. These starquakes send out a blast of gamma rays!

6. They spin very very fast

About that rotation… Thanks to conservation of angular momentum, neutron stars rotate extremely rapidly. When they form, the bulk of a star is compressed down so fast that the newborn neutron star can rotate several times every second. If another star strays too close and starts to lose material to the neutron star, it can speed up even more, potentially reaching several hundred revolutions per second. This is actually so fast that, despite all of that gravitational force, they can start to bulge in the centre due to centrifugal force. Neutron stars do slow down eventually, albeit very slowly. Even after a million years, they’ll still only have slowed by a few hundredths of a second.

7. They aren’t actually made of neutrons

Well… not entirely, anyway. In fact, material at the surface of a neutron star is believed to be made of regular atoms. No one’s quite sure what though. Some think they could be iron atoms (one of the most stable types of atom), while others believe that iron atoms might “drown” beneath the surface, leaving only lighter atoms like helium. As you venture beneath the crust, you find atoms which are heavier and heavier. Unusual elements which belong underneath the bottom line of the periodic table. These nameless elements would fall apart in nanoseconds on Earth, but inside neutron stars, they’re kept stable by the intense pressures.

Eventually you reach the delightfully named “neutron drip”; a region where neutrons actually start to leak from atomic nuclei. From here inwards, the actual atoms start to become smaller and smaller, immersed in a superfluid sea of neutrons and electrons. Eventually, by the time you reach the star’s core, no atomic nuclei remain. Just a superfluid of degenerate matter. Actually, no one’s quite sure what kind of matter would be in the core of a neutron star. No one’s even sure if it would still be a fluid of neutrons. Some have even suggested cores of strange matter or quark degenerate matter.

Incidentally, while sci-fi authors like to use the term “neutronium”, most physicists don’t really like the word. If you find yourself at a dinner party with a physicist, it’s best to avoid using it.

8. Life? On neutron stars? No way…

Well, ok, no one’s made any serious suggestions about looking for life on neutron stars. Astronomer, Frank Drake put the idea forward, tongue in cheek, to highlight how neutron stars are much more planet-like than star like. In a short article (which I’ve searched high and low for, but haven’t been able to find a copy of), he posited the idea that such creatures would be microscopically small, with metabolisms driven by nuclear reactions instead of chemical reactions. Being as nuclear reactions are much faster, these nuclear creatures would be much shorter lived than us, their chemical brethren. Hypothetically speaking, anyway.

Science fiction Author, Robert Forward liked the idea so much, he developed it into two novels, Dragon’s Egg and Starquake. I really should try and get copies of those sometime, too!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
This entry was posted in astronomy, Imported from Livejournal and tagged , , . Bookmark the permalink.

31 Responses to 8 Neutron Star “Facts”!

  1. lbradley96 says:

    Reblogged this on Lauren Bradley and commented:
    My current cosmic obsession is the incredibly dense neutron star-

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  11. Anonymous says:

    I always found the term “dwarf planet” rather bizarre; it implies that it is still technically a planet, but is…not.
    “Pluto is a planet.” “No, it’s a dwarf planet.” “‘Dwarf planet.” Oh astronomy and your inconsistent, vague terminology you.

  12. Anonymous says:

    In my experience, the use of cartoon for diagram seems to be pervasive in all fields of science. What surprises me is that I thought it was a uniquely North American phenomenon.

  13. Anonymous says:

    weight
    i ran some math and found out that a neutron star weighs adout 4.4X10 to the 27 power or 4,400,000,000,000,000,000,000,000,000tons the earth is only adout 6,000,000,000,000,000,000,000 tons

  14. Anonymous says:

    me either???
    yeah i didnt know that either…im actually doing a project on neutron stars in science so i needed to know a little bit about them…. lol

  15. Yep, that’s the one!

  16. invaderxan says:

    Oh nice! I’m intrigued (and I do love retro sci-fi). I may have to pay a visit to my old friend, Amazon… It’s the book by Olaf Stapledon, right?

  17. invaderxan says:

    Re: gravity
    Actually, I believe it’s less than a cubic metre. The approximation used to be that a spoonful would weigh approximately the same as the entire human race!
    Pretty scary stuff…

  18. Oh, now there’s an interesting thought. An entire star, or star-like object being an entity in itself…
    The sci-fi/speculative fiction (more philosophy than science, actually) book Star Maker has just such a concept. Stars that are living, sentient, beings. They considered planets to be embarrassing blemishes, and biological life to be a terrible shame.
    It’s a rather interesting book, although, being written in the 30s, the science is highly dated. :-) There are some really cool alien life forms in it, though!
    He also proposed that the “great nebulae” that the galaxies formed from were also living beings, who slowly died as the stars were formed.

  19. Anonymous says:

    gravity
    yeah …. its pretty amazing … i heard that if you take a cubic meter of the stuff, you could fit all of humanity in it ,…… and if an object fell from just a meter, it would have accelerated to a speed of 2000 km/s before it hit the ground … if im not mistaken

  20. invaderxan says:

    Re: comment
    Hmmm… I guess it’s evident that I’m no particle theorist. Thanks for the correction.

  21. Anonymous says:

    comment
    neutrons are not bosonic. they are spin half fermionic particles.

  22. invaderxan says:

    Re: Nr.3
    Well spotted!
    I’m pretty sure that meant to read 100 million kilometres per hour. Thanks! :)

  23. Anonymous says:

    Nr.3
    Are You sure about that number:
    Nr. 3 “…As this matter fell further towards the star, it would be accelerated to a speed of around 100 million kilometres per second…”
    Thanks!

  24. invaderxan says:

    Glad you enjoyed it, dude. :)
    And yeah — confusing, isn’t it? Actually, the answer to both is to do with neutron degenerate matter (and I’ll admit, I don’t know an awful lot about that…).
    I do know that neutron degeneracy is actually a bizarre superposition of proton and electron degeneracy (at such high pressures, they’re more stable combined as neutrons — which normally decay spontaneously into protons/electrons/neutrinos with a lifetime of around 15 minutes). Also, because neutrons are bosonic, they can form a Bose-Einstein condensate. Extremely dense, due to their tiny DeBroglie wavelength…
    But then, that ridiculous density is why some people hypothesise things like quark degenerate matter (which may or may not even exist).
    I think the term “Neutron Star” is probably more historic these days. I’m pretty sure they were originally believed to be purely composed of degenerate neutrons. Names tend to stick, I guess… :)

  25. Fantastic post; learned a lot.
    #2 confuses me a bit, mostly cause I can’t figure out how something CAN be denser than atomic nuclei, yet still consist of them…
    #7 was the biggest shocker since I assumed that’s why they were named as such.

  26. invaderxan says:

    Oh, now there’s an interesting thought. An entire star, or star-like object being an entity in itself… The most interesting thing is, we’d really have no way of knowing.
    Mind you, let’s not forget that we really don’t know much about subatomic particles, not least thanks to good old Heisenberg and his principle. Protons and neutrons could actually form some kind of structures. Actually, there’s a concept in nuclear physics called the Island of Stability which proposes something along those lines to propose superheavy nuclei.
    (And feel free to stop by if ever you feel like procrastinating! :)

  27. invaderxan says:

    Surprising, isn’t it? They’re such strange objects…

  28. invaderxan says:

    Re: Nifty stuff
    Yeah, a neutron star would be nigh on unstoppable, at leaast for us. Hell, they’re heavy enough that the whole solar system could go into orbit around one. :)
    As for actually detecting them using lensing, I’m not so sure. They cause some lensing, but nowhere near the amount caused by a black hole, and black holes are hard enough to spot as it is. More likely we might eventually pick out a few as all-sky surveys improve.
    Any ideas of life are, it’s true, no more than amusing thought experiments. Though they’re amusing nonetheless! I should really check the library for a copy of Dragon’ Egg…

  29. helen99 says:

    A nuclear life-form on a neutron star (or anything on a neutron star, for that matter) would be an integrated part of the star. The densities and gravitational forces would not permit it to be separate. If one considers a sub-atomic reactor (I’ll call them “subnuclites” for short) to be a life form, then the neutron star itself could be considered a larger, nuclear-based life form whose energy source is its subnuclites (kind of like mitochondria only different – instead of converting neutrients to energy through ATP, the subnuclites convert strange matter to energy through the forces at work at high densities. I’d like to get copies of Starquake and Dragon’s Egg too – assuming I don’t have them already in the back of my closet. I have a huge collection of old sci-fi that someone left me, and very often I have books I never knew I had until I look for them…
    Enough babbling for now – back to work with me.

  30. That’s pretty cool! I don’t really know anything about neutron stars. I didn’t realise they were so small!

  31. maxdwolf says:

    Nifty stuff
    I’m guessing the lensing is probably the best bet to detect a neutron star that’s not sending us radio waves or chewing on another star or a nebula. Nothing we could do if it’s headed for us if we’re not ready to move yet though. Certainly no stopping that sucker.
    Not surprising that there’s no serious suggestions for life under those conditions. We still are only getting ideas for how it occurred here. And there’s no real way to test any hypotheses we come up with for now anyway. I read excerpts from “Dragon’s Egg” in analog I think. It was a fun excursion, worth reading if you like thinking about that sort of stuff.

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