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.
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