Space is rather more prosaic than we usually see in fiction. Star Trek postulates a galaxy stuffed to the brim with exciting bumpy-forehead humanoids; we’re much more likely to find pond scum. The more nightmarish sci-fi visions are probably off the table, too—a lifeform from a totally different planet would be unable to parasitize a human, as we saw in Alien, and there is no evidence to suggest that black holes are actually portals to Hell.
Nevertheless, there’s still some freaky stuff out there, even if it’s not always on a scale humans can visualize or comprehend. Case in point: neutron stars, the subject of this week’s post. Ever wanted to know more about some of the universe’s most extreme objects, stars with smooth solid crusts and 200 billion times Earth’s gravity? Well, now you’ll find out.
Neutron stars are formed when large stars (more than 8 M☉) collapse in on themselves. The star will lose most of its mass in the process, since the sheer violence of its collapse will propel vast quantities of gas outwards in a supernova, and the remainder will become a neutron star, provided it is below a certain threshold (3 M☉; beyond this point you generally get a black hole instead). This leftover core can be anywhere in size from 1.1 to 2.1 solar masses and 15 to 30 kilometers across. Due to the conservation of angular momentum, the modest rotation of the original star is converted to an absolutely staggering figure during the collapse, with some neutron stars taking only 1.4 milliseconds to rotate fully.
And it gets weirder! We are, in fact, just scratching the surface of the weirdness to be found here. Unlike most stars, which comprise boring stuff like hydrogen, helium, and a smattering of heavier elements, neutron stars are made up of exactly what the name implies: neutrons. Such are the pressures during the supernova that electrons and protons fuse together, forming neutrons, and while normal matter is still present—especially at the crust—as you go deeper into the star you’ll find it to be scarcer and scarcer. The content of the core itself is a mystery, though there are a number of theories. Quarks? Pions and Kaons? Superfluid neutron-degenerate matter? God Himself? Nobody knows for sure.
Densities are on the order of 10^17 kg/m^3. In other words, a teaspoon of this stuff would be 900 times as massive as the Great Pyramid of Giza. This is what happens when you cram two solar masses into something twenty kilometers across. You break physics. It’s not quite the same level of physics-breaking as a black hole, mind you, but it’s up there.
It should be noted that, while these things are the collapsed, burnt-out husks of stars, they are very hot. They start at around 10^12 K and gradually cool to a “mere” 10^6. Nevertheless, the insane gravity of a neutron star means that it indeed has a solid crust, probably composed of iron ions with a sea of electrons flowing between them. The crust is theorized to be billions of times stronger than steel, because of course it is. Neutron stars operate exclusively in extremes.
If you stood on the surface you would see a broad, unbelievably flat plain, extending without visible features for kilometers in every direction. And then you would either collapse into sludge from the gravity, billions of times Earth’s, or you’d vaporize from the crazy temperatures, but in reality both of those things would happen at once, unraveling you to your constituent elements long before you had the chance to think about it.
Some neutron stars are what is known as pulsars. These objects are so highly magnetized that they emit intense EM radiation from their magnetic poles. Since the stars’ magnetic poles are not always the same as their poles of rotation, they’re essentially flashlights swinging around the universe, appearing on Earth as regularly pulsing points of light. They are so regular, in fact, that as a timekeeping measure they can rival atomic clocks.
So that’s neutron stars for you, or at least the SparkNotes summary thereof. Thanks for catching my post this week! I’ll see y’all next time, and if I happen to have piqued your curiosity about these strange denizens of the cosmos, here’s some further reading:
Let's Get Off This Rock Already! 
How much of the original mass is turned into neutrinos and photons? I wonder if it is significant or not, in the overall mass budget.
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Hello! I just did some digging on this question, it was surprisingly hard to find a straight answer. Apparently the bulk of a supernova’s energy output goes into a ten-second neutrino burst on the order of 10^46 J. Wikipedia’s a little unclear on how this varies with the size of the collapsing star. Some of it is from fusion and some is from gravitational collapse. My own extremely rough calculations (with the venerable e=mc^2 formula) indicate the conversion of about 10^29 kg, or 0.05 solar masses, but Wikipedia says it’s actually 10% of the star’s rest mass that is turned into energy. Most likely, the conversion is quite significant!
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