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Neutron Stars

An artist's rendition of a neutron star. Note the jets being emitted from the top and bottom, as a result of strong magnetic fields.

From the death of a supergiant star,
came a smaller star shining oh so bright.
In cold, dead space with darkness like tar,
neutron stars fight back, this world's last light.

You may have heard of neutron stars before, but what are they really?

Neutron stars are very nearly the densest objects in the universe. Rarely more than a couple dozen kilometres across, but with masses greater than the sun, they are eclipsed only in extremeness by black holes. There's a reason Hollywood has recently fallen in love with them. But how do we go from a normal star to a neutron star?

Stars have lives, just like the rest of us. They are born, grow, age, and die. For most stars, that's pretty much the whole story. But when giant stars die, instead of going out with a whimper, they can go out with a bang. This is called a supernova. These supernovae (plural of supernova) can briefly outshine entire galaxies. But the aftermath of a supernova is not always the same.

An image of the Crab Nebula, the remains of a supernova, as taken by the Hubble Space Telescope and Herschel Space Observatory. This is a composite image, meaning that multiple images were processed together.

During a supernova, the matter of the star goes through cycles of falling inward, compressing, heating up, and expanding. How exactly all this happens is complicated, but in summary, stellar death begins with the exhaustion of its fuel. When this happens, nuclear fusion can no longer happen, and the energy that was providing heat to the star, keeping it from falling in on itself, disappears. What follows is collapse, compressing the plasma of the star to a huge degree. When a gas (or plasma) decreases in volume, temperature and pressure must increase to compensate (by PV = nRT).

This increase in temperature and pressure triggers another round of fusion, fusing heavier elements like carbon and neon together. Surging outward, the star expands once more... only to collapse, because it can't fuse heavier elements anymore. So go the death throes of the star, progressively and more rapidly fusing elements up the chain. And then, we hit iron. Iron is the most stable element. Fusing iron, rather than giving off energy, drains energy. There will be only one more expansion.

The remains of the star rush in towards the centre, building up insane pressures as they pick up momentum. But even as they build momentum, atoms at the centre push closer and closer to one another, until they can't get any closer. The wavefront of the incoming stellar matter impinges on the hyper-dense core, bounces, and sends shockwaves back out through the dying star.

No matter what happens next, the vast majority of the star is blown off in a cataclysmic explosion that creates what's called a supernova remnant (poorly termed), a new stellar dust and gas cloud. If the star was big enough, then the very insides of the star get so dense that gravity causes the formation of a black hole, a region of space where all matter is drawn in irreversibly towards a singularity, and gravity overcomes even light.

The first image of a Black Hole ever taken, by the Event Horizon Telescope. The orange glow comes from the extremely hot disk of dust and gas orbiting the black hole (known as the accretion disk).

If the star wasn't quite massive enough, though, what's formed is instead a neutron star, and that's where things get weird. All stars are permanently in a precarious balance between collapse and expansion, with the inward force of gravity being perfectly balanced by the outwards pressure of the hot plasma. Neutron stars are different, though. Heat-based pressure being insufficient to overcome gravity, they're stopped from collapsing by the Pauli exclusion principle.

But what is this principle?

References

Images

[1] Kevin Gill, https://www.flickr.com/photos/kevinmgill/14773475650
[2] NASA, https://images.nasa.gov/details-PIA17563
[3] Event Horizon Telescope Collaboration, https://eventhorizontelescope.org/blog/astronomers-reveal-first-image-black-hole-heart-our-galaxy