D-33: The Death Star

The following was first published on the Stellaris forums.


Today we’re going to talk about a dead giant. Specifically, we’re going to talk about a giant star which died a thousand years ago and left a truly beautiful, truly terrifying corpse behind.

Meet my second favourite thing in space, the Crab Nebula.


In the year AD 1054, Chinese, Arab and Japanese astronomers recorded a supernova which lasted approximately two years. At its height, from 4 to 27 July 1054, the Chinese astronomer Yang Weide recorded it as being visible even during the day and spent a lot of time trying to work out what it meant for the Emperor’s reign. Europeans, apparently, didn’t see anything. The dark ages must have been really dark.

In 1928, Edwin Hubble measured the Crab Nebula and discovered that it was expanding at a steady rate. He worked out this steady rate, extrapolated it backwards, and concluded that the Crab Nebula must be what’s left of that supernova a thousand years before.

The Crab Nebula is big. How big? It’s been expanding at 1500 kilometres a second for a thousand years. That’s how big it is. This beautiful filigreed web of light is actually a colossal explosion bursting in every direction as fast as it can, fast enough that it would swallow the entire Solar system in a little over a month. However, that’s not fast enough for it, because the speed of the outer layers is as nothing compared to what’s inside. See that blue glow? That’s synchrotron radiation.

Let me take a brief detour into particle physics, and I promise I’ll keep this simple.

A synchrotron is a primitive particle accelerator which uses an extremely large magnet. When Soviet researchers were using them, they discovered that a charged particle moving very quickly through this magnetic field produced faint radiation. The strength of the field and the speed of the particles determines the wavelength and strength of the radiation. Synchrotron radiation has come to be quite useful in industry, but it’s not cheap: after all, you need a very strong magnet and a particle accelerator.

That blue glow is synchrotron radiation strong enough to be visible from Earth, which is a horrifying sentence to type. Studying it, we can tell that particles inside the Crab nebula are moving at up to half the speed of light, through a magnetic field that is quite literally like nothing we’ve ever seen. If you thought the outside of the Crab nebula was scary, the inside is really bad. However, it’s still nothing compared to the core.

Deep inside the nebula is the remnants of the old supergiant star’s core, which has collapsed down on itself. It’s not quite heavy enough to form a black hole, so it’s formed a neutron star instead. This is where the immense magnetic field comes from. Based on what we know about neutron stars, it’s also going to be pumping out horrific amounts of radiation of various types. If you were to fly a spacecraft into here then pretty much every single field of physics would be attempting to kill you in its own unique way.

Do not go into the Crab Nebula, is kind of what I’m saying here.

Oh, and it’s not slowing as it expands. Most explosions slow down as they expand because the pressure drops as the volume increases. We’ve measured the Crab Nebula and it’s… yeah, it’s decelerating a little bit, but nowhere near as much as it should be. We believe that this is due to the magnetic field, which is forcing new, faster matter out to the edge to keep the pressure up. It potentially has the entire mass of a giant star to hurl outwards, and it’s shown no signs of having any other intentions.

Not only is the Crab Nebula a badass, but the core is also a pulsar: a star which spins in such a way as to send out a regular pulse of light at us. Here it is, winking at us.


This thing looks placid, doesn’t it? Nice and peaceful and happy, blinking like it doesn’t have a care in the world. On this image we can’t see the enormous halo of death that surrounds it, so one could almost be forgiven for thinking that its peaceful act is genuine. Don’t be fooled though: if you ever command a space fleet, and your captains tell you that flying through a nebula would be dangerous, listen to them. Those people are speaking good sense.

However, it’s not just terrifying. It’s also deeply interesting. (Many scientists would say that those two phrases are identical.) If we look at the filaments on the image, we can see what they’re made of by the way they interact with the light. If we then study them over time, we can see how quickly they’re expanding and trace them back to where they may have come from and how they would have unfolded from that colossal star that exploded a thousand years ago. This lets us see inside a star, almost, which gives us priceless information that it’s much harder to get elsewhere.

Haven’t you ever wanted to dissect a star?


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