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Mar 18, 2016
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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.

509586main_crabmosaic.jpg


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.

258px-Crab_Lucky_video2.gif


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: when you play Stellaris, 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. 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?

Next Thread: The Giant
Previous Thread: The Dark World
 
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I'm looking forward to sending science-ships into nebulae such as this one to see what kinds of discoveries can be made :D

Important discoveries... such as "Our shields are insufficient" and "We need stronger hulls" ;)
 
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Important discoveries... such as "Our shields are insufficient" and "We need stronger hulls"

Ha, that'd be cool. It'd be like:
"Okay command, we're approaching the outer shock wave now. Let's see how far in we ge - "
 
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Sorry for the stupid questions (ok, i'm not really sorry about these), but here we go:

1. So, the star went supernova (ie made a big boom) and now, there is pulsar in the center? Did the star transform into a pulsar or did the explosion blow out a star and replaced it with pulsar or do we have no clue how the process goes?
2. Is it usual for a star to keep on exploding or do supernovas usually come to an end

And thanks for making it, interesting read! :)
 
So what your're saying is send slaves to explore this anomaly?

What do you think the practical scientific applications of studying this would be?

I'm not saying send slaves to explore, I'm saying send nobody to explore. Even if you don't value your slaves, the ship they're on is presumably expensive. However, staying at a very safe distance from it and using a telescope would be pretty cool.

I don't know about practical scientific applications in Stellaris. In real life it would tell us a lot about stars, which would help up work out which of the billions of stars in the sky are worth exploring further. It might also teach us something about high-energy physics, which is really not my field of expertise.

orry for the stupid questions (ok, i'm not really sorry about these), but here we go:

1. So, the star went supernova (ie made a big boom) and now, there is pulsar in the center? Did the star transform into a pulsar or did the explosion blow out a star and replaced it with pulsar or do we have no clue how the process goes?
2. Is it usual for a star to keep on exploding or do supernovas usually come to an end

And thanks for making it, interesting read! :)

Firstly, there are no stupid questions as long as they're asked in good faith. Never be ashamed to ask.

1. The star exploded. (In fact, that's not true: the star is exploding. It has not stopped exploding yet.) As it did so, it split into two parts: the outer part and the core, sort of like an egg's yolk and its white. The outer part of the star (which was most of its mass) expanded outwards, forming a shockwave which has continued expanding to form the Crab Nebula we know and love. Meanwhile, the inner core was dense enough not to expand like that. Instead it collapsed in on itself, forming a dense pulsar. We think this is how a lot of pulsars of this type form.

This is very exciting because normally we only see the "before" and "after" of such a process. In the Crab Nebula we're actually seeing the process as it happens.

2. Normally supernovas come to an end. As it expands the pressure drops, which means the outer shock wave slows and cools into interstellar gas and dust clouds (some of which may eventually form new stars.) The core of the supernova will form a neutron star or a black hole. Here, the pressure appears not to be dropping as quickly as we expected it to, which we think is due to its magnetic field.

It's probably worth clarifying that this is not a supernova: the supernova happened a thousand years ago and was recorded by Yang Weide. It was bright enough to be seen during the day. This is a nebula, a cloud of gas, and is what's left over after the supernova happened.
 
You think I would send mere slaves out on anything but a tired old rust-bucket with only the bare minimum to achieve their mission? You insult me.

I don't think you have to be particularly educated on the topic to see how gaining a better understanding about high-energy physics would be useful. Weapons, generators, macroconstructions, the list goes on. Pinpointing where all the useful systems are wouldn't hurt either.
 
Exploding for more than a millenium. Almost sounds more sci-fi than actual physics. Really interesting, thanks :)
 
The way the star dies depends mostly on its mass.
It can eat out all the fuel in it and become brown dwarf (basically big planet, like super-jupiter)
It can expand to become red giant, a big and rather cold star with very low activity (that is what awaits Sun)
It can blow up as a Nova or Super-Nova, creating a big cloud of matter expanding outside and a dense core.
The core may go to neutron star or to black hole, depending on its mass (it needs more mass to become black hole).

But finally everything will just disappear due to cold death of the expanding universe...
 
Excellent work, TBV! It's fascinating to read all this info. If I may offer a small suggestion? Could you put a link to the previous day's post in the first or second post of a new day's thread? This would allow people to easily backtrack through all your hard work. Took me a little digging to find D34 in this busy forum, is all. :)
 
Excellent work, TBV! It's fascinating to read all this info. If I may offer a small suggestion? Could you put a link to the previous day's post in the first or second post of a new day's thread? This would allow people to easily backtrack through all your hard work. Took me a little digging to find D34 in this busy forum, is all. :)

Good idea!

If someone would like to keep an index of all of them, that might be very useful.
 
2. Normally supernovas come to an end. As it expands the pressure drops, which means the outer shock wave slows and cools into interstellar gas and dust clouds (some of which may eventually form new stars.) The core of the supernova will form a neutron star or a black hole. Here, the pressure appears not to be dropping as quickly as we expected it to, which we think is due to its magnetic field.

It's probably worth clarifying that this is not a supernova: the supernova happened a thousand years ago and was recorded by Yang Weide. It was bright enough to be seen during the day. This is a nebula, a cloud of gas, and is what's left over after the supernova happened.

I guess technically the supernova happened around 6500 years ago and the light from it only reached us a thousand years ago, so for a civilization a thousand light years further out from the nebula than we are, they're just seeing the supernova right now.
 
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Firstly, there are no stupid questions as long as they're asked in good faith. Never be ashamed to ask.

1. The star exploded. (In fact, that's not true: the star is exploding. It has not stopped exploding yet.) As it did so, it split into two parts: the outer part and the core, sort of like an egg's yolk and its white. The outer part of the star (which was most of its mass) expanded outwards, forming a shockwave which has continued expanding to form the Crab Nebula we know and love. Meanwhile, the inner core was dense enough not to expand like that. Instead it collapsed in on itself, forming a dense pulsar. We think this is how a lot of pulsars of this type form.

This is very exciting because normally we only see the "before" and "after" of such a process. In the Crab Nebula we're actually seeing the process as it happens.

2. Normally supernovas come to an end. As it expands the pressure drops, which means the outer shock wave slows and cools into interstellar gas and dust clouds (some of which may eventually form new stars.) The core of the supernova will form a neutron star or a black hole. Here, the pressure appears not to be dropping as quickly as we expected it to, which we think is due to its magnetic field.

It's probably worth clarifying that this is not a supernova: the supernova happened a thousand years ago and was recorded by Yang Weide. It was bright enough to be seen during the day. This is a nebula, a cloud of gas, and is what's left over after the supernova happened.

Firstly, thanks a lot for your reply! And secondly, never underestimate my power of asking stupid questions!

From what i remember from school, by definition, explosion is a burning process happening really fast. So, basically, it means that we are currently dealing with part of space burning in all imaginable categories at an unimaginable rate. Like crazed forest fire that has lasted for a 1000 years? Or is anything we do on earth just a bad comparison when dealing with space?

And goddamn, space in interesting. Thanks for doing this.
 
Firstly, thanks a lot for your reply! And secondly, never underestimate my power of asking stupid questions!

From what i remember from school, by definition, explosion is a burning process happening really fast. So, basically, it means that we are currently dealing with part of space burning in all imaginable categories at an unimaginable rate. Like crazed forest fire that has lasted for a 1000 years? Or is anything we do on earth just a bad comparison when dealing with space?

And goddamn, space in interesting. Thanks for doing this.

Well, "to explode" just means "to travel outwards really quickly." You are right that within our atmosphere most explosions are caused by burning which happens quickly enough that the gas created by the burning can't get out of its own way fast enough, creating a pressure shock wave. In space, where there's almost no resistance, an explosion would just continue drifting outwards forever until the pressure was gone.

Here it's a little more complicated. The Crab Nebula is expanding outwards so quickly that the "almost no" resistance in space becomes important. The tiny little bits of gas and other things - what we call the "interstellar medium" - are being hit hard enough by the advancing nebula gas that they can't get out of the way quickly enough. This slows the nebula down, which creates a pressure shock wave in the same way that explosions you're used to do.
 
This entire post is super informative and I really dig your style. Approachable, but I don't feel condescended to.

This is one of my favorite bits:
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.
As opposed to being in space normally, where the fields queue up and try and kill you one at a time.
Part of me feels that astronauts are effing crazy. People want to be astronauts cause space is cool, but actual astronauts understand that it is cool and a horrible death trap and they do it anyway.