D-3: The Tyrant

The following was written after many requests, and was originally published on the Stellaris forums.

Do not go gentle into that good night,
Old age should burn and rave at close of day;
Rage, rage against the dying of the light.

– Dylan Thomas
Throughout this series I’ve had a lot of requests. Some of them are for things I was going to cover anyway, like Gliese 667Cc. Some were for things I hadn’t planned to cover, like Pluto. Some I initially said no to and then changed my mind, like the Great Attractor.

None of them attracted as many requests as this one did. So let’s do it. Let’s talk about Betelgeuse.

(Image courtesy NASA)

The Crab Nebula is deadly, and if you go there you will die far too quickly for the death to be horrible. Sagittarius A* is beyond deadly and you won’t even get close to it without dying horribly. Wolf-Rayet 104 has a very slight chance of being exceedingly deadly. Pluto is deadly in its own way, as is the gravity on Latham’s Planet and the icy cold of the dark side of Zarmina. Even the vast dust fields around Fomalhaut will kill you pretty quickly if you go there. But friends, these are weaksauce compared to Betelgeuse. Betelgeuse is the gold standard of things in space that hate you. When death came for Betelgeuse she wrestled him to the floor, kicked his ass and stole his crown.

Once upon a time, Betelgeuse was a main sequence star like any other, born in the enormous hydrogen nebulae of Orion’s Shoulder, wheremany stars are born and die. She was a big star, bright and blue and short-lived; and for reasons we’ll never know she decided that the best way to spend the few million years of her lifespan was to be violently catapulted out of Orion’s Shoulder and go wandering.

By “wandering” I mean she’s moving so fast that she’s got a bow wave. Space is mostly empty, but not quite: there are a few molecules scattered here and there across the vaccuum, and Betelgeuse is going fast enough that these are offering resistance. O-class stars don’t live long, but Betelgeuse didn’t plan to spend that time amongst her peers.

Not that she has peers.

Eventually, Betelgeuse burned through all her hydrogen. When that happened she swelled up into a giant, started fusing helium, and we can imagine that the galaxy breathed a sign of relief. Surely they were safe now? But no, death just made Betelgeuse angry. She didn’t even lose her blue colour, that’s how hot she was.

Eventually she burned through her helium too. Surely now she would go? Nope. You underestimate just how much Betelgeuse likes burning things. You probably can’t light ashes and smoke on fire: similarly, most stars can’t fuse oxygen and carbon. In this, we see that most stars just aren’t as committed to burning stuff as Betelgeuse is. She swelled up even further and started fusing oxygen. When she ran out of oxygen, she swelled up even further and started fusing yet heavier gases.

Around 100 BC, a Chinese astronomer noticed Betelgeuse and remarked that she was a beautiful yellow colour; so beautiful, in fact, that he proposed that she be used as the standard reference point for a yellow star.

A millenium later, Arab astronomers noticed Betelgeuse and remarked on her beautiful red colour. Not having read the work of their Chinese predecessor, they suggested that she be used as the standard reference for a red star. Her name comes from them: “Betelgeuse” is a corruption of the Arabic word for “a reddish colour.”

In between these two sightings, you might think that Betelgeuse cooled down: red stars are cooler than yellow stars, after all. However, you would be mistaken.

The Arabic astronomers noticed another puzzling thing, which also puzzled Europeans when we finally got our act together and stopped being so backwards: Betelgeuse changes luminosity drastically. She’s traditionally catalogued as the ninth brightest star, but she sometimes goes ashigh as seventh brightest and at other times diminishes to tenth. Many stars change like this, but they normally do so predictably and on a strict pattern. Betelgeuse is utterly unpredictable, sometimes changing in just days.

(Image courtesy NASA)

This isn’t because she hates us. Well, not just because of that. Most variable stars vary because they’re rotating and one side is darker than the other. Betelgeuse, by contrast, is doing it because she’s surrounded by a shroud of her own blasted-off, cooling outer layers. These extend out up to 30AU, which means that if she was in the Solar system they would reach the orbit of Neptune.

Because of her bow wave, sometimes this shroud of gas gets a little thinner, and then we see the hotter, angrier star below it, and we realise that Betelgeuse still burns with a hatred and a hunger that never went away.

(Image courtesy ESA / Herschel)

Even more frighteningly, occasionally a bubble of hot gas comes up from the layers close to her core and reaches the surface; and when this happens we realise that Betelgeuse is still blue on the inside. This whole “giant red star” thing is a costume she’s donned, a ruse to fool us into complacency.

Because of her unpredictability, there is no such thing as a safe distance from Betelgeuse. If you are orbiting her then she might kill you at any moment. If you don’t want to be killed at any moment, then stay away from her. Stay light years away from her. This is the only warning that you will get.


D-12: Journey to the Core, Part Two

The following was originally published on the Stellaris forums.


Welcome back! Last time we were in the midst of a journey to the centre of the galaxy, and had just passed into the molecular gas cloud known as Sagittarius B2. This is the point, beginning around 120 parsecs from the centre, beyond which the stars are dense enough and gas is common enough that we can no longer think of space as being empty. The gas is very thin by atmospheric standards, but if we tried to travel through it at high speeds then it would create enough resistance to slow us down. It’s also bright enough that it gets harder to see beyond it.

What’s beyond it is terrifying, but the stars get even denser and I have no pictures for you. Astronomy into this mass of light and gas is like staring into fog.

When we get within one parsec of the centre, the density of stars is so great that every cubic light-year contains, on average, seven stars. (Compare that to where we are, out in the spiral arms, where the stars have empty light-years between them.) Even worse, the proportions of star types have changed: while red stars are still the most common, big blue stars are no longer fractions of one percent of the stellar population: they are now a full ten percent. These are dense enough that their radiation has ionised all the gas we find within this area. (This means that it is far beyond lethal.) Even worse, many of these are what we call Wolf-Rayet stars: they’re right on the cusp of supernovadom and are massive enough to cause a gamma ray pulse when they finally get there. This is a bad celestial neighbourhood to be in. But even that isn’t the worst thing.

The worst thing is that all these unusually-common massive stars are the same age. Not “around the same age” but “close enough that we think something caused them all to form at the same time.” I am not a superstitious man, and so I do not believe that it was witches (of the Hoag’s Object sort) who caused this to happen. Others may be more superstitious than me.

Let’s leave the puzzling stars behind us and travel even further. Deep inside this hedge of stars we find another structure, called Sagittarius A. It’s a confusing lump which is composed of two visible parts and one hidden. One of the visible parts is the remnants of a supernova; the other visible part is an accretion disk. Over a long period of time the two have partially joined together. We’ve met a lot of deadly things in the course of this series, but Sagittarius A is the worst. Each of these two monsters can devour stars without pausing for breath, and they’re so close they overlap.

They’re not friends either. We have evidence that something colossal blasted away enormous amounts of matter from the accretion disk in the past, in a way which suggests that it was the supernova that did it; and we can see the accretion disk feeding on the supernova remnant.

If you’ve ever played that childhood game of “who would win a fight between Cthulhu and Yog-Sothoth”, then what you are seeing now is evidence that the very gods themselves have played that game too.

In the middle of the accretion ring is the invisible reason why they’ve joined together to form Sagittarius A. We call this invisible thing Sagittarius A* (pronounced A-Star in a way that will make British people think about A-level marks.)

Before I tell you about Sagittarius A*, I want to tell you about a party I went to once. A new album by Muse had come out and it got played, because my university friends and I liked Muse. I vividly remember hearing the lyrics for the first time:

I’ll never let you go
If you promise not to fade away
Never fade away

I just wanted to hold
You in my arms

I asked what the song was called, and found out that it was Supermassive Black Hole. I distinctly remember thinking that this is in a way the most perfect description of a supermassive black hole that has ever been written. That’s what it does: it just tries to hold onto everything, up to and including light. General Relativity tells us that anything which passes the event horizon of a black hole will, to an outside observer, never experience time passing again. This means that light which goes past the event horizon will quite literally never fade away.

That’s what Sagittarius A* is: a supermassive black hole, lurking at the core of the galaxy.

I found out several months later that those weren’t actually the lyrics to Supermassive Black Hole: that’s the chorus from Starlight, off the same album. It’s also a good song but the two tunes are entirely different and it’s difficult to mistake the two. Weirdly, to this very day I can vividly remember hearing those words to the wrong tune. Memory is a strange thing.

Black holes are strange things too. It takes a while to appreciate how heavy they are. If you’re a scientist then 4.1×10^6 solar masses is meaningful to you. If not, then “four million suns” is just a number: it’s big, yes, but it could be three or five million suns and you wouldn’t feel any differently about it. How heavy is it? Very heavy. It is, by itself, 0.1% of the total visible mass of the galaxy. It is one-quarter of the mass of all the other black holes in the galaxy put together. That’s how heavy.

Once you’ve got a grasp on this number, the next surprise is how big it is in size terms rather than mass terms. It must be immense, right? Well, I’m sorry to disappoint you. We know that it’s less than 0.08 AU across, 17 times the size of the sun, which would still fit comfortably inside the orbit of Mercury.

Black holes are actually quite small.

What they are not, however, is quiet. Remember when I said that Sagittarius A* was invisible? That’s true only for people in the spiral arms of our galaxy who can’t see (or hear) radio waves. If you can hear radio then you’ll be able to hear it quite clearly as the loudest source in the galaxy.

Most of the radio emissions, however, are being directed directly out of the galactic plane in two tight beams. If you are in another galaxy and can pick up radio waves, then Sagittarius A* is the most easily-detectable thing in the Milky Way. (We think.) There are other galaxies like ours which are so far away that the only detectable thing is a radio source like that, which probably also comes from a supermassive black hole at the centre.

(Properly, the name Sagittarius A* refers to the radio source itself, not the black hole. The only people who really care about this are the same sort of people who will correct you when you say things like “I weigh 70kg” instead of “I mass 70kg”.)

I wish I could show you a picture of the beast lurking at the centre of the galaxy, but I can’t. I can’t even show you a picture of the vast duelling pair of titans that make up Sagittarius A. The core is too dense and too bright for us to see into it, let alone travel there. This is no place for mortals.

Here be dragons. Stay away.

D-25: Luck

The following was originally published on the Stellaris forums. By the way, when I say that, I mean that it was originally written by me on those forums. This is my own content.


Good news everybody! We’re all going to die!

Yes, that’s right, it’s time for another terrifying thing in space which may absolutely kill every human being at once without warning. If you thought the Crab Nebula was scary, you might want to skip this one. If you thought the Crab Nebula was awesome, read on.

It’s time to talk about WR104.

WR104 is a binary system; that means it consists of two stars which are orbiting around a shared centre of gravity. One of these stars is an O- or high B-class main sequence star, which means it’s very big, very hot and would appear blue to the naked eye. The other star is extremely big, unbelievably hot and would appear OH GOD THE PAIN coloured to the naked eye. They are 2,300 parsecs away and are not visible with the naked eye, which is fortunate for us.

That second star is close to dying. We know this because of the gases it’s fusing. Main sequence stars fuse hydrogen into helium in their cores. You may have been taught at school that when they run out of hydrogen they start to fuse helium, but this isn’t true – they generally start to fuse helium and heavier elements while there are still some scraps of hydrogen left. We can see which gases a star is fusing by looking at what are called emission lines; put simply, we look at the quirks of its colour.

In the 19th century a pair of magnificent Frenchment, Wolf and Rayet, discovered that some very large stars have emissions lines which indicate that there is basically no hydrogen left in their cores because they have fused it all. This means two things: firstly they are about to turn into a supernova, and secondly when they do it will not be as gentle as most supernovae are.

(If you’re not an astro nerd, then a “supernova” is a Latin phrase meaning “a star exploding”, and that’s a pretty fair description of them. Some supernovae are known as “superluminous supernovae” because they’re bright enough to give off deadly radiation rather than merely light; these are also sometimes called “hypernovae.”)

The outside of a Wolf-Rayet star is made of hydrogen even if the core isn’t, and as it approaches its death this hydrogen is streaming away from it in all directions. It might not be hot enough to fuse, but it’s definitely hot enough to glow.

More importantly, it’s coming off hard enough to blast gas off the outer layers of its companion star, giving this companion an immensely long “tail” of cooling blue-hot gas. This looks unbelievably pretty because both stars are, remember, orbiting one another.

(Picture courtesy Keck observatory. This is an animated gif morphed from three images taken as it rotates.)

Pretty, huh?

That pinwheel effect is very large. The visible part – that is, the bit which is glowing hot enough to be seen from Earth – is 160 AU across. (For reference, Neptune is 30 AU from the sun; this means that this pinwheel is almost three times as wide as our solar system.) This is thought to be surrounded by a much larger disc of gas which has cooled too much to be visible.

This beautiful pinwheel is made of blasted-away but still very hot stellar gas. Do not go there. It will not be a pleasant place to go. This is thought to be surrounded by a much larger disc of gas which has cooled too much to be visible (or deadly.)

But wait, there’s more.

Sometime in the next few hundred thousand years, that Wolf-Rayet star will come to the end of its life with a bang. Normal supernovae are very big and fierce, and Wolf-Rayet stars have even bigger and fiercer supernovae, but this one is especially a problem because it spins. A star that spins concentrates its emissions into two beams that come out of its poles. This is known as a “pulsar”, and looks like this.


(Image taken from the video game Stellaris, which has pretty stars in it.)

When a massive star turns into a superluminous supernova, it announces it to the world with a pulse of radiation of a type that’s called gamma radiation. This travels at the speed of light and goes straight through most things. Most of the time, a star the size of WR104 would have a gamma ray burst that goes in all directions and is merely deadly to those near it. However, if it’s spinning fast enough, then it will be focused into two beams, one pointing in each direction from it.

This is probably a good time to tell you that WR104 does indeed spin like that, and its beam is often pointed in our direction, although it wobbles a bit.

This is probably also a good time to remind you that this could have happened any time in the last 7500 years and we wouldn’t know about it. The first warning we would have is everyone dying.

Close your eyes. Take a deep breath. Open them again. Are you still alive? Good. WR104 has decided to delay our deaths.

Hang on. Are we going to die? Seriously?

Of course. Humans are mortal.

I mean, is WR104 going to kill us?

If the beam is pointing at us when it happens and the beam is still strong enough by the time it gets to us, absolutely. We will not only all die in an instant, but Earth will lose such trivial and unimportant things as her atmosphere and her oceans. Merely being on the other side of Earth will not save you either.

The gamma ray burst beam will slowly lose power as it travels, because the beam will gradually widen. The faster WR104 is spinning, the narrower the beam will be and so the more powerful it will be – but also the less chance it will have of hitting. On the other hand if the star spins slowly enough then the beam will be broader and less powerful, meaning that our atmosphere may be able to withstand the radiation.

So how fast is it spinning? Is it fast enough to kill us? Don’t leave me in suspense!

We don’t know. Wolf-Rayet stars are rare and gamma ray burst events are very rare, so we haven’t had much experience with them.

What are our chances?

Pretty good. The wobble of WR104 is enough that it’s not pointing at us most of the time. In order for it to have a real chance of hitting us, the beam will have to be broad enough that the burst will have lost most of its power when it gets to us. Astronomer Peter Tuthill has done some serious study and concluded that it will probably miss.

This means that we will probably not all be wiped out by a totally unannounced, unforeseen and unpredictable death ray from the heavens. By which, of course, I mean we’ll all die of something else first.

Isn’t that a cheerful thought?

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?