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.

[IMG]
(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.

[IMG]

(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?

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