D-2: The Gift of Giving

This piece was written by Murmeldjuret as part of the Astroknowledge series and is reproduced here with their permission. It was originally published on the Stellaris forums.


Today I will be continuing The Beautiful Void’s astroknowledge series.

Right now you are emitting radiation. So are your walls. Don’t worry it is perfectly normal and won’t kill you. You should actually be glad the walls radiate back to you, if they wouldn’t you would start freezing. The same reason the walls radiate heat to you is the way we find the temperature of suns millions of lightyears away.

All light and heat is created when an electron loses potential energy. It is how lightbulbs, LEDs, x-ray cathodes, and forest fires emit light. Whenever an electron moves from a high energy state to a low energy state it emits the difference as electromagnetic radiation. Short jumps become low energy radio waves, and the longest jumps become gamma rays.

X-ray cathodes work by sending high velocity electrons at a target material, knocking out electrons from their orbit. Higher energy electrons fall into these holes, and emit X-rays while doing so.

Atoms and molecules also have kinetic, rotational, and vibrational energy levels. When they interact with each other, they change their internal energy levels. This is what we see as temperature. Hotter objects have more kinetic, rotational, and vibrational energy. Temperature is their ability to give off this energy to other things. When they touch another colder object, they will lose heat energy and the colder will gain that heat energy. Temperature equalizes because something that is better at giving than something else gives back will lose. This seems natural and is something humans notice quickly. Touching cold objects lowers your temperature, while nobody wants to touch hot coals because they love to give.

Heat has another way of transferring than simple touch, namely as radiation. The side facing a hot fire will become hotter than the side facing away, which is not due to the heat transfer via the air between. The vibrational energy can be transferred via photons, as well as normal matter. All things above absolute zero has vibrational energy and as soon as it interacts with another electron it will change its vibrational energy. Any loss here is emitted as electromagnetic radiation. Much of it is contained inside the object, but anything close to the surface has a fair chance to emit it out of the body.

This is then also related to temperature, or willingness to lose energy. The heat you radiate is absorbed by the walls and their radiation is absorbed by you. Heat radiation is for most parts marginal inside earth’s atmosphere. Air is a better conductor for heat than radiation. This is not the case in space. In space, all heat is exchanged through radiation.

The type of radiation is not dependent on the object, or the shape, or the substance. It is only dependent on temperature, the willingness to give energy. This is Planck’s Law, and describes how much and of what type of radiation an object emits. It is always shaped this way, as the total number of atoms is so incredibly large any oddities get marginalized.
We can from this estimate the peak temperature of emission, and it follows the very simple Wien displacement law. Wavelength = constant/Temperature. It is a good approximationexcept for really cold temperatures.

So what does this mean?
It means that whenever we look at an object in its spectrum, we can accurately give its exact temperature. Below is the sun:
We can say that the surface temperature is 5778K within a few degrees of error. Similarly, we cantake the temperature of any distant object that we can spectrally resolve. It also shows why hot suns are blue, as their peak is to the left of the visible spectrum, and why cold stars are red, as their peak is to the right of the visible spectrum.

For those who wonder why we often call it black-body radiation, it is because the actual formula includes an emissiveness term, as radiation from object to medium isn’t perfect. Low emissiveness works like heat mirrors. The heat is never taken up by the object to be re-emitted. If emissiveness is max, it follows the curve exactly and this is called black-body radiation. On earth, almost nothing has true black-body radiation, but in space everything is close to true black-bodies.

If you look at the Planck law curves above you can see that things at room temperature (300K) would be far to the right of the visible (400-700nm). This places it in infrared, which is why we often talk of infrared as heat radiation. Like any radiation wecan see it. IR cameras can photograph things in room temperature, but this is always tricky as the camera itself gives off radiation.

As things grow hotter, their wavelengths become shorter, and eventually what is normally considered heat radiation in infrared turns into visible radiation. When things grow even hotter than the sun they begin emitting their peak in UV, and eventually the hottest things at millions of Kelvin emit X-rays from heat alone. As things grow colder they become redder to eventually be invisible to our eyes. Below is a piece of iron at my guess around 1300K, or 1000C. You can see the hottest part appear white, and as the metal grows colder it grows redder to eventually be outside thevisible spectrum entirely. Light and heat is also reflecting off the hammer above and the anvilbelow.

Spoiler: Ironworking


So this is how we can say how hot a sun is, regardless of distance, because it is the shape/colour/spectrum of the light, not only thestrength that depends on temperature. The lightfrom distant and nearby stars include an indicator of heat and total luminosity in their light.

Heat is wonderful thing and all it wants to do is give. So when a superhot sun is melting your ships, just know that all it is trying to do is share some of its warmth. And your cold metal shell is unable to give the same warmth back.


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-7: The Bride

The following was originally posted on the Stellaris forums.


A big thank you to Murmeldjuret or another fantastically informative weekend! Coming up this week we have all sorts of interesting stuff for you in the astroknowledge series. We’re close to the end now, very close. I’ve been holding back some of the best stuff in space until now, so as to close this series with a bang. As a result, I hope that this week will be as good for you as it is for me.

We’ll also have another guest article from the pen of the delightfully learned Admiral Howe.

Let’s talk about Pollux. This is a star that humans have known about for a very long time; together with Castor it makes up the two bright stars that the Gemini constellation is named after.

(Image courtesy astropixels.com)

Once upon a time, back before humanity existed, Pollux was a white A-class star on the main sequence. Sadly stars of this sort, while very beautiful, don’t live very long. Let’s look at what happened next, to understand what happens to stars when they reach the end of their lives.

Gas falls in
Light comes out
That’s what stars
Are all about

A star is a balancing act. Gravity pulls gas inwards towards the core, and light (and other radiation) that streams out of the core pushes it back. This might seem weird to you if you aren’t used to thinking of light as being something powerful enough to hold up a heavy weight, but this gives you an idea of just how bright the cores of stars are. We call this “thermal pressure.” *

When Pollux exhausted the hydrogen in her core, this light began to fail. Gravity, which had been kept at bay for hundreds of millions of years, is a patient force: the core contracted and enormous amounts of fresh hydrogen poured into it.

What happens when you pour enormous amounts of fresh fuel into a dying fire? It doesn’t just restart gradually: it explodes.

This sudden reignition of the core pushes back against gravity hard. The outer layers of the star are blasted outwards, as is a lot of the still-burning hydrogen that just began to flood into the outer core. We call this the “first dredge-up.”

At this point Pollux became vastly larger, which is where she is today: she’s only twice the Sun’s mass but is almost nine times her radius and thanks to her enormous surface area is a full forty-three times as luminous.

(Image courtesy NASA. That small yellow thing next to Pollux is the Sun, included for size comparison.)

Pollux, which had been a bright white star for a few hundred million years, became a larger, cooler orange giant. Her outer layers are the only part we can see, which create her orange colour. Deep inside, her core has now started to fuse helium instead of hydrogen. This gives her only another few hundred million years of life. When this fuel source ends then it will be like before: her core will contract, bringing in more helium, which will cause a second dredge-up. She’ll get even larger as her outer layers inflate even further, and her surface will be even cooler and redder.

At this point her core will be full of oxygen and carbon, which she isn’t heavy enough to fuse. She’ll continue to glow but this will be from the outer layers rather than the core. Eventually the outer layers will fade and drift away, creating a nebula that slowly cools to black; and a core of oxygen and carbon, still glowing white from residual heat.

Don’t weep for Pollux. Space is a fantastically good insulator, and so she’ll cool extremely slowly. While her life up until this point will have been less than a billion years, her lifespan as a white dwarf is projected to be considerably longer than the current age of the universe. Everything up until this point has really just been her childhood: a brief hydrogen- and helium-burning phase she went through before she settled down to be what makes her truly happy: being small, and beautiful, and white as snow.

Tomorrow we’ll talk about her planet.


* Thermal pressure is a combination of radiation pressure (that is, light) and convection pressure (that is, hot gas rising.) For small cool stars convection pressure dominates; for larger hotter stars radiation pressure dominates.

D-17: The Great Eye

The following was first published on the Stellaris forums.


Today we’re going to talk about a star which has fascinated humanity for ages: Fomalhaut, in the constellation Pisces. Fomalhaut’s name comes from Arabic because it was extensively studied by Arab astronomers of the Golden Age, which tells you just how long we’ve liked it. Many astrologers and other mystics have come up with complex meanings for this brilliant pure-white star. The astronomy-obsessed H P Lovecraft mentioned Fomalhaut as the haunt of a Great Old One. To modern astronomers, however, the planet resembles another fictional franchise entirely.

Meet the Eye of Sauron.

(Image courtesy NASA)

Fomalhaut is surrounded by a huge spinning ring of assorted space debris, mostly dust, which appears red in this false-colour image. This is puzzling because she’s a very hot white star, hot enough for her solar wind to blast the dust away. We believe that new dust is continually being formed by larger lumps of rock and ice slamming into one another, which replaces the dust that she blasts away. It’s certainly beautiful.

(The ring is actually perfectly round; the elliptical effect on this image comes from us seeing it at an angle. The central sphere of black comes from the fact that there’s not much dust close in because Fomalhaut is blasting it away with deadly solar winds.)

There’s a term for this sort of collision between lumps of stuff in orbit: planet formation. And indeed, there are three candidates for protoplanets orbiting Fomaulhaut. Two of them are in the outer reaches and were detected using conventional methods. The first and best, however, was seen in 2008.

Yes, seen. We’ve actually seen it, not just measured its effects. Here’s a photo of Fomalhaut b. This was the first exoplanet we ever saw directly.

(Image courtesy National Geographic)

This planet was named Dagon as a nod to Lovecraft, which irritates those of us who are obsessive enough about our Lovecraft lore to remember that it was Cthugha which was located here. It is also sometimes known as the Mote in Sauron’s Eye as a nod to a classic Larry Niven novel.

This is probably a useful way to tell you how big the Eye of Sauron is. Dagon orbits at a distance of 109 to 245 AU. (For reference, Neptune is 30 AU from the Sun.) A-class white stars like Fomalhaut have a large and distant habitable zone, but even so Dagon is a very long way outside of it.

The normal method of detecting planets tells us their mass but not their volume. Here we have the other problem: we know Dagon’s volume (because we can see the light that it reflects and therefore its surface area) but not its mass. However, the fact that we can’t detect it using normal methods means that Dagon isn’t more than twice the mass of Jupiter. That said, it’s a lot bigger than Jupiter, which means that Dagon is not so much a gas giant as a loose cloud of stuff which is in the process of becoming a gas giant. That loose cloud is reflecting enough of Fomalhaut’s light that we can see it in the above picture.

There may, however, be other planets. As the Eye of Sauron settles down to become a normal planetary ring, it’s easy to imagine other planets forming. The European Southern Observatory didn’t find Dagon but did find signs which suggest that there may be another two planets here.

The difficulty is time: Fomalhaut is a hot A-class star and while it might be young, stars like this only live a few hundred million years. Earth is already 4.5 billion years old. Life may have emerged on earth as early as 3.9 billion years ago, but that means it took 600 million years from the formation of the planet to the formation of single-celled life. On this scale, Fomalhaut will never even have bacteria before she dies.

However, this might be an excellent place for an already space-dwelling species to settle. Fomalhaut has some iron and may have other metals too, which means the Eye of Sauron and its larger bodies may be worth mining. Anyone living here would have to construct sturdy shelters to protect themselves from high-velocity dust and also from the radiation of the white sun.

D-21: The Cradle

The following was originally published on the Stellaris forums.


Good morning, everyone! A big thank you to Murmeldjuret for filling in over the weekend, and I hope everyone enjoyed their updates as much as I did.

Today we’re going to talk about a very, very interesting star system. We’ve discussed several binary systems before, and briefly mentioned one three-star system (Gliese 667.) But what about a four-star system?

Meet HD 98800. If you dislike numbers, this is also called TV Crateris or TV Crt.

(Image courtesy Keck. Apologies for the extremely technical image but I couldn’t find a good visible-light one.)

HD98800 is composed of two pairs of sunlike stars (probably K class, which means orange stars slightly smaller than the Sun). Within each pair, the stars are very close together, close enough to fit within the orbit of Mercury. The two pairs orbit one another at a distance of about 50 AU, which isn’t close on a planetary scale but is very close when you’re dealing with stars. You can think of it as a binary system made up of binary systems; in fact that’s probably the best way of thinking about it.

I know you’re thinking it so say it with me: “Sup dawg, I heard you like orbiting so I put a binary system in your binary system so you can orbit while you orbit.”

50 AU is a distance that’s easily crossable. (For reference, Neptune is 30 AU away.) A species which evolved on a planet orbiting one pair of stars could send a probe to the other with 20th century technology, and could probably send crewed exploration vessels with 21st century technology.

So are there planets there?

Maybe one, but there’ll probably be more one day.

The stars in HD98800 are very, very young. They’re so young, in fact, that hydrogen fusion in their cores hasn’t properly begun. This means that while they’re slightly less massive than the Sun they’re also a little bloated. Give them a few billion years and they’ll be svelte K-class main sequence stars. They’ll also have planets, most likely.

HD98800B (that is, one of the pairs) is surrounded by an enormous ring of debris, mostly rock and gas and ice. We think that our Solar System formed just like this: a ring of debris around a young star. Gradually the debris will form into planets, and if the debris has enough ice in it and the planet is within the Goldilocks zone then maybe it’ll have an atmosphere and oceans.

Even more excitingly, we’ve spotted a gap in the debris ring between 1.5 and 2 AU out. This may seem innocuous, but gaps of this sort are usually made by planets clearing out their orbits. (We’ve seen Saturn’s moons do the same thing in her rings, which is why they have gaps.) The gap is definitely not within the Goldilocks zone right now, given the immature state of the stars, and probably won’t be even when they’re properly on the main sequence, but where there’s one planet there might be more.

HD98800A (the other pair) doesn’t have nearly as dramatic a debris ring (see image above), but it may still have some. Even if it doesn’t, multi-star systems are notorious for having planets migrate from one orbit to another, so it’s not impossible that some of B’s planets may end up orbiting A.

In some tens of billions of years, HD98800 could have multiple habitable planets, possibly around both pairs of stars.

(Artist’s impression. Image courtesy NASA / JPL-Caltech.)

What would it be like to live there?

On Earth, seafaring civilisations tend to arisearound estuaries, bays or clumps of islands. If you’re accustomed to taking a boat across the bay then it’s a natural step to proceed a little further, and a little further again the next day. Thepresence of nearby coasts to travel to provides a stepping stone to help society develop the necessary concepts for exploration and then for colonisation.

A lot of people, including me, think that the Moon played the same role for humans on Earth: it’s soclose and so perfect that it made us realise that something is up there and that up is a direction we can go in. Sadly, the stars are not so near.

The stars are near, however, if you live in HD98800. You could get there in only years, even without any sort of FTL. This might give the localsthe conceptual framework to develop a civilisation based around space travel, which will in turn help them travel a little further, and a little furtheragain the next day.

HD98800 is the cradle of stars, and the cradle ofplanets. One day it may be the cradle of empires.

D-23: The Hunt for Exoplanets

The following is the first guest post on the series, written by a person called Murmeldjuret on the Stellaris forums. It has been reproduced here with their permission.


Today, by request it will be on how to detect exoplanets. I have made it rather exhaustive, so get your coffee ready.
For those of you wanting to learn of the things that can and will want to kill you, there will be moredeadly friendly lightshows tomorrow.

The main problem with detecting planets is that they are very small compared to a star. They don’t give off light, and not being on fire tends to leave them cold. In fact, we have only directly seen a very small set of exoplanets. Wikipedia lists 20, all of them larger than jupiter. Nasa lists 33. All the other thousands of planets we have “detected” we have actually never seen. They have all been inferred from observing the light of the star they orbit. Inferred is mathspeak for qualified guessing, meaning the changes in starlight is most likely from a planet, but we are not 100% sure. Some of them we are really certain about, some like the previously described Gliese581d are less certain and are considered disproven one year to be proven the next.

There are two primary methods to finding planets in other solar systems. Planetary transit and radial velocity also known as doppler shift. Both require the planet to rotate over or nearly over the star, meaning we can only detect planets around stars whose planetary disk aligns with the star and Earth. We will not be able to detect planets from the majority of stars out there. If a star has not yet been proven to have a planet it might still have one, only we are unable to detect it. I should point out that there are methods to detect planets by two other means but we have seen only a dozen this way.

Planetary Transit:

If the planetary disk intersects a star, for parts of that planet’s year it will lie between us and the star. Much like a solar eclipse from our moon blocks sunlight, only much fainter. By measuring the light from the star over a long time we can see how it varies, and if it has any periodic and regular variation. An ideal planet transition is shown in the diagram below, with one period being a year for the detected planet, the y-axis is total luminosity from the star:

  • A: Planet is obscured by star, only the star is visible. Useful as reference point.
  • B: Planet is reflecting sunlight, we could judge albedo (reflectiveness) of planet if we trust the size and orbital distance of the planet. This is useful since the albedo of different wavelengths is dependent on absorption and absorption is dependent on what elements it is composed of.
  • C: Planet is starting to block sunlight, and depending on how sudden the drop in sunlight is we can see if there is an atmosphere. Given enough spectral and spatial resolution we could determine what elements the atmosphere has, especially watervapor. With a sufficiently precise spectral resolution we can also see if it has molecular oxygen (O2) and carbon dioxide. Right now this is only viable for gas giants, not so for rocky planets.
  • D: Planet is obscuring sunlight by a very fine amount. We can also estimate how much heat the planet captures which is a strong indicator of how much atmosphere it has.

All of these combined with total rotation time and solar mass/energy gives us a equation system that can reasonably well give us size, mass, and orbit of planet. This is how we today say how large it is, how far from the star it is, and if it is in the habitable zone. We can also determine if it has an atmosphere and how much atmosphere it has when we get more detailed equipment. With even better equipment we can also detect oxygen and carbon dioxide, and therefore if it has breathable atmosphere and if carbon material for life exists. All without ever seeing the planet.

Radial Doppler:

Most of you have probably heard of the doppler effect. It is when an object moving towards you increases in frequency (gets blueshifted), and what makes things moving away from you get redshifted. It is why police sirens zooming past sound high pitched while approaching and low pitched after they passed. The same is true of light, and since almost everything distant in the universe is redshifted the universe must be expanding.

Another physical phenomena is that two objects in orbit always rotate around a common centre of gravity. It is very noticeable for binary starsystems, less so for Sun and Earth. But this means that every star with a planet orbits a commoncentre of gravity that is not quite the centre of the star. Therefore every star spins in a very small orbit, barely noticeable from just looking at it.

In the event that the planet is on a disk close to being aligned with us and its star, then we can use radial doppler to see the planet(s). This means that the star is at two points going perpendicular to us (not towards us or away from us) and at one point it is moving towards us, and at another moving away from us. This will in turn by a very small degree dopplershift the wavelength of the star. The star and its spectra are wobbling from the planets gravity on the star. This is periodic with the year of the planet and scales with the relative mass of the planet and its star. Since we can know the mass of the star we can also estimate the orbit and mass of the planet. This can unlike the transition method detect planets that are not transitioning directly over the star. Though we will not find the exact mass unless we can compensate by using other means. The European HARPS installation can determine the speed of a stars rotation within an error of 1m/s.

The biggest drawback of both systems is that both are dependent on the relative mass and size of the planet vs the star.

  • Detecting planets around red dwarves is easy, detecting planets around blue supergiants is nearly impossible.
  • Detecting small planets the size of Earth is very hard, detecting gas giants is easier.
  • Detecting planets far away from the star is very hard, detecting a gas giant in close orbit is easy.

See the chart below of EU’s database to see the mass of detected planets, in jupiter masses. And another below with both mass and distance to their star.

So what does this mean?
We have found over two thousand planets so far. Almost all of them in the past two years. Averagingat almost 3 planets per day. We will detect about 70 new exoplanets from today to release.

This has challenged previous assumptions of planet frequency. Previously, planets were thought to only orbit main sequence stars, were uncommon, and multiplanet starsystem are exotic. We have had to revise our understanding of the universe. The estimation is now that there is on average more than 1 planet per star. Meaning that theMilky Way contains something on the order of 100-400 billion planets, probably more.
The local Virgo Supercluster might contain tens of thousands of trillions of planets. (I just took a reasonable number, do not quote me.) If only one planet in a trillion 1/1000000000000 contains life, there are still over ten thousand life bearing planets in our local supercluster. And our local supercluster is one of many millions of detected superclusters.

If the chance that a planet is inhabitable is 1 in a billion, the Milky Way contains 100-400+ habitable planets, reminescent in order of magnitude to a certain spacegame…

In short, there be planets out there.

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?