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:
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.