The following was first published on the Stellaris forums. Since I missed a day posting over the weekend, I’ll post two episodes today to make up for it.
Welcome once again to another episode of what’s been dubbed the Astroknowledge series.
Today we’re going to be talking about the galactic core. What we know about galactic cores comes from two sources: things we can see about our own core despite it being too close and other things being in the way, and what we know about other galactic cores despite being too far away from them.
If we started travelling towards the core in the direction of Sagittarius, what would we encounter? The answer is “nothing, probably.” There are millions of stars out here on the spiral arms but even so they’re spread so thinly that the chances of us encountering any are small. It’s eight thousand parsecs to the core but the chances are that we’ll pass most of it safely in the black.
(Image courtesy NASA.)
The first sign of change will be when we get within three thousand parsecs of the core. Here we’ll start to see two things: firstly the sky stops being dark, and secondly the stars are dimmer to human vision. We’re now within the galactic bulge, and unlike the flat disc shape of the outer spiral arms this is much thicker. In some galaxies the bulge is almost spherical, and in others it has its own sophisticated spiral form. Since the galaxy is “thicker” here, every direction we look will have more stars. If you’ve ever looked up at night and seen the vivid bright slash of stars that’s the plane of the Milky Way galaxy, imagine that but all over the sky.
A star made of fresh hydrogen is very bright but contains no metals. When it dies, its contents (still mostly hydrogen) will spread out and will in time form new stars. Those stars will contain a lower percentage of hydrogen, and as a result more of their light will be put out as infrared rather than as visible light. (If you remember our discussion on Gliese 667Cc, some stars are very noticeable in this regard.) We call this “metallicity.” The stars within the bulge are more metallic: they’re made of the corpses of their ancestors.
Travelling closer to the core, we notice that the stars get steadily more metallic. They also get steadily larger, younger, hotter and bluer. The number of bright O- and B-class stars in the bulge is higher than out in the spiral arms. These stars live extremely short lives, less than 10 million years, too short to form planets. When they reach the end of their life they swell up into giants; some become supernovae and then neutron stars or black holes, and others dwindle into tiny white dwarfs. Either way, when they die they blast huge amounts of gas out into space which then forms the substance for new stars.
Space is now less empty: the stars are denser and they’re filling the gaps between themselves with gas. We are now in the realm of the giants, living their hot brief lives in a frenzy of activity and blasting terrifying radiation at one another when they die.
Most of the stars within the realm of the giants are main sequence M-class red stars: although they are born less frequently here they live thousands of times longer, and so end up being the majority of the population. Still, there are enough O- and B-class stars around to terrify anyone.
Within 120 parsecs of the core, the gas gets so dense that our normal rules about space being empty don’t apply. There are now an average of several thousand molecules per cubic centimeter. This is still a tiny amount when compared to our atmosphere (which has an average of 10^19 molecules per cubic centimeter) but it’s enough that space can no longer be thought of as empty. This is the point past which no spacecraft could possibly go, not least because this gas is hot, agitated and baked by radiation from the stars.
(Image courtesy Hubble.)
We are now in the great molecular gas cloud, also called Sagittarius B2. This is the combined nursery and graveyard of a vast number of stars. This is also the point at which we can’t help noticing something which has been here all along but has been too subtle to detect: we’re orbitting the centre.
Suppose we had a ship which could penetrate Sagittarius B2. What would we find?