In yesterday’s issue (BAN #1094) I wrote about the weird and surprising properties of a geometric solid called Gabriel’s Horn. Specifically, I wrote about how it has infinite surface area while only a finite volume. 

The basic stuff I said is correct, but I got the math wrong: to get pi cubic units to fill the horn, you need to start the solid at x = 1! If you let x approach 0 (so the bell spreads out), the volume does indeed go to infinity, so I was wrong there too. Still, if you start at x = 1 and let it go to infinity, the volume converges while the surface area grows without bound (for the math nerds: it increases as ln(x), which always increases, though slowly), which still means the volume is finite while the surface area is infinite.

My apologies for getting this wrong, and my thanks to BAN reader Nick Fontana for pointing this out to me.

Hey, look, if you know me, you know that I will use the thinnest of excuses to post an image of a globular cluster, because I love them so.

Yeah, well, here you go:

That is NGC 6723, a fantabulous cluster of hundreds of thousands of stars located near the center of our galaxy but just on the other side of it from us, about 27,000 light-years away. Many such globulars are in this part of the sky because they orbit our galaxy at great distances, and when you look toward the center you’re bound to see more of them. About 160 such globulars are known to orbit the galaxy, though there are likely many more that remain undiscovered. 

NGC 6723 is in many ways typical of the breed. It was once thought that the stars in these clusters were born all at once, which made them fantastic laboratories for studying stellar populations. Since they’re all at essentially the same distance from us we can directly compare stars’ brightnesses to see how much energy they give off, and if they’re all the same age that means the models we use to understand them are simpler.

But the universe is rarely so giving. It took a long time, but it slowly became clear that the stars in globulars are not all the same age. Star formation in many globulars happened in waves, with many stars born early on, and then a pile of them formed later, sometimes by hundreds of millions or billions of years. That’s the case with NGC 6723. How this happens, exactly, isn’t well understood.

Confusing things further, some globulars aren’t globulars at all! They’re actually the cores of fully fledged but tiny galaxies that were stripped of most of their stars and gas by multiple close encounters with our much larger Milky Way. The remaining stars form a tight, compact ball that looks just like a globular, but formed and evolved in a very different way. So what was once thought to be a simple structure is in fact a mess.

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