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SYFY WIRE Bad Astronomy

My God. It’s Full of Stars!

By Phil Plait

Look. I’m an astronomer, and a science communicator. I think about this stuff a lot, and it comes up in my life all the time. I’m also an amateur astronomer myself, and spend a lot of time under the stars.

Sometimes I think about how big the galaxy is, or how many stars are in it, and my puny human brain, arrogant and hubristic, imagines it has a pretty good grip on these things.

Puny indeed. For how can any brain even pretend to be able to grapple with this?

You very much want to grab the cromulently embiggened version; I had to shrink this image down wildly to fit the blog.

That is a picture by my friend Adam Block, and shows a region of the sky in the constellation of Sagittarius. The two knots of stars you see are the globular clusters NGC 6522 (upper right) and NGC 6528 (lower left). They are veritable beehives of stars, with tens of thousands of stellar citizens buzzing around their centers.

Yet they are paled into near insignificance by the sheer, crushing multitude of stars in this shot. This part of the sky happens to be near the center of the galaxy, which is where a lot of the action is, cosmically speaking. If the galaxy were a city, Sagittarius would be downtown, so when we look in that direction we see more stars than average.

Not only that, but NGC 6522 is centered in an unusual feature called Baade’s Window, like a tunnel of clear viewing through all the dust and gas that normally obscures our view of the galactic center. Because of this we see many more stars than in parts of the sky just next to it, and in fact this part of the Milky Way can be seen by eye as a bright patch near the spout of Sagittarius’s teapot-shaped outline.

The field of view of this image is about two-thirds as wide as the full Moon on the sky. To your naked eye, any random spot of sky this size would be unlikely to have any visible stars in it at all. Yet look! I wouldn’t hesitate to guess how many stars are in this picture just by looking …

… but like I said, I’m an astronomer, and a curious sort, so I don’t have to wildly guess. I can do some math! I displayed the hi-res version of the image on my computer and randomly placed a small 25 x 25 pixel square in it, counting the stars inside it. I did this several times (to avoid errors due to random fluctuations of the number of stars in the box) and took the average. I found about 25 stars in that box on average—that means there’s about 1 star per 25 pixels in the picture. The picture is 2300 x 2340 pixels in size, which is about 5.4 million pixels total. That means there are something like 200,000 stars in this shot.

That’s a pretty rough estimate, to be sure. But I bet it’s close. And that’s not even counting the stars in the clusters!

I want to point out one more thing. In the hi-res version of the picture, look to the bottom left. In a curved pattern, just below NGC 6528, you can see a dark patch of sky. There are fewer stars there … and if you look carefully, you’ll see that the stars that are there appear to be redder. That’s because you’re seeing a splotch of galactic dust, a cloud of complex organic molecules that are opaque in visible light, blocking our view of stars behind it. That’s why there are fewer stars in it. This dust also absorbs bluer light, only letting red light through, which is why the stars there appear ruddy.

Once you’ve trained your eyes to spot it, you can see a few other such clouds in the image. But in Baade's Window they are too feeble to block the combined might of these thousands upon thousands of stars.

And remember, this is one small part of the galaxy. A tiny fraction, really, of the 200 or more billion stars that call the Milky Way home. That includes our Sun, something like halfway out from the galactic center. From our view, the galaxy stretches across the entire sky. We’re inside it, seeing both its interior and the countless galaxies that lie beyond.

It’s a big Universe, folks. Sometimes it’s nice just to acknowledge that we have the chance to think about it.

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