I read popular physics: The Milky Way

It’s time for another entry in my series where I read through physics articles in Scientific American, now through the eyes of a former physicist. It’s a gratifying exercise because I used to struggle through these when I was in high school. Now, if I struggle, I know it’s not my fault!

Today’s article is “New View of the Milky Way” by Mark J. Reid and Xing-Wu Zheng in the April 2020 issue. Oddly the titles in the print version never seem to match the titles in the online version–also the online version is paywalled this time, that means no images, sorry folks!

The main thrust of the article is that they used radio astronomy to map out the Milky Way galaxy. Now you might think that since we’re inside the Milky Way, we have an especially clear idea of its shape. But it’s surprisingly difficult, because determining the distance of stars is hard, and there are dust clouds in the way. If you’ve ever seen an image of the Milky Way as viewed from the outside, those are all artists’ conceptions and we don’t know what it actually looks like.

So the first thing I see in this article, is an image of the Milky Way, as viewed from outside. At first I thought, this is really neat, finally an image that’s more than just an artist’s conception. But nope, it’s just another artist’s conception. Created to be consistent with the study’s results, but still just art.

So what did this study actually measure? They looked at 200 young stars in the Milky Way, and measured their distance from us. Then they used these to map out the Milky Way. There’s an image plotting the location of each star, superimposed on colored lines representing the locations of each of the spiral arms.

From what I can tell, those colored lines are just drawn in as a “guide to the eye”, and I gotta say, “guides to the eye” make me suspicious. If those lines weren’t drawn in, would I be able to see the spiral arms at all, or would it just look like a bunch of randomly scattered dots? The dots only cover like a quarter of the galaxy too. I trust the astronomers in this case, but this is the skeptical approach to it.

As for how they mapped the locations of these stars, they used the same technique that was used to produce an image of a black hole back in 2019. How convenient that I’ve already written an explanation.  They used radar all across the earth to make a single giant telescope with an extreme zoom capability. The black hole study had a resolution of 0.00004 arc-seconds, while this study had a resolution of 0.001 arc-seconds, but it’s still very impressive.

Their goal isn’t to produce ultra-high resolution images of stars, it’s to measure parallax. Parallax is the effect you see when you’re riding a train and looking out the window. The shrubs in the foreground zip past you, while the shrubs in the background appear to move at a much slower rate. Likewise, when the Earth goes around in orbit, the nearby stars will move a lot, while the faraway stars will move very little. So by measuring the location of stars in the sky with extremely high resolution, they’re able to measure how far away the stars are.

I thought to include a video of parallax, and this music video is the first thing that came to mind, so uh enjoy the music.  Or mute it if you don’t like it I guess.  Contains flashing imagery.

You might be familiar with “parsecs” as the thing that Han Solo was able to make the Kessel Run in less than twelve of. Well “parsec” is short for “parallax second”, and that’s “second” as in “arc-second” (one sixtieth of a degree). If a star appears to move by one arc-second over the course of six months, then the distance of that star is one parsec (or 3.26 lightyears). If a star appears to move by one twelfth of an arc-second over the course of six months, then the distance of that star is about 12 parsecs.

So if the resolution of their telescope is 0.001 arc-seconds, then they can reliably measure stars up to 1000 parsecs away. In truth they can go much farther than that (maybe 10,000 parsecs away), although eventually the error bars get large. We’re about 8000 parsecs from the center of the galaxy, so they have enough precision to reach the center but not enough to map out the galaxy on the other side.


I’ve decided to do the thing that Marcus Ranum does, and insert an image between the main article and postscript. Weee.

This article was pretty easy for me to understand, although there was one thing that threw me. It was explaining why these young stars are particularly good objects of study. Basically, they’re very bright in radio waves, ’nuff said. But the more detailed explanation is that these stars are surrounded by “maser” gas clouds. A “maser” is a laser for low-frequency light like microwaves or radio waves. I get what it is, but I’m just thinking… how??  I briefly looked into it, and the answer is that it works the same way as any other laser (there’s a discrete transition and population inversion), but somehow that explanation is unsatisfying to me.

Anyway, it’s May already, or at least I’ve already gotten the May issue, so best get on it.


  1. publicola says

    Yeah, I sometimes had to slog through articles in S.A., especially if they had a lot of math in them. But I always enjoyed every issue. Sadly, I can’t afford a hundred bucks for a year’s subscription, so I appreciate your post. Thanks.

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