Short Wave - The Stars That Settled The Great Debate

Episode Date: September 1, 2022

It may seem obvious now that other galaxies lie beyond the Milky Way, but less than 100 years ago, some astronomers held a view of our universe that was a little more ... self-centered. In the 1920s, astronomers were locked in the "Great Debate" — whether Earth was center of the universe and if the universe was just the Milky Way. Today, Scientist in Residence Regina G. Barber talks to Dr. Vicky Scowcroft about the stars that ended astronomy's Great Debate. Follow Short Wave on Twitter for more on everything science. See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

You're listening to Shortwave from NPR. When I look into the night sky, I'm in awe of the stars I see lighting up our little part of the universe. But every time, I hope I'm lucky enough to catch some of them twinkling. Which in itself is not rare because starlight gets jumbled in our atmosphere. But that's not the twinkling I'm searching for. What I want is the rare twinkling that comes from the star itself. I want to see the periodic brightening and dimming of the elusive variable star. And in 1920, these unique stars became a distance measuring tool used to argue the great debate between two astronomers.

Harlow Shapley and Herbert D. Curtis. They debated our solar system's place in the cosmos and if our larger Milky Way galaxy was the entire universe. An astronomer Dr. Vicki Skowcroft says, that's what Harlow-Shapley thinks. thought. So Shappley did think that we were it, but he thought it was bigger than we could see, but still not huge. Whereas Curtis thought that our own galaxy was a little bit smaller, but that there were also other galaxies. Those potential other galaxies they were debating were these fuzzy blobs or nebulae, like the Magellanic clouds. No one knew how far away they were or what they were, but Curtis's camp thought that some of these blobs, these other galaxies,

were other universes. So, like, how big did astronomers think the universe was in 1920? Their scale was really, really quite small, as we understand now. They thought we were right in the middle of it. Today on the show, The Great Debate, when humans learned a lesson in being cosmically self-centered and how by the power of starlight, we enlarged our universe and found our place in the Milky Way.

I'm Regina Barber. You're listening to Shortwave, the Daily Science Podcast, Before we get into who won the debate and what that tells us about the cosmos, let's talk about the variable stars a little more. They're Vicky's specialty, and she says they can help us look backwards and forwards into the life of our universe. Most things in the universe are basically pretty boring. They just sit in space doing nothing.

But variable stars are really cool, so they change in brightness over time. We know about variable stars because of John Goodrick. He's the guy who realized what they were back in the late 1700s. He was, like, observing one in his garden every day for months and months, and he could see that it got brighter and fainter, like, every five days. And the problem, though, was that he did this every night in Yorkshire in the north of England. And then he caught pneumonia and he died from astronomy. So, like, variable stars literally killed him. I shouldn't laugh at that, I'm sorry.

So we can laugh about it now because it's been. long enough. It's why I moved from Yorkshire because I don't want to die from stars. You didn't want the same fate. Yeah, it was too much. Vicky says that this blinking, if you will, is because the star itself is changing.

At this point, we know stars are just big balls of gas. If you heat up a gas, it gets bigger and cool things, they get smaller. So as the star fluctuates in temperature, the star... It's pulsating. So you can time it really well. And it gets bigger and smaller, hotter and colder, and colder, and it gets brighter and fainter. At the turn of the 20th century, these changing stars attracted the attention of a group of female astronomers called the Harvard Computers. They were skilled at processing all the data coming from the observatory, categorizing thousands of stars.

Among them was Henrietta Swan Levitt. And she was looking at a galaxy watch. She didn't know at the time that it was a galaxy. It was called the Small Magellanic Cloud. and she found a sample of these variable stars. Back in the day, the tools available to astronomers were photos. And not today's photos either. To get images of these stars she could study,

Henrietta had to point a magnifying lens down on a glass plate coated with a photographic emulsion. Then they used to use these giant glass plates that they stick on the end of a telescope, and they'd have lots of those. And they had this machine called a blink comparator. So they put one photograph on one side and one on the other. and then flick between the two to look for things that were changing.

So people were doing this by eye. And these variable stars, they don't all dim and brighten at the same interval. So the one that John Goodrick was killed by, that took about five days. But some of them are as short as one day, some of them are as long as 100 days. So they were doing like really painstaking work to try and measure. of these for a really long time. And Henrietta realized that a star's brightness is directly related to how long it takes for the star to dim than brighten again.

The longer that takes, the brighter the star. And once we know the average brightness of a variable star, we can do some math to get its distance. Because things look fainter as you get further away from them, knowing how bright something really is and how bright we see it as, we can tell how far away it is. Think of a light bulb. The further you are from a light bulb, the dimmer it looks. That dimming is a standard equation.

And so from that information, we can calculate how far it is to the light bulb. It's the same with stars. So these stars are what we call standard candles. And they work kind of like how lighthouses used to work. They had special candles called standard candles that emit a certain amount of light. Like they knew how much light that candle would make. So people sailing could tell how far away the lighthouse was because they knew how bright the light was,

they knew how bright it looked at them, so they could work out how far away they were. Okay, so now we have this method of measuring stars that are further away than anything astronomers have measured before. Thank you, Henrietta. Especially because this measurement of variable stars is what led us end the great debate. Remember those two astronomers, Harlow Shapley and Herbert D. Curtis?

they were arguing more or less about our galaxy and if it was the entire universe. Shapley thought that the universe was just what we could see. It was bigger than other people did, but we were it. Both of these dudes were taking measurements of these fuzzy galaxies and trying to figure out where in the universe they were, in our galaxy or beyond. When Harlow measured, he got distances so huge he barely believed them. His math pointed to two things. One, our galaxy must be bigger than we realized if it's how large.

these distant blobs. And two, Shapley didn't believe that we were at the center of the universe. He thought that the sun was not right at the center of the galaxy and hence the universe. To which Curtis said. Actually, no, we can't be it. They're probably other galaxies. And so their debate, part of it was,

are we the Iranian galaxy or are there other galaxies? And about the whole center of the universe thing? Curtis did think we were right at the center. See, Curtis was doing his own. own calculations too. And his distances to these little fuzzy blobs were much smaller, so much smaller that he was okay with other galaxies existing. They could be collections of billions of stars just like the Milky Way. So who won? Who got the bragging rights in the end? It's actually a bit complicated. Shappley was right. We are not at the center of the universe or

the galaxy. We're like 25,000 light years from the center. But for the other part, about whether there is only one galaxy in the universe, Curtis won. So Curtis believed that these nebulae were other galaxies. Neither was fully right. And the guy who was able to shut it all down after four years, with even better science, Edwin Hubble. Edwin Hubble was using the variable stars that Hemmiel Levitt had discovered. Edwin had better data.

More accurate distances to those various nebulae, you know the fuzzies from earlier. like the Andromeda Galaxy and the Magellanic clouds. And he got even larger distances than Chappley had. So rather than just being something nearby and small, they were actually a really, really big thing, really far away. They must be something separate. So that confirmed what Curtis thought

that we were not the only galaxy in the universe. The universe was actually made up of lots of galaxies, some nearby, some far away. Astronomy moved quickly after Hubble published his findings, and we started to realize things about our universe that we didn't think possible. I find it quite strange to even contemplate now thinking about this. It seems like one of those things that we must have known forever, but it's less than 100 years since we discovered that there were other things in the universe

other than us, basically. This episode was produced by Rebecca Ramirez, Chloe Weiner, and Rachel Carlson, who also checked the facts. also edited the piece. The audio engineer for this episode was Robert Rodriguez. Our newsroom higher-ups are Terran Samuel, who's our vice president and executive editor. Edith Chapin, who's our vice president and executive editor at large, and senior vice president Nancy Barnes. And before we head out, a quick shout out to our Shortwave Plus listeners. We appreciate you and thank you for being a subscriber.

Shortwave Plus helps support our show. And if you're a regular listener, we'd love for you to join so you can enjoy the show without sponsor interruptions. Find out more at Plus. npr.org slash shortwave. I'm Regina Barber. Thanks for listening to Shortwave, the Daily Science Podcast from NPR.