Star Stuff & Cecilia Payne


quote-do-not-undertake-a-scientific-career-in-quest-of-fame-or-money-there-are-easier-and-cecilia-payne-gaposchkin-91-25-91.jpg

If this quote really is from Cecilia Payne, then she had the right idea—at least for a female astronomer in the 1920s. Women in science back then faced an uphill battle to get recognized for any discoveries they made, and Payne was no different.

What’s so special about Payne, you might ask? Well, she wasn’t just one of the many “unsung heroes” of modern science. She was the one who figured out what stars are made of.

Yeah, that’s right. She sent a probe to the sun, collected a jar of star stuff, and brought it back to her laboratory…

Um, no, not really. It wasn’t that easy.

In fact, it was very difficult. She had far too many roadblocks than were fair. But she wasn’t out for money or recognition. She was just in it for the science. And science was what she got…

In Payne’s time, if you had rummaged through an astronomer’s desk and found his data on what stars are made of, you’d probably see a graph of materials similar to the Earth’s surface—carbon, silicon, iron, aluminum, and other heavy metals.

earth-s-surface.jpg

Yeah…the same kind of stuff you see in this image.

Today, we know that doesn’t make any sense. Stars are the universe’s energy generators, and they’re so good at what they do that they can support all the life and geologic processes in this image. In fact, they can support whole solar systems.

I mean, really, can you picture rocks and metals in something like this?

sun

Well…that’s early science for you. The evidence doesn’t lie, but sometimes it’s just mysterious enough that it manages to fool us. And we’re just as susceptible to it today as we were then.

Anyway, stars are nothing like Earth. They’re millions of times larger, hotter, and denser, and they’re made of hydrogen and helium, not heavy elements.

So how did the scientists of the time get fooled?

Well…the tricky thing is, stars aren’t completely devoid of the heavier elements. They have some. Just not a lot. Why? Because large amounts of heavier elements can’t survive in the kinds of temperatures we find in stars.

What kinds of temperatures? Well…the sun’s surface is about 9980.6°F, and much, much hotter inside.

In those kinds of temperatures, rocks and metals wouldn’t just melt away. Most would have their atomic structure ripped apart until they were reduced back to hydrogen and helium. Some would survive, but not much.

Well, that explains how the early astronomers thought stars had heavier elements in them. They do. But that doesn’t explain how they thought the heavier elements were the most common, and not hydrogen and helium.

The secret lies at the atomic level.

atom Bohr

Remember this guy?

It’s a nitrogen atom, one of the most abundant elements in the universe. It’s made of mostly empty space, but the important bit is the cloud of electrons whizzing around that central nucleus.

I’ve talked about these guys before. And I’ve mentioned how in stars, electrons have enough energy to jump out from the nucleus and hang out in higher energy levels. When they fall back where they belong, they release light at a specific wavelength.

Wait a second…light? At a specific wavelength? What?

I’m talking about the electromagnetic spectrum. It’s the same thing you see whenever there’s a rainbow, except that’s just the visible bit. There’s a lot more to the electromagnetic spectrum that our eyes can’t see.

electromagnetic spectrum

The secret to finding out what any object in the universe is made of is by examining a spectrum—one that looks a lot like this.

emission:absorption

As you can see in this image, different patterns of lines appear in a spectrum depending on what something is made of. Here, you see the lines for hydrogen, carbon, and oxygen. We can do the same thing for stars.

But this is the part that led those early astronomers astray. They knew that the darkness (or strength) of any one line tells you how much of that element there is. But they didn’t realize that a star’s temperature changes the strength of these lines.

See, we usually measure a star’s temperature using the Balmer thermometer. I’ve written about it before. It relies on the energy levels in an atom.

lyman balmer paschen

When an atom is heated up enough, its electrons start going crazy—and they start jumping outward from the nucleus, onto higher energy levels.

If an electron jumps from the second energy level to a higher one, it shows up on a spectrum as a line in what’s called the Balmer series.

lyman balmer paschen hydrogen

Basically, that means it’ll show up as one of the lines in the second group of lines labeled here.

The Balmer series of spectral lines is especially good if we’re looking at hydrogen. Because it only has one electron, we know that if we see a Balmer spectral line, that atom’s really hot—it shouldn’t usually have any electrons on the second energy level.

That means that unless you know what to look for, you won’t see a lot of hydrogen in a star.

Let me explain how we know this.

Here’s a hydrogen atom:

hydrogenatom

A hydrogen atom in its normal state, found on Earth and not in a star, will always have its electron in the first energy level. That’s just the way atoms work.

CHON

But slightly heavier atoms are a bit different. Their electrons can’t all fit on the first energy level. There will always be a few electrons on the second energy level.

If these atoms are heated up to the temperatures found in stars, they will easily produce Balmer lines—because there are at least four electrons all set and ready to jump from the second energy level to a higher one.

Hydrogen, on the other hand, is a different story.

In order to produce Balmer lines, its single electron has to get to the second energy level in the first place before it can jump to a higher one and fall back.

Astronomers in the 1920s didn’t realize this.

They knew how to work with spectra and use it to figure out what the object they were looking at was made of. They didn’t realize that the very nature of stars made it hard to see lighter elements like hydrogen…even though those elements were the most common.

Cecilia Payne realized this, and she made it the topic of her thesis. But as a woman in science, she faced an uphill battle getting any astronomers to believe her. It wasn’t until years later in 1956 that she was recognized for her research.

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