Henrietta Leavitt and the end of the universe

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It's January, it's a new year, we've all made our new year's resolutions.

So we wanted to take a step back and look ahead.

A pretty big step back

and an even bigger look ahead.

Past this year, actually, past the next few thousand years.

We wanted to look ahead as far as we possibly can.

So, we're going to share one of our favorite episodes with you this week.

It starts in a park in Brooklyn and it ends at the end of the universe.

Oh, a bicrophone.

Someone should make a bicrophone.

Bird.

Hello, I'm coming from the lake.

Oh, wait.

Hey, look to your left.

I see you.

Alright.

I'm going to turn on my recorder.

Give me a second.

Okay.

So.

No, I'm.

Here we are.

Prospect Park.

New York City, clear Tuesday night.

And I brought you here because I want you to try a little thought experiment with me.

Okay.

with the approximately four stars, five stars.

Oh, that's a plane.

The approximately four stars.

We can see the sky.

That is a plane.

I would like you to imagine that you are going to make a map of the cosmos.

Okay.

So, that star there, the one that looks really bright, that star next to it, that star over there.

How would you figure out how far away from the Earth or how close to the Earth those stars are?

I'm sure I'm going to get this wrong, but I assume

something about like how big it is, how bright it is.

Wrong answer.

Okay.

So imagine the stars are actually light bulbs.

Maybe that star, the like bright one right there between the trees, maybe it's a 100 watt light bulb, right?

And it's super bright.

Or maybe it's, you know, a 30-watt light bulb.

It's actually quite dim, but it's just really close.

Right.

Or like one of the dim stars over there could actually be super, super bright, like 100 watts, but just really far.

Exactly.

Yeah.

Yeah, that's

conundrum.

Right.

So this is the exact problem that astronomers were facing at the end of the 1800s, right?

They were looking at these stars in the sky.

All they had was the brightness to go on, and they had no way of figuring out what is the map of the cosmos.

And I I assume they eventually figured it out.

Yeah.

So this episode is going to be not just how they figured it out.

Astronomers did develop a yardstick to measure the distance to the stars, but also what happened once they did.

Because

the idea of mapping the cosmos turned into kind of reinventing our entire understanding of the cosmos.

Hmm.

Do you feel set up?

I mean, I feel, I'm intrigued.

I want to hear the rest.

Okay.

That's it.

That's it for the intro.

Oh, you want more intro?

All right.

I'm Noam Hasenfeld.

I'm Bird Pinkerton.

And this week on Unexplainable, the yardstick that reinvented the universe.

The yardstick for measuring the universe came out of a really basic project.

In the 1890s, Harvard University was trying to build a comprehensive database of the southern sky, so everything they knew about every star.

Down at an observatory in Peru, they

were doing a survey trying to photograph all different parts of the nighttime sky in the southern hemisphere.

George Johnson wrote a book about this project.

And they would do this, you know, several months apart so they could see how stars changed over time.

They'd collect all these photographs and then they'd bring them home.

They would pack up these glass photographic plates and then take them down on muleback down the mountain to the harbor and then ship these things by boat to Boston.

Or they would unload them and bring them into Harvard Observatory and put them on a little dumb waiter kind of elevator and bring them up to the level with the computers.

Back in the 1890s, computers meant people, usually women who were doing the hard work and the number crunching that male scientists didn't necessarily want to do.

In this case, they were collecting data about stars.

And they all have these light tables and they would be scrutinizing these glass photographic plates lit from behind and just this meticulous observation.

A lot of the plates were negatives.

White sky with little black dots.

And these computers took measurements of each dot, they compared them to other dots, and then they wrote up these thousands and thousands of dots in neat ledgers.

You know, day after day for all of these stars.

Including some particularly tricky stars.

One thing that astronomers had noticed is, you know, they look up at the sky and they notice that some of these things aren't always the same brightness.

They go brighter, dimmer, brighter, dimmer.

They pulsate, you know, like beacons.

Astronomers call these stars Cepheid variables because they were variable, they changed.

And the first one was seen in the Cepheus constellation.

These Harvard scientists, they didn't know why the Cepheid variable stars were pulsing, but they knew that they were finding them all over the night sky.

So they figured, why not kind of have some projects to track them specifically?

They just said, here are some plates that have just come up from Peru, and we'd like to know what the...

The rhythm was by which they go from bright to dim to bright to dim.

So you can imagine how tedious this would be, like lugging around these heavy glass plates and trying not to break them and

putting them on these viewing frames and measuring these things, jotting them down with ink and a notebook and trying to figure out what the rhythm was of these variable stars.

Back then, being a computer paid better than working in the cotton mill, for example.

And it was actually a pretty good job if you were a woman who was interested in science.

But the women who were making these calculations were definitely not expected to have insights about the stars that they were cataloging.

Like, you don't expect your mechanical calculator to suddenly look up and say that it's noticed a pattern in the math that it's doing.

But these computers were human beings, so that's exactly what happened with one woman in particular.

Henrietta Levitt.

Henrietta Levitt.

Looking very Victorian, you know, with this high collar and her hair up in a bun.

She spent years tracking and measuring Cepheid variable stars.

And then in 1908, she pulled together all her measurements into a paper for Harvard Observatory's publication.

21 pages, 15 pages of tables.

So again, you just imagine all of the work that this took.

Henrietta had focused on one cluster of stars that were all together in the sky, kind of like a

bunch of cosmic grapes.

And this paper was a detailed list of 1,777 of the Cepheid variable stars in that cluster.

And almost as an afterthought, it says, it is worthy of notice that the brighter variables have the longer periods.

It is worthy of notice that the brighter variables have the longer periods.

This was Henrietta's big insight, and it was the first step to solving the problem that I mentioned at the beginning of the episode, the one that astronomers were grappling with as they stared at the night sky.

Because remember, they were looking at the stars and they were wondering, is that really a dim star?

Is that a 30-watt light bulb?

Or is it just far away?

Like, how do we tell?

But because Henrietta looked at Cepheid variable stars in a single star cluster, she could kind of compare them against each other.

So she was writing down, All right, this Cepheid variable star overall, on average, it's brighter than nearby stars and it's pulsing slowly.

Bright to dim, bright to dim.

This one over here, it's dimmer overall, and it's pulsing very fast.

Bright, dim, bright, dim, bright.

This bright one

is going very slow.

Bright to dim.

And eventually, Henrietta saw a pattern.

She noticed that there was a relationship between how fast the star pulsed from bright to dim and what its brightness was.

The slower a Cepheid variable star pulsed, the brighter it was.

This was a huge realization.

This was the first time that anyone could tell whether a particular star was actually bright or just looked bright because it was close to us.

And conveniently, you can find Cepheid variable stars all over the observable universe.

So astronomers could use them to tell how far away from Earth lots of star clusters are.

They could look at any given Cepheid variable and say, okay, this star might look dim to me, but because it's pulsing really slowly, it must actually be bright but far away.

So everything in that star cluster must also be far away.

Henrietta's one kind of tossed off sentence gave astronomers a way to say, this star is closer to us than that one, but it's further away than that one over there.

And that's really still still the basis of the whole cosmological yardstick, as they sometimes call it, that we use today, or these measurements that Henrietta Lovitt made.

This one idea from Henrietta's paper, it eventually cracked astronomy wide open.

Henrietta herself got sick and had to leave Harvard for a long stretch, but her male boss wound up publishing sort of a follow-up paper using her work.

And after that, other scientists, like Edwin Hubble, for example, they took her scale and they gave it concrete numbers.

So you could put a firm distance on the stars.

But really, it's her discovery that sits at the basis of all of the distance scales we use to reach further and further out into the universe.

And this ability to measure the universe completely transformed how we understood it.

We didn't just learn how big it was, but also that it was growing and changing.

So this yardstick, it helped rewrite textbooks.

And as early as the 1920s, a mathematician tried to nominate Henrietta for the Nobel Prize for her contributions.

At that point, she'd already died of cancer, so she didn't get that recognition then.

But even now, she's kind of a footnote.

For other influential scientists, we have papers and journals and their diaries.

Very, very little of that existed for Henrietta Levitt.

I did find this one letter where she refers to embarking on a ship from

America to Europe.

And,

you know, and I remember reading that and thinking, God, I wonder if she was with someone.

You know, I hope so.

But,

you know, we just don't know.

We do know some small details.

So we know that Henrietta went deaf at one point, and we know that she got really sick several times in her life.

We also know from her ledger books that she had very neat handwriting.

But unfortunately, we don't really know all that much more than that.

Henrietta discovered a way for astronomers to unlock countless mysteries of the universe, but at the end of the day, she herself remains kind of a mystery.

After the break, we're going to get into all the cosmic mysteries that Henrietta's yardstick helped unlock.

Because ultimately, she didn't just give astronomers a tool to measure the distance to stars, she helped scientists completely rethink the universe.

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You can keep it classic and cozy this fall with long-lasting staples from Quince.

You can go to quince.com slash unexplainable for free shipping on your order and 365-day returns.

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You are about to take a journey out of this world into the world of the future.

Forget the world around you.

Forget the people around you.

You are entering Unexplainable, alone with your own thoughts.

Unexplainable, we're back here with senior reporter Brian Resnick.

Hey.

So Bird was talking about Henrietta Levitt and how she kind of came up with this yardstick to measure the universe.

And Brian, you were actually working on this story with Bird from the start.

Where did this yardstick end up taking science?

Like,

I mean, basically, what happened?

What happened next?

Yeah, like a lot happened next.

Okay.

Scientists kept improving on this yardstick.

And, you know, once you have a yardstick, you use it to measure stuff.

Right.

So every time scientists took measurements with this yardstick, it was like the whole universe fundamentally changed.

It was not until we started to be able to measure distances in the cosmos that we knew that other galaxies existed.

I talked to Katie Mack about all this.

She's an astrophysicist.

She's written a great book that gets into a lot of this, like the power of measuring things in space.

And she's saying basically that before all of this, before this kind of massive measurement, scientists didn't know there were other galaxies.

Yeah, so there were

these observations of what were called spiral nebulae or island universes, these little smudges in the sky.

And those were for a long time thought to be perhaps clusters of stars within our own galaxy, or nebulae, or some kind of object nearby.

Do you know Edwin Hubble?

Sure do.

Hubble Telescope.

In the 1920s, he was an astronomer at the Mount Wilson Observatory in California, and he was looking at this smudgy area of the sky called Andromeda.

And Hubble is studying it, and he's looking out for one of these pulsating stars, one of the Cepheids.

And because of Henrietta, he knows if he sees a Cepheid, he can see how far away Andromeda is.

And he sees a Cepheid.

He sees one of these variable stars, puts in the calculations and XTRI, X-Tree, read all about it.

Realizes Andromeda is so far away, it just must be another galaxy.

Another universe seen by astronomer this was big like front page headline grabbing news dr hubble describes massive celestial bodies 700 000 light years away so the idea that we can use these variable stars to measure distances to nearby galaxies like the andromeda galaxy that lets you know that these galaxies are actually outside of our own, that they're actually far away.

This was astounding.

Yeah, that old-timey newsboy was saying that this was like seeing another universe.

Right.

That newsboy and the New York Times headline he was reading wasn't right.

Okay.

It was just another galaxy, but that just gives you a sense of just how much bigger this whole discovery made the cosmos feel.

Okay, so the first big thing that Henrietta's yardstick did was it allowed scientists to make the known universe way bigger.

What's the next thing?

So Hubble, once he starts, you know, he has this one measurement of this galaxy, he doesn't stop.

He measures more galaxies.

And then he realizes something even more astounding.

What Hubble discovered by looking at a lot of distant galaxies is that other galaxies are moving away from us.

This was actually really huge.

Up until this point, a lot of scientists had this pretty clear consensus about the universe.

There was the idea that the cosmos is static, that it's an eternal, unchanging thing.

But these new measurements,

they showed something completely different.

The universe is expanding.

There's just more space all the time.

There's more space all the time?

Yeah, this is just wild.

Imagine you have an unfilled balloon out in front of you and you draw dots along the surface of it.

When you blow the balloon up, you would see that the number of dots remain the same, but the space between them keeps expanding.

So like the galaxies aren't growing, but the space between them is growing all the time.

That was a big deal to find out that the universe changes, that it's not just stuck there and everything's in its proper place forever.

So

just by measuring the universe, we realized that we totally misunderstood everything about it.

Yeah.

And then when you see that the universe is in motion, that just creates so many other questions.

So if it's expanding now, does that mean the universe used to be different in the past?

And particularly when we start rewinding this picture, this brings us to thinking about, wait, how did the universe begin?

So is this how scientists got the idea for the Big Bang?

I think the very simplified story about the science of how we got to the Big Bang is like, we see the universe is expanding, expanding.

We just imagine it going in reverse.

Okay, so let me just make sure I have this straight.

We had Henrietta Levitt, who just by being part of this large catalog project of the brightness of stars, she sort of invented this yardstick to measure the universe.

Then Hubble ended up using that yardstick to figure out that our galaxy, the Milky Way, is not the entire universe, that there are other galaxies out there.

And then Hubble again figured out that the universe was actually expanding, which showed us that the universe wasn't static, that it actually had a beginning.

Yes.

And, you know, if there's a beginning,

there might also be an end.

Okay.

So this also comes from the yardstick measurements.

And it brings us to really the biggest mystery in all the universe.

So we get here in the 1990s.

Today's issue of the journal Science reports new information about the evolution of the universe.

So

what these groups were doing is they were looking at the rate of expansion of the universe in order to find out how quickly the expansion is slowing down.

Our expectation was that the universe would continue to slow down after expanding.

The reason they thought it was slowing down is because

there's all this stuff in the universe.

You see, there's gravity in the universe.

Everything is pulling on everything else, all the stuff in there, the planets, the stars, and the galaxies.

And the gravity should be kind of trying to pull things back together and sort of putting on the brakes.

What we've actually found is a very strange result.

What they found was that the expansion was not slowing down.

It was speeding up.

That's about as weird as if you take a ball and throw it up into the air and it slows down for a little while and then just shoots off into space.

You know, like there's,

you really don't expect that.

And these scientists really did not expect to see the expansion of the universe speeding up.

Yeah, so why is this happening?

How could the universe be expanding faster, especially when we'd expect gravity to be slowing it down?

We don't know.

Properly, whatever is causing the expansion, we call it dark energy.

We don't know what dark energy is.

And this is separate from dark matter?

Yeah, dark is kind of physicists talk for mysterious or, you know, we don't know.

So dark matter, scientists are pretty sure it's matter.

Dark energy is even

more of a blank space.

Scientists have no idea what it is.

I mean, we have ideas.

We have lots of ideas.

We don't know if any of them are right.

But what we do know is that dark energy is kind of bad news.

If something unexpected and unknown does not get us first, dark energy is what will end the universe.

So, okay, hold on.

You're saying that we don't know anything about this thing.

Yeah.

And it seems like the only thing we know about it is that it's going to end everything.

Pretty much.

You got it.

Okay.

But the better question here is, how?

How will it end everything?

And that leads us to some really fun speculation.

Oh, fun.

Katie says there are two main possibilities.

Okay.

Option one for the end of time is the heat death of the universe.

The universe will continue expanding, the expansion will continue accelerating, and the universe will get emptier and emptier, and colder, and darker, and more diffuse.

And so, you get to a point where only in about 100 billion years or so, distant galaxies are invisible.

They're so far away, they're moving so quickly that light can't catch up and we cannot see them.

And so, if you had the Hubble Space Telescope in 100 billion years, looking out at the night sky, you would not see those beautiful spiral galaxies.

This is basically saying that the universe is just getting bigger and bigger.

Why does that mean the universe would end?

Because when things get so far away from each other, they can't interact with each other anymore.

And everything on its own, every individual object in the universe, every galaxy, everything falls in part in time.

This is a key idea in physics and in our understanding of everything.

It's entropy.

It's

the progression from order to disorder.

You know, you have these nice structures, stars and galaxies and planets, and those are going to decay.

And more of that energy is going to be radiated out as this kind of waste heat as things are falling apart.

And eventually you get to a point where everything's basically done.

Everything is so far apart that there's no like...

Nothing can jumpstart anything.

Nothing's interacting.

All that's left is this just waste heat.

We call that the heat death, the ultimate heat death of the universe.

It's a state where the universe has a tiny amount of radiation in it that is the waste heat of everything and

that's it.

Not great?

Yeah.

Well...

It could be worse.

How could it be worse than nothing happening forever?

This is where we get to the second scenario.

The big big rip.

The big rip?

This is like what Hollywood would choose.

Okay.

So in the previous scenario with heat death, galaxies and planet and things that are like, that they're held together fine with gravity, they stay together.

But in this scenario,

everything starts to rip apart.

So it would first start to pull apart clusters of galaxies.

So galaxies that are orbiting each other in a clump, they would start to wander away from each other.

And then it would start to pull apart galaxies.

So the stars at the edge of the Milky Way would start to kind of wander off.

And then the planets would start to wander off of our solar system.

And then it accelerates from there.

So then you start to get to a point where there's dark energy inside the Earth that's kind of pulling the Earth apart and it explodes the planet.

And then it gets down to molecules and atoms and nuclei and eventually just tears apart space

Okay.

So we got two scenarios.

I mean, they're equally terrible.

Which

one is more likely to happen?

Like, how do we know if we're going to get this increasing isolation or everything will be ripped apart?

We don't know.

Okay.

Great.

Katie told me, though, there is a lot more consensus around the heat death scenario, that it's just more likely based based on the theoretical models that exist.

But really, all of this depends on what dark energy actually is.

And right now, dark energy is just such a mystery.

We could just be fundamentally wrong about so much here.

So, the real way to get closer to an answer is just to keep measuring.

If we really understand the expansion rate of the universe, if we understand the expansion history of the universe very, very well, which is based partly on all these measurements of the distance and so on then

we should be able to compare different ideas about dark energy and say which ones are more likely

what this all drives home for me is just the the power of measurement i mean just by measuring a thing you see this thing in a new light you even end up changing the actual thing, like changing how you understand the thing.

Like even the tiniest calculations, Henrietta sitting in a room measuring the brightness of stars, those calculations, they gave us the cosmic yardstick.

And then that yardstick showed us just how big and weird and constantly surprising the universe is.

Like this all comes back to measurement.

Yeah.

All this, this incredible story of scientific progress, it just starts with the simplest question.

How far away is that?

How bright is it?

I love how simple questions can lead you sometimes to extraordinary places.

It could lead you to the beginning of the universe.

And it can even lead you to the end of time.

This episode was reported and produced by Brian Resnick and me, Bird Pinkerton.

We had edits from Meredith Hodnott and Noam Hasenfeld, who also wrote the music, mixing and sound design by Christian Ayala, and fact-checking by Manding Nguyen.

If you want to read more about The End of Everything, Katie Mack has an excellent book called The End of Everything, astrophysically speaking.

And if you want to read more about Henrietta Lovitt, I cannot recommend enough George Johnson's book, Miss Levitt Stars.

If you have thoughts about this episode or ideas for the show, please email us.

We are at unexplainable at vox.com.

We'd also love it if you left us a review or a rating.

Unexplainable is part of the Vox Media Podcast Network, and we'll be back next week.