wavelength

What is the Hubble Law?

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Last week, I teased you with the idea that it’s actually easy to estimate distances to galaxies.

I do mean estimate–and distance indicators are still important.

The Hubble Law is named for Edwin Hubble, the astronomer who was first able to settle the debate over what galaxies were–using the new Hale Telescope, the largest in the world at the time. But the Hubble Law is undoubtedly what he’s most famous for.

In order to understand the Hubble Law, though, we first need a little review of the Doppler effect…

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How Were Atoms Discovered?

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Welcome to my fourth “Science Answers” post! If you have a question, you can ask it in the comments here, or ask it in an email. Or find me on Facebook!

Q: (1) How did scientists find elements in the first place? Could there be more undiscovered elements?
(2) How did scientists create the periodic table?
(3) How do we know that everything is made up of atoms, when atoms are so small that they can’t even reflect light (a necessity for seeing them)?
(asked by Mukesh Garbyal)

Really good questions! I was asked these in a comment on my post “Types of Atoms,” and chose to answer them in a post of their own.

Let’s take this apart. I actually want to address the third part of the question first, since it contains a misconception: atoms can reflect light. Their interaction with light is actually why we can see anything in the world.

How?

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Extinction and Reddening of Starlight

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Take a wild guess: What do you think this image is showing you?

If you said it looks like a giant black hole in space, I don’t blame you. I also don’t blame you if you thought it looks like a giant outer space blob…and the funny thing is, that’s actually closer to the truth.

This isn’t a hole in space. We can’t see any stars in this region, but not because there aren’t any. In fact, there are just as many there as there are flanking the giant space blob.

What you’re seeing is evidence of the vast interstellar medium, the galaxy’s backstage. The interstellar medium is the stuff between the stars, often invisible since it’s not hot enough to produce its own light.

Sometimes we can see it as a pale blue reflection nebula, or a bright pink emission nebula. But in this case, we’re looking at a dark nebula—visible only because it blocks the light from stars beyond it. It appears to be a hole in space.

It’s closer to being an outer space blob. But what exactly is it?

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What Makes a Star Blue?

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Albireo is the distinctive double star in the head of the constellation Cygnus. You can find it yourself if you look for the Summer Triangle amid the dusty trail of the Milky Way across the night sky.

The brighter, orange star of Albireo is a K3-class bright giant. That means it’s just a few thousand Kelvins (Celsius degrees plus 273) cooler than the sun. But it’s also larger—70 times the sun’s radius—and that makes it brighter than you would expect.

The blue star, on the other hand, is a B8-class dwarf. It has only about 3.5 times the sun’s radius, although it’s hotter by about 7422 Kelvins.

Neither star in Albireo is particularly unusual. There are doubtless millions, even billions, of other stars similar to each one. But Albireo certainly offers us the most striking contrast. Bright blue and red stars don’t often appear so close together.

But what exactly gives these stars their distinctive colors?

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Spots on the Sun

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Have you ever looked at the sun, and seen something like this?

Now, before you decide to look at it right now and see what you see, it’s my responsibility as an amateur astronomer to remind you of the safety risks. Focusing your eyes on the sun is dangerous—there’s a reason our eyes automatically flinch away.

How dangerous, you ask? Dangerous enough to burn and even scar your retinas, permanently damaging or even destroying your vision.

Yes, I’m serious.

Now, all this is not to turn you off solar observing entirely. There are safe—and cheap—ways to look at the sun, and see its spots.

But what exactly are sunspots?

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Energy Flow from the Sun’s Core

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Ask any climate scientist how we should power our world without fossil fuels, and they’re bound to tell you about wind and solar power.

You might be surprised to know that both of these come from the sun. Solar panels collect the sun’s energy directly, but we wouldn’t even have wind if not for the sun.

Why? Because in order to move, you need energy. And not just you. I’m talking about every speck of material on Planet Earth that shifts an inch. It’s because it has energy.

That energy can come from a lot of places. Earth is still a dynamic world with a hot interior, but it’s not hot enough to sustain all the life and other movement on its surface. A lot of our planet’s energy comes from the sun.

But here’s the big question. How the heck does it get here?

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Our Sun: Helioseismology

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We can’t see below the surface of the sun.

That makes sense, really. We can’t see below the surface of the Earth, either—we have to get creative if we want to find out what goes on below the crust.

In the sun’s case, we can’t see below its photosphere because the gases within are so dense, light can’t escape. And we depend on light to see anything.

So…if we can’t see inside the sun, how do we study it?

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The Doppler Effect

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Have you ever heard the ice cream truck?

When I was little, I remember hearing the ice cream truck all the time. Just the sound of the opening notes of “Pop Goes the Weasel” were enough to propel me to the door, where I’d beg my parents to let me go out.

Of course, I didn’t always make it out front in time. But one day, my dad found a way to solve that problem—by actually getting in the car and chasing the ice cream truck.

I remember us driving around the neighborhood, following that white truck around. A few times, it slowed and stopped, but when we stopped too, it kept going again. It took a while for the driver to realize we were following him!

Eventually, we caught it, and had a good laugh over it. But the moral of the story is…have you ever noticed that you can tell if something is moving toward you or away from you, just by if it’s getting louder or quieter?

The same trick works for stars…sort of.

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The Atomic Spectrum

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Astronomers know that if white light passes through a prism and is bent, it’s separated out into its component colors—the colors of the rainbow.

Astronomers also know that when light interacts with atoms, the building blocks of the universe, the atoms absorb photons of light and reemit them—but in a different direction.

Put these two bits of knowledge together, and astronomers now have everything they need to understand spectra (the plural for spectrum).

A spectrum is something I’ve covered in previous posts. In astronomy, it means the wavelengths of electromagnetic radiation spread out so we can analyze them individually. And it’s an astronomer’s most valuable tool.

So, what exactly is a spectrum, and how can we use it to analyze radiation from space and learn more about the universe?

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Stars and Radiation

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Stars are hot.

Really hot. Hot enough to have energy to spare for their planets. If our star wasn’t hot, we couldn’t live on Earth. And our star isn’t even particularly hot for a star. It’s a middle-aged star of low mass, so it’s relatively cool compared to other stars.

You might also notice that stars aren’t all the same color. There are redder stars and bluer stars and more whitish stars.

We know stars are hot. They’re also bright. And they’re different colors. But how does that all translate to radiation—and how can we see it?

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