What is the Hubble Law?

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…

The Doppler effect is one of the most powerful tools in astronomy. It’s how we can tell if stars, interstellar clouds, or even galaxies are moving towards Earth or away from Earth. We can even use it to detect rotation in an object–such as a galaxy or even our sun.

But would it surprise you to learn that you have experience with the Doppler effect in your own life?

For me, my most memorable experience comes in the form of the ice cream truck.

Once when I was little, I heard the familiar tune of “Pop Goes the Weasel” and raced to the front door of my house–but I didn’t make it to the street in time.

Thinking fast, my dad herded me and my little brother into the family van…and started chasing the ice cream truck around the neighborhood.

At first, the driver didn’t actually realize we were following him. He’d stop the truck, offering the car behind him a chance to pass, only for us to stop too. Then he’d pull away from the curb and keep driving again–only for us to follow him!

Eventually, he figured it out, and I got my ice cream.

But the moral of the story is…how can you even tell when an object is moving towards or away from you?

In the case of sound…the pitch changes.

(This diagram uses an ambulance instead of an ice cream truck–but the science is the same!)

When an object moves toward you, it sounds like it gets louder. And when it moves away from you, it sounds like it gets quieter. But its actual pitch doesn’t change at all. If you were keeping pace with the object, like I kept pace with that ice cream truck, the pitch would stay the same.

No, this is an apparent pitch change, produced by the Doppler effect.


In the image above, a car is seen within concentric circles. Each circle represents a single sound wave.

The sound waves emanate from the vehicle one after the other. The time between waves is the pitch. A shorter time means a higher pitch. A longer time means a lower pitch.

When the car is stationary (not moving), the sound waves are perfectly centered around one another, each with the same physical distance between them.

But when the car moves…

Let’s say that at a certain time, one sound wave is emitted from the car. A set time later, another wave is emitted. But the car has moved from its original position. The center of the new wave is in a different spot–and the distance between the waves is no longer the same.

An observer standing behind this moving car will observe the distance between the sound waves to increase. This means the observer will observe an increased wavelength, which means a lower pitch. The sound from the vehicle will seem to get quieter as it moves away.

On the other hand, an observer watching the same car but from in front of the vehicle will observe the distance between the sound waves to decrease. The sound waves approaching this observer will have a decreased wavelength and a higher pitch. The car will seem to grow louder as it approaches.

So…what does that have to do with astronomy?

In astronomy, we don’t observe the sound of objects–sound waves must travel through some kind of physical material like air, water, or rock (in science speak, a medium), but there just isn’t enough of that in space.

Instead, we observe the light wavelength.

The graphic above shows the electromagnetic spectrum–the spectrum of all light in the universe. Visible light, the range of wavelengths that humans evolved to see, is a very narrow portion of the entire spectrum. Astronomers use all the wavelengths above to study the universe.

The light emitted or reflected by objects can get stretched or compressed just like sound waves. But the effect looks a little different.

The striped rainbows you see above are stellar spectra. I covered these in detail back here–but to fully understand spectra, you might want to go back to my posts on atoms and how they work, particularly how they interact with light. And while you’re at it, you might also want to take a peek at, specifically, how this relates to stars.

(You can find all of these posts and more related ones under the “Interaction of Light & Matter” dropdown on my Astronomy page.)

Anyway. All you need to know for this post is that when astronomers study data about any object in the universe, that data involves spectra. Everything we know about any one star can be found in its spectra. The same goes for interstellar clouds and galaxies.

In fact, here’s an example of just how useful the Doppler effect can be…

Here, astronomers use the Doppler effect to observe how one star in a binary system approaches Earth and the other recedes. The Doppler effect reveals the orbits of binary stars.

Not to mention, by observing the approach of one side of the sun and the receding of the other side, we can observe its rotation.

It’s important to note that the Doppler effect does not change the overall color of any object. It only describes those dark lines on the object’s spectrum–spectral lines. Specifically, we’ve been looking at absorption spectra in this post, and the lines are absorption lines.

When we see spectral lines shifted toward shorter electromagnetic wavelengths, we know that the source of the light is moving towards Earth. And when we see spectral lines shifted toward longer electromagnetic wavelengths, we know that the source of the light is moving away from Earth.

But what does all that have to do with galaxies?

Because, of course, galaxies are great wheels of stars, emitting the combined light of billions of stars.

The light that reaches Earth from any one galaxy is the combined light of all its stars, so the spectrum of a galaxy is essentially a mix of all the data from all those stars.

And what does that mean?

Yup, you guessed it–we can detect a Doppler shift in the spectra of galaxies.

Here, two human observers watch a galaxy–from opposite sides of the galaxy.

Let’s ignore for the moment that this situation can’t exactly happen. Humans are grounded on Earth. We don’t have telescopes stationed on the far sides of distant galaxies. We can only observe galaxies from one side–the side where Earth is.

Still, this works to demonstrate the concepts involved.

When a galaxy moves toward us, its spectral lines are shifted toward shorter (bluer) wavelengths, which we call a blueshift. And when a galaxy recedes from us, its spectral lines are shifted toward longer (redder) wavelengths, which we call a redshift.

Okay, I know what you’re going to ask…what the heck does all this have to do with the Hubble Law?

In the 1920s, Edwin Hubble used Cepheid variable stars to determine the distances to the nearest galaxies. And he discovered something astonishing.

The spectra of most galaxies are redshifted, indicating they are receding from Earth. But even more incredible, there is a straight-line relationship between the magnitude of the redshift and the distance to the galaxy.

In other words…nearer galaxies have a smaller redshift, and distant galaxies have a larger redshift.

Here we are, people. That was the first evidence astronomers discovered that the universe is expanding. But we’ll come back to that in future posts!

Right now, I want to focus on one important fact of this relationship between redshift and distance…

It’s very difficult to measure the distance to the most distant galaxies because we can’t resolve enough detail. Distance indicators–typically objects with known luminosities–are invisible.

The relationship between redshift and distance is a surefire way to estimate how far away these galaxies really are. And it’s called the Hubble Law.

The Hubble Law relies upon measurements of the distances to nearer galaxies using distance indicators like Cepheids. But with those distances known, it empowers us to learn a lot more about the galaxies at the farthest reaches of the universe.

There’s just one really weird thing about these redshifts…

At great distances, their magnitudes would seem to indicate that galaxies are moving faster than the speed of light.

That’s not possible. The explanation lies in the discovery that these redshifts are not ordinary redshifts. They look just like Doppler shifts–but they’re not produced by the Doppler effect. And we’ll delve into that very soon.

Next up, we’ll explore two important properties of galaxies–how to measure their diameters and luminosities.

Questions? Or just want to talk?

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