3d. Radial Velocity

As we’ve seen in previous posts, light is able to give many wonderful pieces of information such as temperature, color, brightness, age, and chemical composition. But now we’ll explore how light is able to give us a property known as radial velocity.

First off, what is radial velocity? In short, it’s the speed of an object towards or away from the observer. Whether or not you realize it, you’re already familiar with a very common use of this: Doppler radar. This device uses radar pulses which are reflected by water vapor and then their speed, towards or away from the detector, is measured.

But how?

The answer is actually right in front of your eyes. It’s due to something known as the Doppler effect. It sounds pretty exotic but again, it’s something you’re most certainly familiar with. If you’ve ever listened to a train passing as it blows its whistle, you’ve heard the Doppler effect. As the train approaches you, the pitch of the whistle seems higher but drops off as it speeds past. If you can’t picture what I’m talking about try here.

So now that we know what the Doppler effect is, what causes it? You’re probably well aware the sound is actually a wave. We’ve already had a good look at what waves are and how they’re characterized for light, so we’ll apply some of the same concepts here before moving on.

With sound, the pitch of something like a train whistle depends on the distance between successive waves, called the wavelength. If you have a long wavelength, this would be a low pitch sound. If there’s a short wavelength, this is a high pitch.

A train whistle has a single tone, so the distance between waves should be constant which means a constant pitch. If you stand by a train that’s holding still as it blows its whistle, you’ll realize this is true.

But let’s imagine that the train is now moving towards you. The crest of one wave is emitted. In the time before the next one is emitted, the train moves towards you, catching up a bit to the wave it just gave off. Thus, when it gives off the next one, it will be closer to the previous one than if the train was remaining stationary. This gives the sound that you’d hear a shorter wavelength, and thus, a higher pitch.

The opposite is true if the train is moving away from you. Since the train is moving the opposite direction of the wave that you’re hearing, each successive wave takes longer to reach you, meaning a longer wavelength, and therefore, a lower pitch.

Here’s a nice image to sum that all up:


Source

So how does that have anything to do with light?

Conveniently enough, light is also a wave and the same rules apply. If an object that’s giving off light is moving towards you, the wavelength gets shortened. With light, this means that it looks bluer. If it moves away, the opposite is true and it looks redder.

But how do we tell if that observed color is due to the actual color of the star, or if it’s due to some sort of shift? To figure this out, we’ll need a reference point that we know what the wavelength should be.

Fortunately, this isn’t too hard. In my last post concerning spectroscopy, we explored types of spectra called absorption spectra in which dark lines were taken out. These lines were due to transitions of the electrons in atoms. Since these transitions have a fixed energy, that means they have a fixed wavelength.

Unless something happens to shift that wavelength that is.

The easiest absorption lines to find generally are two prominent ones due to hydrogen, known as the Hα and the Hß lines. When the source is at rest with respect to the observer (neither moving towards or away) these lines appear at 656.3 nm and 486.5 nm respectively.

So since we know where these lines should be, we can compare them to where they are. The further they’re shifted, the faster the object is moving towards or away from us. If they’re shifted to the blue, it’s moving towards us. If it’s shifted towards the red, it’s moving away.

With this, we’ve examined many of the important tools that astronomers frequently use. With these tools, we can generate a huge number of facts about our universe. However facts are actually pretty useless in science. It’s what those facts mean that is important. To get this meaning, we have to tie facts together with something that is actually useful: a theory.

Thus, in my next series of posts on this topic, I’ll show you a few ways these tools are put to use to make some of the conclusions (read: theories) that are both fundamental and exciting to astronomy.