Doppler's original idea about measuring speeds of stars eventually did work out, but it took a generation for the technology to catch up to the vision. A critical extra idea for it to work was the discovery of spectral lines in starlight. For many sources of light, if you look closely at their color spectrum through a prism, you can see black lines where a color is missing (Fraunhofer lines if you're taking a test). These lines depend on the exact nature of the stuff giving off the light, so they turn out to be a really useful study in their own right, but for our Doppler purposes, the key thing is that these lines are very sharp and easy to measure, so any change in them over time can be tracked.

Well, of course, astronomers did see the lines changing their position in the spectrum in many stars, or we wouldn't be talking about it here. In fact it's a slight movement, but the lines clearly do all move together, first towards one end of the spectrum, then turning around and moving towards the other end, then repeating over and over.

In fact, the situation is just like the Doppler demo above, with a sound ball on a string being whirled around. The frequency oscillates, moves up and down in a regular pattern, just as if the star were moving in a circle. Well, it really is moving in a circle, as we can see in telescopes. The reason is that it's one of a pair of double stars which orbit each other, like a dumbbell spinning around its middle, or two ice skaters holding hands and spinning. Double stars, called binary stars by the pros, might seem unlikely, but actually they're common: we see more of them than single stars like our Sun. If you have Java, here's a simulation

The Doppler shift is useful to astronomers because they can measure it even in stars which are too far away to see the circular motion directly in the telescope. We can infer the stars are binaries from the Doppler patterns. This gets at why Doppler thought of the idea in the first place: we can't tell very much about the stars, so we have to be really clever and squeeze as much information as we can from what data is available to us. The Doppler effect doesn't actually do much in nature: its significance is as a tool to sense what's moving when more direct methods won't do. (And occasionally to change that motion... Doppler ultra-refrigerators win 1997 Nobel Prize, and a undergrad thesis by a football-playing physicist physician! Dare ya to understand them!)

Doppler is still a cutting-edge research technique in astronomy, but now with ultra-high-precision measurements. You've probably heard of planets having recently been found around stars other than the Sun. Astronomers hunt them by just this method! The planets are way too dim to be seen directly, but as they orbit their star, they cause their star to move in a small circle, to wobble. This is an even harder version of the double star Doppler method. Here's a detailed history of the search and here's a professional planet discoverer's scoresheet (his "introduction" link has popular articles).

There're other astronomical uses of Doppler too. One of the unlikely tricks is to find out about the interior of stars (including our Sun) by very careful study of the waves sloshing around on the surface. These waves can be measured by, you guessed it, the Doppler Effect! As the Sun's surface rises and falls, the surface light and its spectral lines shift frequency ever so slightly. Spacecraft instruments track this, and computers draw what's actually called a Dopplergram, a picture of the Sun colorcoded to show velocity, like the radar maps. This has yet another convention: the range of velocity is mapped to a range of brightness, dark to light. Since the Sun is also rotating, there's yet another example of our circular motion Doppler pattern superimposed on the smaller-scale sloshing.

This picture changes over time, and by applying some ferocious math, astronomers can see the Sun ringing like a bell. Exactly how it rings depends on the whole volume of the Sun, not just the surface, just like a bell or any vibrating object. More math allows them to figure out what the interior must be like to ring exactly that way, information unavailable by any other means. This works for other stars too. It's like using earthquakes to figure out what the interior of the Earth is like (in fact, it's called helioseismology -- sun-shake-study) but since the sun is fluid, there are strange issues. For example, the Doppler patterns tell us that the Sun is rotating at different rates at different depths below the surface!

An authorial note: the Doppler Effect is usually taught as "cultural literacy" in the sense that the student is expected to know what it is, in general terms, but not to have much practical grasp. These pages have been an exploration of how it could be used more seriously to teach kinematics.

We started Doppler's story with music, Mozart and trumpeters, so we can end it that way. If you're hearing music, it's the menuetto from Mozart's "A Little Night Music" as midized by a self-described Mozart maniac. If you'd like more than eine kleine, try the University of Pennsylvania student project Music for Engineers! Can't get enough Doppler? Astronomers sing about it!

Wave goodbye, now.

Return to CPO Home Page