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Doppler Shift

On occasion, I will take a few weeks to explore a theme in astronomy on these pages. In 1997 that theme was the solar system. This time, for the next few weeks, I invite you to take a look at the Universe In Motion.

We think of the skies as static, unchanging. The only motions we see easily are the rising and setting of the Sun, Moon and stars, but that is a reflection of our own Earth's rotation. The stars themselves don't seem to move at all among themselves, and it takes a keen eye to discern the motion of the planets, which takes days or weeks to become obvious.

Yet objects in the sky are in constant motion, and some move with an incredible intrinsic velocity. Usually, these objects are so far away that the distance itself shrinks the apparent motion, the way distant mountains hardly seem to move at all even though you may be driving past them at 100 kilometers an hour. Sometimes it does take many years to perceive the movement of heavenly bodies, and sometimes it happens in the blink of an eye. Every Monday, we'll take a look at some of these celestial travelers.

Week of March 7, 1999

Two Snack's back I talked about parallax, which can make stars appear to move in the sky, and proper motion, which is the direct movement of a star in the sky. However, there is another kind of motion, though oddly it doesn't make the star actually move in the sky at all! As a star moves, you can decompose that movement into two components: one that is moving across our line of sight, and another that is moving towards or away from us.

Imagine two people pushing on a big rock. One is standing on the south side, the other on the east side. If the two people push equally hard, the rock moves northwest. If south pushes harder than east, the rock moves more north, and so on. All you see is the combination of the two movements (northwest = northward + westward) but in reality it's two different forces adding up.

So it is for a star. It might be moving directly toward you, or directly across the sky, but most likely it's some combination of both. If it is heading straight toward you, it won't appear to move at all! Yet we do have a way to detect this motion. Time for another analogy! We have all heard the sound of something like a car or train move up in pitch as it approaches us and then drop drastically when it goes past us. This is because what we perceive as sound is actually a wave emitted by the object. The pitch we hear is determined by how many waves per second reach our ear. The more waves, the higher the pitch. When the object moves towards us, the waves pile up, so we get more waves per second, and the pitch goes up. When the object passes us, the waves stretch out; we get fewer per second, and the pitch drops. The first person to quantify this relationship between speed and pitch was a man named Doppler, so we call this the Doppler shift.

diagram of Doppler shift from Ned Wright's web page

The same is true for light. Light is a wave, and what we think of as color is really just the number of waves that hit our eye per second. Something red has fewer waves hitting our eyes (that is, the frequency, the waves per second, is lower) then something blue (which has a higher frequency). In a vacuum, light always travels at one speed: the speed of light (duh). When an object approaches you, the light still has the same speed (contrary to our perceptions of everyday life), but the waves still pile up. The light has a higher frequency, and the color changes. Astronomers call this a blueshift. Mind you, it does not mean the light gets bluer! In this sense, we use the word ``blue'' as a relative term, it always means ``higher frequency''. If you blueshift blue light, it goes to the ultraviolet. When you blueshift ultraviolet, you get X rays! Astronomers sometimes use terms that confuse laymen; we do this to feel superior and ensure further grant money. ;-) The flip side of blueshift is of course red shift. You'll read more about this in next week's Snack.

The fun thing is, since light always travels at the same speed, if we can measure the amount of color shift of an object, we can determine its speed. Usually, we can do this. We know that certain types of stars emit light in certain amounts that vary with color; the Sun, for example, puts out a lot of light in the green and yellow parts of the spectrum, and relatively little in the blue. Better yet, the pattern of this light (called the spectrum) has a very specific shape which acts like a fingerprint, identifying the type of star. If we see that pattern, but shifted a bit, we can actually tell if the star is moving towards or away from us, and even how fast it's traveling! This adds the third dimension to the sky; even though a star may not appear to be moving at all, you can measure its speed toward or away from you with the correct equipment by taking a spectrograph, or picture of the spectrum of the star. It has been said that astronomy was reborn with the invention of the telescope, but astrophysics, the physics of astronomy, was born with the invention of the spectroscope. You'll see next week that the ramifications of all this are indeed Universal.

More info about the Doppler shift can be found easily on most astronomy web sites. I got the image above from Ned Wright's excellent web pages. You can also scan through my list of astronomy links for more pages as well.

©2008 Phil Plait. All Rights Reserved.

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