Doppler Astronomical
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Description:	Scientists calculate the shift in the spectrum of light which emanates from those objects. Learn how light years are considered a unit of distance.
To try this simulation:	Left-click and hold anywhere on the screen. A body will accelerate either towards or away from the object in the center. The faster the body is moving away from the center, the more red-shifted its color will be. The faster the body is approaching the center, the more blue-shifted its color will be.
What's Going On?	It's important to understand that light waves of different lengths are distinguished by different colors. A green light wave is longer than a blue light wave and a yellow wave is even longer. When an object in the universe moves away from the earth, the light waves emitted from it are shifted to longer wavelengths. This is called the Doppler effect. The amount of the shift depends on how fast the object is moving. For instance, an object moving away from us at 10 percent of the speed of light would have its blue light lengthened in wave length until it appeared green, its green light lengthened until it appeared yellow, and so on. Since the Doppler shift depends on how fast the object is moving, if we can measure the Doppler shift, we can determine how fast the object is receding from the earth. The Doppler effect also happens with sound waves. As a train roars by you and moves away from you, the sound waves emitted by its whistle shift to longer wavelengths, so the apparent pitch of the train whistle decreases. If the train is moving towards you, the sound waves are shifted to shorter wavelengths and the pitch sounds higher.
There's More!	Doppler Shift and Distance One of the great discoveries of 20th century science is that the universe is expanding. That means that objects that are further from the earth are moving away from the earth faster. Since they are moving away faster, they have a bigger Doppler shift. In the 1920's Edwin P. Hubbell established that the observed Doppler shift doubled as the distance doubled -- the Doppler shift was proportional to distance. So astronomers can use the measured Doppler shift to determine the distance as well as the speed of an astronomical object. Objects close to the earth will have tiny Doppler shifts, but objects very far away from earth will have large Doppler shifts. The game provides examples of both kinds of objects. The distances involved in astronomy are gigantic. For instance, if we make a scale model of the solar system with the sun represented by a tennis ball, the next nearest star, also the size of a tennis ball, would be located several thousand miles away. For this reason it is important to have a really big unit of distance in astronomy. So astronomers use the light-year, the distance that light would travel at a speed of 186.282 miles/sec or 299,743 km/sec in the course of a year (which is more than 30 million seconds long)! A light year is 9,460,000,000,000 km. Time Astronomers are now convinced that the universe began in a big bang roughly 15 billion years ago. Before the big bang, there weren't any stars and galaxies in the universe, so we can't expect to see light coming back from them. But looking at very large distances from earth, we can see back into the distant past of the universe. The telescope is like a time machine! Because it takes such a long time for light from an object very far away from us to reach earth, we see that object as it was in the distant past. For instance, when we look at the star called NGC1068 we are seeing light that left it 60 million years ago, back when dinosaurs still roamed the earth. It has taken that full 60 million years for that light to reach the earth now. Some of the objects in the game are much more distant than that - the light left them 10 or 12 billion years ago, even before the solar system was formed, and has been traveling for most of the history of the universe to arrive now! By calculating the distance of the most distant objects in the universe, astronomers are able to estimate the age of the universe. They also are able to view objects as they existed billions of years ago. (Remember that the light which is seen here on Earth from these objects, actually left its source billions of years ago, traveling across the vast reaches of space.) By determining the age of the universe, scientists hope to gain insight into the process which formed the universe and into the nature of matter itself.
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