Doppler Shift Interactive (70.0K)
Astronomers use the Doppler Effect to determine the motion and speed of galaxies and other distant objects. This Interactive shows you what the Doppler Effect is: how the frequency and wavelength of light or sound waves change as the source or the observer (or both) move relative to each other. Click on the buttons to make the Observer, the Source or Both approach, and observe the waves on the graph. Or take matters into your own hands by clicking and dragging on the spaceship to change its velocity.

1		Randy Johnson's fastball has been measured by sound waves bouncing back off it with a "radar gun", similar to the highway patrols' radar speed detectors. What is the speed if the gun transmits at 600 Hz, and the reflected signal comes back shifted to 540 Hz? Need a Hint?
			A)	50 mph.
			B)	75 mph.
			C)	98 mph.
			D)	110 mph.

Here we will use radio waves to study the motions of nearby objects, in this case the rotation of planet Venus, always hidden from direct visual observation by a thick veil of sulfurc acid clouds. But radio waves can penetrate the map the surface.

2		The rotation of Venus was first found by radar reflections off the limbs of Venus from Arecibo. When astronomers found Venus was rotating at an incredibly slow rate of only 1.9 meters per second, and that its size was almost identical to Earth, with a circumference of 40,000 km, this gave us a rotation rate for Venus of: Need a Hint?
			A)	59 days
			B)	225 days
			C)	243 days.
			D)	365.25 days

Since 1995 we have used the Doppler Shift to detect planets orbiting other stars. The first instance of this, the finding of a "hot Jupiter" orbitting 51 Pegasi, rocked the scientific establishment.

3		51 Pegasi Undergoes a periodic shift in its spectral lines from red to blue and back to red every 4.2 days. What is the best present explanation for this pattern of shifts? Need a Hint?
			A)	This star rotates in 4.2 days, and we are seeing the shifts created by its limbs.
			B)	This star precesses every 4.2 days, and the jets shooting off its poles create the recurrent pattern.
			C)	51 Pegasi has a "hot Jupiter" orbitting very close to it every 4.2 days.
			D)	51 Pegasi is trapped by a black hole, and spirals around the massive body every 4.2 days.

Few principles in science are applied on as varied a scale as the Doppler Principle. Since vast clouds of dust hide about 90% of the Milky Way from us visually, we must rely on radio observations of Doppler shifts in the hydrogen clouds to map the layout of the spiral arms of the galaxy.

4		If we are observing a hydrogen cloud in the Cygnus Arm of the Galaxy at 20.0079 cm wavelength, and the normal wavelength of H I neutral hydrogen is exactly 21 cm., how is this H I region moving. Need a Hint?
			A)	This H 1 region is approaching us at 30 km/second.
			B)	This H 1 region is approaching us at 300 km/second.
			C)	This H 1 region is receding from us at 300 km/second.
			D)	This H I region is receding from us at 3,000 km/second.

Few principles in science are applied on as varied a scale as the Doppler Principle. In the 1930's Edwin Hubble and Milton Humason used it to discover the expansion of the universe, setting the stage for the Big Bang Theory.

5		In studying hundreds of spectra and photos of galaxies, Hubble and Humason found that larger red shifts were ususally associated with images of smaller and fainter galaxies. What could this mean? Need a Hint?
			A)	The smaller galaxies were less massive, and would move faster, just as Mercury moves fastest around the Sun.
			B)	The smaller and fainter galaxies were usually more distant; there was a linear relationship between the speed of a galaxy and its red shift, which became known as Hubble's Law. The farther away a galaxy lay, the faster it appeared to be receding from us.
			C)	More massive galaxies produce a larger gravitational red shift.
			D)	The larger the red shift, the older and redder the stars in that galaxy.