A quick look at “Average Orbital Velocity” in Clark Planetarium’s Solar System Fact Sheet will reveal that the farther a planet is from the Sun, the slower it moves. In contrast, stars and gas in the outer regions of galaxies all have roughly the same speed regardless of their distance from the center. How do astronomers measure their speed? Using the Doppler Effect. Below is an example from a nearby galaxy known as M33. The image on the right is a radio telescope image showing the distribution of hydrogen throughout the galaxy (hydrogen atoms give off radio waves with a wavelength of 21 centimeters). Colors in the image show the Doppler shift of the radio waves. Blue shows hydrogen that is moving toward us. Red shows hydrogen that is moving away.

Right: M33 Galaxy (credit: NOAO/AURA/NSF/T.A.Rector). Left: M33 Galaxy showing Doppler shift (credit NRAO/AUI)

Since Doppler measurements reveal that stars and gas in the outer regions of galaxies all have similar speeds, this implies that mass in a galaxy must increase with increasing distance. But visible matter in most galaxies appears to decrease with increasing distance from the center. So, unless our understanding of the basic laws of physics needs a tweak (as has been proposed by some) most of the mass in galaxies cannot be seen, hence the name dark matter.

A number of possibilities have been suggested for this unseen mass, from dim stars, brown dwarfs and black holes to exotic and not so exotic sub-atomic particles. Recent searches seem to favor sub-atomic particles. However, one of the exciting things about astronomy research into the unknown is that an unexpected discovery or a better observation can revise current thinking.

Hubble composite showing ring of dark matter in the galaxy cluster C1 0024+17

One of the best evidences for the existence of dark matter comes from the Hubble Space Telescope. Astronomers used gravitational bending of light from faint distant galaxies to map the distribution of mass in what appears to be the aftermath of a collision between two galaxy clusters. The blue color in the Hubble image shows the distribution of matter based on these measurements. In addition to the mass of the cluster in the center, a ring of unseen mass surrounds the galaxies. Computer simulations suggest that such a collision, occurring along Earth’s line of sight could produce this ring of dark matter.

What is dark matter? Does it really exist? Stay tuned.

How does it happen? Sound waves move outward from the source of the sound though the air in all directions. The pitch of the sound results from the spacing in the sound waves. If the waves are closer together, the sound has a higher pitch. If they are farther apart, a lower pitch. This spacing between waves, or the distance from the crest of one wave to the next, is called the wavelength.

Let’s look at two different situations. The first diagram below represents a car that is stopped at a light. We’ll concentrate on a single sound, perhaps the hum of the engine. Each circle represents a crest of the sound wave moving outward.

Sound waves moving outward from a stationary car

If we were to examine the same car a little later, each circle would be bigger, but distance between each circle would remain the same. As the wavelength of the sound is the same in all directions, anyone that is stationary relative to the car will hear the same pitch.

In the second diagram below, the car is moving to the right. Because the source of the sound wave (the car) moves between the times when two wave crests leave the source, the wave crests end up closer together in the direction of motion and farther apart in the opposite direction.

Sound waves moving outward from a moving car

So, someone standing at point A will hear a higher pitch and someone standing at point B will hear a lower pitch. Since the pitch that they hear depends on the car’s speed, they could find out how fast the car is moving by measuring the shift in the sound’s wavelength.

The Doppler Effect also works for light. In the case of light, different wavelengths of light are different colors. Blue light has short wavelengths and red light has long wavelengths. If an object is moving toward us, particular colors of light given off by that object have a shorter wavelength than they do when stationary and we say the light is “blue shifted.” Likewise, if an object is moving away from us, particular colors of light given off by the object have a longer wavelength than they do when stationary and we say the light is “red shifted.” By carefully measuring the apparent shift in the wavelength of the light, astronomers can determine how fast an object is moving toward or away from us.