Measuring distance by stars

Sunday, September 2, 2001


A reader from Jackson, Mo., asks, "How do astronomers know the distances to the various objects they talk about?" The answer is that astronomers use a "boot strap" process to determine distances over ever larger distance scales. We first determine distances to nearby stars and then use that information to develop techniques for even further objects.

For nearby stars, astronomers use a technique called parallax. Parallax is the apparent shift in an object's position caused by motion of the observer. A simple experiment can serve as an example. Close one eye and hold a finger up in front of you. Note its position relative to some background object. Now, close that eye and open the other. You will notice your finger appears to shift its position with respect to the background object. If you know the distance between your eyes and the apparent shift of your finger, you can use simple geometry to solve for the distance. Astronomers take images of the sky six months apart so they can view them from opposite sides of Earth's orbit. This method can find accurate distances for several dozen nearby stars.

Once you know the distances to a few of the closest stars you can use that information to find distances to more distant stars. If you find another star just like the one you know the distance to, you can compare the brightness of the new star with the known star. Since you know that the brightness decreases as one over the distance squared you can solve for the new distance. This techniques can be used for stars half way across our galaxy.

To get to the next distance scale requires a fortuitous circumstance.

There is a class of variable stars called Cepheid variables. These stars are a bit unstable and tend to pulsate at a rate that depends on their brightness. The brighter ones pulsate slower. Measuring the period of their pulsations is easy, as is measuring their brightness. Once we know how bright they really are, we can again solve for their distance. It was this technique that allowed Edwin Hubble to determine the some of the fuzzy blobby things seen in the telescopes were actually individual galaxies as big as our own Milky Way and led to the realization that our galaxy was just one of billions in the universe.

Closer to home, the sun appears to cross the equator as it flies south for the winter. The autumnal equinox occurs on Sept. 22 at 7:04 p.m. This is the official start of fall and is the time when every point of the Earth has 12 hours of daylight and 12 hours of darkness.

Mars continues as the brightest object in the sky at sunset. Its ruby red glow can be seen low in the south. Saturn rises about midnight on the beginning of the month and about 10 p.m. at the end. On Sept. 10, Saturn will put on a special show as it disappears behind the moon. Although this happens in the daytime, a modest sized telescope should be able to see the event which starts 7:50 a.m. and ends 9:12 a.m.

Jupiter rises about 2 a.m. Venus is up in the east at sunrise and makes a pretty picture with the moon on the 15th. The summer Milky Way is nearly overhead at sunset.

The summer constellations will stay with us for a while as the earlier sunset times compensate for the earlier setting times of the stars. Thus they appear to remain frozen in their positions for several weeks to come.

Dr. Michael Cobb is a physics professor at Southeast Missouri State University.

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