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    Articles > October 2000
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A Self-Teaching Guide

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  An Introduction to Variable Stars
Stars are not always the unchanging points of light that they may appear to be. Some stars, known as variable stars, change brightness over a period of time--from a few seconds to years. Over 30,000 variable stars are known and many more are suspected to be variable. A few of the brighter variable stars are shown on our sky maps.

There are many different types of variable stars. One type is the eclipsing binary star, a close pair of orbiting stars in which one star periodically passes in front of the other. The combined light from the two stars that reaches the Earth dims when the faint companion eclipses the brighter star.

The Eclipsing Binary Star known as Algol
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Algol in the constellation Perseus is a famous eclipsing binary star that shines most of the time at magnitude 2.1. However, every 2.867 days (that is, 2 days 20 hours and 49 minutes) Algol will dim to magnitude 3.4 and brighten again when the eclipsing star moves out of the way. The eclipse lasts for some 10 hours in total and is easy to detect with the unaided eye. The first predicted minimum for Algol this month occurs on October 3 at 1:58 UT. Add 2.867 days to get the next date/time and so on.

Another type of variable star are the pulsating variables that periodically swell and shrink due to internal forces. A particular type of pulsating variable known as a Cepheid has the remarkable property that its pulsation period depends upon the luminosity of the star. Measuring the pulsation period of a Cepheid enables the actual brightness, or luminosity, of the star to be determined no matter its distance. Because the apparent brightness of a star depends on its distance from us, Cepheid variables provide a "standard candle" by which intergalactic distances can be measured.

Astronomers recently used the Hubble Space Telescope to observe hundreds of Cepheids in far-off galaxies. The results give the best estimate yet of the size and hence age of the Universe now estimated to be 12 billion years ±10%.

The web site of The American Association of Variable Star Observers has lots of information about variable stars including how anyone can join an international observing program that will contribute to the scientific knowledge of these stars.

On another topic, Saturn and Jupiter make a welcome return to the evening skies this month. Both planets shine bright in the east with Jupiter below and to the left of Saturn. The Moon will join them 15-16 October. And Venus is visible about 30 minutes after sunset low in the southwest. Clear skies!

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