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Cepheid Variable Stars and Measuring Distances in Space

Astronomers use pulsating stars to determine how far away other galaxies are from the Milky Way.

Cepheid variable stars are, as defined by The Facts on File Dictionary of Astronomy, very luminous yellow supergiants that pulsate with periods ranging from 1 to 50 days. The luminosity variations of these stars are continuous and extremely regular, allowing the periods of the stars to be accurately measured.

Using Stars as Standard Candles
Astronomer Henrietta Leavitt discovered that there was a relation between the period of pulsation in a Cepheid star and its brightness. Brighter stars were shown to have longer periods. This period-luminosity relation could be used to measure distances to nearby galaxies. The time it takes for a star to brighten and fade can be calculated to find the star's intrinsic brightness. Compare this to the star's measured brightness to find the distance.

The name Cepheid variable comes from the prototype star for which this was discovered, Delta Cephei. This star in the constellation of Cepheus the King varies between magnitude 3.5 and 4.3 over the course of five and a half days.

Henrietta Leavitt was studying Cepheids in the Small Magellanic Cloud (SMC) when she made her discovery. The SMC is currently believed to lie 200,000 light-years away.

NGC 247
Image of galaxy NGC 247
Credit: ESO

Problems with Cepheids and Measurements
One of the problems that occurs with using Cepheid variables as yardmarkers is that some of the light from the star can be absorbed en route to Earth. If the star loses some light due to intervening dust, it will appear fainter and therefore further away than it really is. This was the case for the galaxy NGC 247.

A team of astronomers has been working to eliminate such miscalculations and has found that NGC 247 is more than a million light-years closer to the Milky Way than previously believed. NGC 247 now appears to lie a little more than 11 million light-years distant. NGC 247 is part of the Sculptor Group of galaxies, one of the nearest group of galaxies to our Local Group. The Local Group consists of more than 30 galaxies, including the Magellanic Clouds and the Andromeda Galaxy.

Limitations with Cepheid Variables
Cepheid variables can be used to measure distances out to about 30 or 40 megaparsecs with the help of the Hubble Space Telescope. There are also two different types of Cepheid variables, and these two types have different period-luminosity relationships. Nevertheless, these stars are the best tools we have now to measure cosmic distances.

Sources:
Illingworth, Valerie and Clark, John E. O., The Facts on File Dictionary of Astronomy, Checkmark Books, 2000.
European Southern Observatory Press Release dated February 28, 2011: The Dusty Disc of NGC 247

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Author: Kelly Whitt

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