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Cepheid Variables as Cosmic Yardsticks

Cepheid stars oscillate between two states: In one of the states, the star is compact and large temperature and pressure gradients build up in the star. These large pressures cause the star to expand. When the star is in its expanded state, there is a much weaker pressure gradient in the star. Without the pressure gradient to support the star against gravity, the star contracts and the star returns to its compressed state.

Cepheid variable stars have masses between five and twenty times the mass of our Sun. The more massive stars are more luminous and have more extended envelopes (the outer layers of gas in a star are sometimes called its "envelope"). Because these envelopes are more extended and the density in their envelopes is lower, their variability period, which is proportional to the inverse square root of the density in the layer, is longer.

Difficulties in Using Cepheids to Determine the Size of the Universe

There have been a number of difficulties associated with using Cepheids as distance indicators. Until recently, astronomers used photographic plates to measure the fluxes from stars. The plates were highly non-linear and often produced faulty flux measurements. Since massive stars are short-lived, they are always located near their dusty birthplaces. Dust absorbs light, particularly at blue wavelengths where most photographic images were taken, and if not properly corrected for, this dust absorption can lead to erroneous luminosity determinations. Finally, it has been very difficult to use ground-based telescopes to detect Cepheids in distant galaxies: Earth's fluctuating atmosphere makes it impossible to separate these stars from the diffuse light of their host galaxies.

Another difficulty with using Cepheids as distance indicators has been the problem of determining the distance to a sample of nearby Cepheids. In recent years, this problem has lessened. Astronomers have developed several very reliable and independent methods of determining the distances to the Large Magellanic Cloud and Small Magellanic Cloud, two of the satellite galaxies of our own Milky Way Galaxy. Since both of the Magellanic Clouds contain large numbers of Cepheids, they can be used to calibrate the distance scale.

Recent Progress

Recent technological advances enabled astronomers to overcome a number of the other past difficulties. New detectors called CCDs (charge coupled devices) made more accurate flux measurements possible. These new detectors are also sensitive in the infrared wavelengths. Dust is much more transparent at these wavelengths. By measuring fluxes at multiple wavelengths, astronomers were able to correct for the effects of dust and make much more accurate distance determinations.

These advances enabled accurate study of the nearby galaxies that comprise the "Local Group" (the group of galaxies including our own Milky Way galaxy and our neighbor the Andromeda galaxy). Astronomers observed Cepheids in both the metal rich inner region of M31 (Andromeda) and its metal poor outer region. This work showed that the properties of Cepheids did not depend sensitively on chemical abundances. Despite these advances, astronomers, limited by the Earth's atmosphere, could only measure the distances to the nearest galaxies. In addition to the motion due to the expansion of the Universe, galaxies have "relative motions" due to the gravitational pull of neighboring galaxies. Because of these peculiar motions, astronomers need to measure the distances to distant galaxies so that they can determine the Hubble constant.

Over the past few decades, astronomers, using different data sets and methods, have reported values for the Hubble constant which range between 50 km/s/Mpc and 100 km/s/Mpc. Resolving this discrepancy is one of the most important outstanding problems in observational cosmology.


Thank you to the MAP project for contributing to this article. Find out about the Microwave Anisotropy Probe at http://map.gsfc.nasa.gov/.

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Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

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This page last updated: Monday, 27-Sep-2004 11:26:11 EDT