Cepheid variable

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A Cepheid (pron: ˈse-f(ē-)id or ˈsē-f(ē-)id) is a member of a particular class of variable stars. The relationship between a Cepheid variable's luminosity and pulsation period is quite precise, securing classical Cepheids as viable standard candles and the foundation of the Extragalactic Distance Scale. This period / luminosity connection was discovered by Henrietta Swan Leavitt while investigating hundreds of classical Cepheid variables in the Magellanic Clouds. Harlow Shapley would later use the variables to place initial constraints on the size of our Galaxy. Edwin Hubble would discover Cepheid variables in M31 and settle the Island Universe debate, which centered around the mystery of whether our Galaxy and the universe were synonmous, or was the Milky Way merely one in a plethora of galaxies that constitutes the universe.^[1] Cepheid variables remain pertinent in a variety of ways, including placing cosmological constraints on the expansion of the Universe through the determination of distances to Galaxies.^[2] Cepheid variables are also used to determine the Sun's height above the Galactic plane, to establish the distance to Galactic Center, and to interpret local spiral structure in the vacinity surrounding the Sun.^[3]

Contents 1 Type I and Type II Cepheids 2 Discovery 3 Period-magnitude relation and Use as a "standard candle" 4 Notes 5 Examples 6 See also 7 References 8 External links

[edit] Type I and Type II Cepheids

The variables are divided mainly into two subclasses, Type I or classical Cepheids, which are young massive stars, and Type II Cepheids, which are fainter old metal-poor low-mass stars.^[4] Classical and Type II Cepheids follow different period-absolute magnitude relationships. The luminosity of a Type II Cepheids is, on average, less than classical Cepheids by about 1.5 magnitudes.

[edit] Discovery

On September 10, 1784 Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Cepheid variables. Although the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later.

[edit] Period-magnitude relation and Use as a "standard candle"

The relationship between a Type I Cepheid's period P, and its mean absolute magnitude M_v has been empirically derived by many astronomers throughout the Twentieth century. One such calibration was published by Michael Feast and Robin Catchpole in 1997 using trigonometric parallaxes determined by the Hipparcos satellite. The resultant period-luminosity relationship was:

with P measured in days.^[5][6] The following relations can also be used to calculate the distance and reddenings to classical Cepheids:

^[7]

Where J is on the 2MASS system. There are several sources of error inherent to distances established by means of Cepheid variables, one of which is the actively debated effect of metallicity.

[edit] Notes

• Some Cepheid stars (for example Polaris), have shown rapid period change.
• The distance to the Cepheid RS Puppis is estimated using light reflected off a large nebula surrounding the star.^[8]
• Cepheid stars are visible from great distances because of their high luminosity. The Hubble Space Telescope has identified classical Cepheids out to a distance of some 100 million light years.

[edit] Examples

Some Cepheid variables with fairly bright apparent magnitudes and variations in brightness large enough to easily distinguish with the naked eye include Eta Aquilae, Zeta Geminorum, Beta Doradus, as well as the prototype Delta Cephei.

[edit] See also

• Instability strip
• Hubble constant
• RR Lyrae variable
• W Virginis variable

[edit] References

• SEDS: Variable Stars

1. ^ Hubble, E. (1925). Cepheids in spiral nebulae, OBS
2. ^ Freedman, W. et al. (2001). Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant, ApJ
3. ^ Majaess D. J., Turner D. G., Lane D. J. (2009). Characteristics of the Galaxy according to Cepheids, MNRAS
4. ^ Wallerstein, G. (2002). The Cepheids of Population II and Related Stars, PASP
5. ^ Feast, Michael W. & Robin M. Catchpole. "The Cepheid period-luminosity zero-point from Hipparcos trigonometrical parallaxes". Monthly Notices of the Royal Astronomical Society. 286 (1997) L 1-5.
6. ^ Allen, Nick. "The Cepheid Distance Scale: A History". Aug 2005. http://www.institute-of-brilliant-failures.com.
7. ^ Majaess D. J., Turner D. G., Lane D. J. (2008). Assessing potential cluster Cepheids from a new distance and reddening parameterization and 2MASS photometry, MNRAS
8. ^ Kervella, Pierre: Light echoes whisper the distance to a starhttp://www.eso.org/public/outreach/press-rel/pr-2008/pr-05-08.html

[edit] External links

• Cepheid Variable Photometry and Radial Velocity Archive