Classical Cepheid variable

Classical Cepheids (also known as Population I Cepheids, Type I Cepheids, or Delta Cephei variables) are a type of Cepheid variable star. They are population I, variable stars that exhibit pulsation periods in the order of a few days to months, are 4–20 times more massive than the Sun,^[1] and up to 100,000 times more luminous.^[2] Classical Cepheids are yellow supergiants of spectral class F6 – K2 and their radii change by (~25% for the longer-period l Car) millions of kilometers during a pulsation cycle.^[3][4]

There exists a well-defined relationship between a Classical Cepheid variable's luminosity and pulsation period,^[5][6] securing Cepheids as viable standard candles for establishing the Galactic and extragalactic distance scales.^{[7][8][9][10]} HST observations of Classical Cepheid variables have enabled firmer constraints on Hubble's law.^{[7][8][10][11][12]} Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.^[9]

Over 700 classical Cepheids are known in the Milky Way Galaxy,^[13] and several thousand extragalactic Cepheids have been discovered. The Hubble Space Telescope has identified classical Cepheids in NGC 4603, which is 100 million light years distant.^[14]

Discovery

On September 10, 1784 Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of Classical Cepheid variables. However, the namesake for classical Cepheids is the star Delta Cephei, discovered to be variable by John Goodricke a few months later. Delta Cephei is also of particular importance as a calibrator for the period-luminosity relation since its distance is among the most precisely established for a Cepheid, thanks in part to its membership in a star cluster^[15][16] and the availability of precise Hubble Space Telescope/Hipparcos parallaxes.^[17]

Period-luminosity relation

A Classical Cepheid's luminosity is directly related to its period of variation. The longer the pulsation period, the more luminous the star. The period-luminosity relation for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds.^[18] She published it in 1912^[19] with further evidence. Once the period-luminosity relationship is calibrated, the luminosity of a given Cepheid whose period is known can be established. Their distance is then found from their apparent brightness. The period-luminosity relationship has been calibrated by many astronomers throughout the twentieth century, beginning with Hertzsprung.^[20] Calibrating the period-luminosity relation has been problematic, however, a firm Galactic calibration was established by Benedict et al. 2007 using precise HST parallaxes for 10 nearby classical Cepheids.^[21] Also, in 2008, ESO astronomers estimated with a precision within 1% the distance to the Cepheid RS Puppis, using light echos from a nebula in which it is embedded.^[22] However, that latter finding has been actively debated in the literature.^[21]

The following relationship between a Population I Cepheid's period and its mean absolute magnitude was established from Hubble Space Telescope trigonometric parallaxes for 10 nearby Cepheids:

with measured in days. ^[17][21] The following relations can also be used to calculate the distance to classical Cepheids:

^[21]

or

^[23]

and represent near infrared and visual apparent mean magnitudes, respectively.

Uncertainties in Cepheid determined distances

Chief among the uncertainties tied to the Cepheid distance scale are: the nature of the period-luminosity relation in various passbands, the impact of metallicity on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Classical Cepheid distances. All these topics are actively debated in the literature.^{[2][8][11][24][25][26][27][28][29][30][31][32]}

These unresolved matters have resulted in cited values for the Hubble constant ranging between 60 km/s/Mpc and 80 km/s/Mpc.^{[7][8][10][11][12]} Resolving this discrepancy is one of the foremost problems in astronomy since the cosmological parameters of the Universe may be constrained by supplying a precise value of the Hubble constant.^[10][12]

Examples

Some fairly bright Classical Cepheids which exhibit variations discernable with the naked eye include: Eta Aquilae, Zeta Geminorum, Beta Doradus, as well as the prototype Delta Cephei. The closest Classical Cepheid is the North Star (Polaris), although the star exhibits many peculiarities and its distance is a topic of active debate.^[33]

See also

• Type II Cepheids
• RR Lyrae variables
• Stellar pulsation theory - Regular versus irregular variability

References

External links

• The Cepheid Distance Scale: A History, by Nick Allen
• SEDS: Variable Stars
• McMaster Cepheid Photometry and Radial Velocity Data Archive
• American Association of Variable Star Observers
• OGLE Atlas of Variable Star Light Curves - Classical Cepheids

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