delta Cephei

The namesake star in the very important class of stars known as Cepheid variables, this star formed part of the original study in which Henrietta Leavitt first discovered that the periods of luminosity were related to their absolute luminosity. This has proved to be an important distance measuring tool.

Analysis of the spectrum of delta Cephei suggests that along with the variation in brightness there is a velocity of somewhat over 20 km/s associated with the orbit, a swing in temperature between 5500 K and about 6600 K, and a change in diameter of about 15% (Kaufmann).

Cepheid Variables

Named after delta-Cephei, Cepheid Variables are the most important type of variable because it has been discovered that their periods of variability are related to their absolute luminosity. This makes them invaluable as a contributer to astronomical distance measurement. The periods are very regular and range from 1 to 100 days.

The shape of the Cephiad luminosity curve is often referred to as a "shark fin" shape when plotted as magnitude vs period. It should be noted that the smooth curve is an average behavior. There is considerable scatter about such a curve, at least in the observations.

The above period-luminosity curve plotted as a function of multiples of the Sun's luminosity (Bennett, et al.) shows the kind of scatter in the dependence of absolute luminosity on period. A Cepheid variable nevertheless gives a good indication of distance when used as a standard candle. The distances to 273 such Cepheid variables were measured directly by stellar parallax by the Hipparcos satellite.

There is some scatter in the measured luminosity as well. Consider a collection of data posted by the AAVSO from their international database:

This composite light curve is described as being composed of almost 750 observations from 35 observers. Despite the apparent scatter, the long term reproducibility of this pattern has led to a value of 5.366 days for the period of delta Cephei. Many Cepheids have periods that are known to the second according to AAVSO.

The Cepheid variables described above are from Population I stars and are sometimes called Type I Cepheids. There is another class of variable star called a W. Virginis Cepheid which is about 4 times less luminous. They are from older, metal-poor star populations (Population II) and are sometimes called Type II Cepheids.

References:

http://www.aavso.org/ American Association of Variable Star Observers home page.
http://www.aavso.org/vstar/vsots/0900.shtml Document celebrating delta Cephei as "Variable Star of the Month" in September 2000.

Cepheid Variable Distance

Cepheid variable stars have proved to be one of the most valuable methods for distance determination because their period of variability has been shown to be related to their absolute luminosity by a period-luminosity relationship.

They can then be calibrated as standard candles for distance calculation. This led Campbell and Jacchia in their 1941 book The Story of Variable Stars to give the description "Cepheids are the most useful stars in the sky." Measuring their apparent luminosity then allows a straightforward calculation of their distance. Cepheid variables can be seen and measured out to a distance of about 20 million light years, compared to a maximum distance of about 65 light years for Earth-based parallax measurements and somewhat over 100 pc (326 light years) for the Hipparcos space-based instrument. The brightest cepheid variable stars have absolute magnitudes of about -6.