Observ Fing Project 12D The Pulsations of Cepheid Variables

Because its luminosity is directly tied to its period of variability, the Cepheid variable is the most valuable measuring stick that astronomers have available to them. Their awesome brightness allows astronomers to find them in distant galaxies and thus determine the distance to those galaxies by comparing apparent magnitude against an easily and precisely determined absolute magnitude or luminosity. Here are three that are visible at differing times of the year.

Delta Cephei is the prototype for this type of star. It falls from a peak brightness of +3.5 to +4.4 over a period of exactly 5.37 days. Unlike with the eclipsing binaries, there is no secondary peak with the Cepheids. What is something of a surprise is that the light curve is not symmetrical. It takes Delta Cephei much longer to fade than it does to recover. The fall of the star's brightness takes almost four days. It recovers in less than a day and a half before starting the cycle all over again.

Zeta Geminorum is another example of a Cepheid variable, ranging in magnitude between +3.6 and +4.2. The cycle of the star's brightness takes 10.15 days to complete. Unlike with Delta Cephei, the light curve of Zeta Geminorum is symmetrical, taking about five days to fall and five days to rise back to full brightness.

Eta Aquilae is a late summer and fall variable located near Altair. Its light curve closely resembles that of Delta Cephei in that the star fades from a peak of +3.5 down to +4.4. The period is somewhat longer than Delta Cephei at 7.18 days and exhibits a similar type of asymmetric decline. There is one little oddity in the light curve of the star that you might notice: a small temporary reversal during the decline before the fall off resumes again.

Note the periods of the stars carefully as they brighten and dim and make pictures of them each night of the cycle that clear weather permits. The pulsations of the Cepheid-type variables are very important to astronomers and their regularity is one of the amazing things in the universe to watch, especially considering the fact that the reason for the pulsations is the lack of equilibrium in the star's core.

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