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Delta Cephei

August 3, 2015

A recent discovery about a well-known star was like finding out that your long-time best friend has a little brother you never knew about.

Delta Cephei is one of the leading lights of Cepheus, the king, which is in the north and northeast at nightfall.

Astronomers have been paying close attention to Delta Ceph for more than two centuries. That’s because its brightness varies over a period of a few days. At its peak, Delta Ceph is roughly twice as bright as at its faintest.

In fact, it’s the prototype of an entire class of variable stars. Cepheids are unstable, so they pulse in and out like a beating heart. There’s a relationship between the length of each beat and the star’s brightness. Astronomers can use that relationship to measure the star’s distance. And Cepheids are bright, so they can be detected in other galaxies. That makes them valuable tools for helping to measure the scale of the entire universe.

A few months ago, a team of astronomers reported the discovery of a small companion to Delta Ceph. The two stars are so close together that the astronomers couldn’t see them as individual stars. Instead, they detected the companion’s effect on Delta Cephei’s motion through space.

The two stars follow an elongated orbit that brings them close together once every six years. They get so close that Delta Cephei may bulge out toward the companion. Astronomers will have to take that into account as they use Cepheids as cosmic yardsticks.


Script by Damond Benningfield, Copyright 2015

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