From the July 2009 issue

Track down the original Cepheid variable

The first known Cepheid variable star lies in the southeastern corner of Cepheus the King. Here's how to find it.
By | Published: July 27, 2009 | Last updated on May 18, 2023
September 2009 WE Cepheus
The house-shaped constellation Cepheus harbors one of the sky’s most famous stars: Delta (δ) Cephei. You can find it just off the house’s southeastern edge.
Astronomy: Roen Kelly
The Andromeda Galaxy (M31) lies nearly 3 million light-years from Earth, while the Virgo cluster of galaxies extends 50 million light-years into the “local” universe. Have you ever wondered how astronomers measure such vast distances? The answer lies in the autumn sky with the extraordinary star Delta (δ) Cephei, which at first glance appears unassuming.

In 1784, astronomer John Goodricke noticed that Delta Cephei varied from magnitude 3.6 to 4.3 and back again in a period of 5.366 days. Over the years, observers came across a host of other stars showing similar patterns of variation, although the periods ranged from a couple of days up to about 50 days. They became known as Cepheid variables, named after the prototype found in the constellation Cepheus.

The breakthrough came in 1912 when astronomer Henrietta Leavitt discovered 25 Cepheids in the Small Magellanic Cloud, a satellite galaxy of the Milky Way. She found that the brighter a Cepheid appeared, the longer it took to go from maximum light to minimum light and back. Once astronomers calibrated this so-called period-luminosity relation through observations of a few nearby Cepheids, they could calculate the distance to any Cepheid. They simply had to measure the star’s period and compare the observed brightness with the absolute brightness derived from the relation. Because all Cepheids are luminous supergiant stars, we can see them across vast distances, even tens of millions of light-years away in other galaxies.

Autumn evenings provide a good opportunity to follow Delta Cephei’s changing brightness. First locate Cepheus, which passes nearly overhead on fall evenings. Delta lies just off the southeastern corner of this house-shaped constellation. Watch the star every clear night during the course of a week or two, and compare its brightness with those of the surrounding stars. With practice, you’ll notice its brightness changing with a predictable pattern, the pulsating heart of the universe’s most important yardstick.