Delta Cephei
Delta Cephei, the orange-colored star in the center of this image, is the original classical Cepheid variable star. It is located in the constellation of Cepheus. Hold your mouse cursor over the image to compare images of it near its minimum and maximum brightness. Its luminosity varies from magnitude 3.48 to 4.37 over a period of 5 days, 8 hours, 47 minutes, and 32 seconds because of pulsations in the outer layers of the star. Its variability was discovered by John Goodricke in 1784. It is easy to observe Delta Cephei and estimate if it is near minimum or maximum, even with the unaided eye, by comparing Delta with Epsilon and Zeta Cephei which form a close triangle with Delta at the southeastern corner of the constellation. Epsilon is a variable that shines at magnitude 4.15 to 4.21, close to Delta's minimum, and Zeta shines at magnitude 3.34, close to Delta's maximum. In 1912 Henrietta Swan Leavitt discovered the relationship between the period of a Cepheid variable and its brightness. The brighter the Cepheid, the longer its period. This period-luminosity relationship can be used to determine a Cepheid's absolute magnitude. The inverse-square law can then be used to determine distance based on the star's apparent brightness. The inverse square law says that a star that is twice as far away will be one-quarter as bright. The relationship between Delta Cephei's period and luminosity was very important in establishing a standard for measuring distances in the universe. In 1923 astronomer Edwin Hubble took photographs of Messier 31, then called the "Andromeda Nebula", with the 100-inch Hooker telescope at Mount Wilson Observatory. Studying plates taken at different times, he discovered Cepheid variable star V1 in M31. V1's period was 31.45 days, and from this he calculated its distance at 900,000 light-years. In the next year Hubble had found 12 Cepheid variable stars in M31. Hubble's study of Cepheid variables in M31 proved it was too far away to be part of our own galaxy. Harlow Shapley had previously measured the Milky Way to be 100,000 light-years in size. If M31 was 900,000 light-years away, it could not be part of the Milky Way, and must be, in fact, another separate galaxy. Later Hubble would use other Cepheid variables in other galaxies to measure their distance and motions. These studies led to the discovery that the universe was expanding. These revelations caused a profound rethinking of our concept of the size and structure of the visible universe. Subsequent studies revealed that there were two types of Cepheid variable stars, with one type being brighter. Hubble had compared the bright kind in M31 to the dimmer type in the Milky Way, leading to an error in its distance estimate. It turned out that M31 was twice as far away as Hubble initially thought. Today, with improved measurements, scientists think that M31 lies at a distance of 2 million to 2.9 million light-years. Delta Cephei is a spectral-class F5 supergiant when it is brightest. When it is dimmest, its spectral class changes to G2 as its temperature changes from 6,800 degrees Kelvin at F5 to 5,500 degrees Kelvin at G2. Delta Cephei is also a wide double star comprised of the pale orange primary and a magnitude 6.3 pale-blue spectral-type B companion separated by 41 arcseconds. The Delta Cephei system is located more than 900 light-years from Earth. Oddly, Delta Cephei does not have a proper name. The first image above was taken on July 30, 2013 at 2:24 a.m. EDT, close to the time of the star's minimum brightness. The second image was taken on August 4, 2013 at 11:52 p.m. EDT, close to the time of star's maximum brightness. More information Delta Cephei and an ephemeris for the maxima of Delta Cephei can be found on the American Association of Variable Star Observers (AAVSO) web site. North is to the bottom in the above image.
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