A tale of two stars

 Two stars of historical and cosmological significance are currently well situated in the northern evening sky: Beta Persei (Algol) and Delta Cephei.  These stars share two things in common: they are both first-of-kind variable stars and they were both subjects of study by the young English astronomer John Goodricke, FRS.

Algol, aka the "Demon Star", is an eclipsing binary whose variability was likely known as far back as the ancient Egyptians.  Goodricke was the first to determine its regular period of 2.86 days and in 1783 (at age 19)  proposed an eclipsing mechanism as the cause.  For this work he was awarded the Copely Medal by the Royal Society.  The next year he discovered the variability of the the star Delta Cephei. Delta Cephei is the prototype of the class of variable stars known as "Cepheids", which are used to determine the distance scale of the local universe.  Goodricke was elected as a Fellow of the Royal Society in 1786 at age 21, but died of pneumonia only four days later and never learned of the honor.

Algol is marked by the white circle.  The Pleiades star cluster is on the right edge and the star Mirfak (Alpha Persei) is on the left. E-M1iii + Sigma 30mm f/1.4  + softon filter. ISO 1600, 30 s.

 Algol (Beta Persei) varies in magnitude from 2.1 to 3.4, a factor of 3.3 in brightness.  It is easily visible to the unaided eye.

The constellation Cepheus.  Delta Cephei is marked by the white circle.  E-M1iii + Sigma 30mm f/1.4 + softon filter.  ISO 1600, 30 s.

Delta Cephei varies in magnitude from 3.5 to 4.4 over a period of 5.4 days.  The brightness changes are caused by radial pulsations of the star's atmosphere. In 1908 Henrietta Swan Leavitt discovered a relationship between the pulsation period and absolute brightness for this type of star.  Her findings were published in 1912.  This relationship is a crucial tool used to establish the distance scale of the local universe (within 20 million ly).

Another interesting star in the above image is the reddish star about a quarter-frame below Delta Cephei.  This is Mu Cephei, Herschel's "Garnet Star".  It s a red supergiant similar to Betelgeuse and is one of the largest and brightest stars in our galaxy.

As usual, click to get access to the full-size images.