Cepheus

by Chris Lancaster

This constellation crosses the meridian at 9pm during the middle of October and can be found in the northern sky between the cross shaped constellation of Cygnus and the "W" pattern of Cassiopeia. Its boundaries stretch all the way to Polaris, the north star, so since it is situated so near the pole, most of the constellation remains above the horizon at all times. Even its most southerly stars rise again no more than 4 hours after setting.

Cepheus was king of an ancient land in northern Africa with Cassiopeia as his queen. These two in addition to their daughter Andromeda form a family portrait of sorts in the sky, all sitting side by side in the area of the heavens north of +30 degrees declination and between 21h and 2h RA.

In 1784, John Goodricke discovered that Delta Cephei, in the extreme southern part of the constellation, exhibited variations in brightness which modern studies have precisely measured to be 5.366341 days. The variability results from an actual pulsation of the star rather than other phenomena like eclipses or outbursts. A multitude of these pulsating stars in the Milky Way and other galaxies have since been discovered, and are all called Cepheid variables after this star. This stellar class is very important to Astronomers since the period of pulsation, which generally ranges from a few hours for the dimmest Cepheids to as long as 50 days for the brightest ones, correlates to the star's luminosity. Thus, Cepheid variables can be used as accurate measuring sticks for determining distances to other galaxies when the period and apparent magnitude of these stars are known.

Although Delta is an easier star to see, its magnitude range of 3.6 to 4.3 is less noticeable than U Cephei, which ranges from 6.8 to 9.2 magnitudes. U is an eclipsing binary at RA 1h 2.3m Dec +81d 52.5', or if you look in the area one-third the distance from Gamma Cephei to Polaris and move 1h 25m east of that line (a short distance at this declination - about 1.5 degrees), you will see a group of four stars forming a crooked line 23' long and all near 8th magnitude. U is the southernmost of this group and if glowing at its brightest it will outshine the other three. If at its dimmest, it will fall short of the others. Its period is about 2.5 days and takes only 8 hours to dim and return to normal after spending 2 of those hours in its eclipsed phase. The finder chart below indicates magnitudes without the decimals. North is left, south is to the right.

Now, point your scope to Mu Cephei. This is the famous "Garnet Star" which gets its nickname from the rich orange-red color it shows. This is an irregular variable with fluctuations spanning hundreds of days, but it always remains visible to the naked eye under suitable conditions. It forms a wide triangle with Alpha and Zeta Cephei, and if you have the patience, you may notice its variability from magnitude 3.7 to 5.0. As a point of interest, Mu is the north polar star of the planet Mars.

Among the brighter deep sky objects in Cepheus are NGC6939 and NGC6946. NGC6939 is an open star cluster made of a scattering of dozens of stars ranging between 12th and 15th magnitude in an area about 8' in size. NGC6946 is a magnitude 11 spiral galaxy of about the same size to the southeast. It looks slightly oval and is generally of uniform brightness across its face. I mention these together because they are only about 40' apart and present a contrasting duo in an eyepiece capable of about a 1 degree field of view. Center your scope at RA 20h 33m Dec +60d 20' (or about 2.5 degrees south of Theta Cephei and slightly east) and you'll see this pair.

Sharp eyed observers will see the faint nebulosity of NGC7023. It surrounds a 7th magnitude star at RA 21h 0.5m Dec +68d 10', or 3.5 degrees southwest of Beta Cephei. It's central glow is roughly rectangular in shape and fades quickly to the edge of its total area of 18'.

There are many more galaxies, nebulae, and star clusters in Cepheus. You may use a star chart with which to guide you or simply browse the constellation on your own journey of discovery.