Although the constellation Cepheus consists only of rather faint stars, its distinctive shape is easily found in the sky. There are three regions of star formation and a number of dark nebulae in this constellation. Due to the precession of the Earth‘s axis, it will become more famous in the next few millennia – in 2,000 years γ Cephei will take over the role of the North Star, and in 7,500 years the northern celestial pole will be located at α Cephei.
δ Cephei is the namesake of an important class of variables – the δ-Cepheids have a fixed relationship between their period and their absolute brightness. If the apparent magnitude and the period between two brightness maxima are known, the absolute brightness and thus the distance can be determined. These stars are therefore used for the distance measuring of nearby galaxies. In the case of the 950 light-years distant δ Cephei, 5 days, 8 hours and 47 minutes pass between two maxima. During this period its brightness varies from 3.6 to 4.3m and its diameter from 32 to 35 solar diameters.
δ Cephei is less known as a double star. 41 arc seconds or 0.2 light-years separate the yellowish variable from a 6.1m bright, bluish white companion. The two stars form a wide, physical double star, although their position has not changed since the discovery by FGW Struve in 1835. The two stars are best separated when δ Cephei is in its minimum, as it will not outshine its companion then.
μ Cephei is also called Herschel’s Garnet Star because of its intense red color – no star visible to the naked eye appears redder. The reason for this is huge dust clouds between us and μ Cephei, which also attenuate the light from the star by 2.5 magnitudes. It owes its name to William Herschel. The brightness of the 5,300 light-years distant star varies with a period of 730 days between 3.4 and 5.1m, a second period complicates the brightness prediction – it lasts 100 to 4,500 days. Just like the fainter Betelgeuse (α Orionis), μ Cephei is also a supergiant. Its brightness equals to that of 44,000 Suns, and its diameter is 2,000 times greater than that of our Sun. If it took the place of our Sun, it would extend beyond the orbit of Saturn. In some old catalogs the stars 13 and 14 Cephei are designated as μ Cephei. Along with IC 1396 to the south, Herschel‘s Garnet Star marks the Cepheus OB2 association.
NGC 188 is one of the oldest known open clusters. It was created five to seven billions of years ago, so that only M 67 in Cancer may be even older. Its position 200 light-years above the galactic disk have it well-protected from the gravitation of other stars, so its members could stay together over such an extremely long period. It consists of 200 suns, but its brightest members reach only 10th magnitude. Therefore, even telescopes struggle with resolving this 15 minutes of arc large star cluster. Its total luminosity is 8.1m; the surface brightness is correspondingly low. The 5,000 light-years away NGC 188 is located only 4° away from Polaris – no cluster is closer to the northern celestial pole than NGC 188.
NGC 6939 is one of the richest star clusters in the northern sky, but with a diameter of only eight minutes of arc it is very compact. Since its brightest stars shine only at 12m, it appears only as untextured, 7.8m bright nebula in binoculars. NGC 6939 is 4,000 light-years away and 800 light-years above the galactic plane. With an age of 1.6 billion years it is very old.
The emission nebula IC 1396 is one of the objects which are rarely observed – with a diameter of nearly 3° it is much too broad for most telescopes. In 10 × 50 binoculars, however, it may be seen at least with averted vision as a gray shimmer – UHC filters increase the visibility clearly, if you can use them. Binoculars with a slightly higher magnification are at an advantage because the sky background is darker with them. But the object still remains hidden for observers close to a city. The nebula is located south of μ Cephei in a 1.5° wide field of stars. Its fluorescence is excited mainly by a 5.6m bright star 1.5° SSW of μ Cephei. Star and nebula are 2,500 light-years away, so the nebula has a diameter of 100 × 125 light-years.