Although Gamma Cephei – also known as Errai – rates as only a third-magnitude or moderately bright star, it is easy to find and quite visible in a dark country sky. To many stargazers, the constellation Cepheus the King looks like a child’s depiction of a house, with Gamma Cephei marking the peak of the roof. This is a fascinating star – a future North Star. It also plays an important role in this history of our understanding of extrasolar planets, that is, planets orbiting distant stars.
How to find Gamma Cephei. Do you know the M-shaped or W-shaped constellation Cassiopeia? If so, then draw a line between the star Caph at one end of the M (or W) toward Polaris, our present-day North Star. Gamma Cephei – aka Errai – is just to one side of that line, a bit more than midway along it.
Think of it this way. Cepheus the King is not a particularly prominent constellation, but you’ll know that you’ve found Cepheus, because you’ll see his more striking wife – the M or W-shaped constellation Cassiopeia the Queen – standing at his side.
Or use the familiar Big Dipper asterism to find Gamma Cephei. The two outer stars in the Dipper’s bowl are Merak and Dubhe, sometimes called the Pointers, because a line between them extended northward points to Polaris. Then jump one fist-width – held at arm’s length – beyond Polaris to Gamma Cephei.
For most of the Northern Hemisphere, orange-colored Gamma Cephei shines as a circumpolar star. Circumpolar stars are stars that neither rise nor set, but always appear above the horizon.
But our present-day Polaris won’t remain the North Star forever, due to a motion of Earth known as the precession of the equinoxes. Gamma Cephei stands next in line to inherit the North Star title. This star will be closer to the north celestial pole than Polaris around 3000 CE. It will most closely mark the north celestial pole around 4000 CE.
But – due to precession – Earth’s northern axis will continue to trace its great circle among the northern stars. Around 7500 CD, Alderamin – Cepheus’ brightest star – will become the North Star. And ultimately, of course, our present-day Polaris will be the North Star once more.
Gamma Cephei has the first planet found in a close binary system Gamma Cephei is a binary star – two stars revolving around a common center of mass. One component is an ordinary main-sequence star, somewhat similar to our sun. The other star has less than half our sun’s mass and is considered a red dwarf.
In 2002, astronomers with the McDonald Observatory Planet Search project found a planet for Gamma Cephei. It was the first planet orbiting a star in a close-in binary star system. The discovery had implications for the number of possible planets in our galaxy, because unlike our sun, most stars are in multiple systems. However, planets in multiple systems have their own inherent challenges. For example, some orbits for planets of multiple star systems are not possible for dynamical reasons; a planet would be ejected from the system, or transferred to a more inner or outer orbit.
That said, there are indeed many planets in multiple star systems known today. As of September 28, 2013, a total of 986 planets in 750 planetary systems have been found, including planets in 168 multiple planetary systems. (Source: Exoplanet.eu)
But Gamma Cephei’s planet will always be the first in a close binary!
Bottom line: The star Errai or Gamma Cephei is a binary star system with at least one planet. This star – at the peak of the “roof” in the house-shaped constellation Cepheus the King – will someday be a North Star for Earth.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.