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Gamma Cephei: A future Pole Star

Gamma Cephei will someday be a Pole Star for Earth.

Image via Tim Jones/McDonald Observatory.

Artist’s conception of Gamma Cephei’s planet, found in 2002, and its view of the two stars in the Gamma Cephei system. The planet, shown here with rings, orbits the bright yellow star on the right every 2.5 years. This was the first planet found in a close binary system. Image and caption via Tim Jones/McDonald Observatory.

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

Gamma Cephei as a future North Star

Gamma Cephei has the first planet found in a close binary system

Cepheus can be found in the northern sky.  It looks very much like a child's drawing of a house.  The star Gamma Cephei, or Errai, marks the peak of the roof of the house.

Cepheus can be found in the northern sky. It looks very much like a child’s drawing of a house. The star Gamma Cephei, or Errai, marks the peak of the roof of the house.

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.

The 26,000-year precession cycle causes the north celestial pole to move counter-clockwise in front of the backdrop stars at about one degree every 72 years

Animation showing 26,000-year precession cycle relative to backdrop stars. This cycle causes Earth’s northern axis to point out at an ever-changing succession of North Stars. Image via Wikimedia Commons

Gamma Cephei as a future North Star. Our present North Star, which we know as Polaris, will continue to reign as the northern pole star for centuries to come.

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.

Polaris: The North Star

Bruce McClure