Brightest Stars

Gamma Cephei, aka Errai, a future North Star

Chart showing Cepheus with its house shape and Errai at peak, near constellation Cassiopeia.
Look for the constellation Cepheus in the northern sky. It resembles a child’s drawing of a house. The star Gamma Cephei, or Errai, marks the peak of the roof of the house. It can be found about 2/3 of the way along a line between the star Caph in Cassiopeia and Polaris, the North Star.

Gamma Cephei, also called Errai, is a moderately bright 3rd-magnitude star in the northern constellation Cepheus the King. Cepheus looks like a child’s drawing of a house, with Gamma Cephei marking the peak of the roof. This fascinating star has played a major role in our understanding of exoplanets, especially in multiple star systems. Interestingly, by 4,000 CE, Gamma Cephei will move into position – due to the Earth’s axial precession – to become our North Star.

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Gamma Cephei: Large bright star with lens rays on black field with scattered stars.
An image of Gamma Cephei, taken at Palomar Observatory in visible light. Image via Digital Sky Survey/ Mikulski Archive.

How to find Gamma Cephei

For much of the Northern Hemisphere, orange-yellow Gamma Cephei shines as a circumpolar star. Circumpolar stars are stars that closely circle the Pole Star, neither rising nor setting because they’re always above the horizon.

Do you know the W-shaped constellation Cassiopeia? It’s visible in northern fall and early winter evenings from northerly latitudes. If so, draw an imaginary line between the star Caph, at one end of the W, toward Polaris, our present-day North Star. Gamma Cephei is about 2/3 of the way along that line, located very slightly westward of 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 W-shaped constellation Cassiopeia the Queen – standing at his side.

Another way to find Gamma Cephei, on spring and summer evenings, is to use the familiar Big Dipper asterism. 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.

A future North Star

The Earth’s north rotation axis points to the north celestial pole, which is very close to Polaris, our present North Star. However, a wobbling motion of Earth known as axial precession changes the north and south celestial poles’ locations with respect to the stars, tracing a circle in the sky of their respective celestial hemispheres approximately every 26,000 years.

A line from Earth's axis. Its far end moves around a circle on the stellar background.
Animation showing 26,000-year precession cycle. Image via Tfr000/ Wikimedia Commons.

Polaris will continue to reign as the North Star for several more centuries. Axial precession will gradually move the celestial poles in the sky. Gamma Cephei stands next in line to inherit the North Star title in around 4,000 CE.

As axial precession continue to trace a circle, other stars will take over the mantle of North Star. Around 7,500 CE, Alderamin – Cepheus’ brightest star – will become the North Star. And ultimately, of course, in about 26,000 years, the circle traced out by axial precession will return to Polaris.

Crowded chart with stars, constellation lines, precessional circle, and dates on circle.
The circular path of the north celestial pole (in orange), due to axial precession, relative to the stars. Over the course of about 26,000 years, the Earth’s rotational axis, projected onto the sky, will trace a circle in northern and southern skies. Positive numbers in yellow show dates in CE (Common Era) where the north celestial pole will be located relative to the stars. Negative numbers in yellow represent BCE (Before Common Era) dates. Polaris is shown near the top of the circle at 2,000 CE, our current time. Image via Tau’olunga/ Wikimedia Commons.

The science of Gamma Cephei

Gamma Cephei is about 45 light-years away from us. It’s a binary star, with two stars revolving around a common center of mass every 67 years. The larger star, about 1.4 times the mass of our sun, is an ordinary main sequence star, somewhat similar to our sun. The small star, less than half our sun’s mass, is a red dwarf star.

First exoplanet discovered, lost and rediscovered

Astronomers announced they had found a planet around the Gamma Cephei stellar system in 1988. Over the years, confusion about what type of stars existed in the stellar system called into question whether the planet really existed, so the scientists retracted their finding in 1992. But in 2002, new observations allowed astronomers to refine their data on the two stars in Gamma Cephei, and they realized once again that a planet indeed circled the binary pair.

The planet, named Tadmor, or Gamma Cephei Ab, has a rotation period of about two years around the bigger star in this binary system. Tadmor is about 1 1/2 times the size of Jupiter. If it were placed in our solar system, Tadmor would circle the sun in an orbit between Mars and the asteroid belt.

A round purplish-grey sphere with faint darker purple bands across it.
An artist’s concept of Tadmor, the planet in the Gamma Cephei binary system. Image via NASA.
Ringed planet in foreground with two distant suns.
Another artist’s concept of the exoplanet in the Gamma Cephei system. Image via Tim Jones/ McDonald Observatory.

Bottom line: Gamma Cephei – or Errai – is a binary star system that will someday be Earth’s North Star. Gamma Cephei is home to the first exoplanet ever discovered.

September 22, 2021
Brightest Stars

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