Cepheus the King (left) and Cassiopeia the Queen as depicted in an 1825 star atlas. In Greek mythology, they represented the king and queen of Ethiopeia
Anybody can find the W of Cassiopeia the Queen. Even if you’ve never seen the constellation, all I have to do is point and say “See that zigzag of five stars?” and it’s instant recognition. Not so with Cassiopeia’s husband and long-time companion, Cepheus. Five stars arrayed in a pentagon outline the form of Cepheus (SEE-fee-us) the King, who occupies the royal throne at the Queen’s side in the northern sky. It wouldn’t be the first time a king is overshadowed by his queen.
Because Cepheus isn’t far from the ever-present, never-setting North Star, it’s visible nearly year round for northern hemisphere observers. Here in Duluth, the he never sets, making him a member of the select group of what astronomers call the circumpolar constellations. Cepheus is overlooked by many, because his stars are fainter than Cassiopeia’s. Once you learn proper way to approach the king, I trust you’ll pay him a visit the next clear night.
Cepheus is wedged between the North Star and the zigzag W of Cassiopeia in the northern sky in early September. The hazy band is the Milky Way. Stars of interest are marked. Created with Stellarium
September is a perfect month to get to know the constellation as it’s high in the northern sky. Start with the W of Cassiopeia, now about halfway up in the northeast at nightfall, and focus on the star Gamma in the center of the W. Reach your fist to the sky and mark off three fists to the upper left to find Polaris, the North Star. Three more fists to the upper right will bring you to Cepheus’ brightest star Alderamin. Notice that Gamma Cas, Polaris and Alderamin form a nearly perfect equilateral triangle.
Now work your way via eyeball from Alderamin back toward Polaris to Arrai, a moderately bright star that forms the point of the pentagon. From there, follow back up the other side of the figure to Delta Cephei. Got it? That’s all there is to it.
Like any king, Cepheus keeps a trove of treasures and freely shares them with any sky watcher who makes the effort. Below you’ll find gems of interest for the naked eye, binoculars and telescope:
Arrai (or El Rai) — Notice that this star is just a short hop from the North Star. Because Earth’s axis slowly wobbles, it traces out a circle in the northern sky over a 26,000 year period. At the moment, the north polar axis points to Polaris, but about the year 4000 A.D., it will be pointing to Arrai in Cepheus. Quietly waiting in the background for now, Arrai will be crowned Pole Star on that future day. The star also is home to an extrasolar planet. Called Gamma Cephei Ab, the Jupiter-like world has about 1 1/2 times Jupiter’s mass and orbits the star in 2 1/2 years.
Xi Cephei — Pronounced ‘Zye SEF-ee-eye’, this is one of the most beautiful double stars and a personal favorite. Any telescope magnifying around 60x will split what appears to be a single star to the naked eye into a tight pair of stars, one white and the other orangish. The stars are magnitudes 4 1/2 and 6 and separated by 8 arc seconds. Xi is also known as Kurhah and located about 100 light years from Earth. Xi is also a great jumping off point to explore the “top” of the constellation in binoculars. The Milky Way passes through this area offering up rich star fields to delight the eye.
The Garnet Star — Also known as Mu Cephei, this star is a red supergiant (similar to Betelgeuse in Orion) and one of the largest stars in the entire sky. Sure doesn’t look impressive at 4th magnitude, but that’s because it’s over 1,800 light years away. If we could haul Mu to our solar system and put it in place of the sun, its surface would reach all the way to Saturn’s orbit. Mu’s other name – the Garnet Star – refers to its lovely, fiery red-orange color. Last night I looked at this monster in 10×40 binoculars and the color was obvious. A telescope shows it well, too.
Delta Cephei — Remember the throbbing, pulsating variable star Eta Aquilae we examined over the weekend? You don’t? Well, here’s the link. Delta is another star of the same ilk, and not just any one, but the prototype for the whole class. That’s why they’re called Cepheids after Delta Cephei. As Delta expands and contracts, it brightens and dims from magnitude 3.5 to 4.4 over a period of 5.4 days. That’s a big enough range to easily follow the changes with your naked eye. By good fortune, two neighboring stars of magnitude 3.4 (Zeta) and 4.2 (Epsilon) make ideal references. Last night Delta was sitting pretty at 3.9.
Comparison of the sun and the supergiant Mu Cephei (left). The time exposure photo of Cepheus at right shows some of the gems described in the blog. Credit: Windows2theUniverse (left); Bob King (right)
Delta is also an easy, bright double star like Xi but with a wider separation requiring only about 20x to split. Any telescope is up to the job. Look for the 7.5 magnitude companion star just 41 arc seconds away. As you bid the king adieu at night’s end, please offer a word of thanks for sharing his treasures.