Cassiopeia’s “W” Delights
This time of year is ideal for taking in the sights around the constellation Cassiopeia, the distinctive crooked “W” of five bright stars situated high the northeastern evening sky during October. Cassiopeia is circumpolar — so close to the North Celestial Pole (and therefore Polaris) that, for Canadians and other high latitude observers, it never sets. Nowadays, in mid-evening, you’ll find Cassiopeia oriented sideways, about halfway up the northeastern sky, with the broken end “dangling” below the rest. I’ll post a sky chart here.
In Greek mythology, Cassiopeia was queen of Ethiopia and boasted of her daughter Andromeda’s unrivalled beauty. After a long tale involving Perseus, Pegasus, and Cetus the sea monster, the queen and her husband Cepheus were banished to the stars, ever clinging to the spinning north celestial pole. Many cultures have interpreted this set of bright stars. The Inuit see a blubber oil lamp and stand. The Navajo saw a female form too, albeit upside down. Biblical scholars replaced the queen of Ethiopia with Bathsheba, Mary Magdalene, and others.
Cassiopeia is depicted as sitting in a chair, her hand combing her hair. The brightest star, second from the top, is named Shedir, Arabic for “breast”. It’s a giant star, 229 light-years from Earth. In binoculars, you’ll see that it’s an orange-ish star with a small companion. The second brightest star, named Caph (“hand”), is at the top of the “W” and is white, 54 light-years away. The fourth star from the top is named Ruchbah (“knee”). It’s a variable brightness eclipsing binary star that dims when an orbiting dimmer companion star partially blocks its light every 25 months.
The star marking the queen’s waist, in the middle of the constellation doesn’t have an Arabic name. Instead it is designated Gamma Cas, using the third Greek letter for the third brightest star in Cassiopeia. It also has the unofficial nickname Navi, which is Apollo astronaut Gus Grissom’s middle name spelled backwards. This star is also variable due to a high rate of spin and periodic gas ejection. The last of the five stars marks Cassiopeia’s foot. Its nickname, Segin, is of unknown origin. This modest looking star is actually a blue-white giant star about 440 light-years away. It’s about six times the mass of our Sun, but 2,500 times more luminous!
The thinning outer edge of the Milky Way passes directly through Cassiopeia, meaning that the area is rich in interesting objects and star fields. One of my favorite objects can be seen in binoculars or a telescope. It’s a cluster of stars called the Owl Cluster (or ET Cluster or Dragonfly Cluster). It consists of two prominent yellow stars that form the eyes. A sprinkling of dimmer stars forms the owl’s body, and two curving chains of stars define upswept wings. Be aware that the critter is positioned with head to the lower right. This will be the same in binoculars, but your telescope will flip things around depending on the type. It is located by taking Navi and Ruchbah and making them the two vertices of a right angle triangle. The cluster sits at the third vertex, where the 90 degree corner is. It’s about four finger widths to the lower right of Navi — as if the queen is bouncing a baby owl on her knee!
The cluster was discovered by William Herschel in 1787 and is more than 7,900 light-years away! Other astronomical names for the object include NGC 457 (from the New General Catalogue) and Caldwell 13.
While you’re gazing at that part of the sky, use the top three stars of Cassiopeia as an arrowhead that points directly at the great Andromeda Galaxy. The galaxy is approximately 1.5 fist widths (at arm’s length) from the tip of the arrow. At 2.5 million light years away, its faint smudge is among the farthest objects visible to unaided human eyes. Under dark skies, you might be able to detect its glowing patch spanning more than two finger widths across, or two Full Moon diameters. In actuality, it’s six Moon diameters across!
Early October is the best time in the year to see the dim glow of the zodiacal light in the pre-dawn eastern sky. It appears as a triangular pillar of faint light reflected from millions of interplanetary particles. It lies along the ecliptic, which tilts towards the south. Don’t confuse it with the Milky Way, which is much further south and visible all night long.