The Andromeda Galaxy, M31, is almost over head for observers in the northern latitudes, although for those in the southern hemisphere it remains disappointingly low. There are few bright stars on display at this time of year, but the Great Square of Pegasus is easy to see from both hemispheres. Other constellations include Taurus the bull and Cygnus the swan in the north, and Dorado the goldfish and Vulpecula the fox in the south.
This month we feature 3 constellations: Cepheus the king, Cassiopeia the queen, and Perseus the hero.
This large constellation north of Aquarius and Pisces adjoins Andromeda. It represents the upper body of the winged horse that, in Greek mythology, sprang from the body of Medusa when she was beheaded by Perseus. The most distinctive feature in Pegasus is the Great Square formed by Alpha, Beta, and Gamma Pegasi, and Alpha Andromedae. The area inside the square is relatively barren, containing no stars brighter than 4th magnitude.
Two open clusters known as the Double Cluster, mark the hand of Perseus. When seen with the naked eye, the pair resembles a knot that appears brighter than the surrounding Milky Way. They are a spectacular sight to see through binoculars. The Double Cluster was known since antiquity as a faint cloud in the northern Milky Way and was discovered by William Herschel. The pair are also known as NGC 869 and NGC 884. Each cluster contains roughly 300 stars each, a few of which are around 50,000 times more luminous than our own Sun.
The most distant object visible to the naked eye is a spiral galaxy similar to our home galaxy. Also called the Andromeda galaxy, it can be seen with the naked eye in rural skies, but its full extent is better appreciated through binoculars. M31 is about 2.5 million light-years away, so the light by which we now see it left when mankind's ancestors still roamed the plains of Africa.
At an apparent magnitude of 3.4, the Andromeda Galaxy is notable for being one of the brightest Messier objects, making it easily visible to the naked eye even when viewed from areas with moderate light pollution.
Perseus represents the mythological Greek hero who decapitated the fearsome Medusa, whose gaze could turn men to stone. On his way back from this exploit, Perseus rescued Andromeda from the jaws of a sea monster. In the sky, Perseus lies next to Andromeda and her mother, Cassiopea, forming part of a great tableau depicting this most famous of Greek myths. Perseus is represented by brandishing his sword in his right hand, marked by the twin star clusters NGC 869 and NGC 884, while in his left hand he holds the head of Medusa, marked by the star Beta Persei, better known as Algol. A rich part of the Milky Way runs through Perseus, making it an attractive constellation for binocular users.
Points of Interest
A yellow-white super giant of magnitude 1.8, the brightest star in the constellation. It is a prominent member of a large and loose cluster of stars known as Melotte 20, visible with binoculars, which lies about 600 light-years away.
A famous eclipsing binary star consisting of a close pair of stars in orbit around each other. When the brighter star is eclipsing the fainter one, every 2 days and 21 hours, the magnitude drops from 2.1 to 3.4 for about 10 hours.
A variable red giant. Due to changes in size, it varies from magnitude 3.3 to 4.0 in a cycle lasting about 7 weeks.
An open cluster about 1,500 light-years away. It is visible with binoculars or a small telescope and appears about the same size as the full Moon. The cluster's brightest stars are of 7th magnitude.
NGC 869 and NGC 884 (Double Cluster)
Two open clusters just visible to the naked eye; an excellent sight through binoculars or a small telescope. Each cluster covers about the same area of sky as the full Moon.
This attractive constellation represents the mythical Queen Cassiopeia. Her husband and daughter are represented by the adjacent constellations Cepheus and Andromeda. Cassiopeia was notoriously vain and is depicted sitting on a throne, fussing with her hair. Cassiopeia's brightest stars form a distinctive W-shape. Epsilon Cassiopeiae, at one end of the W, marks the queen's ankle while Beta, at the other end, lies in her shoulder.
Points of Interest
A variable star, currently about magnitude 2.2. Its rapid spin causes rings of gas to be thrown off its equator, changing its brightness temporarily.
A double star. It consist of a yellow star of magnitude 3.5 and an orange companion of magnitude 7.5 that can be seen with a small telescope. The two stars lie 19 light-years away, forming a genuine binary pair with an orbital period of 480 years.
A highly luminous yellow supergiant. As a result of pulsations in its size, it varies between about 4th and 6th magnitudes in a cycle that lasts just under a year.
An open cluster. It is visible with binoculars, covering an area about one-third the apparent size of the full Moon, although a telescope is needed to show its individual stars. It lies 5,200 light-years away. A 5th magnitude star that appears to be a member of the cluster actually lies much closer to us.
An elongated open cluster, about one-third the apparent size of the full Moon and visible through binoculars or a small telescope. The cluster's appearance has been compared to an owl, with its two brightest stars marking the owl's eyes. The brightest star is the 5th magnitude Phi Cassiopeiae, a luminous supergiant.
Cepheus is a far-northern constellation, adjoining Cassiopeia and extending almost to the north celestial pole, representing the mythical King Cepheus, the husband of the vain Queen Cassiopeia and father of Andromeda. Its brightest star is Alpha Cephei, of magnitude 2.5.
Cepheus is home to the open cluster known as NGC 188, and is the nearest open cluster to the north celestial pole. It is also one of the oldest known open clusters.
POINTS OF INTEREST
A double and variable star. The brighter component is a blue giant of magnitude 3.2, about 600 light-years away. It is a pulsating variable star (the prototype of a group called Beta Cephei stars), although its variations are so small that they are barely perceptible to the eye. A small telescope shows a companion of magnitude 7.9.
A double star and a famous variable, is the prototype of so-called Cepheid variables, which astronomers use for finding distances in space. Such stars change in brightness as they pulsate in size. Delta Cephei itself varies between magnitudes 3.5 and 4.4 every 5 days 9 hours. It lies about 1,000 light-years away. A small telescope shows a wide companion of magnitude 6.3.
A variable star with a strong red color, hence its popular name, which is noticeable through binoculars or a small telescope. It is a red supergiant that pulsates in size, varying from magnitude 3.4 to 5.1 about every 2 years.