The most apparent constellations found overhead in January’s evening skies are Orion, Taurus, Auriga and Gemini. The western horizon still has Cygnus standing upright but setting quickly as night falls. The Great Square of Pegasus can be found high in the western sky. On the eastern horizon you can find Sirius; the brightest star visible from Wyoming. High in the eastern sky look for the bright glow of Pleiades, a gorgeous and bright star cluster.
January is a good time of year to hunt down Perseus the Hero. The constellation of Perseus can be found high overhead during the winter evenings, but his form is not distinct or easy to pick out. If you find Cassiopeia, which looks like a big W or M and then find the Pleiades in Taurus, Perseus will be found about midway between the two of them. The two brightest stars of Perseus are Algol and Mirfak.
Mirfak is a yellow giant star shining at magnitude 1.8. It is located a little over 500 light years away, contains 7.3 times the mass of the Sun, has a diameter 60 times larger than the Sun and is 5,000 times more luminous than the Sun. If you look at Mirfak in binoculars, you will clearly see the Alpha Persei star cluster.
Since ancient times, Algol has been considered an unlucky or demon star. The name means “the ghoul” and it is indeed an unusual star. Once every three days it suddenly dips in brightness by more than a magnitude going from 2.1 to 3.4 magnitude. Algol is a prime example of an eclipsing binary or a double star system. The more massive primary star is a brilliant blue-white giant and its companion is an even larger yellow star but it is not as bright as the primary. The two stars orbit each other around their common center of mass every 3 days. The orbital plane is oriented in such a way that from our perspective the stars eclipse each other. The sudden dips in brightness occur when the dimmer companion passes between us and the primary, blocking the light of the brighter star. There is a less dramatic dip when the secondary star is eclipsed. The two stars are about 6 million miles apart and their atmospheres may intermingle.
Perseus lies directly along the Milky Way and is in an area of the sky rich with stars and star clusters. One of the spiral arms of the Milky Way Galaxy takes its name from this constellation.
If you have binoculars, you will want to scan the sky in Perseus. You will be able to see dozens of sparkling stars to the southeast of Mirfak. This is called the Perseus III Association, a group of stars slowly moving away from each other. A few millions years ago, it may have appeared as tight as the Pleiades.
As you look around the area between Mirfak and Cassiopeia you may see a glint on the sky that’s barely visible to the unaided eye, this is the famous Perseus Double Cluster. The Double Cluster shines at 4.3 magnitude but you’ll need dark skies to see it with the naked eye. This double cluster of stars pops in to view in binoculars but is magnificent with wide-field telescopes. The clusters go by the names of NGC 869 and NGC 884.
The clusters are very young; perhaps no more that a million years has passed since they condensed from a giant nebula of gas and dust. There are thousands of stars in the clusters, but we see only the blue and white giants with a few red giant stars.
While gazing at the Perseus Cluster, look over at the nearby Pleiades Star Cluster. Both clusters are very similar in that they are very young star clusters and contain hundreds of stars more massive and luminous than our sun, but Pleiades appears brighter because it lies a mere 400 light years away while the Perseus Double Cluster is a much more distant object located 7500 light years away.
Perseus had a Nova appear in 1901, which became as bright as the star Capella. After appearing, the brightness of the nova steadily dropped for a few weeks then became variable before fading away. Today we can see a shell of expanding gas from this stellar flareup. We now know this is a reoccurring nova on a star located 1500 light years away.
An interesting telescopic object in Perseus is Messier 76, known as the little Dumbbell. It is a planetary nebula that got its name since it resembles the better known Dumbbell Nebula.
Long exposure photos of Perseus will reveal NGC 1499 also known as the California Nebula. This emission nebula resembles the shape of the state of California and has a surface magnitude of 6. You may be able to catch a glimpse of a gray spot on the sky with binoculars, but only under really dark skies.
The Perseid Meteor Shower is one of the most reliable and active meteor showers of the year. It peaks on about August 13th and averages about 60 meteors an hour. The radiant of the Perseid Meteor shower is found near the Double Cluster.
Another thing to ponder when gazing towards Perseus is that you are looking at another spiral arm of our Galaxy. We are located on the edge of the Orion spur in the Cygnus spiral arm; when looking at the constellation Perseus we are looking through our spiral arm to the next nearest spiral arm known as the Perseus arm. If you trace the Milky Way in Perseus through the sky, you will trace it back toward our galactic center.
Looking past our galaxy Perseus also contains the Perseus Cluster of Galaxies, which contains thousands of galaxies.