Perseus: Hero of the Autumn Sky

Perseus is a rich constellation of the autumn and winter Milky Way, home to the famous variable star Algol, the stunning Double Cluster, and the radiant point of the Perseid meteor shower. Positioned between Cassiopeia and Taurus, Perseus is part of a connected family of mythological constellations that spans the autumn sky. Find it with StarGlobe.

How to Find Perseus

Perseus lies in the Milky Way between the W-shape of Cassiopeia to the north and the bright star Aldebaran in Taurus to the south. Its brightest star, Mirfak, sits at the center of a curving line of second and third-magnitude stars that forms the hero's body.

From Cassiopeia, follow the Milky Way band toward the southeast, and the next concentration of bright stars is Perseus. Alternatively, from Capella in Auriga, look to the west-northwest. Perseus is roughly midway between Capella and Cassiopeia.

The constellation's most famous star, Algol, marks the head of Medusa held in Perseus's hand. It sits somewhat apart from the main body of the constellation, to the south and west of Mirfak.

Algol: The Demon Star

Algol (Beta Persei) is the prototype eclipsing binary star. Every 2 days, 20 hours, and 49 minutes, a dimmer companion star passes in front of the brighter primary, causing Algol's brightness to drop from magnitude 2.1 to 3.4 over about five hours, then return. This regular dimming was noted by ancient observers and gave Algol an ominous reputation.

The name Algol derives from Arabic "ra's al-ghul," meaning the head of the demon, referring to its position representing Medusa's head. Ancient Hebrews, Chinese, and Egyptian astronomers all associated the star with misfortune, likely because they noticed its strange behavior. Modern astronomers understand the system as three stars, with the primary eclipse caused by a cooler subgiant passing in front of a hotter main-sequence star.

Watching Algol fade and brighten is one of the most accessible variable star observations. No telescope is needed; simply compare Algol's brightness to nearby stars over several hours on a night when an eclipse is predicted.

The Double Cluster

The Double Cluster (NGC 869 and NGC 884) is one of the most spectacular sights in the sky through binoculars or a low-power telescope. Two rich open clusters sit side by side about halfway between Perseus and Cassiopeia. Each cluster contains several hundred young, hot stars, and together they present a breathtaking field of diamonds scattered on black velvet.

The clusters are about 7,500 light-years away and are physically related, having formed from the same giant molecular cloud roughly 14 million years ago. They are visible to the naked eye as a hazy patch in the Milky Way, and ancient observers including Hipparchus noted them. Curiously, Messier did not include them in his catalog.

Other Deep Sky Objects

The Alpha Persei Cluster (Melotte 20) is a large, loose cluster of stars surrounding Mirfak. It is best appreciated with binoculars, which frame the cluster perfectly and reveal dozens of blue-white stars scattered around the brighter yellow Mirfak. This group lies about 600 light-years away and is one of the nearest open clusters to Earth.

M34 is a bright open cluster visible in binoculars as a hazy spot and resolving into about 80 stars in a telescope. NGC 1333 is a reflection nebula and active star-forming region, while IC 348 is a young cluster still partially embedded in its birth cloud. The California Nebula (NGC 1499), a large emission nebula near Xi Persei, is faintly visible in wide-field photographs.

The Perseid Meteor Shower

The Perseids, peaking around August 11-13, are the most popular meteor shower of the year. They produce 60 to 100 bright, fast meteors per hour under dark skies, many leaving persistent trails. The shower is caused by debris from Comet Swift-Tuttle, which orbits the Sun every 133 years. The radiant point lies in northern Perseus, and the warm summer weather during peak nights makes the Perseids the most widely observed shower worldwide.

Mythology and Cultural Heritage

Perseus was one of the greatest heroes of Greek mythology. Son of Zeus and the mortal Danae, he was tasked with slaying Medusa, a Gorgon whose gaze turned living creatures to stone. Using a reflective shield, winged sandals, and a helmet of invisibility, Perseus beheaded Medusa and later used her head to rescue Andromeda from a sea monster.

The constellation shows Perseus holding Medusa's head (marked by Algol) in one hand and a sword in the other. The surrounding constellations of Cassiopeia, Cepheus, Andromeda, and Pegasus (born from Medusa's blood) complete this mythological ensemble.

Best Time to Observe

Perseus is best observed from October through February. It transits the meridian during December evenings, when it passes nearly overhead for observers in the northern United States and Europe. The constellation is circumpolar from latitudes north of about 50 degrees. From the Southern Hemisphere, only the southern portion of Perseus is visible, and it remains low on the horizon.

Neighboring Constellations

Cassiopeia lies to the north, Taurus to the south, and Andromeda to the west. Auriga with Capella borders Perseus to the east, and Aries and Triangulum sit to the southwest. Map out the hero's sky with StarGlobe.

Quick Facts

Perseus covers 615 square degrees, ranking 24th in size. It contains one Messier object (M34) plus the Double Cluster and numerous other clusters. Mirfak shines at magnitude 1.8. The constellation's right ascension ranges from about 1h 30m to 4h 51m, and its declination spans from about +31 to +59 degrees.