Perseus

A hero in the Milky Way

Perseus is a hero of the northern autumn and winter sky. Its main stars form a curving line, sometimes called the Segment of Perseus, that runs through the Milky Way from Cassiopeia toward Taurus.

Because it sits in the band of our galaxy, the area is crowded with faint stars and clusters, and slow sweeps with binoculars are rewarding. From about 40°N its northern stars barely set, so part of Perseus is visible on most nights of the year: it skims low across the northern sky on late spring evenings and returns high overhead in winter.

How to find it

Start with Cassiopeia's W. Extend a line from the W toward the east-southeast and you reach Mirfak, the brightest star in Perseus, at the center of a curved arc of stars. The arc bends away from Cassiopeia, toward Capella in Auriga.

Algol lies about 9 degrees south of Mirfak, off the side of the arc, marking the head of Medusa that the hero carries. Continue south-east and you reach the Pleiades in Taurus. The Double Cluster lies between Mirfak and the W of Cassiopeia, slightly closer to Cassiopeia.

The main stars and their names

Mirfak (Alpha Persei), also called Algenib, means 'elbow' in Arabic. It is a yellow-white supergiant of magnitude about 1.8, roughly 500 light-years away, though estimates vary.

Algol (Beta Persei) is the most famous eclipsing binary, a pair of stars whose orbit, seen nearly edge-on from Earth, makes one pass in front of the other. Normally about magnitude 2.1, it drops to about 3.4 every 2.87 days, about 2 days and 21 hours. The dip and recovery take about 10 hours. The system is about 90 light-years away.

Its name comes from the Arabic ra's al-ghul, 'the demon's head'. The Italian astronomer Geminiano Montanari recorded its changes in 1667, and in 1783 the young English astronomer John Goodricke measured the period and suggested that a darker body passes in front of the star, which is essentially correct.

Deep-sky objects for binoculars and small telescopes

The Double Cluster (NGC 869 and NGC 884) shows as a hazy spot to the naked eye from a dark site. Binoculars reveal two rich clusters side by side, each with hundreds of stars, about 7,500 light-years away. In a small telescope at low power, a few orange giants stand out among the blue-white stars.

Around Mirfak, the Alpha Persei Cluster is a group of bright blue-white stars spread over several degrees. It is too large for most telescopes but perfect for binoculars. M34, about 1,500 light-years away and around magnitude 5.5, is a bright open cluster halfway between Algol and Almach in Andromeda.

M76, the Little Dumbbell Nebula, is a small planetary nebula of magnitude about 10 that needs a telescope. The California Nebula (NGC 1499) is a long cloud of glowing gas north of the Pleiades; it is a popular target for photography but very hard to see with the eye.

The Perseid meteors

The Perseids are one of the most reliable meteor showers of the year. They peak around August 12–13, when Earth passes through dust shed by comet 109P/Swift–Tuttle, and under dark skies they can bring 50 to 100 meteors an hour.

The radiant, the point in Perseus from which the meteors seem to come, rises in the north-east in the evening and climbs higher through the night. The best rates come in the hours before dawn. Check the Moon's phase for the year: a bright Moon washes out the fainter meteors.

Mythology

In Greek myth Perseus was the son of Zeus and the mortal princess Danaë. Sent to bring back the head of Medusa, a Gorgon whose gaze turned people to stone, he received help from the gods: winged sandals, a helmet of invisibility and a polished shield. By watching Medusa's reflection in the shield, he could approach and cut off her head.

On the way home he rescued Andromeda from a sea monster, so he stands in the sky next to her and her parents, Cassiopeia and Cepheus. In star maps he holds a sword in one hand and Medusa's head, marked by Algol, in the other. In Turkish the constellation is known as Kahraman, the Hero, or simply Perseus.

Practical observing tips

Perseus is best on autumn and winter evenings, high in the north-east to overhead. For Algol, look up the predicted times of minimum for the coming week; several astronomy magazines publish them. Compare Algol with nearby Almach in Andromeda, which is about the same brightness when Algol is normal. A few hours before minimum, you will see Algol become clearly fainter.

For the Perseids, find a dark site away from lights, lie back in a reclining chair and watch a wide area of sky rather than the radiant itself. Give it at least an hour, because meteors come in irregular bursts.