Perseus
Perseus the hero, holding the winking "Demon Star" Algol. The hero who beheaded Medusa became a constellation.
Science
The scienceThe most famous star in Perseus is Algol, known as the "Demon Star." Algol is an eclipsing binary of two stars orbiting each other; every time the dimmer star passes in front of the brighter, its brightness drops sharply and then recovers on a cycle of about 2.9 days. This change, as if the star were winking, can be seen with the naked eye, and the ancients gave it the ominous name "the head of the demon (ghoul)." This constellation is also the radiant of the "Perseid meteor shower," which peaks each August and is counted among the most spectacular meteor displays of the summer nights.
Mythology
How it became a constellationThe hero Perseus was given the task of slaying Medusa, the monster who turned to stone anyone who looked at her. By reflecting her in his mirror-bright shield, he beheaded her without looking directly at her. On his way back he found the princess Andromeda about to be sacrificed to a sea monster, and drawing out Medusa's head, he turned the monster to stone and saved her. In the constellation, Perseus is drawn with a sword in one hand and Medusa's head (Algol) in the other.
Observing Guide
how to observe🔭 How to find it
On an autumn evening, shift your gaze from the "W" of Cassiopeia eastward along the Milky Way (to the lower right) to find Perseus, its stars scattered like a figure with arms and legs spread. It lies over the Milky Way between bright Capella in Auriga and Cassiopeia, so look about midway along the line joining those two to find it easily. Its brightest star, Mirfak, anchors a gentle curve of stars.
✨ What to look for
Over the Milky Way between Cassiopeia and Perseus shines the "Double Cluster" (NGC 869/884, h and χ), two open clusters glowing side by side — two hazy patches to the naked eye and, through binoculars, a spectacular cascade of hundreds of stars. To the south lies the bright open cluster M34, and Algol, "the Demon Star," is an eclipsing binary that changes brightness over a period of about 2.9 days, a variation you can track with the naked eye.
📍 Where to see it (US · Canada · UK · Australia)
From the US, Canada and UK, Perseus rides high on autumn and early-winter evenings (October–January); with stars reaching declination +54° it is largely circumpolar from Canada and Britain, visible in all but the height of summer, and every August its famous Perseid meteor shower radiates from here. From Australia, Perseus barely rises above the northern horizon on late-spring evenings and its northern parts stay hidden, so it is poorly seen.