Perseus is a large northern constellation crossed by the Milky Way and surrounded by some of the best-known figures in Greek mythology. Its stars include a “Demon Star” that visibly fades every few nights, while every August the constellation becomes the apparent source of one of the year’s most spectacular meteor showers.
How to Find Perseus in the Night Sky
The easiest way to find Perseus is to first find Cassiopeia. Its bright W-shaped pattern is one of the most recognizable sights in the northern sky, with Perseus sprawling just below it. Look for Mirfak, the brightest star in Perseus at magnitude 1.8, lying roughly between Cassiopeia and the bright star Capella. From around Mirfak, the stars of Perseus stretch away in two loose, roughly parallel chains, giving the constellation a long, branching appearance. Algol, its second-brightest star, lies along the southern part of the pattern.
Perseus covers about 615 square degrees, making it the 24th-largest of the 88 constellations. It is best seen on fall and winter evenings in the Northern Hemisphere and is visible from roughly 90°N to 35°S.
The Sky Around Perseus Tells His Story
The myth of Perseus does more than add a story to the stars—it gives you a memorable way to navigate an entire region of the night sky. Start with Perseus and the princess Andromeda beside him, whose stars stretch away toward the Great Square of Pegasus. Above Perseus, the familiar W of Cassiopeia marks Andromeda’s mother, with her father Cepheus nearby, while Cetus, the sea monster she was meant to be sacrificed to, lies farther south. Pegasus, the winged horse born after Perseus beheaded Medusa, completes another part of the story and provides one of the region’s easiest landmarks in its Great Square. The story ends back inside Perseus. Traditional star maps show him carrying Medusa’s severed head, marked by Algol, the “Demon Star.”
Brightest Stars in Perseus
The two loose chains that give Perseus its shape are traced by a handful of bright stars, led by Mirfak and Algol.
- Mirfak (Alpha Persei) – The brightest star in Perseus at magnitude 1.79, Mirfak is a yellow-white supergiant about 590 light-years away, around 40 times wider than the Sun and thousands of times more luminous. It is also the brightest member of the Alpha Persei Cluster, a loose group of young stars surrounding it. Its Arabic name means “Elbow of the Pleiades”—a name that comes from an old Arabic star figure that stretched across a much larger area of sky.
- Algol (Beta Persei) – Usually the second-brightest star at about magnitude 2.1, Algol lies roughly 93 light-years away and is actually a triple-star system. Its Arabic-derived name means “the Demon’s Head,” reflecting its association with Medusa in Perseus.
- Zeta Persei – Shining at magnitude 2.84, Zeta Persei is a massive blue supergiant roughly 1,000 light-years away, around 25 times wider than the Sun and tens of thousands of times more luminous.
- Áldu (Epsilon Persei) – A hot blue-white star of magnitude 2.90, roughly 500 light-years away and more than 10 times as massive as the Sun.
- Gamma Persei – A magnitude 2.9 binary system around 260 light-years away. Its principal star is a yellow giant accompanied by a hotter companion.
- Sarvvis (Delta Persei) – A magnitude 3.0 blue giant roughly 530 light-years away and another member of the wider Alpha Persei Cluster.
The “Demon Star” Winks Every Few Days
There is something wonderfully eerie about Algol’s place in the Perseus story: a star marking Medusa’s severed head really does appear to “wink.” Every 2 days, 20 hours and 49 minutes, Algol noticeably fades from its normal magnitude of about 2.1 to around 3.4 before brightening again. The change is large enough to see with the naked eye. The explanation isn’t supernatural, of course. Two of the stars in the Algol triple-star system orbit in such a way that the dimmer star regularly passes in front of the brighter one from our viewpoint. The resulting eclipse, including the fading and return to normal brightness, lasts about 10 hours.
Perseus Lies in a Rich Part of the Milky Way
Because Perseus lies along the Milky Way, its part of the sky is particularly rich in star clusters and nebulae. The showpiece is the Double Cluster, two neighboring groups of young stars that appear to the naked eye under dark skies as a single hazy patch between Perseus and Cassiopeia. Binoculars transform that patch into two separate, densely packed fields of stars.
Perseus also contains M34, a loose open cluster bright enough to approach naked-eye visibility; the enormous California Nebula, a glowing cloud of hydrogen several times wider in our sky than the full Moon but surprisingly difficult to see; and M76, the Little Dumbbell Nebula, the expanding glowing remains of a dying star about 3,400 light-years away.
The Perseids Come From a 16-Mile-Wide Comet
Each August, Perseus becomes the apparent starting point for one of the year’s most spectacular meteor showers. The Perseids are best seen with the naked eye from a dark location, where around the shower’s peak you may see dozens of meteors an hour streaking across different parts of the sky.
The meteors appear when Earth plows through dust and debris left behind by Comet 109P/Swift-Tuttle, a roughly 16-mile (26-km)-wide comet that orbits the Sun about once every 133 years. Tiny particles strike our atmosphere at around 37 miles (59 km) per second, producing the bright streaks we see. Although they can appear anywhere in the sky, tracing their paths backward leads to a point in Perseus—the radiant that gives the shower its name.
Swift-Tuttle won’t return to the inner Solar System until 2126, when it is expected to pass about 14 million miles (0.15 AU)) from Earth—close by astronomical standards, but still around 60 times farther away than the Moon.