Perseus
What is Perseus?
Constellations· Autumn–winter (N) · between Cassiopeia and the Pleiades. The hero holding Medusa's head. The head is marked by Algol, the "Demon Star", which visibly dims every 2.9 days. Perseus also hosts the Double Cluster below Cassiopeia and lends its name to the August Perseid meteor shower.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where Perseus is in your sky right now: https://nowsky.org/solar/sky?sel=con:Perseus — location and time set automatically, no login.
Stars of the figure
| Star | Magnitude |
|---|---|
| Mirfak | 1.8 |
| Algol | 2.1 |
The 2 brightest of the 7 stars used in the figure (6 lines). Lower magnitude means brighter.
Deep-sky objects near this constellation
Constellation basics
| Season | autumn to winter evenings, November–January |
|---|---|
| Brightest star | Mirfak (1.8) |
| How to find | the curved line of stars between Cassiopeia's W and the Pleiades |
| Area rank | 24th of 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Frequently asked
- When can I see Perseus?
- autumn to winter evenings, November–January.
- How do I find Perseus?
- the curved line of stars between Cassiopeia's W and the Pleiades.
- What is the brightest star in Perseus?
- Mirfak, magnitude 1.8.
- Where is Perseus in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
Algol's period, written on papyrus three thousand years ago
The first person recorded to notice Algol's variation is Montanari in 1667, but there may be a far older trace. The Cairo Calendar, an Egyptian papyrus from around 1200 BC, lists each day as lucky or unlucky, and when researchers at the University of Helsinki analysed the recurrence of those judgements statistically in 2015 a period of 2.85 days emerged, almost the 2.867 days at which Algol dims.
A slight shortening of the period over three millennia could be explained by mass transfer between the two stars, and the team read the result as the Egyptians recording Algol's winks as the doings of a god. The hypothesis is still debated, but it leaves open the possibility that the eye of Medusa in Perseus's hand is the longest-observed variable star. That Algol's name means demon in Arabic is often cited as further hint that the ancients knew of its change.
The constellation in depth
Perseus beheaded Medusa and, on the way home, rescued Andromeda from the sea monster. In the sky he holds a sword in one hand and the Gorgon's head in the other, and the head's eye is Algol. Ancient observers found a star that dims on a schedule unsettling and named it the Demon Star. Perseus stands where the Milky Way runs thick, so the whole region rewards a slow sweep with binoculars.
Algol is an eclipsing binary: every 2 days 20 hours 49 minutes one star passes in front of the other and the brightness drops from magnitude 2.1 to 3.4. John Goodricke first explained the mechanism in 1783. Toward Cassiopeia lies the naked-eye Double Cluster, NGC 869 and 884, and around 12 August each year the Perseid meteors radiate from this constellation.
On autumn evenings find Cassiopeia's W in the northeast, then follow the curved chain of stars down toward the Pleiades to reach Mirfak. Compare Algol, south of Mirfak, on several consecutive nights and you can catch its fade with your own eyes. Set the Sky view to a December evening and Perseus rides nearly overhead. Mirfak lies about 500 light-years away, while the Double Cluster is some 7,500 light-years off — still close enough to resolve easily into individual stars.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.