Perseus
What is Perseus?
Constellations· Autumn–winter (N) · between Cassiopeia and the Pleiades. The hero Perseus holding Medusa's head. The head is marked by Algol, the Demon Star, which clearly dims about every 2.9 days. Perseus is also known for its Double Cluster and the August Perseid meteors.
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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.
Is Algol's rhythm hidden in a 3,000-year-old Egyptian papyrus?
The first person on record to notice Algol's changes was the Italian astronomer Geminiano Montanari in 1667. There may, however, be a much older trace. The Cairo Calendar, an Egyptian papyrus from around 1200 BC, marks each day as lucky or unlucky. In 2015 researchers at the University of Helsinki analysed the pattern statistically and found a repeating period of 2.85 days, almost the same as the roughly 2.87 days at which Algol dims.
The researchers noted that material flowing between the two stars can slowly change the period, so it could have been slightly shorter three thousand years ago. They interpreted the calendar as the Egyptians recording Algol's winks as the actions of a god. Not all scholars accept this conclusion. If it holds, though, Algol would be the variable star that people have watched for the longest time.
The constellation in depth
Perseus cut off the head of the monster Medusa and, on the way home, rescued Andromeda, who was about to be given to a sea monster. In the sky he holds a sword in one hand and Medusa's head in the other, and the star at the head's eye is Algol. Ancient observers found a star that dims on schedule unsettling and gave it an Arabic name meaning the demon's head.
Algol is a pair of stars orbiting each other. Every 2 days, 20 hours and 49 minutes the dimmer star passes in front of the brighter one, so the brightness drops from magnitude 2.1 to 3.4 and returns a few hours later. The English astronomer John Goodricke first proposed this explanation in 1783. Comparing Algol with nearby stars on several nights in a row shows the change to the naked eye.
Near the border with Cassiopeia lies the Double Cluster, a hazy patch even to the naked eye. Two clusters about 7,500 light-years away sit side by side, and binoculars show two dense crowds of stars. Around 12 August each year the Perseid meteors also radiate from this constellation. With the Sky view set to a December evening, Perseus rides nearly overhead.
Frequently asked
- When can I see Perseus?
- Perseus is best seen in autumn to winter evenings, November–January.
- How do I find Perseus?
- To find Perseus, look for the curved line of stars between Cassiopeia's W and the Pleiades.
- What is the brightest star in Perseus?
- The brightest star in Perseus is Mirfak, at magnitude 1.8.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.