M45, the Pleiades, is the open cluster most familiar to the naked eye: a compact knot of hot blue stars on the shoulder of Taurus, long known as the Seven Sisters. It lies about 444 light-years away. The stars you notice by eye occupy a bright core roughly 16 light-years across, while the cluster’s gravity holds members out to a tidal radius of about 43 light-years.
The brightest members are young B-type stars, about 125 million years old. Nine of them carry names from Greek myth — Alcyone, Electra, Maia, Merope, Taygeta, Celaeno, and Asterope, together with their parents Atlas and Pleione. A dust cloud the cluster is passing through scatters their light, wrapping the group in a blue reflection nebula. The glow around Maia is NGC 1432; the veil beside Merope is NGC 1435, also called Tempel’s Nebula. More than a thousand stars are confirmed members. Galileo, viewing the cluster in 1610, already recorded 36.
On the sky the Pleiades cover about 2°, roughly four times the width of the full Moon, at a combined magnitude of 1.6. They stand near right ascension 03h 47m and declination +24° 07′, and they are the nearest object in Charles Messier’s catalogue. Their rising has marked seasons for cultures across the world, including Matariki in Aotearoa New Zealand.
Facts
- Open cluster in Taurus, the Seven Sisters (Messier 45)
- About 444 light-years away (136 parsecs), the nearest Messier object
- Bright core about 16 light-years across; tidal radius about 43 light-years
- About 125 million years old, with more than 1,000 member stars
- Magnitude 1.6 and about 2° across, near RA 03h 47m, Dec +24°
- Blue glow is reflected starlight in NGC 1432 and NGC 1435
