Alcyone (Eta Tauri)
Alcyone (Eta Tauri) is a star in the constellation Taurus, magnitude 2.9.
The brightest of the Seven Sisters — a spinning, flattened giant once hailed as the centre of the Milky Way.
In 1846 the German astronomer Johann Heinrich von Mädler proposed that the stars of the Milky Way all circled one central body: Alcyone. He was wrong, but the star at the heart of his idea is remarkable enough. It is the brightest member of the Pleiades, the open cluster named for the seven daughters of Atlas and Pleione in Greek myth, and Alcyone was one of them.
Alcyone is a blue-white giant about 1,750 times as luminous as the Sun, and it spins so fast that it bulges: its equator is more than ten times the Sun’s radius, its poles only eight. The spin has flung off a disc of gas that circles its equator, making it a Be star, with bright emission lines in its spectrum. A 2017 study of Kepler space telescope data caught it flickering by just a thousandth of a magnitude, with a 2.3-day rhythm that is probably its rotation.
It is also a puzzle. The Pleiades are about 130 million years old, yet Alcyone looks younger — under 100 million. It may be a blue straggler, or the models may simply struggle with stars like it.
Alcyone (Eta Tauri) is 400 light-years away: the light we see from it set off around 1625, 17 years after Galileo first turned a telescope to the sky.
Did you know?
- The Hairy Head. Chinese astronomers called the Pleiades Mǎo, the Hairy Head. Alcyone is its sixth star, Mǎo Xiù liù.
- Alone after all. Lunar occultations once hinted that Alcyone had very close companions. A 2021 interferometric study concluded that it is a single star.
- Hotter at the poles. Because Alcyone is flattened by its spin, its poles are hotter than its equator. Averaged over the whole star, its surface is about 12,300 kelvins.
The brightest star of the Pleiades, near the heart of the cluster’s little dipper shape. Use binoculars or a low-power eyepiece: a small group of fainter stars sits close beside it, while the other Sisters spread out around it.
When and where to see it
It is best placed in the evening sky around December, when it crosses the meridian at about 10 pm local time (around midnight in November). It can be seen from everywhere north of 66°S and never sets north of 66°N.
At its highest it stands 64° up from 50°N, 79° up from 35°N, 66° up from the equator, 31° up from 35°S.
Facts
- Type
- Star
- Constellation
- Taurus
- Designations
- Eta Tauri, η Tau, 25 Tau, HR 1165, HD 23630, HIP 17702
- Magnitude
- 2.9
- Position
- RA 3h 47.5m, Dec +24° 6′ (J2000)
- Spectral type
- B7III
- Colour
- white (B−V -0.09)
- Distance
- 400 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Alcyone (Eta Tauri) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.