Alkaid (Eta Ursae Majoris)
Alkaid (Eta Ursae Majoris) is a star in the constellation Ursa Major, magnitude 1.9.
The tip of the Plough’s handle — a hot young star that does not travel with its neighbours.
Alkaid is a hot young star, only about six million years old and just starting its main-sequence life. With about five times the Sun’s mass, it spins at around 240 km/s, bulging slightly at the equator. Its surface, between about 16,300 K at the equator and 17,700 K at the poles, gives it a crisp blue-white colour.
Most stars of the Plough travel together through space as the Ursa Major moving group. Alkaid does not — it simply lies in the same direction. Since 1943 its spectrum has been one of the standards by which other stars are classified. Astronomers have looked closely for planets or companions, and found none.
Its name comes from the Arabic qā’id banāt na‘sh, ‘the leader of the daughters of the bier’. The four stars of the bowl were a funeral bier; the three handle stars were the mourning maidens, with Alkaid at their head.
Alkaid (Eta Ursae Majoris) is 100 light-years away: the light we see from it set off around 1925, around the time Edwin Hubble showed that other galaxies exist.
Did you know?
- The military breaking star. In Japan and Korea Alkaid is known as Hagunsei and Mukokseong, ‘the military breaking star’ — names that came from ancient China.
- A starfish namesake. A fossil starfish, Alkaidia, is named after the star.
The blue-white star at the end of the Plough’s handle, the last of its seven stars. Follow the arc of the handle onwards from Alkaid to reach orange Arcturus.
When and where to see it
It is best placed in the evening sky around May, when it crosses the meridian at about 10 pm local time (around midnight in April). It can be seen from everywhere north of 41°S and never sets north of 41°N.
At its highest it stands 89° up from 50°N, 76° up from 35°N, 41° up from the equator, 6° up from 35°S.
Facts
- Type
- Star
- Constellation
- Ursa Major
- Designations
- Eta Ursae Majoris, Alcaid, η UMa, 85 UMa, HR 5191, HD 120315, HIP 67301
- Magnitude
- 1.9
- Position
- RA 13h 47.5m, Dec +49° 19′ (J2000)
- Spectral type
- B3V
- Colour
- blue-white (B−V -0.19)
- Distance
- 100 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Alkaid (Eta Ursae Majoris) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.