Achernar (Alpha Eridani)
Achernar (Alpha Eridani) is a star in the constellation Eridanus, magnitude 0.5.
The end of the river — a hot blue star spinning so fast it bulges at the waist.
Achernar spins so fast that it is squashed: its equator is about 35% wider than its poles. As of 2015 it was the least spherical star known in the Milky Way, as flattened as the dwarf planet Haumea. The spin makes its poles far hotter than its waist — over 17,000 K against under 13,000 K — and a wind streams off the poles. Around its middle it throws off a disc of gas that grows, shrinks and sometimes falls back into the star.
It is a blue star six to seven times the Sun’s mass and about 3,000 times as bright — the hottest and bluest of the ten brightest stars in our sky. A smaller white companion circles it every seven years. Its name comes from the Arabic for ‘the end of the river’, the last star of Eridanus.
Achernar (Alpha Eridani) is 140 light-years away: the light we see from it set off around 1885, 7 years after Edison lit his first practical light bulb — the start of light pollution.
Did you know?
- Missing from Ptolemy. Precession kept Achernar so far south in ancient times that Ptolemy could not have seen it from Alexandria. It is the only first-magnitude star missing from his Almagest; Europe learned of it from Pieter Dirkszoon Keyser on the first Dutch voyage to the East Indies.
- Heading north. Achernar is slowly moving north in our sky. Between the 8th and 11th millennia AD it will be visible as far north as Germany and southern England.
- The echidna. The Wardaman people of Australia call Achernar Gawalyan, the echidna. To the Boorong people of Victoria it was Yerrerdetkurrk.
A bright blue-white star marking the end of Eridanus, the long winding river of stars. It stands alone in a sparse patch of sky, so it is easy to pick out. The south celestial pole lies roughly halfway between it and the Southern Cross.
When and where to see it
It is best placed in the evening sky around November, when it crosses the meridian at about 10 pm local time (around midnight in October). It can be seen from everywhere south of 33°N and never sets south of 33°S.
At its highest it stands below the horizon from 50°N, below the horizon from 35°N, 33° up from the equator, 68° up from 35°S.
Facts
- Type
- Star
- Constellation
- Eridanus
- Designations
- Alpha Eridani, α Eri, HR 472, HD 10144, HIP 7588
- Magnitude
- 0.5
- Position
- RA 1h 37.7m, Dec −57° 14′ (J2000)
- Spectral type
- B6Vpe
- Colour
- blue-white (B−V -0.16)
- Distance
- 140 light-years
- Discovered
- Naked-eye star; first reported in Europe by Pieter Dirkszoon Keyser
Open AstroPilot to see when Achernar (Alpha Eridani) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.