Achernar
What is Achernar?
Stars· Distance 139 ly · size 7–10× Sun (oblate). The blue star at the end of the celestial river Eridanus. It spins so fast that its width at the equator is 1.4 times its height from pole to pole. It never rises from Europe or most of the United States.
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Distance and spectral type
Achernar is about 139 light-years away, a blue-white main-sequence star (spectral type B3Vp), in the constellation Eridanus. Its apparent magnitude is 0.5, visible to the naked eye.
Stellar figures
| Distance | 139 ly |
|---|---|
| Magnitude | 0.46 — ninth brightest |
| Type | B6 blue main-sequence |
| Note | flattest known star, equator 1.4× poles |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Arabic "end of the river". The flattest star known — it spins so fast its equator bulges 56%.
Why Achernar's poles are hotter than its equator
On a fast-spinning star, the surface temperature is not the same everywhere. The equator bulges outward, so it lies farther from the star's centre. Farther from the centre, the star's grip is weaker, and less light and heat come out. The poles, closer to the centre, are hotter and brighter. This effect is called gravity darkening. Calculations put Achernar's poles at about 17,000 K and its equator at about 12,700 K, a difference of more than 4,000 degrees across a single star.
Because of this, Achernar's brightness and colour depend a little on the direction it is seen from. Seen from over a pole it would look brighter and bluer; seen from the equator, somewhat dimmer. From Earth, its spin axis appears tilted by about 60 degrees. So the brightness we measure shows only part of the star's true nature. To find the real output of a star like this, astronomers must first measure its shape and tilt.
The star in depth
Achernar is the brightest star in Eridanus. At magnitude 0.46 it is the ninth-brightest star in the night sky, 139 light-years away. Its name is Arabic for 'the end of the river'. Eridanus is drawn as a long river winding across the sky, and Achernar sits where that river ends in the far south.
Achernar is much bigger and hotter than the Sun. It has about six times the Sun's mass, and its surface is around 15,000 K, more than twice as hot as the Sun's, so it glows blue. It is still in the prime of life, burning hydrogen in its core. Even so, it is more than 3,000 times as bright as the Sun, because heavier stars burn their fuel much more fiercely than lighter ones.
Achernar's most striking feature is its shape. Its equator spins at about 250 km/s, more than 500 times faster than Earth's equator moves. At that speed, the outward push of the spin makes the equator bulge. As a result the star is squashed, 1.4 times wider across the equator than from pole to pole. Gas thrown off by the fast spin forms a thin disc around the star, and a small companion star circles it every seven years.
Achernar lies so far south that it never rises north of about latitude 33° N, so it cannot be seen from Europe or most of the United States. In the southern hemisphere it stands high on evenings from October to December, during the southern spring. With few bright stars around it, it is easy to pick out. In the Sky view, switch the location to Sydney and choose a November night to see Achernar shining blue almost overhead.
Frequently asked
- How far is Achernar from Earth?
- The distance of Achernar is 139 ly.
- How bright is Achernar (apparent magnitude)?
- The magnitude of Achernar is 0.46 — ninth brightest.
- What kind of object is Achernar?
- Achernar is a B6 blue main-sequence.
- What is notable about Achernar?
- The note of Achernar is flattest known star, equator 1.4× poles.
- Which constellation is Achernar in?
- Achernar lies in the constellation Eridanus.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.