Achernar is the brilliant blue star marking the southern end of the constellation Eridanus. Located about 139 light-years away, it is the ninth-brightest star in the night sky — but its extraordinary brightness is far from the most unusual thing about it. Here are some interesting facts about Achernar.
The Ninth-Brightest Star in the Night Sky
With an apparent magnitude of about 0.46, Achernar is the brightest star in Eridanus and the ninth-brightest star visible in the night sky. That’s particularly impressive considering it lies around 139 light-years from Earth. Achernar only appears so bright from that distance because it is intrinsically powerful, producing several thousand times more energy than the Sun. Its blue color also gives us our first clue to the kind of star it is: Achernar is considerably hotter and more massive than our Sun.
Achernar Means “the End of the River”
Achernar’s name comes from an Arabic phrase meaning “the end of the river,” a fitting description for the star at the southern end of the long, winding constellation Eridanus. The celestial river begins near Rigel in Orion before flowing far into the southern sky, making Eridanus one of the longest constellations.
Achernar’s far-southern position also means many northern observers never see it. It remains below the horizon from locations north of about 33° N, while south of roughly 33° S it becomes circumpolar and never completely sets.
It Spins So Fast That It Isn’t Round
Achernar rotates extraordinarily quickly, with material around its equator traveling at close to 300 km/s (670,000 mph) — roughly 150 times the Sun’s equatorial rotation speed. Despite being many times larger than the Sun, Achernar completes a rotation in only about two days.
This extreme rotation has physically distorted the star. Achernar bulges dramatically around its equator, making it roughly a third wider across its equator than from pole to pole. Astronomers have measured this distorted shape using interferometry, making Achernar one of the most striking examples of how rapid rotation can reshape a star.
Its Poles Are Much Hotter Than Its Equator
Achernar’s extreme rotation doesn’t just change its shape — it also changes the temperature across its surface. Its poles are thought to reach around 18,000 K, while the swollen equatorial regions may be closer to 10,000 K.
The difference is caused by the star’s rapid spin. Around the equator, the outward effect of rotation partly counteracts gravity, while the equatorial bulge also places the surface farther from the star’s center. At the poles, these effects are much weaker, so the effective surface gravity is stronger and more energy escapes through the surface. The result is hotter, brighter poles and a cooler, dimmer equator, an effect astronomers call gravity darkening.
Its Extreme Rotation Creates a Disk Around It
Achernar belongs to a class known as Be stars — hot B-type stars that sometimes eject material from their surfaces. In Achernar’s case, its extreme rotation is thought to play an important role, helping material escape from around the equator and form a thin, rotating disk of gas around the star.
The disk isn’t permanent. Achernar goes through periods when it develops and loses this surrounding material, allowing astronomers to watch the process change over time.
Only Around 60 Million Years Old
Achernar has around six times the mass of the Sun and is estimated to be only about 60 million years old. Despite its young age, it has already reached the end of its main-sequence life because massive stars consume their nuclear fuel much faster than smaller stars like the Sun.
Achernar therefore won’t enjoy anything resembling the Sun’s roughly 10-billion-year main-sequence lifespan. Its greater mass gives it vastly more fuel to work with, but its much hotter, more energetic interior burns through that fuel at an enormously faster rate.
Achernar Has a Companion on a Seven-Year Orbit
Achernar isn’t actually a single star. Its companion, Achernar B, is an A-type main-sequence star with about twice the mass of the Sun. It follows a highly elongated orbit around Achernar A, completing one circuit roughly every seven years.
Because the orbit is so eccentric, the distance between the two stars changes dramatically. Achernar B comes within about 2 AU of the primary at its closest approach but retreats to nearly 13 AU at its farthest — roughly comparable to moving from the region of Mars out beyond Saturn in our Solar System.