Achernar – a binary star system in the constellation Eridanus

Achernar is a bright, binary star system in the constellation Eridanus, and is the ninth-brightest star in Earth’s night sky. It is the name of the primary (or ‘A’) component of the binary system designated Alpha Eridani (α Eridani). It is the 9th brightest star and flattest star known. The two stars, Achernar A (seven times the mass of the sun) and Achernar B (which is smaller), rotate about 6.7 Earth-sun distances or astronomical units apart. It’s famous as the southernmost bright star in the constellation Eridanus the River.

Achernar is a blue star with an apparent magnitude of about 0.50 that is the brightest star in the constellation Eridanus. It is a star of the first magnitude in the constellation Eridanus.

Achernar visual magnitude is 0.46, making it the 9th brightest star in the sky. Achernar’s apparent magnitude — how bright it appears to us — is 0.46. Despite its position in the top 10 stars in the sky, Achernar is sometimes not cited by Northern Hemisphere observers, perhaps because it is invisible above 32 degrees north latitude (approximate location of El Paso, Texas, Nagasaki, Japan, and Tel Aviv, Israel), according to University of Illinois astronomer Jim Kaler. Thanks to its high brightness, Achernar is clearly visible when observed from locations with dark skyes, and should be also quite easily visible from light polluted areas.

Of the ten apparent brightest stars in the night-time sky, Alpha Eridani is the hottest and bluest in color, due to Achernar being of spectral type B. It is the brightest star in the constellation Eridanus and the ninth brightest star in the sky. Achernar (Arabic for “end of the river”) is 144 light-years from Earth. Achernar has an unusually rapid rotational velocity, causing it to become oblate in shape. It is a binary star with a B-type star, Achernar A, as its primary and a much fainter A-type star, Achernar B, orbiting the primary at a distance of 6.7 astronomical units (1 billion km, or 621 million miles) with a period of about 15 years.

Achernar means “the end of the river” in Arabic — although the star’s official name is Alpha Eridani. The secondary is smaller, of spectral type A, and orbits Achernar at a distance of about 12 astronomical units (AU). Achernar is situated south of the celestial equator, as such, it is more easily visible from the southern emisphere. It is so far south in Eridanus that it originally was not considered a part of the constellation, according to Kaler. Achernar’s location is:

  • Right ascension: 1 hour 37 minutes 42.9 seconds
  • Declination: -57 degrees 14 minutes 12 seconds.

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