Theta1 Eridani - Acamar

Acamar or θ1 Eri is a star in the constellation Eridanus, the ancient Greeks' "River Ocean", which unlike real rivers has two ends. Eridanus begins at β Eri (Cursa), and now terminates at α Eri (Achernar), whose name comes from an Arabic phrase that means "the end of the river." But Achernar is too far south to be visible from ancient Greece. The original end of the river was the star we now call Acamar; European southern hemisphere explorers in more modern times extended the River down to Achernar. Acamar's name derives from the same Arabic phrase as Achernar's ("Akhir an-nahr", meaning "the last of the river"), but with Achernar's "rn" mis-transposed to Acamar's "m" in Roman-alphabet handwriting.

At magnitude 2.90, Acamar cannot compare in the brightness to Achernar at the "new" end of the river, which shines brilliantly at magnitude 0.45. Interestingly, Ptolemy described Acamar as a first-magnitude star, and Al Sufi in the 10th century listed it as one of the 13 brightest stars in the sky; Ulugh Beg from Persia in the 15th century also listed it as a first-magnitude star. They probably overestimated of the amount of dimming by the Earth's thick atmosphere, as the star appears very low in the sky from mid-northern latitudes. Acamar is known as the Sixth Star of the Celestial Orchard in Chinese.

Components

Acamar harbors a very nice surprise for telescopes: it is a beautiful double, separated by 8.3" for easy observing. Both components are white class A stars. The brighter one (θ1 Eri) is magnitude 3.24; the fainter (θ2 Eri) magnitude 4.42. The pair has been called "one of the gems of the southern sky." There is some evidence that θ1 Eri may itself be a close double. θ1 Eri is a class A4 giant, while θ2 Eri is a hotter A1 main-sequence star. Shining at a distance of 160 light years, from surfaces with temperatures of 8200 and 9200 K, they have luminosities of 96 and 36 suns, respectively. The giant, with a mass of 2.6 sun, has just begun to evolve off the main sequence, is now expanding to become a red giant. Its fainter main-sequence companion has a slightly lower mass of 2.4 suns, resulting in a longer hydrogen-fusing stage. At the end of their lifetimes, however, both components will retire as pair of white dwarfs: dead high-density stars that have shrunk to the size of the Earth.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]