ACAMAR (Theta Eridani). Eridanus, the River, the embodiment of the Greek's "River Ocean," unlike real rivers has two ends. It now terminates in the brilliant first magnitude star Achernar (Alpha Eridani), the name from an Arabic phrase that means "the end of the river." But Achernar is too far south to be visible from Greece. The original end of the river was the star we still call Acamar, which derives from the same phrase and means the same thing. Only in modern times did the explorers of the southern hemisphere skies stretch the river down to Achernar. Only a modest mid-third magnitude (2.90) star, Acamar cannot compare to the brightness of star at the "new" end of the river, and was given the rather-well- down Greek letter "Theta" by Bayer. The star harbors a very nice surprise for the telescope, however: it is a beautiful double, both components white class A stars separated by a near-perfect (for easy observing) 8.3 seconds of arc, the brighter one (magnitude 3.24) called Theta-1, the fainter (4.42) Theta-2. The pair has been called "one of the gems of the southern sky." There is some evidence that Theta-1 may itself be a close double. Curiously, Ptolemy called the combined pair first magnitude, probably from an overestimate of the amount of dimming by the Earth's thick atmosphere, as the star appears very low in the sky from mid- northern latitudes. Theta-1 is a class A4 giant, while Theta-2 is a hotter A1 dwarf. Shining from a distance of 160 light years, from surfaces with temperatures respectively 8200 and 9200 Kelvin, they have respective luminosities 96 and 36 times that of the Sun. The giant, with a mass of 2.6 times that of the Sun, has just begun to evolve with a dead helium core, and is now expanding and on its way to "red-giant-hood." The dwarf has a slightly lower mass of 2.4 solar, and as a result is living a bit longer in its stable hydrogen-fusing stage, but will before very long follow its just-bigger mate. Acamar vividly shows the odd nature of astronomical nomenclature. The two were born as dwarfs that were white (and one still is). At the end of their lifeline, however, they will retire as pair of "white dwarfs," not the same thing at all, "white dwarfs" (which are not necessarily white) being dead high-density stars that have (like Sirius-B or Keid-B) shrunk to the size of Earth.
Written by Jim Kaler. Return to STARS.