ALGORAB (Delta Corvi). Many stars come in pairs. Some come in pairs in different ways. Algorab lies at the upper left corner of Corvus, the Crow (or Raven), and with Gienah Corvi (Gamma Corvi) points eastward to Spica in Virgo. Even its name is paired with Gienah, as both come from an Arabic phrase that means "the raven's wing," "Gienah" from the word for "wing," "Algorab" from that for "raven." Unfortunately, it is not all that tidy, as Algorab used to belong to the star now called Gienah. Though the third brightest star in the constellation, it was called Delta by Bayer. The two stars, however, are paired only as the raven's wings and as pointers. Gienah is 165 light years away, whereas Algorab is a mere 88, so the two really have nothing to do with each other. But then look closely at Algorab, and it itself is a pair, a beautiful and easy-to-see double star that consists of a third magnitude (2.95) white class B star (B9.5, just on the edge of class A) and an orange class K star just over the edge of ninth magnitude (8.51). The contrast gives them interesting colors (quoted ages ago as "yellowish and pale lilac," "pale yellow and purple"). The brighter, Algorab-A, is a very normal 2.5 solar mass dwarf (hydrogen-fusing) star with a temperature of almost exactly 10,000 Kelvin (a nice reference point), and shines 48 times more brightly than our Sun, its radius about double solar. The orange companion, Algorab-B, with a mass three-fourths solar and a surface temperature of about 5000 Kelvin, radiates much more dimly, producing only 0.3 times the energy of the Sun. The contrast immediately shows how only a small change in stellar mass leads to a huge difference in brightness, Algorab-A 160 times more luminous than Algorab-B. At a minimum distance of 650 astronomical units from A, little B takes at least 9400 years to orbit. From B, A would shine with the light of 500 full Moons. The real focus, however, is actually on Algorab-B. A high level of infrared radiation shows that the system is involved with a great deal of dust that is left over from the formation of the stars. A stars is created by the collapse of gas within a cold dark, dusty cloud, the dust blockin out heating background starlight. "T Tauri stars" are very youthful stars that have just been created from the interstellar gases, and as such are heavily surrounded by disks of dust (from which planets may be born). Algorab-B is a "post-T Tauri star" that has just passed through that phase and is now settling down on the main sequence to fuse its hydrogen. It appears to be less than 110 million years old (as therefore is Algorab-A), and has yet to clear the dust out of its system. Algorab-B therefore makes Algorab-A one of the more unusual stars of the naked-eye night sky, acting not just as a pointer for Spica, but also as a pointer for rather remarkable Algorab-B.
Written by Jim Kaler 4/14/00. Return to STARS.