BETA MON (Beta Monocerotis). Dim constellations commonly hold remarkable sights. The luminary of Monoceros, the Unicorn, a modern constellation, is among them. None of the stars within the constellation carry proper names; the brighter ones are called out only by their Greek letters. While "Alpha" usually denotes the brightest star of a constellation, in the Unicorn -- as in neighboring Orion -- the brightest is fourth magnitude (3.92) Beta, which just barely nudges out Alpha Monocerotis (magnitude 3.93). Beta offers a bit of a subterfuge, however, as the telescope shows it to be a glorious triple star. If we take Beta's stars singly, then Alpha quite wins the brightness prize. At a distance of 690 light years, the three are lettered from west to east A, B, and C. B and C make a double 2.8 seconds of arc apart, while A stands off from them by 7.4 seconds of arc. B and C mostly likely orbit each other, while A orbits the pair. And most likely the distances are foreshortened, so A is really farther away in the background or foreground. The wonder of the trio is that they are close to identical. B and C span the border of sixth magnitude (5.4 and 5.6), while A tops them a bit at bright fifth (4.6). All are hot, blue-white B (class B3) stars with temperatures near 18,500 Kelvin. Star A, the most luminous, shines with 3200 solar luminosities, while B and C come in at 1600 and 1300 Suns. The masses of the three (in order A, B, C) are 7, 6.2, and 6 solar. All are hydrogen-fusing dwarfs. Born only 34 million years ago, A (the most massive) is a bit farther along in its evolution, and has another 9 million years left before it gives up core-hydrogen- fusion and starts to become a giant. Given the minimum distances of 590 Astronomical Units that separate B and C, and 1570 that separate A from B-C, the B-C pair takes at least 4200 years to orbit each other, while A takes at least 14,000 to circuit the closer pair. Viewed from A, the B-C pair could be up to 20 degrees apart. The identity of the three continues with their rotations. A, B, and C rotate with respective minimum equatorial speeds of 346, 123, and 331 kilometers per second (compared with the Sun's 2 km/s). As a result, all three are "B-emission" stars that are outfitted with circulating circumstellar disks. A 12th magnitude star lurks within the system, though it is most likely a line-of- sight interloper.
Written by Jim Kaler. Return to STARS.