Beta Monocerotis

At magnitude 3.92, Beta Monocerotis is the brightest star in Monoceros, the Unicorn. However, β Mon is a triple star - if we take its components singly, then α Mon is actually the constellation's brightest.

Components and Orbit

Beta Monocerotis was discovered by Sir William Herschel in 1781. The triple system consists of three equally bright stars, an unusual configuration among multiples. This triple star forms an acute triangle of bluish-white stars. The three stars are lettered from west to east β Mon A, β Mon B, and β Mon C; they have magnitudes of 4.6, 5.4, and 5.6, respectively. β Mon B and C make a double 2.8" apart, while A stands off from them by 7.4".

β Mon B and C mostly likely orbit each other, while A orbits the pair. Most likely the perspective foreshortened, so A is really farther away in the background or foreground. At a distance of 690 light years, the minimum separation of B and C is 590 AU, and the separation of A from B-C is 1570 AU. 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.

Properties and Evolution

The wonder of the trio is that they are close to identical. All are hot, blue-white class B3 stars with temperatures near 18,500 K. β Mon A, the most luminous, shines with 3200 solar luminosities, while B and C have luminosities of 1600 and 1300 suns. The masses of the three (in order A, B, C) are 7, 6.2, and 6 suns. All are hydrogen-fusing main-sequence stars.

Born only 34 million years ago, β Mon A (the most massive) is a bit farther along in its evolution, and has another 9 million years left before it finishes core hydrogen fusion and becomes a giant. β Mon A, B, and C rotate with respective minimum equatorial speeds of 346, 123, and 331 km/sec. As a result, all three are "B-emission" stars with circumstellar gas disks.

A 12th magnitude star lurks within the system, though it is most likely a line-of-sight optical companion, unrelated to the others.

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