Double Star of the Month:
Beta Monocerotis
AKA: 11 Monocerotis; Herschel’s Wonder Star; STF 919.
Position: 06 hr 28.8 min -7 degrees 01 min 59 sec
Due south at 22:48 (GMT) on 15 January 2021.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Having observed a multiple star system in December, we now turn to a fairly nearby triple star system, namely Beta Monocerotis. The designation of stars in the modern constellation of Monoceros is idiosyncratic. Most stars in the constellation are called by their Flamsteed numbers rather than their Bayer letters. So the other famous double in Monoceros is usually called 8 Monocerotis rather than Epsilon Monocerotis, but Beta Monocerotis is more common than 11 Monocerotis. Beta Monocerotis can be found by following the line formed by the Belt of Orion downwards pass a fairly dim star (Gamma or 5 Monocerotis) until you reach another fairly dim naked eye star (magnitude 3.7) above Sirius, which is Beta Monocerotis. You should be able to see the double fairly easily as they are 7.1 arc seconds apart and have almost identical brightness (4.6 and 5.0), but you will need to push the magnification up to around 70-100 times. Now continue to increase the magnification to at least 140 times and you should see B separating out into two distinct stars, C being almost the same magnitude at 5.4, which are 2.6 arc seconds apart. They form a “fishhook” shape like a flattened V. They are all white although I find they are bathed in a greenish glow. Their appearance is very similar to Mizar in Ursa Major. Even now, it is not clear if the stars in the group are physically connected or not. Above them is an unconnected faint star HD 45709, mag. 7.5. It is known as Herschel’s wonderful star as he described it as “one of the most beautiful sights in the heavens”, although strictly speaking he was referring to the BC double rather than the whole triple. He recorded seeing them on 5 December 1779, but it has been argued that he saw the BC double in 1780.