Located in the rich star clouds of Sagittarius is the gossamer nebula M20, also known as the Trifid Nebula. A beautiful sight in its own right, the object of our challenge this month is actually the multiple star system HN 40, which lies at the heart of the Trifid.
In small telescopes at low power, HN 40 appears as a prominent double star at the tip of one of the lobes of nebulosity. It is near the geometric center of the Trifid. The brightest member is magnitude 6.9 (star “A” in my sketch which is mirror-reversed because a star-diagonal was used), and about 11 arc-seconds to it south is the second brightest star at magnitude 8.8 (star “B”). These are the two stars most observers are familiar with. However, anyone who has taken a closer look at the heart of the Trifid may have noticed something else. Examine this double at medium or high power. I would recommend at least 200X. If the seeing is good, you might be able to pick another fainter star just to the north of star “A”, the main star in this system. This star, labeled “C” in my sketch, is magnitude 10.6, and located only 5 arc-seconds from the primary. That makes this at least a triple star system. However, we are not done yet.
While examining this system with my 6.1″ refractor, I also caught glimpse of a fourth star! Decidedly more difficult, I was nonetheless certain of its existence. Located just about 2 arc-seconds to the west of star “B” in the sketch is a faint companion, about magnitude 10.5. This may seem pretty bright, but keep in mind it is deeply embedded in the brightest part of the Trifid’s nebulous glow. You will only see this star if the seeing is good, and your telescope has clean, well-collimated optics. Maximizing contrast is the name of the game to detect the 4th member of this interesting system! When I first observed this star last summer, I was unaware of its’ existence. This shows it should be visible with some luck in 6-inch and larger telescopes.
Using the 36-inch refractor at Lick Observatory, S.W. Burnham found a total of 6 stars in this system. The remaining two are much fainter than star “D”, and I suspect they would be challenges for telescopes in the 12-inch range and above.
Contributed by Myron Wasiuta