DSO of the Month
Messier 42
AKA: Orion Nebula; NGC 1976.
Position: 5 hr 35.3 min -5 deg 23 min 28 sec
Due south at 23:56 (GMT) on 15 December
Image of Messier 42 (and Messier 43 underneath) by HAS Member Martin Gill.
Used with permission.
Messier 42 is the well-known large bright fuzzy patch underneath the Belt of Orion in the middle of the three stars that make up Orion’s sword. The nebula is a diffuse nebula which can also be called a reflection-emission nebula. Even in small binoculars or a very small telescope it is an amazing sight. M42 is basically a stellar nursery situated 1,344 light years away from us. The stars are formed within the swirling clouds of M42 shielded from our eyes. When they turn their nuclear fusion on and grow hotter, their stellar winds and ultraviolet radiation drive the obscuring gas away and become visible. M42 is about 3 million years old, but some of the stars in the nebula may be only 10,000 years old. In about 100,000 years, most of the gas will be pushed away and the Orion Nebula will become a star cluster. The core of the cluster is already visible in the form of a group of four stars at the heart of the nebula called the Trapezium from its shape. It is very small and the glare of the nebula makes it hard to see, but with a high enough magnification, it is perfectly clear. Galileo discovered three of the stars in the Trapezium in 1617 and the fourth one was known by 1673. There are other stars in the cluster, eight were identified in total by 1888, and a larger telescope (125mm or larger) should be able to show five or six of them, although from personal experience they are not easy to see. Three of the stars in the Trapezium are B stars (one is a Herbig Ae/Be star) and one is an O star. It is possible that there is a black hole in the centre of the Trapezium.
The Trapezium is only a small part of the whole cluster, which may contain up to 2,800 stars. Eventually just as babies grow up and leave home, stars will eventually be expelled from the cluster, and three stars -- AE Aurigae, 53 Arietis, and Mu Columbae – may have already left the cluster at velocities of over 100 km/sec. 53 Arietis is a 6th magnitude B star roughly halfway between Beta Arietis (Sheratan) and Aldebaran (Alpha Tauri). AE Aurigae is also a 6th magnitude O star, between Beta Tauri (Elnath) and Capella (Alpha Aurigae). Although the Orion Nebula is now obvious to the naked eye, even in light-polluted areas, it was not recorded by any observers before the 17th century. Even Galileo, who first noted three stars of the Trapezium , did not mention the nebula. It is not clear if there was a mental block that prevented earlier observers from conceiving of the notion of such a nebula or whether the nebula may have suddenly brightened in the early 17th century. Finally, like me you may have wondered what is the boundary between M42 and M43? If you look at an image of M42 and M43 (they are often imaged together), M42 looks like a spread eagle with a body and outspread wings and M43 is the head of the bird; it also looks like a comma in many images. With the naked eye or a small telescope, M43 is hidden in the glare of M42 (it is only 9th magnitude) and you really only see M42. In reality they are both part of the same nebula.
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