The HII region NGC 281 (also known as Sharpless 184) is a classic star forming complex consisting of an ionization front formed on the edge of a molecular cloud, and illuminated by its progeny of young stars. The complex is located in the Perseus arm of our galaxy and its visible structure spans some 60 light years. The molecular cloud counterpart has been busy in the last few million years forming the massive young stars of the central cluster IC 1590. IC 1590 is the source of ionization for the visible nebula and an excellent example of a very young cluster where pre-main sequence stars can be found in abundance, indicating active and ongoing star formation.
The visible nebula contains a variety of elephant trunk-like pillars and dark globules and a conspicuous incursion of foreground dust into the southwestern quadrant of the visible nebula. NGC 281 is a relatively bright HII region which formed between one and two million years ago as a thin ionization front on the edge of an extensive molecular cloud complex. The molecular cloud complex has been mapped using the tracer CO (carbon monoxide) and has been found to have two distinct components, a southeastern component N281A associated with the visible nebula, and a southwestern component N281B. The southwestern component of the molecular cloud is foreground to our line of site and forms the dusty incursion into the southwest region of the nebula. Within the southwestern cloud convincing signs of active star formation have been identified. Bipolar outflows of hot gas ejected from infant protostars and a radio source identified as a "water vapor maser" are convincing evidence of ongoing star formation in the southwestern cloud. The formation of water masers requires a high energy source to form molecules of water from the ambient gases in the cold cloud. The energetic outflows of protostars provide this source which explains why water masers are found in areas of active starbirth.
IC 1590 is a fascinating young cluster with a central core of OB stars within its center known as ADS 719 (Aitken 1932). The core is a trapezium-like system of closely spaced O and B type stars confined to a small region of about 8000 AU (astronomical units). The brightest star of the system is the O5.5 giant HD 5005. This star is the dominant ionization energy source for the surrounding HII region. Compact groupings of bright O and B type stars tend to form at the center of open clusters. Known as "trapezium systems" (named after the prototypical trapezium cluster of the Orion Nebula) they were once thought to be unstable because of their narrow separation distances. They are now known to be long lived stable stellar systems that exist in several well known clusters. The formation of the stars of IC 1590 began some 3.5 million years ago and likely continues today in the southwest quadrant of the complex.
Several compact globules known as Bok globules are projected against the background of the emission nebula. These compact dark structures were first described by the astronomer Bart Bok (1906-1983) in the 1940's. Bok Globules are small dark clouds of gas and dust that are found in HII regions and typically have a mass of about 10 to 50 solar masses. They often span a region of about one light year across. Their contents are predominantly molecular hydrogen, carbon oxides and helium but also contain about 1% silicate dust. Infrared observations in the 1990's detected the existence of protostars within the dense confines of Bok globules confirming the hypothesis of Bok, who strongly believed that these globules represented cocoons of starbirth. The larger Bok globule seen in the image is about 2.6 light years in diameter.