Tuesday, 17 February 2015

Where the wild things are: Cygnus OB2

This week I want to tell you about one of my favourite astronomical objects, and one which I have spent many years studying, the Cygnus OB2 association.

Cygnus OB2 is a loose group of young stars known as an association, a sort of less-dense version of a star cluster. As the name would suggest, the Cygnus OB2 association is the 2nd OB association catalogued in the constellation of Cygnus. This constellation lies very prominently in the plane of our galaxy where the majority of stars and nearly all young stars in our galaxy are found.

The Milky Way, showing the constellation of Cygnus and the position of the Cygnus OB2 association
(Original image credit: Axel Mellinger)

From the image above you wouldn't believe anything particularly exciting was happening in Cygnus, as the region appears mostly dark, particularly around Cygnus OB2 itself. The reason for this is that between us and Cygnus OB2 there is a vast cloud of obscuring dust that absorbs the majority of starlight emitted by the association, hiding it from us. This cloud of dust is sometimes referred to as the Cygnus Rift or the Great Rift, and can be seen extending across much of the Galactic Plane, as the above image shows.

Optical photons are readily absorbed by dust particles, but infrared photons are less susceptible to this problem and can penetrate these obscuring dust clouds. An infrared view of the Galactic Plane, such as that shown below thanks to Japan's Akari satellite, reveals the prominent sites of star-formation activity in our galaxy, radiating brightly thanks to the many young stars forming within them.

The Milky Way in infrared light, as seen by the Akari satellite, with major infrared-bright regions labelled
(Credit: ESA)

The brightest and most prominent of all these star forming regions is probably the Cygnus region, despite it being more distant than many of the other star forming regions shown on this image. This brightness is testament to the intense amount of star formation that has, and still is occurring in this region. In fact when this region was first studied at radio wavelengths the bright structure was so prominent as to be marked with an 'X', hence leading to the name of Cygnus X for the whole star forming complex.

Detailed infrared view of the Cygnus X region, with
Cygnus OB2 in the centre (Credit: NASA)
Zooming into this image we can begin to see some of the amazing structures present within the Cygnus X giant molecular cloud. Vast clouds of gas and dust can be seen collapsing to form young stars, while huge pillars are constantly being sculpted and eroded by the recently-formed stars. This is a place where star formation takes place at the extremes!

Right in the centre of the Cygnus X giant molecular cloud is the Cygnus OB2 association, a massive group of young stars, as populous as some of the most massive young star clusters in our galaxy, yet nowhere near as compact.

The diffuseness of Cygnus OB2, coupled with the obscuring dust clouds, led to it being maligned for many decades. Only in the last 15 years, thanks to the revolutions in infrared and X-ray astronomy, have researchers been able to penetrate the extinction and uncover the thousands of massive, young stars in this huge OB association, including some of the most massive and luminous stars known!

As I said at the beginning of this post, Cygnus OB2 is a region I have studied for many years, hoping to better understand its origins and its content, and therefore to appreciate its role in the continual evolution of our galaxy. Next week I hope to share some exciting news and discoveries about Cygnus OB2 that we will be publishing very soon, so stay tuned!

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