Wednesday, 8 August 2012

Whither the star clusters

Today’s astronomy is all about the growth of structure in the Universe – how stars and planets form, how star clusters form, how entire galaxies form...  The mid-scale of clusters is as perplexing as any spatial scale. We know from observation that most star clusters are destined to extraordinarily short lives, in that 9 out of 10 vanish by the time the dense dust and gas in which they are born has dispersed (a timescale of no more than a few million years, typically – a blink of an astronomical eye). We also don’t know what to make of the larger related structures known as OB associations.  Are they star clusters on steroids, or something different?

Image Courtesy of Nick Wright
To solve this, we have to get down to the details - learning all we can about the space velocities of the stars making clusters up. Are the stars flying apart, falling in, orbiting ...moving around chaotically, or what?   If we can discover what the stars are up to now, we can foresee what will happen next.   But this is much easier said than done, as it requires the wholesale measurement of tiny stellar motions in the plane of the sky, as well as the line of sight motion revealed by the Doppler effect.  But we are on the verge of doing it, and that’s what the newly published collaborative paper The Dynamics of an Expanding OB Association” is about.

The tiny stellar motions have to be deduced from high quality sky images obtained years apart. We at the University of Hertfordshire are at the forefront in this area, via our Galactic Plane surveys. Here, data from IPHAS (INT Photometric Hα Survey of the Northern Galactic Plane) steps in. This is the first comprehensive kinematic study of the massive northern OB association, known as Cyg OB2.  The results portray a complex hierarchical structure, at odds with simple dispersal.

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