Thursday, 10 December 2015

What are OB associations and why should you care?

Over the last year I've talked a lot about different types of star cluster, but I've also mentioned another type of stellar group known as an OB association, and you may be wondering what are these OB associations? In this post I'm going to talk about OB associations, what they are, and why they're so important for understanding star clusters.

OB associations are very similar to young star clusters in that they are a group of young stars. They're not as compact as star clusters though, and the stars in them are spread out over a larger area of space than for star clusters.

OB associations were first identified just over a century ago by Arthur Eddington who noted the presence of groups of stars distributed across large areas of the sky but sharing a common motion. The actual term OB association was first penned by Victor Ambartsumian in 1947, who noted that these associations appeared to consist mostly of bright and blue stars known as O- and B-type stars. The image below shows the distribution of these stars near the Sun, showing how they appear to fall into a number of rough groups.

The distribution of nearby O-type (filled circles) and B-type
(empty circles) massive stars from Hipparcos. The boxes show the
positions of known OB associations (Credit: Preibisch & Mamajek 2008)
Ambartsumian noted that the low density of the OB associations meant that they weren't gravitationally bound (meaning that they weren't held together by their own gravity - in the same way that dense star clusters are). This means that they must be in the process of expanding and dispersing, and also implies that they must be relatively young structures if we are able to observe them before they have dispersed.

This revelation came at the same time as other scientists were beginning to understand how long stars of different types lived for. This revelation came from understanding the nuclear fusion that gives stars their energy, and then comparing the source of energy that each star has with the rate at which it is radiating that energy away. The O- and B-type stars are amongst the most luminous of all stars (as well as being the most massive), meaning that they are radiating away their energy the fastest and therefore have the shortest lives of all stars. This discovery confirmed the youthfulness of the OB associations that Ambartsumian had hypothesised.

If OB associations are not gravitationally bound then this means that they're probably in the process of expanding and dispersing into the Galactic field. And if OB associations are expanding then this means they were probably smaller and more compact in the past. This has led some astronomers to suggest that OB associations are the expanded remnants of compact star clusters that have been disrupted by some process.

The most common explanation for this disruption is that young star clusters are thought to be held together by the giant gas clouds that they formed in, and when star formation finishes this gas is blown away by powerful winds that come from massive stars. Without this gas the star cluster doesn't have enough mass to hold themselves together by gravity, and so the cluster begins to expand and disintegrate. The schematic below illustrates this sequence of events.

The sequence of events leading to the disruption of a star cluster: (1) The star is born embedded within a cloud, (2) the winds from the young stars disperse the cloud, and (3) without the gravitational potential of the cloud holding the cluster together the star cluster disperses and is briefly visible as an OB association. (Credit: Nick Wright)

There are other suggestions for how star clusters might be disrupted, such as tidal heating of the cluster, or possibly that some of the young star clusters that appear so ubiquitous may not actually be gravitationally bound in the first place and would therefore naturally expand and disperse.

This all means that OB associations are quite important objects to study, because by studying them we can effectively observe the process of star clusters being destroyed. Unfortunately they're not easy objects to study because the stars in an OB association are often spread over a large area of the sky and they can sometimes be difficult to distinguish from the older stars that make up the Milky Way Galaxy. For this reason our knowledge of OB associations, and our census of those that exist in our galaxy is rather slim.

The Scorpius Centaurus association
(Credit: Akira Fujii)
The most well studied OB associations are those nearest to us (this is often the case in astronomy). Associations such as the Scorpius-Centaurus association (see image on the left), the Perseus OB association, and the Lacerta I association were all discovered thanks to the bright and blue O- and B-type stars in them.

Other, slightly more distant OB associations include a number of slightly larger and more populous OB associations in the constellation of Cygnus (some of which I've discussed before here and here), as well as the Orion I association that surround the bright clusters in that constellation. Sometimes OB associations include a number of smaller clumps or clusters of stars within them, for example the double clusters h and chi Persei are part of the larger Perseus OB associations discovered in 1943.

There are a few OB associations that you can see without the aid of a telescope, but not many unfortunately because they are often very diffuse. The Alpha Persei cluster in the constellation Perseus is part of the larger Perseus OB association and is easy to observe. And while you may not be able to see most OB associations there are a number of young clusters you can observe that may one day become OB associations!

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