David Stevenson - The Complex Lives of Star Clusters
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Presents up-to-date coverage accessible for lay readers of the universes most glamorous and frequently-observed objectsTackles the breadth of knowledge about how the universes most dynamic stellar gatherings operate in a highly accessible and visually attractive formatConsiders the fate of star clusters and any planets that form in association with them As with the authors recent books Extreme Explosions and Under a Crimson Sun, the complex topic of star clusters is broken down and made accessible with clear links to other areas of astronomy in a language which the non-specialist can easily read and enjoy. The full range of topics are addressed regarding how star clusters are formed. Why is it some are dense conglomerates of stars while others are looser associations? Are the young, brilliant clusters seen in neighboring galaxies such as the Large Magellanic Cloud, M33 or M82 analogous to the ancient globulars seen in the Milky Way? How will these clusters change as their stars wane and die? More interestingly, how does living in a dense star cluster affect the fates of the stars and any attendant planets that accompany them?Star clusters form many of the most dazzling objects in the astronomers catalogs. Many amateur astronomers are interested in exploring how these objects are created and what it would be like to live among these objects. From the historical views of how star clusters came about to the most recent assumptions about how stars within these clusters evolve, different strands of science, from observation to theory, are woven together into a compelling investigation specifically targeted at amateur astronomers.
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Springer International Publishing Switzerland 2015
David Stevenson The Complex Lives of Star Clusters Astronomers' Universe 10.1007/978-3-319-14234-0_11. Initial Observations
David Stevenson 1
(1)
Sherwood, UK
Introduction
Star clusters are glorious objects to look at. The closest, such as the youthful Pleiades or the more mature Hyades, form little villages of light woven amongst the constellations and naturally attract the eye. With binoculars the six or seven stars of the Pleiades that are visible to the naked eye resolve into dozens of stars encased in a wispy nebulosity of gas and dust that shimmers with a pallid blue glow. It is only when the full glare of a moderate or larger scope is brought upon these objects that their full glory emerges. Hundreds of stars, the majority far dimmer than the Sun, appear. Through the steady increase in our ability to resolve the stars we have begun to understand how these hypnotic objects have emerged from the cold darkness of space. It is this story that The Complex Lives of Star Clusters explores.
Historical Observations
Star clusters have been known since antiquity; after all many are easy to spot by casual observation of the sky. The first recorded, systematic cataloging of these stellar aggregates was done by done by Galileo Galilei in 1610. Galileo turned his attention to the faint patches of fuzziness recorded by Aristotle, the Persian astronomer Al-Sufi and others. Galileo resolved the peach-fuzz of light into dozens of stars and developed the idea that the Milky Way was a vast metropolis of stars. It was the progressive development of the telescope, as well as the burgeoning curiosity of an increasingly affluent (and influential) academia that exposed the true number, distance and scale of star clusters.
The nearby open clusters Pleiades and Hyades were obviously observed since the dawn of mankind, but it wasnt until 1665 that Abraham Ihle discovered the first globular cluster, later named M22. However, Ihle only had access to a telescope with a small aperture. Consequently, individual stars within this globular cluster were not resolved and the object was assumed to be some form of nebula. Over a century later Charles Messier observed M4 and recognized that the nebulosity was really a tightly knit cabal of stars. Using his larger aperture telescope, Messier could resolve hundreds of stars. In the intervening decades increasing numbers of open clusters were cataloged but the true nature of them was not fully understood. In 1767 an English naturalist, the Reverend John Michell, calculated the probability that the spatial associations of light in the Pleiades might be chance alignments. Michell surmised that the chance these were a fluke alignment was one in 496,000: these associations had to be real, physically bound groupings of stars. Between 1774 and 1781 Messier published his catalog of comet-like objectsnebulous associations of light that were, in some cases, only barely resolved in his telescopes. William Herschel then examined many of these, teasing the light into further agglomerations of stars. Although we now reject Herschels idea that star clusters were the handiwork of gravity pulling stars together, his observations provide the framework for later classification schemes.
Towards the end of the nineteenth century astronomers worked with instruments vastly superior to those commonplace in the seventeenth and eighteenth centuries. It was apparent at the higher resolutions that the globular clusters formed some sort of spherical cloud centered towards the constellation of Sagittarius, while the open clusters lay in all directions in the plane in which the solar system was located.
As technology advanced the science of astrometry developed with improvements in the measurement of proper stellar motion. In 1943, nearly 170 years after Michell deduced the likelihood that the members of the Pleiades were physically associated, Adriaan van Maanen measured the proper motion of its visible stellar population. Comparing the motion of individual stars within the cluster to the mean motion of the cluster as a whole across the sky, van Maanen confirmed that they were physically associated and moving together as a coherent object.
In the 1940s it was certainly not clear how these associations were produced and whether the stars formed at the same time. The basic idea that star formation commenced when a cloud of gas and dust collapsed was well-established. However, despite this understanding, without an ability to deduce the ages of the stars within the visible association, it could not be clear that the stars had comparable ages. This understanding would come soon afterwards when astronomers began to interpret the pattern of stars in the color-magnitude diagrama more useful and quantitative form of the Hertzsprung-Russell diagram developed three decades earlier.
Open, Globular or Simply Associated?
So, what exactly constitutes a star cluster, and what is the difference between a globular and an open cluster? These are seemingly trivial questions and the former is rather easy to answer. The second question seemed easy to address but as with many areas of science, the more you look the more complicated the picture becomes.
Lets start with the most diffuse affiliationsthe stellar associations. These are loose groups of stars that began life in a single cloud of gas and move through space together. They are typically separated by 1 or more light years and are not bound together by gravity. These are simply cars on a race track, moving at a similar pace. Sooner or later the individual members of the association will fall out with one another and drift apart. There are many well-known examples of associationsperhaps most notably the Orion OB1 association, which includes the stars of Orions belt and numerous other, nearby, luminous O or B-class stars, as well as a multitude of much dimmer objects. Associations have lifetimes measured in millions or tens of millions of years. Their undoing is a combination of internal strife such as supernovae blowing material out of the cluster and physically removing stars, as well as the more insidious pull of all the other stars and matter within the galaxy. The Sun may well have begun life in some loose and, long ago, abandoned association. The one, famous, exception to the rule that stellar associations lead short lives is the Plough. This asterism is a collection of fairly luminous stars that are physically associated with one another, but that are (somewhat) more than 200 million years old.
Moving up the ladder somewhat leads to open clusters. As with associations, there may be hundreds or thousands of stars present, but in this case gravity does hold them at least loosely together. Like those in an association, the members of this grouping of stars have approximately the same age. Associations may contain one or more open star clusters as part of their make-up, much like villages congregating along a river valley.
Within an open cluster there are a few hundred to around 10,000 stars, which are located inside a space of 1020 light years in diameter. They are usually found with a distinct and considerably denser core, measuring a few light years across. This is surrounded by a more diffuse halo of stars. Within the central core of an open cluster stars may be separated by less than 1 light year or roughly the distance between the Sun and its distant cloud of cometsthe Oort cloud. Although close, this is still around nine trillion kilometers between each star, so the chance of collisions will be rare. As the stars within open clusters are only loosely bound by gravity, tides generated by any nearby gas clouds or other groups of stars will tend to pull stars out of the cluster (see Chap. ). Consequently, open clusters have relatively short life-spans that are measured in tens or hundreds of millions of years, the exact figure depending on the mass and density of the clusterand its location within the galaxy. Naturally, there are exceptions to this rule but these are rare. The most prominent exception is an ancient cluster called Berkley 17 (Be17). This ancient open cluster is likely 8 billion years old.
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