Claim 3 - Globular Clusters are Young

Star clusters generally occur in two distinct types. That is, globular clusters and open clusters. Globular clusters are large collections of between ten thousand and one million stars and are usually, at least in our galaxy, extremely old. Open clusters are generally much smaller, consisting of a few hundred to a few thousand stars, and are intrinsically much younger. Some are still in the process of forming, and many are probably not sufficiently tightly bound to last more than a few hundred million years. Star clusters have been discovered around virtually all nearby galaxies, and have been detected out to distances of several hundred million light years.

Formation of globular clusters is rather well understood, and extremely well studied in large numerical supercomputer simulations, as well as analytically and by observation. They form when a large gas cloud collapses under its own gravity, then breaks into many smaller pieces. These smaller pieces then collapse individually to form stars. The earlier stars 'sweep up' most of the remaining gas, and the remaining interstellar material is then pushed away by radiation pressure as soon as the first generation of stars 'switches on'. This whole process takes several million years, but it is worth noting that in terms of the ages of the globular clusters we see today, this is an insignificant length of time.

It is therefore safe to assume that these enormous collections of stars are coeval, that is to say that the stars within them formed at roughly the same time. We can also assume that the other various factors which affect the lives of stars, such as the initial level of heavy-element enrichment, are similarly equal amongst all the cluster members.

Large supercomputer simulations can now model upwards of 100,000 stars for the entire life of a globular cluster in very high detail. The physics which governs the orbits of individual stars is very well understood, and requires no complicated processes whatsoever, just simple gravity. At any one point, every star in the cluster is in some point of its orbit around the cluster centre, with its distance from the centre either increasing or decreasing. Almost all stellar orbits in this type of system will be highly elliptical, as opposed to the orbits of the planets of the solar system which are, for the most part, accurately circular. That means that at some points in their orbit they are found closer to the centre than other times. It's extremely simple to show that the average orbit of a star in a typical globular cluster takes a few million years. The globular clusters in our galaxy are remarkably spherical bodies, showing that they have clearly been in existence for many orbital time periods so that the orbits of the stars can settle down into regular patterns.

Several Creationists claim that one can take an individual star and show that it is moving outwards at a speed which would take it away from the cluster within a few million years. This is an entirely incorrect argument. That star will eventually be dragged back towards the main body of the cluster by the gravitational attraction of the remaining stars. Occasionally a star manages to achieve the velocity needed to escape the cluster's pull, normally by means of close interactions with other stars, but this is reasonably rare.

The argument presented by Hovind, amongst others, is akin to looking out of the window at night and noticing that it is dark, then remembering that it was much lighter just a few hours ago and continuing this trend backwards and concluding that the Earth is only one day old because otherwise the sunlight would have blinded every living creature on the planet. It is attempting to analyse a simple consequence of an event without even the most fundamental understanding of the physical processes governing it. It is an example of incorrect extrapolation, as explained on the main page.

Age Estimates for Clusters

The lifetimes of stars are governed by many different parameters. By far the most important factor is their initial mass. The more massive a star when it begins its life, the shorter that life will be. Stars like the Sun are expected to last approximately ten billion years (ten thousand million years). Stars ten times more massive than the sun can expect to last a few tens of millions of years. Stars one hundred times more massive than the sun will only last a few million years at most. There are many reasons for this. The more massive stars have, obviously, much more fuel, so one would expect them to last much longer. However, they use up this fuel at a much quicker rate than the less massive stars, which more than cancels this out. In addition, more massive stars are significantly less stable and go through many more catastrophic stages in their lives than do the more well-behaved low-mass stars.

Stellar models are now very accurate and well-studied. The important stages in the lifetime of the average star are well-known, and can be simulated accurately inside a computer. This means that, given the mass of a star, together with a few less-important parameters such as its metal abundance, we can predict exactly what that star should look like at any point through its life. Reversing that process, if we see a star we can tell its metal abundance, mass and age just from its visual properties.

Looking at globular clusters, two things strike us. Firstly, they are extremely stable systems, hinting that they have been in existence for a significant number of stellar orbits. Secondly, there are no massive stars whatsoever in any of the milky way's globular clusters. Several of them don't even have stars as massive as the Sun. Now, if we remember two things; Firstly, that globular clusters are coeval systems, formed during a relatively short time period, and secondly that we already know how long we expect stars to live given their various observable properties, then it should be obvious that we can get an estimate for the age of a globular cluster by examining the most massive stars within it. The cluster must be approximately the same age as the average lifespan for its most massive members.

Now this is by no means an easy task. There are also many other similar methods for analysing globular cluster populations, but all rely on approximately the same theory - analyse the stars present in a globular cluster and compare them with our accurate model predictions. This process has been repeated many times for the globular clusters that we see around the Milky Way, which number around 150. We reach two conclusions;

(1) Globular clusters seem to have differing ages, with some younger populations, mainly located in the disk and bulge of our galaxy, and many more older populations, primarily located in the outer parts (or 'halo') of our galaxy.

(2) Even the younger populations are significantly older than our sun (4.5 billion years), and the older populations are mostly between 10-14 billion years old.

The study of globular cluster populations around nearby galaxies gives us a fascinating insight into how galaxies formed. We believe that these oldest globular clusters formed at the same time as the first fragments of our galaxy began to merge together. Elsewhere in the local universe, we see examples of globular clusters being formed in large galaxy mergers which take many millions of years to occur. All around us, globular clusters point towards the inescapable conclusion that these processes only take place over the course of billions of years. Yet another example of the truly enormous age of the Universe.

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Last Update : 17th March, 2004