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.
Is this a fair representation? If not then drop me an email. Address below.
This page maintained by Colin Frayn.
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Last Update : 17th March, 2004