Star Cluster Expansion

How Good Are Those Young-Earth Arguments?
A Close Look at Dr. Hovind's List of Young-Earth Arguments and Other Claims
by Dave E. Matson
Copyright © 1994-2002

Young-earth "proof" #8: At the rate many star clusters are expanding, they could not have been traveling for more than a few thousand years.

Without more detail, I can only guess as to the nature of Dr. Hovind's argument! As it is worded above, it remains something of a mystery.

I believe that some creationists have argued that many of the stars in a typical globular star cluster are moving outward, thus limiting the cluster to a certain age before it dissolved. Such an argument would betray a gross ignorance of globular clusters. A given star moves away from the central area of a globular cluster for a time, slows down, reverses direction, and falls back through the central region of the cluster and out the other side. Thus, stars move back and forth through the center of the cluster. There is no net expansion.

There are some stellar associations, which are loose groups of stars whose members are moving fast enough to overcome their mutual attraction. However, there is no particular reason to believe that stellar associations have been together for long periods of time!

Star clusters do, however, present a fascinating proof of great age! To reasonably understand the details of this proof, you should read Dalrymple (1991, pp.365-375) or else consult a good astronomy text. I'll quote from Dr. Alan Hayward to sum up the central idea.

[Scientific] techniques have enabled astronomers to work out the life span of each particular kind of star. They have found, for example, that the hottest and brightest blue stars were endowed with only enough energy to keep them going for a few million years, whereas the coolest red stars have a life span of many billions of years.

With this background in mind, we must now take note of a most remarkable fact about the star clusters...

Some clusters contain stars of all life spans, from the shortest to the longest. Some contain all except the very shortest-lived types. Some contain all except very short-lived and fairly short-lived types. And so on, all the way to those clusters where only the long-lived types are present.

But never do we find a cluster without a selection of the long-lived types. The missing ones are always from the shorter end of the range. We can look at the data for each cluster and say, 'This particular cluster contains only those types of stars with life spans greater than x years', where x has a different value for each cluster.

(Hayward, 1985, p. 103)

The basic idea is quite simple. Originally, when each star cluster formed it was populated by a variety of star types as might reasonably be expected. As it aged, the first stars to disappear were the shortest-lived ones, the massive giants which spent their fuel prodigiously, and they were followed by the short-lived stars until, in the very oldest star clusters, only the very old red stars remained.

On a more technical level, the above facts are reflected in the Hertzsprung-Russell (Hertzsprung-Russell)H-R) diagram. The vertical axis of an H-R diagram plots a star's true brightness while the horizontal axis plots its surface temperature, which determines the star's color. When stars are thus plotted the points are not randomly scattered about but fall into various meaningful groups. A surprising amount of information is present in the H-R diagram. "The existence of fundamentally different types of stars is the first important lesson to come from the H-R diagram. ... the H-R diagram [also] reflects an understanding of the life cycles of stars: how they were born and mature, and what happens when they die." (Kaufmann, 1994, p. 353). One would never guess that so much information was locked into something as simple as the H-R diagram! (If you feel lost, that's okay as this subject requires some study. I'm just trying to install the basic landscape.)

It turns out that the majority of stars plotted on the H-R diagram fall on a diagonal strip known as the main sequence. Main sequence stars are those stars that are burning their primary fuel, namely hydrogen. For a typical star that is a stable condition, and it accounts for most of that star's lifetime. Hence, the reason most randomly chosen stars plot on the main sequence. When a star exhausts its primary fuel, its plot on the H-R diagram drifts off the main sequence. Therein lies the key.

If we plot a random bunch of stars on the H-R diagram, we, of course, get all the patterns associated with the H-R diagram. However, if we plot just the stars in a cluster, it being very likely that they all formed at about the same time the cluster originated, we get something very different. The super-heavy, gas-guzzling stars, which burn up their primary fuel first, will be the first to leave the main sequence. If you plot the stars in a cluster that is fairly young (as clusters go) you would find that only the heaviest stars, which normally plot at one end of the main sequence, had left the main sequence. The heavy stars, which burn their fuel not quite as fast, are the next to run out of hydrogen gas. Consequently, if you plot the stars in a cluster that is a little older than the above, you would find that the heavy stars no longer plot on the main sequence. The next stars to leave the main sequence would be the moderately heavy ones, and so forth, until the lightest stars are all that remain on the main sequence. To make sense of all this you need a couple of more clues.

The more rapidly a star burns its fuel, the hotter it is, and the hotter it is the more its color is shifted to the blue end of the spectrum. The heavier a main sequence star is, the hotter it burns. Consequently, you can now see why the blue stars are the first to disappear from a cluster and the small, red ones the last to remain. It's a natural consequence of age. Let's now tie this fact to the H-R diagram.

The main sequence, a diagonal strip on the H-R diagram, not only represents stars burning their primary hydrogen fuel but also sorts them by weight. For various reasons, the heavier a star is the further it plots to one end of the main sequence strip. (I told you there was a lot of information hidden in the H-R diagram!) Consequently, starting at the small-red-star end of the main sequence of an H-R diagram, the older a cluster is the sooner its stars will turn off from the main sequence. At that point the star plots will start drifting to the right as they leave the main sequence. By noting where the turn-off point is, astronomers can estimate the age of a star cluster. Such a pattern in the star clusters, as revealed by the H-R diagram, has only one intelligent meaning. Those clusters of stars have aged! Amen, Brother Ben!

If all the star clusters had been created recently at the whim of God, any combination of stars would be just as reasonable as any other. Star clusters without the small, red end of the main sequence would be just as reasonable as clusters representing only the middle of the main sequence or clusters with only the white and blue portions of the main sequence. The possible combinations are practically endless, and the creationist must explain how it is that God decided on the improbable, peculiar pattern we actually observe, which plainly suggests that the ages have been at work. Do they believe in a deceptive deity?

Since [the above] is based upon a great mass of experimental data it seems inescapable, unless we are prepared to write off the extraordinary distribution of star types in clusters as a mere coincidence. And the odds against that have been calculated to be countless millions to one.

(Hayward, 1985, p.104)

In summary, the odd distribution of stars in the star clusters is a result of great ages at work. Among the oldest star clusters are the globular clusters, some of which may be older than 10 billion years! (Chaisson and McMillan, 1993, p.411). Far from being an argument for a young universe, star clusters (especially globular clusters) are a showcase for an old universe.

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