Sunday, July 09, 2006

Globular Clusters and The Age of the Universe

Usually, when asked how we know the age of the universe, one refers to the expanding of the universe that we see when looking at distant galaxies. When this expanding is tracked back, we arrive at an age of around 15 billion years. But that is not the only method. Here I want to explain an independent method: the age of globular clusters.

Globular Clusters (GCs) are clusters of a huge number (a few hundred thousands) of stars, all at about the same age. They surround our and other galaxies, and were formed at about the same time the galaxies formed, when star formation was much heavier than it is now. Nowadays, GCs do not form any more in our galaxy, but they do in other galaxies where there is much more star formation going on. They should not be confused with Open Clusters, which are much smaller, and contain a lot less stars (a few thousands). Those still do form, and a prominent cluster is for example the Pleiades.

Why are they important to understand the age of the universe? Well, if they are old, than the universe must be at least that old. So their age gives a lower limit to the age of the universe. They are also convenient because all members are of about the same age, and of the same composition. Also, because they are clusters, all members are at about the same distance to us. Now, it happens that the evolution of a star is determined by just two factors: the mass and, to a lesser extent, the composition. A heavy star is relatively short lived, and a lighter star has a much longer life. The sun will live for about 10 billion years in total, but a star with a few tens the mass will live only a few million years. Lighter stars live much longer, even longer than the current age of the universe - so we will not see any of those at the end of their life.

To classify a star, that is to determine its type and therefore its stage in its development, we just need two basic properties: their luminosity, and their color. The color is a more or less direct measurement of the surface temperature. When these observed properties of stars are plotted against each other, we do not see them randomly distributed, but we see a very significant pattern. This is the Hertzsprung-Russell diagram. Follow the link (and others I give here), there is a nice picture (HRD). You see that most stars line up in that diagram in a diagonal line. That is the main sequence. A few others cluster in the upper (brighter), right (cooler) region. In the main sequence, brighter stars are hot and dimmer stars are cool.

Okay, but what about the age now? Well, we can make simulations of the evolution of a star. Simple models assume a spherical symmetry, and use four differential equations. The models also require a lot of input from other sources, for example the opacity of the elements, which are derived in part from nuclear bomb testings. The important thing: the input is not coming from observations of other stars, apart from determining their initial chemical composition, and occasional reality checks. What do we learn from these models? We see that a star on start very quickly moves in the HRD to the main sequence, and spends most of its time there. Its exact position on that line depends just on its mass (and a little bit on its composition). The heavier the star, the brighter and hotter. But also, the heavier the shorter its lifetime. As soon as the end is near, the stars move to the right, upper corner, so it gets cooler and brighter at the same time (which means that it also gets bigger, that's why they are then called giants, but that's another topic).

Okay, to summarize:

  • the heavier the star, the shorter its time on the main sequence
  • all stars of a GC are of the same age
So this means that if we plot all stars of a GC in the HRD, at some point in the main sequence we see stars moving off it into the upper, right direction. All heavier (brighter, hotter) stars have already disappeared - the exact point where that happens depends on the age of the GC. And this is plotted in this post on the Globular Clusters Blog, which shows an age of 13 billion years for the GC M55. It turns out that all GCs of our galaxy are of that age. Which means that the universe must be at least that old.

2 Comments:

Blogger Sprittibee said...

So if all the stars of a GC are the same age, are they also the same type of stars? In color and luminosity?

And in your simulated models to "test the simulation of evolution", you say you are not using observable data from other stars, but rather are using assumptions from many sources (some being chemical traces from bombs). How do we know that the elements of the star are the same as the elements of our bombs? Do we have a sample of the elements? Why do we not use observable data?

SCIENCE:
"Systematized knowledge derived from observation, study, and experimentation carried on in order to determine the nature or principles of what is being studied." (Webster's New World Dictionary of the American Language)

How, then, is this "hypothesis" of the age of a star Science?

Do you believe then in a Big Bang? If so, the evolutionist material in modern school textbooks explain it as a creation of matter from nothing. So nothing (or in some textbooks, a point as big as a period on the page) containing all the matter we see today in the universe was spinning and exploded. The law of angular momentum states that if something is spinning and comes apart, all it's pieces will spin in the same direction.

How then do you explain that some planets and moons have opposite rotations?

I believe that evolution is just as much of a religion as creation science. Erasing the line between your hypothesis and the facts and declaring that it MUST be SO does not necessarily MAKE it so. That is a MANTRA, not a reality.

11:07 AM  
Blogger oku said...

So if all the stars of a GC are the same age, are they also the same type of stars? In color and luminosity?

No, they are not, as explained in the post. It depends on their mass. The type of any random star we see depends mainly on three paramters: the age, the mass and the composition (last one is the least important). In GCs, though, the initial composition and the age is the same, so the only free parameter left is the mass Masses of stars are in a range from about 0.1 to 120 solar masses (the less massive, the more common, but that's not important).

How do we know that the elements of the star are the same as the elements of our bombs

The initial composition is assumed to be 23% helium, 75% hydrogen, the rest heavier elements (this is what varies among individual clusters). These values are derived from observations of insterstellar clouds and the spectra of stars, and lots of other observations.

It is found that the development of stars does not vary much with different compositions of the heavy elements (for example the ratio of Mg to Fe), but more on the total amount of heavy elements, but even that is of less importance.

The most important thing is that the developmemt of stars can be simulated by just using physics, and that it matches the observations. There is _no_ way to explain the observations we do make of GCs by assuming a age of 6000 years. After 6000 years, no star would have even reachec the main sequence, not to speak of leaving it again. We would still observe very, very massive stars (we that those were there, because they are there in other, younger clusters). What we see is that stars which leave the main sequence at an age of 15 Gigayears indeed do so in the GCs.

You may object that the initial composition might have been different, although they do come from observations. But even if it were, we wouldn't see the nice match of theory and observations.

Do you believe then in a Big Bang? If so, the evolutionist material in modern school textbooks explain it as a creation of matter from nothing. So nothing (or in some textbooks, a point as big as a period on the page) containing all the matter we see today in the universe was spinning and exploded. The law of angular momentum states that if something is spinning and comes apart, all it's pieces will spin in the same direction.

Sure, I do believe in the big bang, because observations lead to that conclusion: we see distant objects receding from us, in all directions, so extrapolating this into the past, it all came from one point (it is more complicated than that, it is the space that expands, not the objects receding, but that's not important for now). See http://brahms.phy.vanderbilt.edu/~rknop/blog/
for a good blog on that subject.

But you cannot conclude from that that therefore in the solar system objects cannot revolve in anothe direction. The total angular momentum of the universe is most likely zero (it probaly has to be, because there is no reference point to measure the rotation against, but that's an area I am not that ATM qualified to discuss. See General Relativity).

But the formation of the solar system is another matter. First, the solar system is a very tiny part of the universe. Our galaxy is a tiny part, and the solar system is a tiny part of the galaxy. It formed 10 billion years after the Big Bang. Before that, the galaxies formed, and generations of other stars and solar systems formed before ours.

There were turbulant motions in the gas cloud which formed it, which could explain that for example Uranus rotates on its side. Venus either had a large impact by a mini planet, or it had undergone tidal locking. Moons rotating areound their planets in the opposite direction have been captured (this can be simulated, too). All this can be checked on the TalkOrigins site Zeteo linked to, especiually http://www.talkorigins.org/indexcc/CE/CE260_1.html

We do observe though that all planets revolve around the sun in the same direction, nearly in a plane, with Pluto being a little bit of an exception because its orbit is tilted.

But in any case, even if they were not, the law of conservation of angular momentum states something different. I do not want to get into that now, but you may want to look that up in the wikipedia. If you want to teach this to your kids, you definetely want to read some basic physics book first.


The most important thing, and that is one reason I wrote about the GCs: science is sa lot like a puzzle. The pieces fit neatly into each other. Sure it isn't complete. But starting the assumption that the universe is 6000 years old is like trying to fit together pieces that do not fit, and thereby destroying the whole picture. You do not get another picture: you do not get any picture at all.

Look at the refutation of the creationist claims, Zeteo gave a few links. But you will find lots of other ressources in the internet. Read some books about that matter. It is interesting and fascinating.

1:21 PM  

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