31 July 2009

Measuring The Age of The Universe- An Example of How We Know

Posted by Dan Satterfield

Inside view of a globular cluster from The Harvard-Smithsonian Center for Astrophysics.

Inside view of a globular cluster from The Harvard-Smithsonian Center for Astrophysics.

I love Astronomy, and many times I write something about it here to teach MYSELF something. Atmospheric Science is my field, but I thoroughly enjoyed the two undergrad astronomy courses, and the graduate course I took. This means I know just enough about Astronomy to be dangerous! Pilots are taught just enough Meteorology to be dangerous!

A "globular cluster" of stars. (Astronomers aren't any better than Meteorologists in thinking up names for things!)

A "globular cluster" of stars. This one was first discovered by Edmund Halley who later got into comet predictions. (Astronomers aren't any better than Meteorologists in thinking up names for things!)

Frequently, I hear someone say something along the lines of “how do we possibly know what the temperature was like 10,000 years ago? There were no thermometers around!”

A similar refrain is often made with regard to the age of the Universe. I’m sure you have thought of other examples. I know I have. How do we know what dinosaurs ate for instance? No, we have not found any ancient menus from the town of Bedrock!

You put enough pieces together though, and you can have a real Yabba Dabba Doo moment!

In Science, one piece of information can be very valuable, but no one would state something like the examples above based on just one piece of the puzzle. What Dinosaurs ate is based on hundreds if not thousands of independent pieces of evidence. Same for the temperature 10,000 years ago. Fossil trees, ice cores, seabed cores, etc.

That’s how we know.

These are the fascinating things about science. What we know can be fabulous, but learning HOW we know, now that is the fun stuff!

Picture 1

Andromeda Galaxy from Hubble Space Telescope. It has globular clusters orbiting it. At least 500 of them!) Our own Milky Way galaxy has a bunch as well. M31 G1 is a globular cluster. M31 is Andromeda. Picture courtesy NASA McDonald Obervatory/Stardate.com

Take globular clusters. I want to use it as an example of how we can get a glimpse of how old the Universe must be. Many times in Science, you cannot get an exact answer, but you can constrain the number you are looking for. Globular clusters for instance tell us the Universe is “at least x billion years old”.

Read on- it’s a neat piece of deduction!

First, you ask, WHAT’S A GLOBULAR CLUSTER???

Nothing more than a cluster of stars. A mini galaxy in the shape of a ball. Thousands and more likely millions of suns. Some like our own, and others much larger and smaller than ours. Over 150 globular clusters are known to orbit our own Milky Way Galaxy.

Globular clusters formed out of Hydrogen and Helium like all the galaxies. In the very early Universe, that is all there was. Look around you. EVERYTHING you see that is not Hydrogen or Helium, was made in a star. Do you have a gold ring on? If so, it was made in a supernova. Look at yourself in the mirror. Yup, you too are made of star dust.

So is the computer I am typing on, but the good folks at Apple in California have done a lot with the raw elements the stars made!

Now, imagine a huge vast cloud of Hydrogen and Helium that slowly starts clumping together due to gravity. As it compresses it heats up. Knots of gas form. Some big knots and some small ones. Eventually the pressure is so great that nuclear fusion begins and stars form.

Some giant, hot blue stars and some like our own sun. Many more smaller “red dwarf” stars.


The Hertzsprung-Russell Diagram. Big stars burn bright, but live short lives. (From: http://cse.ssl.berkeley.edu/bmendez/ay10/2002/notes/pics/bt2lf1509_a.jpg)

Here is the neat part. All stars are not created equal. You might think it would be great to be a giant blue star visible for a billion light years in all directions. Not so! Big stars use up their fuel very rapidly. They may burn for only a few million years. Smaller stars like our sun, can burn for 10 Billion years. Smaller stars still can burn even longer.

Now imagine that we use one of the giant telescopes to look at a bunch of globular clusters. We can look at them in visible light and infrared and even in x-ray light using the Chandra X-Ray telescope.

It turns out that most of the globular clusters around our own Milky Way Galaxy and indeed many of the ones we see elsewhere, are all missing something. They are missing the giant stars.

Where did they go? Did the globular cluster just not form any when it formed from the giant cloud of Hydrogen and Helium and dust?

Not likely.

They were there. They lived out their lives, and either exploded as some big stars do or turned into white dwarfs, a fraction of their old size with such low luminosity, that we cannot see them anymore. (The deaths of stars is one of the most fascinating parts of astronomy)

If we know how long stars of a certain size live, (and we think we do!) then we can look at a globular cluster, and by seeing what stars are missing, we can estimate it’s age! Astronomer Alex Filippenko says its like walking into a room with a burning candle. If you know how fast it burns, and how tall it was originally. It’s easy to figure out, how long ago it was lit!

Now if that doesn’t make you go Gee Golly, then you have no imagination what so ever!

Many globular clusters are measured to be over 12 billion years old! Since the Universe they are in has to be at least that old itself, we can now say the Universe is at least 12 Billion years old!

Techniques like this (and many others using different astronomical objects) have allowed Astronomers to say with growing confidence that they have constrained the age of the Universe to 13.7 Billion years old. Plus or minus 120 million years.

I just ordered my first pair of astronomical binoculars and am told you can see globular clusters with them. Look for a guest post soon on how to buy your first telescope.


Note: If any experts in Astronomy read this, and find that I have gotten something wrong, PLEASE email me. I will post it, and correct the error, and we can all learn more about the stars. Keep in mind that I write this for a general audience, so let’s not get into deriving the Hubble Constant- but I am anxious to know more. I would be honored to have your thoughts and to share your knowledge.

Sources: Astronomy 10- Alex Filippenko UC Berkely
The COSMOS- Astronomy in the New Millenium A. Filippenko Jay Pasachoff
McDonald Observatory/Stardate