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Explosive Measurements
Science >
This is the concluding article of the suspenseful best-selling action trilogy: Measuring the Fantastic Distances of Space. The first two were pretty straightforward; this one can get weird.
You may recall that for close-by stellar critters we use "parallax." For objects farther out we utilized the cosmic object called a Cepheid variable. You can still read those articles at http://firstlightastro.com/icolumn.
But those methods only took us to millions of light years out. Problem? The visible universe is billions of light years - thousands of times bigger - in all directions.
How can we measure things out to such great distances? One trick is to use another relatively reliable source of light --- the supernova.
Recall that a supernova is the explosive death of a gigantic star. One type of supernova --- Type Ia --- pop off everywhere in the universe with pretty much the same amount of light as each other. And they are really, really bright; as bright as the galaxy they resided in before their timely death.
If we know how much energy they are supposed to throw when they blow, and then measure the amount of energy that actually manages to make it this far, we can estimate how far away they were (via last week's inverse-square law).
These explosive bad boys can help us measure distances out to about 8 billion light years!
But there's a cosmic ruler that can take us way out there, to the edge, 13 billion light years away. This is the special child in the family of measurements. And to understand it in this extra short explanation will require an extra large Thinking Cap. Ready?
Through the combined efforts of three great astronomers --- Vesto Slipher, Milton Humason, and Edwin Hubble --- it was discovered early last century that the universe was expanding!
They had discovered that essentially all other galaxies were moving away from us. But the closer ones were moving slowly away. Ones farther moved faster. The farthest galaxies were racing away from us. The only explanation of this was that the universe was getting bigger.
Imagine a deflated balloon with dots drawn randomly on the surface. As one blows up the balloon the dots all move away from each other. Focus on any one dot --- any dot you want --- and you'll discover that other dots close to it are moving away slowly, the dots farther away are moving faster.
The beauty of it is this: Say a dot at a certain distance is moving a certain speed away from your Home Dot. Then a dot twice as far will be moving away twice as fast. A dot three times as far will move away three times faster.
Now without getting bogged down in the math, suffice it to say now that we can do the same basic thing with galaxies. If we know how fast the universe (balloon) is expanding, and we can measure how fast a certain galaxy (dot) is riding away from us (the Home Dot) on that expanding universe, then we can estimate its distance.
Now, it's considerably more uncertain than our balloon example. Getting a handle on how quickly our universe is expanding is one of the great challenges of modern-day astronomy.
But as techniques and measuring devices continue to improve at amazing rates, we're starting to get some very precise measurements. And as we get more precise in our distances we can start building giant maps of the universe.
And just as we can learn a lot about the history of planet Earth by looking at a detailed globe, we can learn much of our universe by studying the Great Map of the Heavens.
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2003.03.15 01:45 PM | Comments (0)
