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Not Aware?

Week of July 13, 1998

A lot of problems in astronomy are very complex, so much so that we need to simplify them a bit to be able to get a handle on them. In most cases, the person trying to solve the problem makes a couple of simplifying assumptions, and works the problem out in that easier way. Then, once that solution is found, they try to eliminate one of the assumptions, making the problem a little harder, but still solvable. This process is repeated until the assumptions are eliminated and you hopefully have the answer to your complicated problem.

Those kind of assumptions are pretty nice. They help solve a problem, and as long as you are aware of them, they can help. The problem is, what about assumptions of which you are not aware?

image of a GC

This image is a small portion of a Hubble Space Telescope image of stars in a globular cluster (called M15).

I ran across one of these internal assumptions just last week. Globular clusters (GCs) are giant collections of stars, roughly spherical (hence the name) and usually pretty old. They have as many as a million stars in them, and although they are terribly far away, a couple are visible to the naked eye. Many are easily seen by a small telescope. Anyway, I was asked about what the sky would look like inside a globular, and it turned out to be a tougher question than I anticipated. I knew that many stars in globulars are intrinsically pretty bright; after all, you can pick out individual stars in them with a small 'scope, so they must be bright to be visible that far off!

And that's where my internal assumption came in. I am used to bright stars being very massive. In stellar terms, the more massive the star, the faster it burns its fuel and the brighter it is. But when I asked a colleague about stars in globulars, I was surprised to learn that the average star masses less than the Sun! As soon as he said it, I realized I had been walking around with an astronomical prejudice. You see, we know that if a more massive star burns faster, it runs out of fuel faster. We also know globulars are old, like 10 billion years or so, so any really massive star would have long ago used up its fuel. I knew that, but I had never really thought about it. So I has always been making an assumption about something, and it turned out to be wrong. Once I knew it, it was an easy thing to switch mental gears and work from there. I have worked on the problem, and with my new knowledge I have reached an answer I think is correct.

Sometimes I wish ``real'' life were more like that!



©2008 Phil Plait. All Rights Reserved.

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