Hmm. When I was in school, the width of the milkyway was roughly 100,000 light years, with an estimated 100,000,000,000 stars. Its actually a decent sized spiral galaxy as they go, though our closest spiral neighbor, the Andromeda galaxy is a bit larger at 200,000 light years and an estimated 400,000,000,000 stars in it. Our two closest galaxies, almost ugly twins of the Milkyway are the Magellanic clouds.These figures being estimates are in constant revision as more exact means of estimating based on evidence are found. Currently, the figure for the Milkyway's population of stars has in some circles been increased to roughly 400 billion, but I've never heard the 1 trillion figure before. I've also heard estimates of 120-175,000 lightyears for the diameter, but never 600,000 lightyears (twice a radius of 300,000) even relating to the galactic Halo, which is usually put at 130,000 light years when related to the 100,000 light year figure. That would put the galactic halo extending beyond both the magellanic clouds which are about 200,000 lightyears out.
None of these figures are exact, because they cannot be measured directly using our current methods and astronomical knowledge. Distances and sizes and numbers are based on statistical analysis of the areas around us that we know with greater precision, applied to areas we know with lesser precision. In other words, its a lot of guesses built on top of each other and checked against each other for accuracy. If they don't contradict, we figure they're accurate, and error margins are measured by the degrees of difference between various methods of arriving at the figures that somewhat contradict.
Edit: I found this site -
http://community.silverlink.net/celestial/astronomy/proj/milkyway/milkyway.htm
has some details that are kind of interesting, it is written by school kids for school kids, but the data is pretty accurate to what I learned in school. They also give some descriptions of things in the FAQ.
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Old farts never die, they just get blown away.
This message has been edited by Hellraiser on October 28, 2001 at 10:39 AM