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If you're in the 95th percentile at school, that means your grades are better than the grades of 95% of the other students. In other words, you're in the top 5%. Something that's in the 75th percentile is in the top 25%, etc.

Wikipedia has a pretty interesting article on supernovae, SB. Check out the "Current Models" section.

JB's correct about the likelihood of a collision. The analogy I've heard, iirc, goes something like this: Suppose the continental United States were as flat as a pool table and three basketballs were set to rolling around at random on it, none of them within a thousand miles or so of its nearest neighbor, and each moving in a random straight line direction with a speed of a couple of miles per year. When a basketball encounters a coastline or the Canadian or Mexican border, it bounces back toward the interior of the country. (1) How long would you have to wait, on average, before two of them collided? (2) Call one of them the Sun. How long would you have to wait (i.e., what's the expected value?) before it collided with one of the other basketballs? Longer than the age of the universe.