"Where do stars go when they die?"
As soon as I set up my desk at COSI, I put up a small whiteboard in front and labeled it "Ask me anything!” I'm happy to report that it wasn't long before someone took advantage of the opportunity. And what a question it was! "Where do stars go when they die?" It's a beautiful question because it evokes certain imagery. We're used to thinking of the stars as just...there. Even ancient astronomers noticed the occasional "new" star (which we now identify as nova, by the way). But nobody ever noticed a star going away. I mean, they last so dang long, especially compared to the puny handful of millennia we've been recording the heavens. Indeed, most stars don't go away for a *very* long time. That's because most stars are smaller than the sun, and the smaller the star, the longer it lasts. Stars are powered by fusion: the intense gravity from the star's own weight pressing in on itself shoves atomic nuclei together, leaving a little bit of energy behind in the process. More massive stars press that much harder, increasing the rate of nuclear burning, using up their fuel faster. Smaller stars keep the weak fire going, burning dimly for trillions of years. In contrast, the biggest stars last only a few million years. I know, either way it's a stupid-long time, but relatively speaking, there's a big age gap. So small stars just kind of peter out. Medium stars like our sun leave behind white dwarves, which are balls of carbon and oxygen. The biggest stars blow up in spectacular fashion, leaving behind one of three dreary options: 1) nothing, 2) a neutron star, or 3) a black hole, depending on their exact mass and exactly how they go off. Given enough time (and the universe has plenty of that), the galactic neighborhoods eventually fill up with these leftover remnants, which just kind of...float around.