how often do they appear?
HOW OFTEN DO THEY APPEAR?
These processes are so slow that human history hasn't been long enough - nowhere near - to notice new stars forming, or older ones disappearing. The exceptions are novae and supernovae. Those explosions happen very quickly, and many are seen every year, although it's been a long time (centuries) since one happened nearby enough to be a big naked-eye-visible event.
Supernovae happen all the time, but you only notice if you're checking other galaxies all the time. There are people (hobbyists and professional astronomers) who are into that, though. The explosion is so gigantic that the single flash is bright enough to see across millions of light-years between galaxies.
The last time there was one relatively nearby was 1987, when a star exploded in one of the Magellanic Clouds, small disordered galaxies near our own. There's been a huge amount of scientific study of that one, even though it's still quite far off. A nearby one, say, in our own spiral arm and visible clearly in the sky, would be a bright, unmistakeable naked-eye object, the brightest thing in the sky other than the sun and the moon, and would keep a lot of professional astronomers employed for a long time. But nothing like this has happened in over 400 years.
Supernovae happen all the time, but you only notice if you're checking other galaxies all the time. There are people (hobbyists and professional astronomers) who are into that, though. The explosion is so gigantic that the single flash is bright enough to see across millions of light-years between galaxies.
The last time there was one relatively nearby was 1987, when a star exploded in one of the Magellanic Clouds, small disordered galaxies near our own. There's been a huge amount of scientific study of that one, even though it's still quite far off. A nearby one, say, in our own spiral arm and visible clearly in the sky, would be a bright, unmistakeable naked-eye object, the brightest thing in the sky other than the sun and the moon, and would keep a lot of professional astronomers employed for a long time. But nothing like this has happened in over 400 years.