What is a star the explodes temporarily increasing its brightness 10000 times or more?

Answer 1

Near the end of a star's life (a star similar to our own Sun), when the star has consumed most of its hydrogen by fusing the hydrogen into helium, the declining energy output of the star is no longer quite enough to keep the star's outer layers up against the star's gravity. So the star begins to collapse under its own gravity.

The gravitational collapse provides enough energy to kick the nuclear fusion reaction into overdrive, fusing the helium into carbon. The helium fusion releases far more energy, causing the star to explode abruptly.

Early observers watching the sky would occasionally observe a new star appearing suddenly where no star had been visible before. "New star" in latin is "nova stellarum"; so we call these ordinary stars dying "Nova". The "nova" stars shine brightly for weeks or months before fading into insignificance and disappearing again.

Now with powerful telescopes such as the 200-inch reflector at Mount Palomar, we can observe the stellar remnants. There is one in the constellation Orion, the Crab Nebula.

Answer 2

any star that explodes. That explosion is a supernova and possibly a hypernova.
A supernova.

Answer 3

This is a "supernova" which is the gravitational collapse of the central mass and the violent expulsion of the outer layers of an old large star.