Violently exploding stars which shine very brightly for days or weeks. They occur when the fuel for nuclear reactions is used up, and a star cools. Gravity pulls all the matter down toward the star's center. If this happens quickly, nuclear reactions may suddenly begin again, detonating the star in a nuclear explosion.
Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run twice as fast as that.
The Red Queen in Lewis Carroll's Through the Looking Glass