Inside a very large star the temperature gets hotter and hotter. Hydrogen turns to helium, and then into other things – carbon, oxygen, and iron. But then, one day, the star becomes too hot, and there is not enough hydrogen left. Then, very suddenly, the star collapses. As it collapses, it gets even hotter still, and then it explodes. In the explosion, the temperature may reach 100 billion °C.
This exploding star is called a supernova. One day in the next million years, Betelgeuse, that big red star in Orion, will explode like this. In fact, it is possible that it exploded a hundred years ago, and is already a supernova. But the light from the explosion has not reached us yet, so we cannot see it.
When the light from a really big supernova does reach the Earth, it is really exciting. Sometimes people can see the supernova in daylight. People in Japan and China saw a supernova like this in 1024 AD, and people in England saw one in 1604 AD.