A supernova is a powerful and energetic stellar explosion that occurs at the end of a star's life cycle. It is one of the most energetic events known in the universe, releasing an enormous amount of energy and often outshining an entire galaxy for a short period of time.
Supernovae can be triggered by different mechanisms, but the most common types are core-collapse supernovae and thermonuclear supernovae. In a core-collapse supernova, a massive star runs out of nuclear fuel and its core collapses under the force of gravity, leading to a powerful explosion. In a thermonuclear supernova, which occurs in a binary star system, a white dwarf star accretes matter from a companion star until it reaches a critical mass, triggering a runaway nuclear fusion reaction.
Regarding why we can't see more than one supernova at once, there are a few reasons. First, supernovae are relatively rare events in the universe. They typically occur in galaxies at a rate of about one or two per century. Given the vastness of space and the number of galaxies, the chances of observing multiple supernovae simultaneously from a single location are extremely low.
Additionally, supernovae are very bright and can outshine their host galaxies for a short period of time. This means that when a supernova occurs, it can dominate the light coming from its galaxy, making it difficult to observe other supernovae in the same region of the sky at the same time.
As for why Earth doesn't get hit by the debris from supernovae, it's because supernovae are usually located at significant distances from our planet. When a star explodes as a supernova, it releases a vast amount of material, including gas, dust, and heavy elements, into space. However, due to the immense scale of the universe, the debris from a supernova is dispersed over vast distances, and the chances of Earth being directly hit by it are extremely remote.
Supernovae typically occur in other galaxies, which are millions or even billions of light-years away from us. This means that by the time the light from a supernova reaches Earth, the debris has already been dispersed over a vast region of space. The exact distance to a supernova when it explodes can vary, but they are generally very far away from us in astronomical terms.