What is a nova?

Sometimes, looking up at the starry sky, you may be surprised to find that there is a bright star in a certain star area that you have never seen before! However, after only a few months or even days, it gradually disappeared.

This "strange" star is called a nova or supernova. In ancient times, it was also called "guest star", which means "visiting star".

Novas and supernovae are a kind of variable stars. When people saw them suddenly appear, they once thought they were just newborn stars, so they were named "new stars". In fact, they are not only newborn stars, on the contrary, they are dying old stars. In fact, they are exploding red giants. We have mentioned more than once that when a star enters old age, its center will shrink inward, while its outer shell will expand outward, forming a red giant. The red giant star is very unstable, and one day it will explode violently, throwing its shell out and revealing the white dwarf or neutron star hidden in the center.

In the Big Bang, the star will lose most of its mass and release huge energy at the same time. In this way, in just a few days, its luminosity may increase by hundreds of thousands of times. Such stars are called "new stars". If the explosion of a star is more violent, its luminosity can even be increased by more than100000 times. Such a star is called a supernova.

The intensity of supernova explosions is unbelievable. It is said that the energy it pours out in a few days is equivalent to the energy emitted by a young star in hundreds of millions of years, so that it looks as bright as the whole galaxy!

The explosion of nova or supernova is an important part of celestial evolution. It is the glorious funeral of old stars and the promoter of new stars. The explosion of supernovae may trigger the birth of countless stars in nearby nebulae. On the other hand, the ashes of nova and supernova explosions are also important materials for the formation of other celestial bodies. For example, many material elements on our planet today come from stars that have long since disappeared. Nova is a kind of cataclysmic variable star, which is a phenomenon of violent nuclear explosion caused by hydrogen accumulated on the surface of white dwarfs. This kind of star is usually very dark and difficult to find. When it broke out, it suddenly brightened and was considered a new star, hence its name. Novas is divided into fast nova (NA), medium nova (NAB), slow nova (NB) and extremely slow nova (NC) according to the speed of luminosity decline. During the explosion, the brightness will increase by tens of thousands, hundreds of thousands or even millions of times, lasting for several weeks or years. However, it should not be confused with Ia supernovae or explosions of other stars, including the glowing red nova first discovered by California Institute of Technology in May 2007.

At present, more than 200 new stars have been discovered in the Milky Way.