Nova is the term used for a previously undiscovered star that displays a sudden increase in its brightness. A supernova is even brighter. It is the result of the collapse of a massive star at the end of its life span.
It was once thought that the sudden appearance of a star where there was none before indicated the birth of a new (nova = Latin new) star. However, a nova is not a new star at all. It is actually a star whose brightness has increased a thousand times over a period of a few hours to a few months. In the course of a few years or decades, the nova returns to its original state.
The reason for this sudden increase in brightness can be found in the nuclear reactions. The production of too much helium causes the outer layers to expand so rapidly that they cannot be held together. A portion of the gas envelope is released, giving rise to the increased brightness.
Novas are relatively common. They occur 10 to 12 times per year in the Milky_Way. Some of them are too far away to be observed or are occluded. They are given names that are a combination of the year and the constellation in which they occurred. Novae belong to the group of eruptive variables.
A supernova is much more rare than a nova. This spectacular event occurs only every few hundred years in the Milky Way. Although their brightness increases suddenly by a factor of several billion, very few supernovae have yet been observed with the naked eye. Most of them have been too far away for a closer examination.
Even records from the past describe no more than three conclusively identified events of this type. One supernova occurred in 1054. Its remains are still visible in the Crab_nebula.
Not very much is left after a supernova eruption. What remains is the expanding outer gas envelope. One example of this is the Crab nebula, where a spent supernova can be seen in the form of a small neutron_star. It is also known that as they heat up the interstellar gases, supernovas contribute to the birth of new stars.
On the 24th February2 1987, a supernova (1987A) appeared in our neighboring galaxy, the Large Magellanic Cloud. A closer examination was possible using the Hubble space telescope. The wave of the explosion was so strong that it caused a gas ring one light-year away to fluoresce.
