Some stars that have been shining steadily for millions of years suddenly undergo a fantastic change. This change comes in a very unpredictable and violent way. Within a few hours or a couple of days their brightness increases by 10,000 times or more. An ordinary observer might think that a new bright star has appeared in the sky. The increase in brightness occurs when an explosion throws up a small amount of the star’s matter—probably less than a hundred thousandth of its matter. This matter is actually a shell of gas that expands brilliantly in the outer space as soon as the ignition and explosion take place. A nova reaches its maximum brilliance in a few hours or a few days, and then after a few weeks or months it returns to its normal brightness. The decline of the brightness begins at various rates which often fluctuate. After a few years the brightness of the novae remnant becomes steady and a gas cloud may be observed around it which expands at a rate of hundred kilometres per second. Some novae have been known to erupt more than once and are termed as recurrent. All recurrent novae flare up at long intervals. About 20 or 30 novae are believed to occur in our galaxy every year.
It has been found that the novae reach absolute visual luminosities to the extent of about 10,000 to 1,000,000 times than that of the sun. The total energy emitted during a large novae outburst is of the order of 
ergs, equal to the radiation from the sun in 10,000 years. Should the sun ever become novae, the earth would be destroyed in a few hours or days. However, Sun is unlikely to become so.
A supernova is a much more spectacular event than a nova. In a supernova explosion, there is a complete self-destruction of the star or at least one-tenth of its matter is thrown off. This may result in an increase in brightness which reveals an entire galaxy as the increase is a billion times more.
The remains of a supernova that occurred in 1054 A.D is still seen today as the crab nebula which has become one of the most fascinating objects in the sky. Some supernovae including the above were bright enough to be seen in the broad daylight. It seems that a supernova occurs once in about every 300 years. All supernovae are shattered to pieces in their explosions, collapsing into neutron stars.