Carl Hagerstrom, age 10, of Winston‑Salem,N.C., for his question;
What causes a nova to shrink?
At its peak, a nova may be 50,000 times as brilliant as the star we call our sun. A supernova may flare up to be some 250 million times as brilliant as our sun. The flare‑up is always sudden. Telescope pictures show that a dim, distant star may become a fully fledged nova in a few days. After this the bright spectacle begins to shrink. However, there may be evidence of t he flare‑up hundreds and maybe thousands of years later nova, meaning a. new star, is a rare event for our telescope to catch.
In our Galaxy, only about 100 of thorn have been studied. A11 of them have been hundreds or thousands of light years from the earth, most of them near the center of the Milky Way. Others have been spotted in our sister galaxy Andromeda and in other neighboring star systems. Studies of these stars show us how a nova behaves, but so far we have not enough evidence to prove what causes the dramatic event.
Nova are classed. as variable stars, though only a few have been known to become novae more than once. The dramatic event seems to be an explosion of the star's outer shell of gases, Before the eruption, the star is usually of a small type(3 hot type.) There are several theories as to what makes it explode. It may brash with another star or collide with a planet, though these accidents are not likely. One theory suggests that the outburst may be caused by subatomic adjustment deep in the heart of the star.
In any case, the terrific explosion seems to relieve the pressure and the star shrinks back to its normal size. However, the exploded. shell of gases does not return to the star. It continues to move outwards and hover around the shrinking star.
In 1054 AD. Chinese astronomers recorded a brilliant new star in the constellation Taurus, the Bull. They called it the Guest Star, perhaps because it did not stay very long. This star was almost certainly a nova or maybe even a supernova ‑ which is a sort of super‑duper nova.
The Guest Star faded back to normal, but our telescopes can still show its results. In Taurus there is a spidery shaped haze of light called the Grab Nebula. Most experts believe this haze is the shell of gases once exploded from a nova. The gases are still expanding out from the center 1,000 years later.
The color of the exploding nova changes from white to yellow to deep rod. This indicates that the outer gases ore cooling as they rush outward at many hundreds of miles a second. For red stars are much cooler than white stars. In a few days or at most a few weeks, the nova reaches its peak. The outer gases separate and the seething; heart of the star begins to shrink beck to about its norm a1 size. This is as mysterious to us as is the nova outburst itself.
The gases continue to rush outward, long after the star is back to normal: In the coldness of space they may solidify into bits of matter.
At least one old nova shows a ring which might be an embryo planet system. For more exciting views of galaxies go to: http://hubblesite.org/