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Stars
Stars change over time. It may take millions to billions of years for a star to live out its life. That is a very, very long time!

A star is a big ball of gas which gives off both heat and light. So where do stars come from? What happens to them as they grow older?

A galaxy contains clouds of dust and gas, as well as stars. It is in the clouds of dust and gas that stars are born. As more and more of the gas (which is mostly hydrogen) is pulled together by gravity into a cloud, the cloud starts to spin. The gas atoms start to bump into each other faster and faster. This creates heat energy. The cloud gets hotter and hotter. Finally, it gets so hot within the cloud that something called "nuclear fusion" happens. The cloud begins to glow. The glowing cloud of gas is now known as a protostar. The protostar continues to grow. Once it stops growing, it is known as a main sequence star. A main sequence star can shine for millions of years or more. The amount of time it lives is determined by how big it is.

The Pleiades

Medium stars

In medium size stars, after the nuclear fusion has used up all the fuel it has, gravity will pull the remaining material closer together. The star will shrink. In fact, it may get to be only a few hundred kilometers wide! The star is then called a "white dwarf". It can stay like this for a long time. Eventually, it will stop producing any light at all. It is then called a "black dwarf" and it will stay that way forever.

Massive stars

In large size stars, nuclear fusion will continue until iron is formed. In stars, iron acts like an energy sponge. It soaks up the star's energy. This energy is eventually released in a big explosion called a supernova. The little bit of matter that used to be at the center of the star before the supernova will then be either a neutron star or a black hole. Which object it becomes depends on the size of the original star. A star that is 1.5 to 4 times larger than our Sun will become a neutron star. Stars that are even bigger than that will become black holes.

(Words)

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A Question

What type of stars will become neutron stars as they are dying out?

1) Stars smaller than our Sun.
2) Stars more than 10 times larger than our Sun.
3) Stars the same size as our Sun.
4) Stars 1.5 to 4 times larger than our Sun.

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The Answer
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