Many scientists believe that between 10 and 20 billion years ago, there was a tremendous explosion. It occurred everywhere in the universe. It is called "The Big Bang."

When this happened, matter was scattered throughout the universe. Small bunches of matter made up such bodies as dust particles, gases, asteroids, meteors, and comets. Much greater amounts of matter bunched together to form planets. Far greater amounts bunched close together and became stars.

One of the stars became the sun, the center of our solar system in the Milky Way galaxy.

The Milky Way is only one of billions of galaxies in the universe. Scientists today are discovering that the universe is bigger than anyone before ever imagined. Galaxies bunch together making super clusters in what may be an even greater system.

We can photograph at least a billion galaxies with our powerful telescopes. Some are nearer to us than others. But when we talk about outer space, "near" means billions of kilometers away!

Today, we can get information about objects so far out in space that it takes many, many years for their light to reach us. When we look at a blinking star that is light-years away, we are seeing something that started a very long time ago.

As we look out in space, we look back in time. Scientists, called astrophysicists, know much more now about the different stages of objects that form deep in space. Gradually, we understand better how stars are born, what happens to them while they are stars, and what becomes of them when they die.


The Life Of A Star

What is a star made of? How is it born? Why does it shine? Will it live forever? These are questions people have been asking for ages.

Stars are huge, very, very hot balls of gas that shine brightly. How did they get that way?

All matter, from the tiniest microscopic particle to the biggest galaxy, starts with atoms.

Oxygen is an element made up of oxygen atoms. Hydrogen is an element made up of hydrogen atoms. Atoms combine to make different molecules. For example, when oxygen and hydrogen come together, they form a molecule we can't live without — water. Elements, alone or combined with other elements, make up all the matter that exists.

Look around you. Whatever you see is made up of elements — rocks, rain, the air we breathe. We are made up of certain elements. So are the stars.

The simplest and lightest element is hydrogen gas. Enormous clouds of hydrogen gas and dust particles form in space. Gravity draws the dust and gas close together forming a huge clump, or mass.

As more and more matter is added to the mass, its gravity increases. This pushes the atoms of hydrogen gas closer and closer together. The mass begins to contract, or get smaller. It also gets denser and hotter. The center is hottest of all because the atoms are being squeezed together. The heat creates tremendous energy. Energy is the power necessary to make things happen.

Finally, the crowding of the atoms, the heat, and the violent energy forces the hydrogen atoms to combine. This makes a new and heavier element called helium. The change of hydrogen gas to helium gas is called thermonuclear fusion. When this happens a star is born.

Throughout a star's life, thermonuclear fusion goes on in its core, creating enormous energy. That energy becomes light and heat. It makes the star shine. The same kind of energy from our shining star, the sun, travels billions of kilometers to reach us here on Earth.

Our sun is a medium-sized star that is medium bright. There are others like it. The universe has stars that are much larger and much smaller than our sun.

The new star will shine in the heavens for billions of years. It will stay that way until all its hydrogen gas has changed to helium.


The Death Of A Star

When a star has finally turned all its hydrogen into helium it is nearing the end of its life. Scientists believe that the amount of mass left in the dead star determines what will become of it.

Because our sun is of average size, astronomers use it to measure other stars. At the end of its life, a star with the same amount of mass as our sun will shrink to about the size of Earth. That's a million times smaller!

Matter gets packed so tightly that the force of gravity pulls the star into itself. The star gets so hot it turns white. This small dead star is called a "white dwarf."

When an even bigger star — a giant star — uses up all its energy, it collapses even more than a white dwarf. Its mass and gravity are much greater. No space is left between the atoms. Electrons and protons inside the atoms are pushed together. They become neutrons and the giant star shrinks to a very small size. It is called a "neutron star" and is usually only a few kilometers in diameter. But its mass is so great that a tiny bit of neutron star — less than a teaspoonful — would weigh about half a trillion kilograms. That's as much as a long freight train loaded with bricks!

Sometimes, when a giant star collapses, particles called neutrinos are forced out of the atoms. As the neutrinos burst forth from the dying star, the star shines more brightly than any other object in the galaxy. It looks like an enormous sparkler. This is a "supernova."

When the amount of mass left in a dying star is three or more times the mass of the sun, something very mysterious happens. A star this massive shrinks much faster than the white dwarf and the neutron star. Its force of gravity is very great. Nothing can escape it — not even light, which moves faster than anything else in the universe. This star becomes a "black hole."

Many astrophysicists believe that there are black holes at the center of most galaxies, including our Milky Way. Although that may be so, our solar system is too far removed from the black hole for us to be influenced by its pull of gravity.


Adapted from A Book About Planets and Stars by Betty Polisar Reigot.