About 80 years ago, American astronomer Edwin Hubble discovered that the universe is expanding.
Other stars and galaxies are moving away from us, and the further away they are, the faster they are moving.
This is called Hubble’s Law.
The fact that the universe is expanding means that it must have started from somewhere.
Astronomers now believe that it was created in the “Big Bang” about 15,000 million years ago and has been expanding ever since.
The Big Bang was an enormous explosion that created all the matter and energy in the universe.
About 75 percent of this matter was hydrogen and about 25 percent was helium, the two lightest elements.
Before the Big Bang, there was nothing.
The Big Bang theory was first proposed by Russian‒ American physicist George Gamow.
A theory like this is impossible to prove.
But it was accepted by most astronomers after American physicists Arno Penzias and Robert Wilson discovered what is called “background radiation.”
This is radiation left over from the Big Bang.
Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.
The Nobel Prize is never awarded to people who have died, so Gamow, who died in 1968, did not share the award.
About 300 million years after the Big Bang, stars and galaxies started to form.
Stars, such as our sun, are enormous balls of hydrogen and helium brought together by the force of gravity.
Gravity forces them to contract and causes very high pressures and temperatures.
The pressure at the center of our sun is over one billion atmospheres and the temperature is over 10 million degrees Celsius.
The high temperatures cause hydrogen atoms to combine and form helium atoms in what is called a fusion reaction.
This reaction releases heat and light energy, which is radiated out into space.
Galaxies are groups of stars.
Most galaxies contain hundreds of millions of stars.
Our galaxy is called the Milky Way.