QUESTION:
Why are there so many stars and where does gravity come from?

ANSWER from Jan Simpson on May 19:
Stars are made when gravity pulls clouds of gas and dust together 
(remember, air is a gas and dust is very small rocks or pieces of planets).
The gas and dust get hot when they fall into the middle and stars 
are made.  The reason there are so many stars is that the universe 
is very big and contains lots and lots of gas and dust, 
so there are lots and lots of stars.

All the matter in the universe (gas, dust, planets and stars) is called mass.
When two masses fall towards each other, it is called gravity.
(This can be a ball falling towards the earth, or a planet falling
around the sun - the planet doesn't fall into the sun because it's 
moving forward too fast, but it is still falling because of gravity.)

Albert Einstein (one of the smartest people who ever lived)
showed that gravity is the effect that mass has on space and time 
(when objects fall, they fall through space and it takes time).
This is called Einstein's Theory of Relativity.
It sounds very complicated and it is.

For studying how planets move around the sun and how spaceships fly to 
Mars and Jupiter, we can use Isaac Newton's theory.  This says that 
two masses attract each other or are pulled towards each other
(that's what you think of gravity - when you fall down, the earth 
is pulling you towards the center).

Isaac Newton is also one of the smartest people who ever lived.
He lived in England from 1642 to 1727.  He was so 
smart that he knew his theory wasn't right, but that it
was good enough for throwing balls, and falling apples, 
and planets and comets.  (Halley used Newton's theory when he discovered
Halley's Comet).

But these theories just say how gravity works.
In fact, we do not know why there is gravity, or why there is mass, 
or light, or even why there is a universe and space and time.
I think that when we know why there is a universe, we will know 
all the other answers.

Jan Simpson