Hi Jimmy and Stephen, and their scribe- whoever you are!
here's the answer to Jimmy. The centre of the Milky way is glowing just
because there are so many stars all together, about 200 billion of
them. They are so far away that we can't see each one separately, but
it's kind of like seeing all of the lights from a city when you are a
long way away, or in an airplane looking down. You know that the whole
city is not glowing, but that hundreds of thousands of lamps are all
shining together, too small and far away to see each one.
And now for Stephen. The Sun is a star because it makes its own light -
it shines with its own energy (not like a planet, which shines with
reflected light). And it makes its own light because it is very hot,
like the centre of a log fire glows shines because it is hot. The next
question is - why is the Sun hot? Thar's because in the middle of the
sun there are nuclear reactions going on, changing mass into energy.
But perhaps that is too much information for 4 years old. :-)