Micro-lesson 1
Birth
A star is a gas ball that squeezes itself until its core gets hot enough to fuse. How fat it is at birth decides almost the whole story.
Stars start in a cold cloud of gas and dust (a nebula).
A clump gets dense. Gravity pulls inward. The center heats up. When it is hot enough, hydrogen begins to fuse into helium — a star switches on.
One cloud can make many stars. The leftover glow around new stars is often the nursery you still see.