What it was
A cloud folds inward, and the gas at its heart gets hot enough to shine. That is the Sun's opening scene. You could not stand on Earth to watch it: Earth had yet to grow from the dust circling this new star.
The Sun makes energy by fusion, joining small atoms into larger ones. Deep inside, hydrogen joins to make helium. That releases energy which pushes outward while gravity pulls inward. The gas can stop falling and hold its shape as a star.
When and where
Before the Sun lit up, the cloud spun slowly. As it shrank, it spun faster, like a skater pulling their arms in. Bumps between bits of gas erased motion up and down, while the shared turn survived. The cloud flattened into a disk with a growing star in the middle.
How big
The Sun weighs about 333,000 Earths. Squeezing all that gas made fierce heat. You can feel a small version of the same effect when you press a bike pump and the barrel warms in your hand. The Sun's own gravity did the squeezing.
How we know
Point a telescope at some young stars today and you can see dusty disks. Dark gaps cut through them. Growing planets may clear many of those gaps, so scientists can test ideas about our own beginning against stars that are still young.
Still a mystery
The young Sun shone less brightly, yet early Earth had liquid water. Scientists still argue about how Earth's air held enough heat. Gases that trap warmth are a leading answer. The surviving rocks give us pieces of that old sky, rather than a complete weather report.