恒星是如何形成的？

A nebula is a giant "cloud" of gas and dust. Over time, gravity attracts particles within a nebula, so they begin to group into massive balls.

It depends on how much mass the balls have! The smaller ones become moons, planets, or random space rocks like asteroids. The larger ones have such powerful gravity that their cores heat to such a degree that nuclear fusion occurs.

The sun formed in the way described above. However, not all of the gas and dust in the nebula went towards creating the sun. Large portions of it formed into planets, moons, and other space objects. (Ok, not large portions. About 99.9% of all the matter went into the sun)

Young Stellar Objects

Young Stellar Objects (YSOs) are stars in the early stages of development, starting from a molecular cloud of gas, plasma and other particles that catalyze star formation. YSOs fall into two major categories: protostars, contracting fragments of molecular clouds that are still accumulating mass, and pre-main-sequence stars, which have completed accretion but haven't started burning hydrogen.

That's enough of an introduction for now. How about a warm-up question?

Balls of dust with mass of less than 0.08 solar mass(the mass of our sun) will become brown dwarfs or planets or moons. Otherwise most will become stars that enter the main sequence.