The tiny particles that made up the fundamental matter began to come together to form larger clumps due to electrostatic forces between particles. As these clumps grew larger and larger their mass was sufficient to cause strong enough gravitational forces to bring the clumps together to form larger clumps. As the clumps became larger and more dense they began to generate heat and light by a nuclear process called nuclear fusion, which eventually become very large bright features that are the stars.
Nuclear fusion is a nuclear reaction in which two or more atomic nuclei become very close and collide at very high speed to form the nucleus of a new element. The Sun is primarily fueled by the nuclear fusion reaction that involves two hydrogen atoms fusing to form one helium atom and releases photons (light and heat energy). Other stars with greater masses (~1.5 times the mass of the Sun) may generate carbon, nitrogen, or oxygen.
After a star forms, it is considered a main sequence star. Our Sun is considered a main sequence star, which is defined by where it plots on a graph of the star’s brightness (luminosity) against its temperature (a function of color), known as the Hertszprung-Russell diagram (H-R diagram). The video below goes into detail about star classification and the H-R diagram.
The more massive a star is, the shorter its lifespan on the main sequence. Given enough time, a star’s hydrogen fuel will be consumed, and it will evolve away from the main sequence in the H-R diagram. How a star evolves depends on its mass; stars with approximately 35% of the mass of the Sun evolve into white dwarfs directly, whereas stars with up to 10 times the mass of the sun pass through a red giant stage. More massive stars can explode as a supernova, or collapse directly into a black hole.
By normal nuclear fusion reactions, stars can only produce elements up to a certain atomic mass (iron with 56 atomic mass units). Because heavier elements (like nickel, zinc, and gold) are found in our Solar System, these elements must have formed some other way. Runaway nuclear fusion during supernova explosion is thought to be responsible for the formation of heavier elements. (Recall cosmic abundances described in Module 1.)
And in fact, our Solar System is thought to have formed ~4.55 billion years ago after the dramatic and catastrophic destruction of a massive star by a supernova explosion.
From a human perspective, supernovae are relatively rare occurrences (only three Milky Way supernovae in the last 1000 years). One historical supernova was first described by Chinese astronomers in 1054 A.D. and the remnants of this event make up the Crab Nebula (located in the constellation Taurus).