Scattered along the spiral arms of our Galaxy are clusters of stars along with cosmic clouds of dust and debris. All stars are born in clusters, though gravitational encounters among them subsequently tend to fling them apart. Clouds of rarefied gas and dust span many light years, loosely bound by the collective gravitation of the myriad grains and molecules which bend space around them just enough to hold the tenuous mass together.
The slightest perturbation can create denser pockets of material, and gravity collects more and more stuff into these pockets over time. As the gas and dust falls toward centers of higher density, random rotation is amplified in the same way a figure skater spins as he pulls his arms toward his chest. If the balance between gravity and vorticity is upset, mass may fall together into dense cores, heating the material by collision and pressure.
The hottest and densest pockets of gas become runaway gravitational traps, the material falling and falling into them until the pressure and temperature is so great that even the mutual electrical repulsion of protons is too weak to hold them apart. Then matter is crushed out of existence by the Strong Force, releasing the energy of nuclear fusion and birthing stars among the dense nascent clouds of gas and dust.
Stars are formed in all sizes by the dozens and the hundreds in these galactic clusters. The biggest ones burn hottest and brightest and bluest, blowing the gas and dust away and carving huge cavities in the dark clouds of their birth. These hottest stars burn through their nuclear fuel furiously fast, as their huge mass crushes hydrogen into helium at a prodigious rate. The resulting ultraviolet radiation shines forth into the darkness, ionizing the surrounding hydrogen gas by stripping its electrons away. When the electrons recombine, the atoms shine fluorescent red, and lovely clouds of amazing shapes emerge to decorate the star cluster with color.
Over time, the biggest stars blow the gas and dust away into interstellar space, and the nebulae dim and go out. Stars more than 10 times the mass of our Sun don’t last long, just a million years or so, before they exhaust their hydrogen. Then they fuse their helium into carbon and oxygen and iron, eventually blowing their guts out into space to create new dust and smoke and soot. The remaining stars whip around one another until they are shot out into the galaxy, carrying their solar systems with them. This is how all solar systems start, and we can see the process all around us on summer nights.