Nebula
A nebula is a vast cloud of gas and dust in space, often a place where new stars are born or the remains of a star that has died.
Definition
A nebula is a giant cloud of gas and dust in space, sometimes a place where new stars are being born and sometimes the leftovers of a star that has died. Nebulae are made mostly of hydrogen and helium: emission nebulae glow because nearby hot stars energize their gas, reflection nebulae shine by reflected starlight, and dark nebulae block the light behind them, while planetary nebulae and supernova remnants are the shed outer layers of dying stars. Their densities range from tens to millions of particles per cubic centimeter, far below any laboratory vacuum. Star formation begins when a molecular cloud core exceeds its Jeans mass and gravity overcomes thermal pressure, and H II regions glow at the $656\ \text{nm}$ hydrogen-alpha line as ultraviolet photons from O and B stars ionize the surrounding hydrogen.
Example
The Orion Nebula, visible in binoculars as a fuzzy patch below Orion's belt, is a star nursery about $1{,}300$ light-years away and roughly $24$ light-years across where new stars are forming right now. The Crab Nebula is the expanding debris of a supernova seen from Earth in the year 1054. A cold molecular cloud at $10\ \text{K}$ with $10^4$ hydrogen molecules per cubic centimeter has a Jeans mass of only a few solar masses, so it fragments into many star-sized clumps rather than collapsing into one giant object.
Key Insight
Nebulae look thick and solid in pictures, but they are far thinner than the best vacuum made on Earth and only look bright because they are so enormous. The word nebula means cloud, and before telescopes improved any fuzzy object was called one, including galaxies; today the word is reserved for gas and dust clouds inside a galaxy. Nebulae are both cradle and grave: the heavy elements ejected by a dying star enrich the cloud that forms the next generation, so each generation of stars begins with more carbon, oxygen, and iron than the last.