What is a nebula?
A nebula is a cloud of gas and dust in space, linked to star formation or the late stages of stellar lives.
How it works and key principles
Star birth: Dense regions of a cold molecular cloud can collapse under gravity. The material heats as it contracts, and disks may form around young stars. Not every bright nebula is a stellar nursery: some glow because nearby hot stars illuminate them, while dusty clouds can block background light.
Evidence, examples and limits
Observation and significance: Different wavelengths reveal gas, dust and young stars hidden within a cloud. Emission, scattered light and dark silhouettes can represent distinct physical processes. A planetary nebula is not a planet despite its name; it consists of outer layers released by an aging star.
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