Short answer
A nebula is a cloud of gas and dust; a galaxy is a gravitationally bound system containing stars, planets, and gas-and-dust clouds. The distinction is cloud versus whole system, not simply a fuzzy object versus a sharply defined one. 1 2
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What each thing is
“Nebula” identifies a kind of cloud in space, rather than one particular origin: NASA describes both star-forming regions and clouds produced by dying stars. “Galaxy” identifies an organized system whose constituents include stars, planets, and clouds. Its definition explicitly includes gravity as the binding relationship. 1 2
Key differences
The important boundary is the level of organization. Describing a cloud’s material or its connection to stellar birth and death concerns a nebula. Describing stars, planets, and clouds together as one gravitationally bound system concerns a galaxy. Gas and dust alone therefore do not separate the categories. 1 2
How to tell them apart
Use the object’s description, not just its picture: does it identify a gas-and-dust cloud, or a gravitationally bound system of stars and other constituents? That is a practical reading rule grounded in NASA’s definitions. Its limit is that an image or a name alone may not supply that information. 1 2
Where they overlap
Both terms involve gas and dust, but that shared material plays different roles. It defines the nebula itself and forms part of a galaxy’s contents. The comparison is therefore not between mutually exclusive substances: identifying a cloud does not mean that gas-and-dust clouds are absent from galactic systems. 1 2
Edge cases
A nebula associated with new stars does not become a galaxy merely because stars are forming there; star formation is explicitly part of NASA’s nebula description. Conversely, galaxies need not have a neat spiral outline: NASA also describes elliptical and less orderly forms. Neither nearby stars nor a tidy shape is a sufficient shortcut. 1 2
Why the distinction exists
The terms distinguish physical objects, whereas an Earth-based appearance describes how an object looks to us. The Milky Way is a galaxy even though we see its disk as a faint band across the sky. NASA connects that appearance to its name; these excerpts do not establish the historical use of “nebula” for galaxies. 2
Common misconceptions
Neither “every nebula is a stellar nursery” nor “a galaxy is only stars” matches the supplied definitions. Some nebulae originate in material expelled by dying stars, and galaxies also contain planets and gas-and-dust clouds. Those differences prevent a single familiar example from standing in for the entire category. 1 2
Examples
Two named cases illustrate the boundary. The Helix Nebula is NASA’s example of a nebula: the label identifies a cloud, not a whole galaxy. The Milky Way is our home galaxy, with Earth located along a spiral arm: the label identifies the larger system containing our location. 1 2