The main difference between a star and a brown dwarf depends on what?

Correct answer: Ability to sustain stable hydrogen fusion

Direct answer: Option A, the ability to sustain stable hydrogen fusion. A star is a very large, hot ball of gas, mainly hydrogen and helium. Its strong gravity squeezes the central gas until the temperature and pressure become high enough for nuclear fusion. In stable hydrogen fusion, hydrogen nuclei combine to form helium and release enormous energy as light and heat. A brown dwarf forms like a star but does not have enough mass to maintain ordinary hydrogen fusion in its core, so it is sometimes called a failed star. Therefore the decisive scientific difference is the sustained hydrogen-fusion ability, not its orbit, appearance, or surface features. Option A is correct because it gives the physical test used to distinguish a normal star from a brown dwarf. Option B is wrong because stars and brown dwarfs are not defined by revolving around Earth; Earth itself revolves around the Sun. Option C is wrong because comet tails are produced when a comet approaches a star and its icy material is heated, not because an object is a star. Option D is wrong because making oceans is not a defining property of stars or brown dwarfs and requires a suitable planetary surface and conditions. Memory cue: star means sustained hydrogen fusion; brown dwarf means insufficient mass for it.