Which of the following stars is classified as a brown dwarf?

The classification of a brown dwarf is based on its mass and temperature characteristics. Brown dwarfs are substellar objects that do not have enough mass to sustain hydrogen fusion in their cores, which typically requires a mass of around 0.08 solar masses or greater. Instead, they fall into a category between the heaviest gas giant planets and the lightest stars.

Sirius B is actually a white dwarf, which is the remnant of a star that has exhausted the nuclear fuel in its core and has shed its outer layers. This means that it has already gone through its life cycle as a regular star, making it unlike a brown dwarf.

In contrast, other options like Betelgeuse, Proxima Centauri, and Algol are all classified as stars. Betelgeuse is a red supergiant, Proxima Centauri is a red dwarf, and Algol is a binary star system, specifically an eclipsing binary.

None of these stars fit the definition of a brown dwarf. Brown dwarfs exist in a unique category distinct from these more well-known star types, emphasizing their inability to sustain fusion processes in the same way that true stars do.