Which star is classified as a white dwarf?

A white dwarf is a stellar remnant that results from the evolution of stars that have exhausted the nuclear fuel in their cores and shed their outer layers. They are generally characterized by high temperatures but low luminosities, and they are typically found in the later stages of stellar evolution.

Sirius B is classified as a white dwarf because it is the remnant of a once massive star that has gone through the red giant phase and lost its outer layers. It was once part of a binary star system with Sirius A, which is a main-sequence star. After exhausting its fuel, Sirius B underwent a transition into the white dwarf phase, leaving a hot, dense core that is now slowly cooling over time.

In contrast, the other stars listed—Sirius A, Rigel, and Vega—are not in the white dwarf stage. Sirius A is a bright main-sequence star, Rigel is a blue supergiant, and Vega is also a main-sequence star. Their evolutionary paths differ significantly from that of Sirius B, which solidifies its classification as a white dwarf.