Which statement about the Sun is true?

• A. It is a red giant
• B. It is a main sequence star
• C. It is a neutron star
• D. It is a black hole

Answer: (B)

Stars spend most of their lives fusing hydrogen in their cores while they stay on the main sequence. The Sun is in this phase, making it a main-sequence star. Its surface temperature is about 5800 K and its spectrum places it in the G-type category, with a luminosity class that identifies it as a normal dwarf star (not a giant). That combination (hydrogen burning in the core and a stable, relatively small size for a star) is what puts the Sun on the main sequence.

It isn’t a red giant, which would be a much larger, cooler, more luminous star in a later stage of evolution after it has exhausted hydrogen in its core. It isn’t a neutron star or a black hole, which are remnants of much more massive stars after dramatic endpoints like supernovae. The Sun simply hasn’t reached those stages yet and will instead, far in the future, become a white dwarf after its red-giant phase.