Final answer:
The Sun is classified as a medium star, or a G-type main-sequence star. Its evolution from main-sequence to white dwarf includes becoming a red giant and shedding mass before collapsing. The Sun's position on the H-R diagram is along the main sequence, and its evolution includes changes in luminosity and surface temperature.
Step-by-step explanation:
The Sun is classified as b) As a medium star, specifically, a G-type main-sequence star (G2V), often referred to more informally as a yellow dwarf. It is not a giant star, nor is it a white star or a white dwarf. The mass of the Sun is considered average in comparison to other stars.
Evolution of a star like the Sun begins in the main-sequence phase, where it spends most of its life fusing hydrogen into helium in its core. As the Sun consumes its hydrogen fuel over billions of years, it will eventually swell into a red giant, ejecting a substantial portion of its outer mass and creating a planetary nebula. The remnant core will ultimately collapse into a white dwarf, a dense, Earth-sized object with a mass less than 1.4 times that of the Sun (Chandrasekhar limit).
An H-R (Hertzsprung-Russell) diagram allows us to visualize the various types of stars, including main-sequence stars, cool supergiants, and white dwarfs. The Sun, on this diagram, is located along the main sequence, indicative of stable hydrogen fusion in its core. A typical star in the Galaxy is likely to be smaller and less luminous than the Sun. Additionally, differentiating between stars, brown dwarfs, and planets involves looking at an object's mass and ability to sustain fusion—stars including the Sun fuse hydrogen, whereas brown dwarfs and planets do not.
The evolution from a main-sequence star to a white dwarf involves significant changes in luminosity, surface temperature, and chemical composition due to the star consuming its nuclear fuel, thereby changing its position on the H-R diagram from the main sequence to other regions where fewer stars are found due to the shorter duration of these later life stages.
Learn more about Classification and Evolution of the Sun