Why is the Sun classified as a yellow dwarf star?

The Sun is classified as a yellow dwarf star because it's a medium-sized main-sequence star of spectral type G.

The Sun, like other stars, is classified based on its spectral type, luminosity, and size. The spectral type of a star is determined by its temperature and the elements it absorbs, which in turn determines its colour. The Sun, with a surface temperature of about 5,500 degrees Celsius, falls into the G spectral type, which is characterised by a yellow-white colour. Hence, it is often referred to as a yellow star.

The term 'dwarf' is used in astronomy to refer to stars that are in the main sequence phase of their life cycle. This is the longest phase in a star's life, during which it burns hydrogen in its core to produce helium. The Sun, being in this phase, is therefore classified as a dwarf star.

The 'yellow dwarf' classification, therefore, is a combination of the Sun's spectral type and its current stage in the stellar evolution process. It's important to note that this is a somewhat informal term, and astronomers more commonly refer to the Sun, and stars like it, as G-type main-sequence stars, or G V stars.

The Sun is medium-sized compared to other stars. There are many stars that are much larger (known as giants or supergiants) and many that are much smaller (known as white dwarfs). The size of a star is related to its mass and temperature, and influences its brightness and lifespan. The Sun's size, temperature, and brightness are all typical for a G-type main-sequence star.

In summary, the Sun is classified as a yellow dwarf star due to its colour, size, and the stage it is at in its life cycle. This classification helps astronomers understand the Sun's properties and predict its future evolution.