What is the classification of our sun?

The sun is what is known as a G-type main sequence star. Specifically, the sun is a G2V star, sometimes referred to more vaguely as a yellow dwarf. Let's break down what each of those three characters means.

The G is the spectral type of a given star, according to the Harvard spectral classification system (http://www.star.ucl.ac.uk/~pac/spectral_classification.html), derived from the emission spectra of a star as seen through a telescope. The emission spectra is dependent on the temperature of the surface of the star. The temperature is mostly, but not completely, dependent on the star's mass. The classifications of the Harvard system are O, B, A, F, G, K, and M, in descending order of temperature (and usually mass). These are often remembered by the phrase "Oh Be A Fine Girl/Guy, Kiss Me." Our sun has an emissions spectra that appears yellow, which correlates to about 5,800 degrees Kelvin, firmly in the G classification.

The "2" in the classification lets astronomers be more specific about the temperature. There are 10 sub-levels of temperature, 0 through 9, with 0 being hottest and 9 being coolest. This more granular classification system let's you make some additional inferences about a star. For instance, our sun is a G2, which is relatively hot for a star of its mass. That's likely because of its age, as middle aged stars like the sun burn hotter than they do in their youth. By contrast, 70 Virginis is a G4 star, cooler than the sun despite being more massive. That is likely because it is much older than the sun and has passed its prime, at least as far as temperature is concerned. It's begun to progress towards the next stage of its stellar evolution, cooling down but also growing in diameter.

Finally, the V is the luminosity class (http://www.spektros.de/lumi.html) of the star. While the spectral classification is dependent on the temperature of the star, the luminosity is dependent on the star's radius. Hence the luminosity class is defined by a Roman numeral with I being a "Supergiant" and VII being a "White Dwarf," a star with a volume comparable to the earth. The V class, including our sun, is "Dwarf," although the sun is not a particularly small star. The next class upwards is sub-giant, which the sun will become as it ages.

You can see from these descriptions how important age is for main-sequence stars like the sun, as their temperature and size changes based on their stage of life. Other important classifications include composition, degree of variability, and certain common spectral peculiarities. These identifiers help astronomers to assess a lot about the life of a star from just a few symbols in a database.