O-class stars are the largest stars in the universe. They are blue and red supergiant/hypergiants stars that are near the end of their life. For every 10,000,000 stars, there is one O-class star. There are around 20,000 class O stars in the Milky Way Galaxy. The largest star in the universe that we know of is UY Scuti. 5 billion suns can fit inside UY Scuti but the surface temperature of UY Scuti is less than our sun.

B-class stars are a very rare type of star in the universe. They make up 0.12% of all stars. B-class stars have a luminosity of 10000-30000 K. An example of a B-class star is Regulus. Regulus is the brightest star in the Leo constellation. Regulus is also the 21st brightest star that we see when we look into the sky. Regulus has 150 times the luminosity of our sun and 3.5 solar masses.

A-class stars are the third brightest stars in the universe. For a star to be an A-class star they need to have 1.4-2.1 solar masses and have a surface temperature of 7500-10000 K. An example of an A-class star is Vega. Vega has a magnitude of 0.03 and lies 25 light years from earth. Vega is apart of the constellation lyra. Vega shines 40 times brighter than our sun, is double the size, and double the mass. 

F-class stars are the fourth brightest stars in the universe. For a star to be an F-class star they need to have 1.2-1.4 solar masses and have a surface temperature of 6000-7500 K. They also glow a yellowish white color. An example of an F-class star is Procyon. Procyon is the brightest star in the Canis Minor constellation. Procyon is 11.4 light years away from earth and emits 7.0 times more energy than the sun per minuet and shines brighter.

K-class stars are the second smallest stars in the universe. They have a range from 0.6-0.8 the solar mass of our sun and a temperature range of 3,700 K-5,200 K. One example of a K-class star is Epsilon Indi. Epsilon Indi has  0.7 solar masses and is located 12 light years from Earth. It is in the southern part of the Indus constellation. It has a magnitude of 4.69 and has a orange glow. 

The smallest type of star in the universe and the most plentiful. Red Dwarfs are considered class M-stars and are the most common star in the Milky Way.  M-class stars last up to 100 billion years and when they die they become white dwarfs. They are to small to undergo the giant phase of a dying star. Once they burn all their fuel they will turn into a white dwarf.