Most stars are small. The smallest stars of all are only ten times bigger than Jupiter. These small stars are quite common. The bigger a star gets, the less of them there are. There are many small stars, some medium sized stars like the Sun, and only a few super giant stars.
Not all stars are the same color. The Sun is yellow, which you can see just by looking at it. (Don’t do this though). Betelgeuse is Red, and if you look around the sky you will see that some stars are blue.
These colors tell us the temperature of the star. The hotter a star is the more blue light it produces. The cooler a star is the more red light it will produce. This means a hot star is blue and a cool star is red. Medium hot stars are yellow. Think of a fire. The hottest part of the flame is blue, the middle is yellow, and the cooler outer edge of the flame is red. Stars work the same way. This is important because we can’t visit other stars to see how hot they are. If we could not figure it out by looking at their color as well as a few other clues in the light they send us, we could never know their temperature.
Scientists use a star’s temperature and a few other bits of information they learn by looking at a star’s light to classify stars into seven different groups. The hottest and therefore bluest stars are called O stars. The reddest and therefore coolest stars are M stars. There are five other groups in between representing the shades of blue, red and yellow. These groups are O, B, A, F, G, K, M.
There is a famous sentence which all scientists use to help them remember these letters.
Oh Be A Fine Girl Kiss Me
The first letter of each word is the name of a star classification.
Assignment # 2:
What do we learn by studying a star’s color?
Is an O star blue or red?
Is an M star blue or red?
The Sun is a G star. What color are G stars? In other words, what color is the Sun?
Check your answer
The temperature of the star.