Stars are Amazing! by Leah Kalimeris
Stars are Amazing! by Leah Kalimeris

Did you know that astronomers can use a star’s temperature to sort it into one of seven classes, from the smallest and coolest, to the biggest and hottest?

The first thing to remember is that a star is simply a huge ball of gas. There is enough force at its centre, however, that atoms are being crushed together – a process called nuclear fusion. Fusion creates incredible amounts of energy, and this is why stars are so hot and bright.

Using a star’s temperature, we can sort them into classes; O, B, A, F, G, K, M.

You might have noticed the same letters along the bottom of the image below, which is called the Hertzsprung-Russel diagram. The letters match with temperatures, but you’ll also notice there are colours, too. In astronomy, colour equals temperature, but with an unexpected twist. That’s because, when it comes to stars, blue ones are the hotter ones (above 30 000C), and the red ones are the cooler ones (around 3000C).

O-type stars are the biggest and hottest, yet they burn their fuel so quickly that they’re the first to die. M-type stars are the smallest and coolest, and the most common, yet they’re so dim our eyes can’t see them. The Hertzsprung-Russel diagram also includes our Sun; a G-type star, average in both size and temperature, yet very important to us.

Stars also change as they age, transforming into giants, but that’s a conversation for another day. For now, remember, as you look up at the night sky, that a star is just as individual as you and me.

If you want to learn more search stellar classification, or for any other astronomy information, head to