Spectral Types Part 2

There is still a little more to discuss when looking at spectral types of stars. Last time, we learned that based on the surface temperature of the stars, the spectral types break down in this way:

• O
• B
• A
• F
• G
• K
• M
• L
• T
• Y

In astronomy terms, we say that O stars are earlier type stars compared to M stars because they are hotter. The idea of calling hotter stars early-type stars and cooler stars late-type stars, evolved from the early days of spectroscopic study of stars when astronomers thought that hotter stars evolved into the cooler stars, i.e. hot stars were in the early stages of their formation while cooler stars were in the later stages of their lives.


Astronomy can even break down stars further in each spectral type. The hottest stars in each spectral type are the 0 (zero) stars and the coolest stars are the 9 stars. For example, G stars that have a 6000 K surface temperature are the G0 stars while stars with a surface temperature of 5200 K are G9 stars. The Sun has a surface temperature of around 5800 so is a G2 star (some even break it down farther into a G2.5 star.

Next time, we will break stars up into their different Luminosity Classes (the y-axis on the HR Diagram).