We don’t get our sunlight from a B-type star, and they’re far from the most common type, yet when it comes to influencing the development of the universe, their role is crucial. They’re also particularly important to astronomers wishing to understand the structure of the galaxy and beyond.

Astronomers trying to make sense of the differences between stars realized that the wavelength of light at which its output peaks is among its most important features. Combined with the presence or absence of certain spectral lines, this wavelength reveals the temperature of the star’s outer layers. For very young or very old stars, that temperature changes relatively quickly, but for most of their lives, stars’ temperatures are a very good indication of their mass, which in turn determines their lifespan.

During the long years on what is called the “main sequence,” temperature is also a good guide to how much energy a star is releasing, from which we can calculate its distance.

It’s arbitrary where we draw the lines between one category of stars and another, but having a categorization system is certainly useful. You’d expect such a system to have A-type stars as the brightest through to G, but for historical reasons, things got scrambled.