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).

Spectral Types of Stars

When we describe stars, we can divide them based on their temperature. However, when the science of spectroscopy was new, astronomers looked the spectra of stars and decided to break them down into the strength of the hydrogen lines.


Initially, Edward Pickering broke them down in this way:

• A stars: strong hydrogen lines
• B stars: strong, but a little weaker than A stars
• C stars
• D stars
• E stars
• F stars: medium strength
• G stars: weaker than F stars
• H stars
• I stars
• J stars
• K stars: really weak hydrogen lines
• L stars
• M stars: very weak lines and very dim stars
• N stars
• O stars: non-existent hydrogen lines, but very, very bright stars

Later on, Annie Jump Cannon took these spectral types and discovered that it would be easier to classify the stars by the temperature of the star, which could be gotten from the spectrum. She reclassified them in the following way:

• O stars: blue stars, very hot, >30,000 K surface temperature
• B stars: blueish white, hot, between 10,000 K and 30,000 K
• A stars: white, between 7500 K and 10,000 K (Sirius is an A star)
• F stars: whitish-yellow, 6000 K to 7500 K
• G stars: yellow, 5200 K to 6000 K (our Sun is a G star)
• K stars: orange, 3700 K to 5200 K
• M stars: red, 2400 K to 3700 K (the most abundant of main sequence stars)
• L, T, and Y stars: brown, < 2400 K (are brown dwarfs, failed stars that cannot fuse hydrogen in the core)

While before, the stars were classified by the strength of their hydrogen lines, Annie Jump Cannon's reordered makes more sense to base it on their temperatures. An easy way to remember the order is to use the pneumonic that I learned when I was a kid:

Oh, Be A Fine Girl, Kiss Me

This helps with the main sequence stars, but does not incorporate the brown dwarfs. You can come up with your own to help you remember.

The spectral types of the stars can actually tell us a lot about the star's past, present, and future. Generally, the hotter the star, the more mass it has. A hotter star also reaches the main sequence quicker than a cooler star, but also ages much more rapidly. To overcome the gravity the hot, massive star experiences, the star must burn hydrogen faster to keep the star in hydrostatic equilibrium.

This also helps us to determine what type of stars to look at when looking for planets that might support life. We don't want a star that is too hot, or the radiation from the star would sterilize any planet orbiting it, and that star would not last too long on the main sequence to have any life to evolve.