In the late nineteenth century, we had collected enough photographs of stellar spectra to realize the
need to classify them. This task was accomplished by
Annie J. Cannon
at the Harvard Observatory. After
sorting through thousands of images
like this one
, she finally arrived at a sequence of just
designated by the letters, O B A F G K M.
How did we end up with that odd sequence of
? The famous way to remember this sequence is Oh, be a fine girl kiss me. There is even a
Can you come up with your own mnemonic
The modern classification system is a
bit more complicated
than the original seven types. There are
for designating a star as one type or another.
Each spectral type has a specific temperature and luminosity.
We have special designations for white dwarfs, for example. There are also luminosity classes which differentiate between red dwarfs and red
giants, both of which have essentially the same surface temperature and
. The color indices for stars
range from about -0.3 to +1.7
depending on the spectral type. There is also a somewhat precise way to
use color index to calculate the temperature of a star.
Soon, it became obvious that we should plot the spectral type of a star and its luminosity, or brightness. The result was the
Looking for a particular star?
Find it here!
Here’s the list of all 88 recognized constellations