A system for classifying stars according to their spectral features. First used in the 1890s by astronomers at Harvard University; pioneered by Edward Pickering and a woman astronomer with the unlikely name of Annie Cannon, among others.

The Harvard system divides stars into seven major spectral types (colors). Arranged by convention in order of decreasing surface temperature, they are:

      Color          Temp (x1000K)
+---+--------------+---------------+
| O | Blue         | 30-40         |
| B | Blue/White   | 11-30         |
| A | White        |  7-11         |
| F | White/Yellow |  6-7          |
| G | Yellow       |  5-6          |
| K | Orange       |  4-5          |
| M | Red          |  3-4          |
+---+--------------+---------------+
There were originally a number of spectral classes from A-Q, designating stars according to the complexity of their emission lines. That classification was dropped in favor of the simplified version we see today, which orders stars according to their color/temperature, but maintains the letter names from the old classification. Recent decades have seen the introduction of a few new spectral classes, to cover interstellar oddballs:
+---+-------------------------+
| W | Wolf-Rayet stars (Blue) |
| C | Carbon stars (Red)      |
| S | Brown Dwarfs            |
+---+-------------------------+
Each spectral class contains subdividions, indicated by suffixing the spectral class with a digit from 0 to 9, 0 being the warmest and 9 the coolest within a class. Stars in the Harvard system are further classified according to their luminosity, a measurement based on the brightness of the star, which gives us some idea of its mass. The major luminosity classes are:
+-----+-----------------------+
|   I | Supergiant (a/b)      |
|  II | Luminous Giant        |
| III | Giant                 |
|  IV | Subgiant              |
|   V | Main Sequence (Dwarf) |
|  VI | Subdwarf              |
| VII | White Dwarf           |
+-----+-----------------------+

Taken by itself, this is known as the Yerkes spectral classification. Two stars may have the same surface temperature (color) but different luminosity (size), according to their age, mass and composition.

Log in or registerto write something here or to contact authors.