Stellar classification is a system of classifying stars based on their appearance. The system that is pretty much universally used today is a combination of the Harvard system of spectral classification and the Morgan-Keenan system of luminosity classes.
Format
To demonstrate, this article will take a look at the Sun's stellar classification, G2V.
Spectral type
The G2 indicates the stellar classification from the Harvard system of spectral classification. It ranks by temperature, but that often correlates with mass and luminosity as well.
For the letter, there are 7 classes for normal stars, which are, from hottest to coldest:
| Type | Temperature[1] | Color | Main-sequence luminosity[1] |
|---|---|---|---|
| O | 54,000 K | Blue | 846,000 L☉ |
| B | 15,200 K | Bluish white | 360 L☉ |
| A | 8,310 K | Slightly bluish white | 9 L☉ |
| F | 6,700 K | Yellowish white | 2.4 L☉ |
| G | 5,660 K | Yellow | 0.86 L☉ |
| K | 4,400 K | Orange | 0.19 L☉ |
| M | 3,200 K | Orange-red | 0.026 L☉ |
Each class is then split into 10 subtypes numbered 0 to 9, with the number appended to the letter, where a higher number represents lower temperature.[2] For example, the temperatures and luminosities shown in the table above are those of the 5-numbered stars (M5, K5, ...). An exception to the pattern is that O0 and O1 don't exist, and instead, the hottest stars are O2.[4] The number can also be a decimal like M4.5 for more precise classification.
Therefore, the least massive stars are M9, then the scale goes M8, M7, ..., M0, then continues with K9, K8, and so forth. Since the sun is a G2 star, it is more on the more massive end of the G spectral type, and is therefore slightly more luminous than the G5 luminosity of 0.86 solar luminosities shown above.
Other letters for stars include W for Wolf-Rayet stars, C for carbon stars, S for stars between M-type and C-type, and D for white dwarfs. Although not actually stars, brown dwarfs also get letters: L has the hottest brown dwarfs, followed by T, then Y.[3]
Luminosity class
The luminosity class, in the Sun's case V, is from the Morgan-Keenan or Yerkes system.
The luminosity classes are, from most luminous to least luminous:
| Affix[2] | Class | Subdivisions |
|---|---|---|
| -0 or -Ia+ | Hypergiants | |
| -I | Supergiants | -Ia, -Iab, -Ib |
| -II | Bright giants | |
| -III | Giants | |
| -IV | Subgiants | |
| -V | Main-sequence stars/dwarfs | |
| sd- | Subdwarfs | |
| D- | White dwarfs |
Names and examples
The names for each combination of the 7 main spectral types and 6 main luminosity classes, at least the ones used here on Polus, along with some examples, are:
Note that stellar classes' names are very debated, and this is Polus' take. For supergiants and hypergiants, the clean grid falls apart, and instead there are merged classes. This is because these stars are rare, and some of these stars even change their spectral type from time to time.