According to the European Space Agency (ESA), there are approximately 1 trillion trillion (10^24) stars in the universe. The number, for sure, will increase as humans get better at technology and explore deep space. But is there any specific way to categorise the stars in the Universe? The answer is yes! The Morgan Keenan Classification System: an amalgamation of the older Harvard System and the Yerkes System. Let us dig into the details.
Harvard Classification System
The Harvard system is a one dimensional system in which the stars are classified into 7 main categories according to their spectrum. This classification is based on the surface temperature of the star. The 7 categories are denoted by 7 alphabets, which, from hotter to colder are, O, B, A, F, G, K, M. So an O type star is the hottest, with surface temperature of about 50,000 K and a M type star is the coldest, with surface temperature of just 2,500 K. The colour of the stars also varies with the surface temperature as shown:
The easy way to learn the order is by associating a word of a sentence to each alphabet:
Oh Boy, A Funny Girl Kicked Me.
The range of temperature for each spectral class is as follows:
- O: ≥ 30,000 K
- B: 10,000–30,000 K
- A: 7,500–10,000 K
- F: 6,000–7,500 K
- G: 5,200–6,000 K
- K: 3,700-5200 K
- M: 2,400–3,700 K
Within the same class, there are 10 more divisions. Each star is assigned a number from 0-9, with lower number depicting hotter star. So a K0 star is hotter than a K7 star. Conventional color descriptions are traditional in astronomy, and represent colors relative to the mean color of an A class star, which is considered to be white. The apparent color descriptions are what the observer would see if trying to describe the stars under a dark sky without aid to the eye, or with binoculars. However, most stars in the sky, except the brightest ones, appear white or bluish white to the unaided eye because they are too dim for color vision to work. Red supergiants are cooler and redder than dwarfs of the same spectral type, and stars with particular spectral features such as carbon stars may be far redder than any black body.
Yerkes classification System
Just assigning an alphabet to each star according to its surface temperature isn't enough. Stars come in all sizes and are in different stages of evolution. There are the main sequence stars that are still burning hydrogen into helium in their core (Sun) and there are white dwarfs that have ended their lives. So we need another parameter to differentiate them. That parameter is Luminosity.
Luminosity, in astrophysics, is the total energy output per second. Denser stars with higher surface gravity exhibit greater pressure broadening of spectral lines. The gravity, and hence the pressure, on the surface of a giant star is much lower than for a dwarf star because the radius of the giant is much greater than a dwarf of similar mass. Therefore, differences in the spectrum can be interpreted as luminosity effects and a luminosity class can be assigned purely from examination of the spectrum. The Luminosity class and its description is as follows:
- 0 or Ia(+): hypergiants or extremely bright super giants
- Ia: luminous supergiants
- Iab: intermediate-size luminous supergiants
- Ib: less luminous supergiants
- II: bright giants
- III: normal giants
- IV: subgiants
- V: main sequence
- sd: sub-dwarfs
- D: white dwarfs
Morgan Keenan Classification System
When the Harvard system and the Yerkes luminosity classes are combined together, we get the current Morgan Keenan (MK) classification system. Each star is designated a spectral class according to its surface temperature and a luminosity class corresponding to its surface gravity (luminosity). So our Sun is a G2V star. Its surface temperature is about 5,900 K (G type) and it is fusing hydrogen into helium in its core, hence a main sequence (V) star. The MK system comes into play while plotting all the stars in the Universe on just one diagram, the Hertzsprung Russell Diagram (see HR Diagram)