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 categorize the stars in the Universe? A well established stellar classification system to say? 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

Firstly, the Harvard system of stellar classification 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 an M type star is the coldest, with surface temperature of just 2,500 K. The color of the stars also varies with the surface temperature as shown:

Harvard Stellar Classification system
A diagram of spectral types with information on temperature, color and what elements and compounds show up in each spectral class. (Eugene R. Zizka)

The easy way to learn the order is by associating a word of a sentence to each alphabet:

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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

In this stellar classification, within the same class there are 10 more divisions. So, each star has a number from 0-9, with a 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. Moreover, 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 for stellar classification. 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.

Also Read: UY Scuti – The Largest Star Discovered In The Universe So Far

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 an examination of the spectrum. The Luminosity class and its description is as follows:

Yerkes Spectral Classification System
The Yerkes Spectral Classification
  • 0 or Ia(+): hypergiants or extremely bright supergiants
  • 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

Finally, when the Harvard system and the Yerkes luminosity classes are combined together, we get the current Morgan Keenan (MK) stellar classification system. Therefore 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.

Also watch: The mystery behind Methuselah star – the star older than the Universe

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Aryan Singla
Aryan Singla

In the MK system, the apparent colors mean the same as Harvard system, i mean that these colors describe what an observer would see from ground without any optical aid

Shivam sharma
Shivam sharma

Sir I want persue my carrier in astrophysics please guide me with some advice and your precious experience


[…] Also read: The spectral classification of stars […]


[…] classified 400,000 stars into 7 major categories based on their spectrum. The system, known as the Harvard Classification Scheme, changed the course of astrophysics and is still used today. It again showed the importance of the […]

Ashutosh Pandey
Ashutosh Pandey

This is actually one of the simplest explanation of such a topic I’ve ever had. Imagine knowing this classification, you have names of trillions of stars in your pocket.

Suryaprakash V Rathnam
Suryaprakash V Rathnam

Fantastic. Easily explained and very helpful to understand the designated classifications.

Bainty Kaur
Bainty Kaur

I love was worth reading and getting knowledge..keep posting more and more so that we can get more important facts and knowledge about our universe.
Thank you!

Harshvardhan Sharma
Harshvardhan Sharma
Reply to  Bainty Kaur

It was very fun to read. Got the whole thing very easily. Thank you so much?

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