by Rishabh Nakra / Astronomy
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:
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:
Oh Boy, A Funny Girl Kicked Me.
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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:
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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10 COMMENTS
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
0 0 Reply
Shivam sharma
Sir I want persue my carrier in astrophysics please guide me with some advice and
your precious experience
1 0 Reply
The EM Spectrum And Its Importance In Astrophysics | BoA 2
[…] Also read: The spectral classification of stars […]
0 0 Reply
Difference Between Astrophysics, Astronomy And Cosmology.
[…] 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 […]
0 0 Reply
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.
0 0 Reply
Suryaprakash V Rathnam
Fantastic. Easily explained and very helpful to understand the designated
classifications.
0 0 Reply
Rishabh Nakra
 Reply to
Thanks! Glad you enjoyed reading.
0 0 Reply
Bainty Kaur
I love this..it 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!
0 0 Reply
Rishabh Nakra
 Reply to
Thank you so much Bainty. I a glad you enjoyed it.
0 0 Reply
Harshvardhan Sharma
 Reply to
It was very fun to read. Got the whole thing very easily. Thank you so much?
0 0 Reply
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Rishabh Nakra
Admin and Founder of ‘The Secrets Of The Universe’ and former
intern at Indian Institute of Astrophysics, Bangalore, I am a science
student pursuing a Master’s in Physics from India. I love to study and
write about Stellar Astrophysics, Relativity & Quantum Mechanics.
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