Basics of Astrophysics | Classification of Stars

A Systematic Classification of Stars

This is the ninth article of Basics of Astrophysics series and the journey so far has been exciting. We started from the basic question: What is astrophysics? We then learned about the basic tools and terminologies that are used in this field, the units of distances, the celestial coordinate systems, the concept of magnitude, the importance of EM spectrum and the types of redshifts.

Read all the articles of Basics of Astrophysics here

Now is the time to go deeper and understand how the Universe works. Today, we begin our journey in the field of Stellar Astrophysics which is one of the most widely researched branch of Astronomy. In the ninth article of the series, we will learn how a trillion trillion stars in the Universe are classified into just 7 groups: The Spectral Classification of Stars.

A Systematic Classification of Stars 2
  Hubble Sky Full of Stars (Image: NASA/Hubble)

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.

Subscribe Us On Youtube

We have started a new video series, Stories of the Great Minds. Each season of the series will comprise 5 episodes on scientists and mathematicians. Make sure you subscribe to our Youtube Channel to get the notifications of the uploads.

Harvard Classification of Stars

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 Systematic Classification of Stars 3
The classification of Stars

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 temperatures and the spectral features for each category are given in the image above. It is yet another example of the importance of spectroscopy in astrophysics.

These 7 types were the one originally developed in the classification scheme. The new spectral types L, T, and Y were created to classify infrared spectra of cool stars. This includes both red dwarfs and brown dwarfs that are very faint in the visible spectrum.

Also watch: How will the Sun Die?

L brown dwarfs have temperatures between about 1,500 and 2,500 K and have spectral lines caused by alkali metals such as rubidium and sodium and metallic compounds like iron hydride. T brown dwarfs have prominent methane absorption in their spectra and temperatures between about 800 and 1,500 K. Class Y brown dwarfs are cooler than 800 K and have spectral lines from ammonia and water.

A Systematic Classification of Stars 4
An artist’s impression of a Y dwarf

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 an 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 of Stars

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:

Spectral classification of stars
The Yerkes Classification of Stars

Morgan Keenan Classification of Stars

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, that we will learn in the next article.

Subscribe Us On Youtube

We have started a new video series, Stories of the Great Minds. Each season of the series will comprise 5 episodes on scientists and mathematicians. Make sure you subscribe to our Youtube Channel to get the notifications of the uploads.

Author’s Message

The spectral classification of stars, along with the Hertzsprung Russell diagram, is one of the most fundamental concepts in Stellar Astrophysics. The entire story of stars revolves around these two concepts. This article was very important to learn about Stellar Astrophysics. Today, in the name of Astrophysics, most of the people just talk about black holes, wormholes, white holes and other pop science concepts. But in reality, Astrophysics is much more than this. It is a very wide branch of Physics.

This is the main aim of the series. I really want you to understand the deeper concepts of this field: concepts, that most people don’t know or don’t talk about. I always tell my young friends, “If you wanna be an Astrophysicist, master physics first. Without Physics, there’s no way you can understand astrophysics in detail.” If you have a strong hold over Physics, Astrophysics will then be a cake walk for you. See you in the next article.

Read all the articles of Basics of Astrophysics here

15 thoughts on “A Systematic Classification of Stars”

  1. Amazing bro I love this article and your explanation is matching the highest peak of deeper cosmos
    Please put more article like this in series because without stars astrophysics is nothing

  2. Very random article!!! And it’s just the basic :0
    I love the space, but the physics are the magic there ?

  3. Pingback: The Hertzsprung-Russell Diagram | The Secrets Of The Universe

  4. Pingback: The Formation Of White Dwarf Stars | Stellar Evolution

  5. Pingback: Types Of Nebulae | The Secrets Of The Universe

  6. Pingback: Top 10 Astrophysicists And Their Contributions That Changed The Course Of Astrophysics.

  7. Pingback: 10 Unsolved Problems In Astrophysics That Are Way Too Interesting.

  8. Pingback: 4 Ways By Which Scientists Can Determine The Temperature Of Stars

  9. Pingback: 170 Years Ago On This Day, The First Photograph Of A Star Other Than The Sun Was Taken.

  10. Pingback: Meet These 8 Brilliant Scientists Who Were Never Awarded The Nobel Prize.

  11. Pingback: How A Severe Solar Storm Can Wreck Havoc On Our Planet?

  12. Pingback: A Star Within A Star: Understanding The Science Behind Thorne Zytkow Objects.

  13. Pingback: A Total Solar Eclipse And Lots Of Passion - The Story Behind The Discovery Of Helium.

Comments are closed.

Scroll to Top

Let's Stay In Touch

Sign up to our newsletter to get the latest and the greatest from our blog right in your inbox.