The Spectral 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. 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 trillion trillion stars in the Universe are classified into just 7 groups: The Spectral Classification of Stars.

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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.

Spectral Classification of Stars

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 color of the stars also varies with the surface temperature as shown:

Classification of stars
The Harvard Classification System. Note the color changes from red to bluish white from M to 0.

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
Table
Temperature vs spectral class. The conventional and apparent colors are also written.

Within the same class, there are 10 more divisions. Stars are numbered from 0-9. The lower number depicts 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.


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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

This classification is based on the spectrum of the star. Remember my words from the second article? "Spectrum is the most important tool for an Astrophysicist to decode the Universe". Now it is showing up!

Morgan Keenan Spectral Classification of Stars

The amalgamation of the Harvard system and the Yerkes luminosity classes yields 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.

Author's Message:

The spectral classification of stars, along with the Hertzsprung Russell diagram, is the most fundamental concept 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 travelling through a worm home, black hole, white hole, time travel, dark matter etc. But in reality, Astrophysics is much more than these concepts. This is what the main aim of this series is. I really want you to understand the deeper concepts of this field: concepts, that most people don't know or won't talk about. I always tell my young friends, "If you wanna be an Astrophysicist, master Spectroscopy, Electrodynamics, Statistical Mechanics, Quantum Mechanics, Optics and Nuclear Physics."

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