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