Stellar Classification : The Diagram of Hertzsprung-Russel

As we all know, there’s a huge universe outside of the world we’re living in. It is mostly unknown, but that’s what makes it mysterious. Many planets, dark holes, stars that we don’t know the existence of. Stars… A great example could be the sun. There are many different kinds of stars though, the Sun is just an example of a G-type star, also known as a “yellow dwarf”. They’re all nice and all but how do we really classify the stars? To what extent are they categorized? You may find most of your answers in this article, I’d suggest you keep reading.

NASA/SDO (AIA), Public domain, via Wikimedia Commons

Contrary to what most people believe, not all the stars are bright. They might also be pale. But not all hot stars are bright. Similarly, not all cold stars are pale. The stars can vary according to their temperature, size and brightness. The diagram of Hertzsprung-Russel summarizes these variances.

The Diagram of Hertzsprung-Russel

Daniel William “Danny” WilsonCC BY 4.0, via Wikimedia Commons

The vertical axis shows the luminosity of the stars while the horizontal axis shows the surface temperature of the stars. Hot, blue-white stars are located in the left part of the diagram whereas the cold and red stars are located in the right part of the diagram. Since the luminosity increases as the diagram goes above the x-axis, small stars should be located in the down-left while the big stars should be located in the up-right. This is because if a star is so hot but pale, it should be very small and if a star is cold but so bright, it should be very big.

As we can see, most of the stars form a sequence of a line in the diagram. Stars spend most of their time in this area which is the time they have hydrogen fusions inside them.

Types of Stars

1) Main Sequence Stars

They are the stars that are fusing hydrogen to helium inside them, which makes up about 90% of the star population.

MerikantoCC BY-SA 4.0, via Wikimedia Commons

2) Red Giants

When main sequence stars collapse and hydrogen fusion expands the outer layers of the star, red giants occur.

KKolaczynskiCC BY-SA 4.0, via Wikimedia Commons

3) White Dwarfs

ESO/L. CalçadaCC BY 4.0, via Wikimedia Commons

4) Neutron Stars

ESO/L. Calçada/M. KornmesserCC BY 4.0, via Wikimedia Commons

5) Red Dwarfs

NASA/Walt Feimer, Public domain, via Wikimedia Commons

6) Brown Dwarfs

NASA/JPL-Caltech, Public domain, via Wikimedia Commons


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