Chapter 5 part 3 — HR Diagrams
Introduction
This is going to be a *very* short post but a very useful one nonetheless. HR Diagrams are extremely important in Astronomy and Astrophysics; they can be likened to the Astronomer’s Periodic Table. Well… the Astronomers’ Periodic Table would be the actual Periodic Table but you get what I mean.
And no! HR Diagrams do not stand for Human Resource Diagrams.
“Just what future the designer of the universe has provided for the souls of men I do not know, I cannot prove. But I find that the whole order of nature confirms my confidence that, if it is not like our noblest hopes and dreams, it will transcend them.”
- Henry Norris Russell
The HR diagram stands for the Hertzsprung-Russell Diagram. The first version of this diagram was started by Danish Astronomer Ejnar Hertzsprung
It was then improved by American Astronomer Henry Norris Russel.
The HR Diagram is extremely useful because it can reveal various properties of Stars and it is also a visual representation of the stellar lifecycle.
We’ll be working with a much simpler diagram for the purpose of this blog.
It is essentially a graph with temperature on the x axis and luminosity on the y axis. However the x axis isn’t conventional; the values increase in magnitude as you get closer to the “origin”.
HR diagrams can be split into three main sections:
- The Main Sequence Line
- The Giant Line
- The End Line
The Main Sequence Line
This is the main logarithmic like curve in the middle. All main sequence stars in our universe would be plotted on that line. Our Sun is somewhat in the middle of that line.
As you can see visually, the stars that have lower temperatures are actually red and orange whereas the blue stars have greater temperatures. This is contrary to popular belief where blue is associated with “cool” while red is associated with “warmth”.
There is a relationship between mass and luminosity for main sequence stars.
The weird infinity sign thing stands for ‘proportional to.’ This means that doubling the mass of a main sequence star would increasing its luminosity by a scale of about 11.
Stellar Evolution
We can start looking at the stellar evolution from the Main Sequence line. If it is an average mass star, there would be a line entering the Giant area. After which, there would be a line that goes straight through to the White Dwarf area. Something like the image below
However for large mass stars, there would be a branch from the Supergiant area that does not lead to any other part of the graph, this is because they evolve via supernovae into either black holes or neutron stars which aren’t represented on the HR diagram.
Conclusion
I hope you guys enjoyed, learning how to interpret an HR diagram is a key skill for any Astronomer. Next week, we’ll be delving into star classes!
Images can be found at:
