The Hertzsprung-Russell Diagram

Rishabh Nakra

Admin and Founder of ‘The Secrets Of The Universe’ and former intern at Indian Institute of Astrophysics, Bangalore, I am a science student pursuing a Master’s in Physics from India. I love to study and write about Stellar Astrophysics, Relativity & Quantum Mechanics.

When you study Astrophysics, especially the stars, it is impossible not to stumble upon the Hertzsprung-Russell diagram. In the tenth article of the Basics of Astrophysics series, we will be learning about the most important diagram in Astronomy, the HR Diagram. Why is it the most important diagram? First of all, you can plot every star in the Universe on it, and secondly, the entire life of a star can be traced to this diagram. So let us learn about the Hertzsprung-Russell diagram and its wonderful applications in Astrophysics.

Read all the articles of Basics of Astrophysics series here

Hertzsprung-Russell Diagram

There are about a trillion trillion (10^24) stars in the Universe. Every star is different in terms of mass, surface temperature, size, density, surface gravity, etc. When Astronomers started studying the stars, they found a particular pattern among them. They wanted to develop a graph on which every star of the Universe could be plotted. The Hertzsprung-Russell diagram is one such attempt. It is a scatter graph, as shown below.

Axes Of The Diagram

If you want to plot something, the first thing required is the axes of the plot. In the Hertzsprung-Russell diagram, the y-axis represents the luminosity that increases with the increasing y. Luminosity is the total energy output of the star. We discussed this in the eighth article (The Concept of Magnitude).

Alternatively, luminosity can also be defined roughly as the brightness of the object. In Astrophysics, we define brightness by the term magnitude. It is a number that tells how bright an object is. Lesser the number, the brighter the object. So in this diagram, as we go upwards (increasing y), the magnitude decreases, and the brightness increases.

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Next, we come to the x-axis. It represents the surface temperature of the star. In the ninth article (Classification of Stars), we learned that stars could be classified into seven main categories according to their surface temperature. In the decreasing order of surface temperature, they are: O, B, A, F, G, K, and M type stars. Now remember one thing, on this graph, the surface temperature decreases with increasing x. So, the O-type stars that are the hottest ones lie on the left of the diagram while the cool K and M stars lie to the right side.

Plotting The Stars

Having understood this diagram’s axes, we can now pick any star in the Universe and plot it here. The dots on the above graph represent individual stars. Note that the stars appear as groups. Let us explore them in detail.

Group A

Most stars appear to be present in Group A. These stars are the main sequence stars. The main sequence star is the one that is burning hydrogen into helium in its core. Our sun also lies in this group. Sun has a surface temperature of about 5,900 K. This means it is a G-type star. Also, the absolute magnitude of the Sun is +4.8. So we know both the parameters, and we can easily place the Sun on our diagram. It lies near the middle. Other notable main sequence stars include Alpha Centauri and Sirius.

Group B

After fusing hydrogen into helium, the stars exit this main sequence band. Most of the stars become red giants and enter Group B. Now red giant means that the star’s surface temperature decreases (as it goes to K or M class) while its luminosity or the energy output increases. So where will the star go? Well, it will move towards the right on the x-axis and upwards on the y-axis (decrease in temperature and increase in luminosity). This region is known as the Red Giant Branch. Notable stars in group B are Aldebaran and Mira.

Group C

The stars in Group C are even more luminous than the giants. These are the supergiants and hypergiants, the largest stars with extremely high luminosities. A red supergiant such as Betelgeuse would extend beyond the orbit of Jupiter if it replaced the Sun in our solar system. Note that most of the red giants and supergiants share the common x-coordinate. Thus, red supergiants differ from red giants mostly in luminosities. But this is not the case with blue supergiant stars that lie in the upper left corner of the diagram. They have a higher surface temperature.

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Group D

The stars in Group D are known as white dwarfs. Once you know the location of the star on this diagram, you can immediately guess its properties. So, lying on the left side, it is evident that they have a higher surface temperature and bluish-white or white color. But, since they are in the lower region, this means their luminosity/energy output is quite less. These characteristics belong to neutron stars and white dwarfs. They are basically dead stars.

Author’s Message

Stellar Astrophysics is a vital branch of Astronomy, and the Hertzsprung-Russell Diagram is the key to it. To understand this diagram, we needed a few basic concepts in our hands. That is why the concept of magnitude and spectral classification of stars were already discussed in the eighth and ninth articles, respectively. On a personal note, the Hertzsprung-Russell diagram was the first thing I ever came across in Astrophysics, and from here, my journey began 5 years ago.

Earlier, I wrote an even more detailed article on the HR diagram. You can find it here.