Journey of Stars On 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.

The Hertzsprung Russell diagram is one of the most important diagrams in Astrophysics. On this single diagram, one can track the entire life of the stars in the universe.

Hertzsprung Russell Diagram- A brief introduction

The Hertzsprung Russell (HR) Diagram is a scatter graph between the temperature ( spectral classification) and the brightness (luminosity or absolute magnitude) of a star.

On the horizontal axis, the temperature is marked. The blue portion on the axis marks the higher temperature while the the red right end marks the lower temperature. The vertical axis mark the Luminosity of the star or its Absolute Magnitude. The brighter the star, the lesser its absolute magnitude. So if a star is at the upper left end of the diagram, it implies that the star is very bright and has very high temperature. Majority of stars lie on a band which starts from the upper left corner and ends at the lower right corner. This portion is known as the Main Sequence. Here the star produces energy by hydrogen fusion at the core.

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Let us explore the Stellar evolution and the Journey of the star on Hertzsprung Russell diagram.

Pre-Main Sequence Star:

A star starts its jorney from the gravitational collapse of large clouds of dust and gas. These clouds are initially at temperatures below 0 degree F. In millions of years these clouds come together and the temperature soars beyond millions of degrees celsius. They ultimately form a large rotating sphere called a Protostar.

The Hayashi Track:

The Hayashi track is a near vertical curve on the HR diagram which gives the luminosity temperature relationship of a protostar whose mass is about 3 times the mass of the sun (M).

In the above figure, the blue vertical lines in the lower right part represent the Hayashi Track. The blue colored numbers represent the mass of the star. ( 0.6 means 0.6 times the mass of the sun). The rapid contraction of the protostar due to gravitational collapse increased star’s temperature and luminosity considerably. When rapid contraction ends, the slow contraction makes the star less luminous but at the same surface temperature. This is the reason why the Hayashi Track is vertical. When the core becomes hot enough to sustain nuclear fusion, the protostar migrates to  the Main sequence.

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The Henhey track:

The stars comparable to the size of the Sun do not enter the main sequence directly after the Hayashi Track. They enter the Henhey track. The Henhey track is a horizonal track ( the horizontal blue lines in the above figure). A Henhey star has constant luminosity and marks the period of near hydrostatic equilibrium. The Hayashi stars are convective where as the Henhey stars are radiative. They too end up on the Main Sequence once the Hydrogen Fusion starts in the core.

The Main Sequence:

The Main Sequence is the most populated region of the Hertzsprung Russell diagram. In the Main Sequence, a star is in complete hydrostatic equilibrium. The inward gravitational force is equal to the outward thermal pressure due to Hydrogen fusion at the core. A star can never be crushed by gravity in main sequence. The star will burn its fuel happily in this period. For how long will the star stay on this band depends on its size. More massive stars tend to burn their fuel quickly and hence will exit the main sequence in some million years whereas mid sized stars will stay on the main sequence for billions of years.

The Sun is the best example of a Main Sequence star. It will happily burn its fuel at the core for another 5 billion years. The Sun is a G2 spectral class star. On the diagram, it lies at the centre of the main sequence.

The Red Giant Branch:

Once the hydrogen fusion at the core stops, the star “runs out” of its fuel and exits the main sequence. In the absence of the outward thermal pressure, the gravity starts to collapse the star and the star enters the Red Giant Branch (RGB) of the HR diagram. The RGB stars have an inert core of Helium. Hydrogen has finished in the core but is still present in the shells around the core. Hydrogen burning in the shell causes the star to expand and cool down. The star is said to ascend the Red Giant Branch.

The Luminosity of the star increases considerably while ascending the RGB as shown in the diagram. But there is a limit to which its mass and luminosity can reach. The star is said to be at the tip of the Red Giant Branch where the core becomes hot enough to start the helium fusion this time. The fusion of helium begins explosively. The core increases in temperature due to the helium fusion but the star decreases in size and temperature. The star then  descends the Red Giant Branch and enters the Horizontal Branch.

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Horizontal Branch:

In the horizonal branch, the star produces its energy by the helium fusion at the core. During this period, the star’s total energy output (luminosity) will remain constant and hence the star will move horizontally along the HR diagram.

Asymptotic Giant Branch:

Once the Helium fusion at the core ends, the star leaves the Horizontal branch and enters the Asymptotic Giant Branch (AGB). The AGB stars have a core of carbon and oxygen and shells of helium and hydrogen.

Deep convective zones develop in the star which take the carbon from the core to the surface. This is known as the second dredge-up. In this way, a carbon star is formed.

Post AGB Phase:

Low to mid sized stars never reach enough temperatures to begin the carbon fusion at the core. The gravity which was trying to collapse the star since its birth gains the upper hand. But for these stars the Electron degeneracy pressure does not allow the gravity to further collapsed the star. As a result the star becomes a planetary nebula and then later a white dwarf.

Random Regions of HR Diagram:

The Hertzsprung Gap:

Hertzsprung gap is a small region between A5 and G0 spectral type and between -1 and +3 absolute magnitude.

When a star crosses the gap it means that it has finished core Hydrogen burning and is yet to start Helium fusion. Stars do exist in the Hertzsprung gap but for a relatively shorter period ( thousand years compared to its lifespan of millions of years).

The Instability Strip:

The instability strip is a region of the Hertzsprung Russell diagram which is populated by the classes of pulsating variable stars such as  RR Lyrae variables, Delta Scuti variables, SX Phoenicis variables and the Cepheid variables.

The Hertzsprung Russell diagram is an important concept that every Astronomy enthusiast must know. This short article was an attempt to tell you about the basics of the Diagram.

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