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The Hertzsprung Russel Diagram

16-17-year-old students

In this Experience, we will learn what type of stars exist in the Universe and how astronomers classify them.


The material for the classroom before coming to ESAC can be divided into two categories:


  • Material for the students:  Videos that can be watched in the classroom for enjoying more this Experience.
  • Material for teachers:Information about the activity that students will carry out during the visit. Click here to download the Scientific Case and the cutouts.

Stars are born from a cloud of dust and gas called the molecular cloud. This cloud starts to generate high-density regions where the protostar will be born. As the protostar collapses it starts to rotate around itself and to accrete mass from the outer layers through the disk. By conservation of angular momentum part of the accreted material is ejected as energetic jets of material being blown out. The protostar phase ends when thermonuclear reactions begin in the nucleus and the object is in equilibrium with the gravitational collapse. The star no longer collapses under its own gravity because it is also trying to expand due to the escaping light produced in the core, as it is shown in the figure below.

Hydrostatic equilibrium. Credits: Standford University


Astronomers use the Hertzsprung-Russell diagram to trace the evolutionary stage of a star. This diagram represents the temperature versus the brightness of a star. As stars are born, live and die, they move in regular patterns around the diagram.

The Hertzsprung diagram where the evolution of sun-like stars is traced. Credits: produceESO


During the life of a star, various chemical reactions take place. The level of complexity of these reactions is determined by the mass of the star: the most massive stars produce the most evolved elements. The composition of the stars is 91% Hydrogen, 8% Helium and 1% heavier metals. When the thermonuclear reactions transform Hydrogen into Helium we say that the star is in the "Main Sequence" phase.

Stars which are  8-10 times the mass of the Sun when they are formed are considered massive by the astronomers. Our Sun is currently at an equilibrium stage called the "Main-Sequence phase" and it will evolve into a "Red Giant" in 5000 million years time.

The evolution of a star along its life depends on its original mass. Credits:students.um.eduç


The internal structure of the stars has an onion shell pattern. At the end of the life of a massive star, the internal structure has the distribution that is seen in the figure below with shells of oxygen, neon, magnesium, silicon and iron. The heaviest elements can be found closest to the nuclei.


The most metallic layers are in the most internal part of the stars. Credits: Astro-Edu.

When many stars form together from the same original cloud they are called "cluster of stars". These regions could be more or less massive and more or less metallic depending on their original cloud.



  • This Experience could be complemented with the following activity in El Planetario de Madrid: El cielo profundo