The general instructions and the material for the are provided on the Space Science Experience Home. In this experience, we explore the Hertzsprung-Rusell diagram.

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Stars, as with the Universe in general, evolve with time. They form in regions of dense gas where accretion is triggered by passing pressure such as from nearby supernova. Where many stars form together (and usually at the same time) we call this a star cluster. These regions, called star-clusters, could be more or less massive and more or less metallic depending on their original cloud.

Astronomers use the Hertzsprung-Russell diagram to trace the evolutionary stage of a star. 

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

This is a plot of temperature versus brightness, and as stars are born, live and die, they move in regular patterns around the Hertzsprung-Rusell diagram.

The first stage is when a cloud of gas and dust starts to collapse on itself. This is called a protostar and can be a very energetic stage with jets of material being blown out as well as in.

The protostar phase ends when thermonuclear reactions begin in the nucleus and are in equilibrium with the gravitational collapse. Star not longer collapses under its own gravity because it is also trying to expand due to the escaping energy produce in the core.

        Hydrostatic equilibrium. Credits: Standford University


The thermonuclear reactions transform Hydrogen into Helium.

During the life of a star various chemical reactions take place. The level of complexity of these reactions are 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. 

The evolution of a star along its life depend on its original mass.


Stars more than 8-10 times the mass of the Sun are considered to be massive. Our Sun is currently at an equilibrium stage called the "Main-sequence phase". It will evolve into a "Red Giant" in a billion years time.


The internal structure of the stars has an anion shell pattern. At the end of the life of a massive star the internal structure have this distribution:


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