Stellar Evolution

Written by: Jahn Fredrik Breivik

Before reading, there are some basic expressions which you will find very useful if you want to understand the text.

Fusion: Lighter atomic nucleuses converts into heavier atomic nucleuses, fusion requires extremely high temperatures.
The most common fusion in the universe is hydrogen merging into helium, when the fusion is in progress, some mass will also disappear as energy (heat and electromagnetic radiation).

Electromagnetic radiation: energy in motion, visible light is also electromagnetic radiation. When electrical charged particles release energy, they release electromagnetic waves.

What is a star?

There have been many different explanations of the starry sky up through times, but there is no wonder why, as the starry sky is probably one of the most beautiful and mysterious views there are. One of the common explanations in former times was that each star was placed on the starry sky by the gods to guide sailors. Today we know that the stars are something else, they are in fact, giant fusion reactors that are held together by enormous gravitational forces. Our sun, for instance, converts 700 million tons hydrogen to helium each second. The disappearing mass releases as energy in form of electromagnetic radiation.

How do we learn about stellar evolution?

Almost everything we know about stars and stellar evolution today, we have learned through observing the electromagnetic radiation stars sends out. As humans, we can not sit back and observe stellar evolution as it takes billions of years, but we’ve learned about it by gathering information from different stars in different phases of the evolution.

The birth of a star

The universe has a very low density, in average 1 atom/cm3, but there are some exceptions. In some places particles attract each other with their low gravitational forces. This effect starts the congestion of dust and gas, and it has now been created a huge cloud, these clouds are called molecular clouds. As the density of the particles is increasing, particle-collisions occur more often, this boosts the temperature incredibly. When the temperature is high enough, fusion of hydrogen to helium starts. The fusion reactor is now alive. There are also two opposite forces; the high temperature has made a pressure out from the core, but the gravitational force makes a pressure towards the core. These forces stabilize each other after a while and a star is now born!

The Orion-Nebula. One of the well known molecular clouds. (picture1)

Red giants

After becoming a stable star, the star will eventually become a red giant; the smaller stars with lesser mass will probably skip the phase as a red giant and become white dwarves instead.

When all the hydrogen in the core of a star has been burned into helium through fusion, the star will absorb the helium and pull back together, the size is decreasing. The density increases and the temperature rise higher; this makes the fusion progress more rapidly than before. The star starts burning hydrogen in an outer shell and the size of the star increases magnificently. The temperature in the outer shell falls down to approximately 2000-4000 Kelvin, this gives the red giant a red glow, hence the name.

The other part of the name, calling those stars “giants” is not without a reason either. In fact, red giants can have a diameter up to thousand times longer than the sun’s diameter.

The phases after becoming a red giant


In the core of the red giant, the temperature rises so high that fusion of heavier elements than helium begins. Helium burns into oxygen and carbon, but after a while, the star runs out of helium. In some stars, all fusion will stop here because the temperature is not high enough to merge the heavy elements into heavier elements. All fusion stops, and the outer shell will be released. The star has now decreased in size, and the remains are called a white dwarf. After billions of years, the stars radiation will slowly fade, and the remains is what we refer to as a black dwarf.

As you can see in the picture, some red giants evolve into supernovas, and further on to black holes or neutron stars.

Supernova – An exploding star

I will only include the most common type of supernova in this text, the type we refer to as “Supernova type 1”.
If the temperature is high enough to merge oxygen and carbon further on, the core will after a while exist of iron. Because iron requires energy to merge further on, the fusion stops. The gravitational forces and the fusion forces will no longer be in balance and the core collapses. While the core is collapsing, protons and electrons will merge and connect into neutrons. This process lasts only 1 second!

The core is now basically a ball of neutrons.
When the core consists of neutrons only it decreases dramatically in size and bumps out again quickly at great ease, the supernova is still not over though. There are some vibrations in the neutron core, after a while, they send out a shockwave that starts fusion-processes of heavy elements like gold and silver in outer shell of the star. When the shockwave hits the outer shell, the temperature reaches 200,000 Kelvin, this makes the star explode and a lot of matter will be thrown out in the universe in a speed reaching up to 40,000 km/second!

So what is left after such an incredible explosion? The most common is that there is a very dense and compact core of neutrons left, a neutron star. Neutron stars are rotating very quickly; some of them rotate almost thousand times each second!

If the remains are large enough (larger than three sun masses), they will become black holes. The black holes have such unbelievable gravitational forces that nothing “slips by”, not even light. Black holes are invisible, because not even light passes by.

A quick summary:
- Stars are born in molecular clouds.
- Stars will eventually become red giants. Stars with lesser mass will skip this phase and become white dwarves.
- Some red giants explode; this is what we call a supernova. These will eventually become black holes or neutron stars, depending on the size of the core.
- Other red giants will evolve into white dwarves and eventually fade to black dwarves.


Ekeland, Johansen and Strand: Nexus Naturfag 5.Oslo: Aschehoug.

Stellar Evolution(19.02.08). URL:

Red Giant(19.02.08) URL:

Supernova(19.02.08) URL

Nasa(24.02.08) URL:

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