The Life and Death of Stars

I know tumblr is mostly filled of awe-inspiring, but physically quite dull nebulas, so here is a crash course in the life cycle of stars. Which are totally way cooler.


Stars begin their lives rather unassumingly as blobs of gas in molecular clouds that slowly grow in density even though they’re still less dense than vaccuum chambers on Earth. Slowly gravity pulls in more atoms and molecules, mostly hydrogen and helium until there’s a gravitational instability that causes this cloud to go over the tipping point and begin to collapse itself. This can be brought about by strong gravitational effects, such as the death of another star as a supernova. The collapse itself is known as a Jeans instability. As this happens the gravitational energy gives way to heat energy and the blob of gas rapidly begins to heat up and a protostellar core forms. This contraction typically takes between 10 and 15 million years.

Main phase

The star spends the majority of it’s lifetime in the main phase, what this is when the heat of the star reaches a high enough temperature to allow a fusion reaction of hydrogen into helium. The beginning of this phase is comprised of relatively small stars known as red and yellow dwarfs (our sun is a yellow dwarf). There is another type of object known as a brown dwarf and is essentially a star that did not reach enough mass or temperature to begin the fusion reaction. As the helium concentration of a star increases it gradually swells and increases in temperature. 

Stars can spend a range of time in the main sequence, our sun is predicted to last 10^10 years, while others may live much shorter or much longer. A red dwarf for example will last hundreds of billions of years, older than the universe is now.

Post-main Sequence

This is the phase characterized by red giants which are stars that begin to expand and cool releasing shells of gas to form planetary nebula. Larger red giants however begin the next stage in their life, they then begin to heat up again in the layer around the core and begin the fusion of helium into heavier atoms like cosmic furnaces. It’s stars like this that are the reason you’re alive. Stars that are yet more massive, about 9 solar masses become what is imaginatively known as red supergiants in their helium fusing stage. Towards the end of their lives these stars are fusing different elements at different layers, helium on the outside and getting progressively larger elements as we head towards the center like an incredibly hot, giant onion.

The collapse

Finally after billions of years we come to the death of a star. This can occur in many different ways, some relatively peaceful, others violent. Most average sized stars begin to shed their outer layers and the core compresses to form a white dwarf, with mass that of the sun and size that of the Earth these guys are pretty dense. They’re also made from electron-degenerative matter which can be thought of as atoms or molecules in which the electrons occupy higher than normal energy levels due to the high amount of pressure and energy concentrated in a small space.

Larger stars which have made it all the way to synthesizing iron have a more dramatic exit, the iron core of the center grows so dense and massive that the atoms the atoms themselves become crushed causing electrons to collapse into their protons. This itself causes the famous event known as a supernova. From here there are two other remaining options, most stars will remain as nothing more than incredibly dense neutron stars (such as pulsars) while even more massive ones will collapse until they occupy no space at all, becoming black holes.

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