Stellar Evolution #1 – Star Birth

Stars have fascinated humans for millenia, however it wasn’t until the last century or so that we started to really know the inner workings of stellar mechanics.

Stars are living beings. Not in the biological sense, but in the sense that they are born and die, and experiences life stages much like a human being does. But how do we define these stages? How is a star born? And what happens when it dies? These are questions that i intend to answer in this series of posts. First i shall discuss star birth and the interstellar medium, then i shall progress to the main sequence, finally finishing with the ultimate stellar climax- star death. All these posts will be basic in their explanation, with more complex discussions to follow in the future.

Star Birth

Interstellar Medium

Everyone with working eyesight knows what a star is, they are those little pretty points of light that twinkle in the night sky. But few actually know what makes up a star. Stars are spheres hydrogen plasma, fuelled by nuclear reactions in their core- reactions that are responsible for sustaining life as we know it.

The start of the stellar journey can be traced back to something called the ‘interstellar medium.’ The interstellar medium is what makes up interstellar space  in a galaxy. It is made up of dust and gas- mainly hydrogen, and can be split into several regions based upon temperature and number density, that is the number of particles per unit volume. Some of these regions are more favourable for star birth than others, and one is particularly suitable- the so called ‘dense clouds.’

The reason dense clouds are suitable for star birth is that they fulfill two requirements – low temperature and high density. These two requirements can be explained through the means that stars are born: gravitational contraction.

Gravitational contraction of a cloud of gas and dust is what triggers star birth. Newton’s laws of gravity show us that a high density cloud will have more gravitational energy than a less dense cloud, and so is more likely to start contracting. Similarly a cool cloud has less kinetic energy than a hot cloud, so again is more likely to begin contracting under their own combined gravity.

It is not just dense clouds that can begin contracting, but also diffuse clouds. These are less dense than dense clouds, however are closer to the requirements than others, and with a suitable trigger mechanism, can be pushed over the edge. Trigger mechanism can include Supernovae, spiral galaxy structures and collisions with other clouds.


As a cloud contracts, the gravitational potential energy of the cloud is decreasing. This means that the energy is converted first into kinetic energy, and thus into heat energy, so the cloud gets hotter. This stage is what makes a protostar. A protostar is a pre-fusion star that is yet to begin nuclear reactions in its core. As the cloud contracts it also spins with increasing speed, flattening out into a circumstellar disc with the main protostar in its center. This is the beginning of a solar system, with the sun in its center (from the protostar) and the planets and other bodies from the circumstellar disc, which explain phenomena such as planets spinning.

This concludes part 1 of stellar evolution, the next post will concern itself with the main sequence of a star.

image: public domain



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