The Life Cycle of Stars

Stellar Evolution

What's not to love about this topic.  If Hubble's red shift data gave us an age of the universe then measurements and analyses of the wavelengths of light and the brightness of stars has given us an understanding of our own solar system, its age and its evolution.


Our galaxy formed around 9 Billion years ago.  there would have been fewer, but much more massive stars!  These stars would have had very short main sequences, during which they turn hydrogen into helium by nuclear fusion, liberating inestimable energies and making heavier and heavier elements.  These stars after only hundreds of thousands of years, expanded to become red supergiants before collapsing under their own gravity and exploding in supernovae.  How do we know this?  Well there are elements that are heavier than iron here in our little region of the milky way, and those elements are only made using the vast energies in a supernova as the outer layers of the star all bounce off one another, and off the dense iron core of the star.


Some of these stars would've left behind neutron stars, an incredibly dense ball of neutrons, formed when the core and inner layers were compacted with such force that the repulsion between protons meant that they were all ejected, leaving nothing but the neutral particles.  Some of them, the most massive, would have left behind black holes, a seemingly infinitely dense object, with such a large gravitational field that it simultaneously absorbs all wavelengths of light and emits gamma jets from either side of its swirling.  We still don't know much about black holes, except that they are left over after the largest of stars explode and that they are found at the centre of galaxies such as our own.



Other smaller remnants of these explosions were scattered sparsely across the milkyway.  This matter, again mostly Hydrogen, but crucially with a percentage or so of other elements, formed gas clouds known as nebulae.  We still see these remnants around our galaxy as we gaze back in time at the light which is just reaching us.  Our nebula, coming together under the mutual attraction of its own gravity, had enough hydrogen for fusion to start.


Nuclear fusion requires immense temperature and pressure to start, but once it starts provides its own heat to keep it going.  This initial thermal energy was provided by friction as the gas cloud became more dense.  This point, with the beginnings of fusion, we call a protostar, the before star, or trial star.  Given high enough density this fusion is established, the outwards forces of fusion pressure are balanced by the inward force of gravity, and the whole mass of hot gas takes a spherical shape.


This period of relative stability, we call the main sequence of the star.  Stars with the mass of our star can have a main sequence of about 10 billion years, and the sun, our star, is about 5 billion years through it.  The other particles in our nebula began to spin around the star, and separated out, first into rings and then clumping together under gravity to form the planets.  Our Earth is about 4 billion years old, and here's where the story goes into the fascinating study of geology, our atmosphere and eventually evolution.


But we'll look forward to what will happen next, after our sun begins to run out of Hydrogen and fusion starts to slow, as heavier heavier elements are formed, but the star cools and expands.  The sun will expand to somewhere near the orbit of Jupiter, the elements will separate into layers, it will dim and become redder in colour.  We call this a red giant star, and again, we can see stars just like this, with similar masses to our sun.  These stars do not have the crucial critical mass to collapse in on themselves under their own gravity, so their end is rather more placid.  These layers will drift out into space leaving behind a planetary nebula, with not enough Hydrogen to form new stars.


We can see small dense, white hot remnants of these less massive stars, within their planetary nebulae.  We give them the apt name, white dwarfs, and this is the eventual fate of our sun.  We expect these stars themselves will eventually cool, all fusion will stop and they will appear much darker in the sky.  We call this a brown dwarf… or rather we would, or will if humanity survives long enough to see one!  The Universe you see is not old enough for any one of these small stars to have completed this section of their stellar evolution!


Stars are fascinating, it makes a lovely story, and this is just the barebones, without the classification, the different magnitudes, the Thermodynamics and rotational dynamics that go into analysing this literal universe of possibilities.  But for your GCSE in Physics you need to know the three possible sequences for stars and some key details around them.  It's a good one for drawing a really awesome flow chart and adding the level of detail that you see in your revision guides, text books and markschemes.




1. By ESA/Hubble, CC BY 4.0,