Now let’s look at what happens when a star reaches the end of its life. Thing is, what happens depends on how big the star is so we’ll look a two stars, an average star like our Sun, and a giant star with several times the mass of the Sun.
A star about the size of the Sun will reach a point where there is no more Hydrogen in the core and as we saw last time, the core is where the action is and now it has run out of fuel. When this happens the amount of radiation streaming out of the core falls dramatically so there is not enough outward pressure to stop gravity pulling the layers just outside of the core inwards. This does two things. First of all it makes the core collapse inwards until it becomes dense and therefore hot enough to start burning Helium. When this happens the engine fires up again with a burst of energy we call the Helium Flash and because the core has shrunk, its gravity has no longer such a tight grip on the outer layers which expand outwards. Because the energy being produced in the star is being radiated out into space over a much larger surface area, the star appears dimmer and this means redder so now the star has become a red giant.
This expansion is bad news for any planets unlucky enough to be orbiting too close. Their atmospheres have long since been blasted away by the increasing solar wind and any water they might have had, has evaporated as the star has burnt hotter and hotter the last millions of years, and now they are simply swallowed up by the expanding outer layers of the ageing star. Outside of the core is a layer where Hydrogen is still being fused into Helium so the star has begun to form layers like an onion. When the Helium is used up the same thing happens again and the core begins to burn Lithium, Beryllium, Boron etc. until the star simply doesn’t have enough mass to compress the core enough to burn the next element up and it stops. The engine cuts out and that’s it for our hypothetical star. So what happens after that? Well what happens is that as soon as there is no more radiation streaming out of the core, there is no longer enough outward pressure to counteract the force of gravity and the core collapses in again but this time it can’t collapse enough to start a new round of fusion. This time it is “Electron Degeneracy” that stops the collapse. Electron Degeneracy, put simply, is the Quantum Mechanical effect that basically says that two electrons can not be in the same state in an atom. When the atoms in a star reach that state it means that the star can not collapse any more. It has become a so called White Dwarf. White because it is still extremely hot from billions of years of nuclear fusion and dwarf, well no surprises for guessing that it is very small, about the size of a small planet. When that happens there is no where near enough gravity to hold onto not just the outer layers of the star, but pretty much all of the star outside of the core and all of this star stuff puffs out into space. This creates what we call a Planetary Nebula, like this-
Gradually all the gas in the nebula will drift out into interstellar space and add to the interstellar medium which may, one day, start the process all over again.
The White Dwarf is still hot so it shines very brightly, but because it is so small, the total amount of energy it emits is actually quite small and as time goes on it will get dimmer and dimmer until it ends its days as a dark lump of ex-star.
OK, now we come to the show off’s of stars, those that live life in the fast lane and go out with a bang, the biggest bang since the big one! Yes, we are talking about super-giant stars, stars with a mass several times that of the Sun. These stars might have many times the amount of Hydrogen fuel as the Sun, but they blow it all away like a lottery winner gone amok. You can think of this as the difference between a small, economy car and a big V8 muscle car. The small car carries maybe 35- 40 litres of fuel and the V8 has 70, 80 , 100 litres, but while the economy car sips gently at its fuel, the big V8 guzzles it greedily. This means that the little, economy car takes way longer to get up to its meager top speed and the muscle car rockets away from the starting line and reaches maybe double the speed of the little car, but the big V8 uses all its fuel much faster and grinds to a halt while the economy car keeps going long after the flash git in the V8 is walking to the nearest petrol station, petrol can in hand. It’s the old story of the Tortoise and the Hare all over again.
So a super-giant lives life in the fast lane and because it has more mass there is a greater force pressing in on the core. This in turn means that it has the power to fuse heavier and heavier elements all the way up to Iron, and then it hits the end of the road. You see, fusing atoms to make larger atoms releases the so called binding energy, the energy associated with holding the nucleus together, but once your get to Iron you need to put energy in. You can see this on this graph-
This is bad for business and no business can survive for long with a loss making product. In the case of a giant star we are talking about seconds and once there is Iron in the core there is no longer energy coming out to counteract the force of gravity pressing in. The core collapses in under the weight of the rest of the star until it reaches such a small size that electrons and protons are crushed against each other and become neutrons. This is like running into a brick wall and the core bounces back, exploding out through the body of the star which until now didn’t even know it was dead, the force of this cataclysmic explosion ripping the star to shreds instantly. Like this-
The force of this explosion is powerful enough to fuse all the elements heavier than Iron and seed interstellar space with the heavy elements that are necessary for the creation of rocky planets and life. If the star was massive enough the core is crushed to such a small size that its gravity is concentrated to the point where not even light can escape and we have a black hole. In the mean time, the remains of the star have been blown out into space in a blast that shines brighter than all the billions of stars in a whole galaxy. In the centre is the corpse of the star’s core in the form of a Neutron Star just 20 or so kilometers across, or a Black Hole surrounded by a beautiful nebula that disperses into the interstellar medium, to one day be part of a new star as the process starts over, only this time with the heavier elements needed to make small rocky planets where living things can evolve to the point where they can write blogs about where they came from. As Carl Sagan said, we are all made of star stuff and he was right, we are the result of the fusion processes in the cores of ancient stars that blew themselves apart billions of years ago and whose atoms now make up our Solar System, our planet and our bodies. So forget about the so-called celebrities, we are all super-stars!