A few weeks ago we talked about the birth of a star, how tiny pieces of matter start clumping together in very cold and dark regions of space, becoming a protostar, and blowing its shell out with the massive amount of energy from fusion.
A star is born in the darkness. It lives anywhere between millions of hundreds of billions of years, then the star dies just as fantastically. We’re new to this party, we’ve only been around for a tiny piece of history, because we are the sons and daughters of a star that died a long time ago.
Like in birth, the way a star dies has everything to do with how massive it is. We start with stars with a low mass.
Structure of an Asymptotic Giant Branch (AGB) star
Low Mass Stars (less than about 4 solar masses)
As a star burns, all the hydrogen in the core is being converted into helium. As the hydrogen is depleted, in a complicated sequence of events, helium will start to fuse together in shells and will start giving the star enough energy to expand. The star eventually becomes a red giant - stars with very large sizes with relatively cool surface temperatures. The star’s core could be the size of the Earth, and its envelope be as big as the Earth’s orbit!
The Dredge-Up
Energy in a star is conducted in two ways: convection and radiation. Convection is what happens in a stove when you heat up water, the water at the bottom gets warm, loses density, and will “float”, and will be replaced with cooler water which will eventually heat up and rise. Water on top of the pot will cool off, get heavy, and sink to heat up again. This causes a “convective loop”.
Radiation is what makes you warm when a fireplace is running, the infrared radiation will excite the molecules on your skin and warm you up. It’s how the sun makes you hot in a hot sunny 4th of July beach day. Good thing you’re sitting on the water, because it’s really freaking hot. Great, now I want to go paddleboarding, but my body will never forgive me.
When a star starts fusing helium in the core, three elements are created: carbon, nitrogen, and oxygen. These elements then rise to the surface of the star through convection. A star’s life may have up to three dredge-ups, the third is very rich in carbon, and when a star undergoes this it’s often called a carbon star.
Mass Loss and Stellar Wind
So the star is now huge, and luminious, and gravity at the surface is very weak – the core is very low in density and spread out over a very large area. Any kind of disturbance would send star-stuff out into empty space. Remember that the outer layer has carbon, nitrogen, and oxygen? These stars actually blow soot into space.
It is this carbon soot that is the building block for our carbon-based life forms, and it’s this nitrogen and oxygen that you’re breathing right now.
Taste that air, feel your skin. You are experiencing what once was the core of a dying star, blown into space during a star’s death.
Planetary Nebulae
We’re going to use the previous picture to guide us for this one.
The helium burning shell is burning, pushing the hydrogen shell away, and creating carbon and oxygen that fall into the middle of the star. As the helium in that shell burns out, the hydrogen burning shell can crush into itself, and creating more helium, which falls into the temporarily dormant helium shell. After a certain point in temperature, a helium-shell flash will occur, which blows star stuff out of the star, increasing the luminosity of the star, and then starting the cycle again. These flashes happen every 100,000 to 300,000 years, and they give rise to the planetary nebulae. We discussed these some time ago, they are very beautiful objects that only live for a few ten or hundred thousand years or so.
The Ring Nebula M57, located 2,300 light years from Earth, is an example of a planetary nebula.
The Eskimo Nebula (or the Clownface Nebula) - notice the small, hot nucleus of the star in the middle. Super hot and about the size of our Earth, it is densely packed with carbon and oxygen in the center, and helium, and hydrogen in its outer shell.
As these stars blow their shells in a stacatto, their hot, bright centers are discovered further and further. These, in turn, become what are known as white dwarfs – hot, bright stars that used to be the heart of a very large red star. They are not very large, sometimes they’re similar to the size of the Earth. The odd thing about them is that the heavier they are, the smaller they are. This is backwards from what we’re used to, it’s due to something called electron degeneracy. To put it simply, in a white dwarf the size is supported by degenerate electrons. The more massive the star, the bigger its density, which creates an increase in pressure, and it overcomes the repulsive force of these electrons, so it shrinks.
It takes a loooong time for a white dwarf to twinkle out, they are very hot, but they are very dim.
This is the end of a star that’s less than four solar masses. It changes fuel sources, starts blowing its shell, increases in luminosity, and the core will start pulsating to release what’s left of it: a carbon-oxygen rich core that will cease to burn fuel and will become dimmer and dimmer over billions of years until it becomes a black dwarfs, stars that have stopped emitting significant heat or light, objects of theory since the universe is not old enough to have produced any.
Soon: a large star dies very differently – you may have heard of it – a supernova. The most cataclysmic explosion since the Big Bang.
