Have you ever thought about what the stars we see in the night sky look close up? Have you ever considered that some of them are baby stars while some are dead? About 275 million stars are born and die every day! But how does a dead star look? Is it cold and dark? Bloody red? Or a shiny white object with a heart of diamond? “The death of a star, is the most beautiful thing.” says Gourav Khullar, a PhD student in Astronomy and Astrophysics at the University of Chicago. He talks about it as if he’s reading poetry, describing a rainbow, or a flower blooming. And we’ll find out soon why the death of a star is so fascinating.
Stars meet their ends in many different ways. Some small stars like our Sun become white dwarfs, while bigger stars become neutron stars and giant stars become black holes. To understand these processes better, let’s get into some details, shall we?
Stars get their energy from nuclear fusion. In that process 2 hydrogen atoms turn into one helium atom and release a lot of energy in the process, like how nuclear bombs work. The energy released by an atomic bomb, however, is just a tiny fraction of that produced in the sun every second.
The energy produced by these nuclear reactions create a constant outward radiation pressure, while the dense core of the star creates a strong gravitational pull, balancing the outward pressure and keeping the star from disintegrating.
To understand this better, think of a point to which a few springs are attached. A fly is attached to each spring. If there were no springs, there would be nothing to keep the flies around the center, but, if the springs were too short and too tight, all the flies would collapse to the point in the middle. The point represents the star’s center and the flies represent the gasses, while the springs work like gravity.
High gravity in the core and tight springs smoosh the flies together.
The gravity of the core and the energy of fusion, work like normal springs and flying flies …pure balance.
No core gravity to hold the star together, no springs to hold the flies…freedom!
Red Giants and White Dwarfs
Let’s say a star has used up all of its fuel, then there is no more fusion taking place and the gas starts to cool down and molecules become slower, leaving very little radiation energy to support the now overpowering gravity. So gravity takes over …flies lose their flight energy while the springs get tighter. However, As the core collapses, gas molecules gain energy gradually increasing the temperature of the star. Surprise! The same star that had no energy left, thanks to the strong gravity, now has a high enough temperature in the middle to help it burn up some of the leftover material.
This process stops the core from collapsing since the star is producing more outward energy. The energy produced by these reactions is so great that it starts affecting the outer layers of the star that are loosely bound to the massive core, causing them to burn and push out and making the star swell. The energy makes the flies move away from the center since they’re all motivated to get away from that terrible heat in the middle. Think of flies on steroids, strong enough to stretch out of the central point. If the central point were the size of a grape, it could puff out to the height of a basketball player or even to the width of a basketball court. This dying star is called a red giant. When our Sun becomes a red giant, it will swell to around the point where it eats up Mercury and Venus and probably Earth.
After a while, the star turns all its helium into carbon in the core and there is nothing more to stop its gravity. The core starts to collapse and shrinks to the size of a planet, imagine a mass comparable to the Sun squeezed into a space equal to that occupied by Earth! Burning hot and white, the dead star is called a white dwarf. So, do we get to have a shining white object with a heart of diamond? Yes, in some cases like V886 Centauri– yes, that’s its name, you should be thankful for your name, scientists later nicknamed it Lucy after Beatles Lucy in the sky with diamonds- the white dwarf shapes crystallized carbon, the same structure as diamond.
Sirius B the tiny star on the left corner is a white dwarf. (NASA)
Our flies are pretty tired, just hanging out around the core of the old star, remember, no energy to fly out and strong gravity of the springs to pull them in. That’s what happens to any star 1.5 times the size of our sun or smaller. But what happens to the more massive stars?
Neutron Stars
These superstars have a much more dramatic death. When a big star runs out of fuel and reaches the white dwarf stage, it doesn’t stop the fusion like smaller stars. The star is massive! The outer layers compress the core which has other leftover elements. This high pressure and heat because the leftover elements to start burning again. The core gets hotter and eventually, the pressure gets so high that the particles of the atom fall into its core, causing the atom to occupy way less space than before.
This sudden collapse in size releases crazy amounts of energy and causes the star to become a supernova. The explosion is so bright that it outshines its own parent galaxy. A supernova is an amazing, colorful, hot cloud of gas, the remains of a massive explosion. At the heart of a supernova is a dense core made of neutrons, known as a neutron star. Just a core with densely squashed flies around it. A tablespoon of a neutron star could have the same mass as a mountain!
A neutron star (pulsar) in the heart of the Crab Nebula, in x-ray light. (NASA)
Black Holes
Finally, if the remnant of a supernova is greater than 3 times the mass of our sun or more the star collapses even further and become a black hole. They are so massive that nothing can stop them from collapsing. They are limitless gravity, falling deeper and deeper inside. The springs are so tight that they take all the flies into infinity with them. A black hole has such strong gravity that not even light can escape it. The collapsed star is now so dense that it is called singularity point. Try to think of a point in which there is infinite density, mass, and flies…Could anyone imagine it?
Artist (me) rendition of a black hole. (not NASA)
Why Does It Matter?
Now we know there are many different ways that stars die, each one wonderful and breathtaking. So why should we care about the death of a star? Studying the death of the stars, the process, and the end result gives us a lot of information about the past and the future of the universe. “Our sun is also a star, and we should care about the death of our sun”, says Dr. Larry Cupick, an astronomer at the Adler Planetarium. “If we want to know the fate of our Sun we should study the death of stars. One important factor is the change in the heat output and how it changes through the life of the star.” He adds that ”The main changes in the life of stars are in their birth and the death, the adulthood is pretty stable. The end of the star could be an explosion and that explosion not only is important to study our own Sun but is also important since if a neighbouring star explodes, it could affect the environment of our solar system.”
Veil nebula, remnent of a supernova explosopn. (NASA)
Scientific Advisor: Aprajita Hajela