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PHY: Stars Part 2 - The death of stars


Hello everyone, David here! I'm back! It's been a while, sorry for being a little bit late with the second part of this awesome series of posts about starts, but I've been pretty occupied with studying for a bunch of stuff at school and some other things. Oh, by the way, if you are reading this and you took the August SAT, good luck with the scores! I know a lot of people who took it, so I had that in the back of my head. I know the reading section was thought to be harder than usual which scares me, as that is my weak point. Jeez, no one cares David, just get on with the post.

Will do! So, in today's post, we will talk about the death of stars! Quick things that are good to remember for this post is that mass highly influences a star's "passing", as we will see in an instant!

Wait, so do different stars die in different ways?

Yes! This is very weird, especially for someone who has just thought of stars as these flaming balls of gas. If you read my previous post, you do know that they have different characteristics, such as their weight, color (which is intrinsic to their temperature, and vice-versa), and apparent brightness. When a star dies, it all really boils down to its mass, as this factor influences the most on what happens after a star dies, and there are Stars with all sorts of different masses. Our Son, for example, has, well, 1 Solar Mass, which is about 2*10^30 Kg - a lot of mass if you ask me. There are, however, much bigger stars with much more mass. Antares, or α Scorpii, the brightest star of the Scorpion Constellation, has a mass of 15.5 Solar Masses, which is, you guessed it, 15.5 times the mass of our own Sun! Yes, it's a dying star, but it still shows how small even the biggest objects of reference in our system can really be when compared to others. The picture below should make it more clear how humble our Sun truly is compared to some other Stars:

Our Sun is the little dot to which the arrow is pointing

Crazy, ain't it? Okay, I think that's enough mass-talk. I said stars die in different ways, but just how many ends are there? Generally, there are 3 ways a star can end. I will not include stars called Brown Dwarfs, as they are in the low end of the mass ruler. Their mass is so low that they don't have enough pressure in the core to ignite the Thermonuclear fusion. Just for comparison sakes, they are usually the size of very big planets, such as Jupiter, and don't become anything remarkable after they die. Although these 3 groups of stars do become 3 different things after they die, they all die in the same way. How is that?

How do stars actually die?

The answer is simple but interesting: they run out of fuel. Literally.

Remember when we said that stars consume Hydrogen to power their Thermonuclear reactions? Well, a very important thing to remember is that the Hydrogen used up becomes Helium after the fusion. When a star starts running out of Hydrogen, it officially starts to die, but stars of different masses do so in different ways. In a general sense, however, the star's core starts to shrink, some crazy reactions - which depends on the star's mass - occur in the core (with even higher pressure), and, eventually, the outer layer is gone. It can go in different ways, again depending on the mass, but it's either a peaceful and slow fade away or a violent and sudden explosion, which we call Supernovas. More on that later.

Tug-of-war of forces acting on a star: Gravity and Fusion outward forces

That is, in a very general sense, how stars die. Let's take a look at the more specific events related to stars of different masses!

Average Stars: Less than 8 times the mass of the Sun

The first "mass-group" we'll talk about is the Average Star. We consider Average Stars to have less than 8 times the mass of our own Sun, as stars on this range usually go through the same death process.

As we talked about earlier, these stars die by running out of fuel. In this case, however, after the core starts to shrink, the pressure raised starts expanding the outer layers of the star, slowly but surely increasing its radius and thus making it bigger and bigger. When this happens, the star becomes a Red Giant. The core, with increasing pressures as time goes by, now has sufficient pressure to make fusion of Helium into Carbon and, sometimes, Oxygen, but nothing further than that. Eventually, when the core is composed of mainly these two elements (Carbon and Oxygen), the pressure in the core is so big that it pushes the outer gas layers out completely, leaving the planet-sized core hot and exposed, which will slowly cool down – as it doesn’t have material to generate fusion - until it is dark and cold. This stage is called a White Dwarf, and around it, formed by the pushed out gas, a nebula is formed, called a Planetary Nebula. The name is counter-intuitive, as in no means the nebula is formed by a planet, and received its name due to confusion when they were discovered. Despite being the deathbeds of previously active, fusion-enabling stars, these nebulae are truly beautiful. Don’t take my word for it, see it yourself!

Cat's Eye Nebula

NGC 6326

This is, in some billion years from now, what will happen to that bright, life-giving star we call the Sun! Yes, it will become a white dwarf engulfed in a beautiful nebula! And yes, it will inevitably expand and engulf Mercury, Venus, and quite possibly Earth in its gazillion degrees of plasma, but that’s something our grandgrandgrand…grandchildren need to worry about, if we’re still around, that is.

High Mass Stars: Between 8 and roughly 20 times the mass of the Sun

When a star has about 8 to 20 times the mass of our Sun, its death is somewhat of a… well, one could say firework.

Again, these stars die by running out of fuel, i.e, Hydrogen. Just like the Average Stars, the core of these higher mass stars begins to shrink, and the outer gas layer expands, forming a Red Supergiant. Notice that it is not a Red Giant, but rather a Red Supergiant, as the star has more mass therefore is capable of expanding its radius even more than the Average Stars.

There is, however, a big difference when it comes to what happens in this now shrunk core: in this case, due to the enormous pressure in the core, the Helium is fused into Carbon, which is fused into Oxygen, which is fused into Neon, which is fused into Silicon and then finally into Iron. Each of these metals is heavier and heavier, and lay deeper and deeper into the core, where they can have enough pressure to undergo fusion. Iron, however, is so stable that it can not fuse. When a star accumulates enough Iron in its core, no energy is emitted anymore (as there is no more fusion). The following events happen in matters of seconds: because there is no outward pressure, the immense force of Gravity is left by itself, which collapses the whole star into itself. This force is so big that electrons are literally forced into neutrons, and the energy released from this – which, again, happens in a matter of seconds – creates a huge explosion, called a Supernova.

Supernova

Supernovae are among the most energetic, powerful events in the whole universe. When a star goes supernova, the explosion generates more light than the whole galaxy combined! If a Supernova were to occur in the Milky Way, anywhere, really, the explosion would most likely be visible with the naked eye here on Earth.

As I said, electrons are forced into protons creating neutrons, which constitutes the star remnant of a Supernova: a Neutron Star. It is nothing more than an incredible number of Neutrons packed together forming a 26ish kilometer diameter enormous “atom”. These stars are incredibly dense, as they contain all of the mass from the previous stage’s core. For example, a teaspoon of a Neutron Star would weigh 10 MILLION TONS… pretty heavy, huh? There is, however, something that has an even bigger density than a Neutron Star that is formed after a star of even more mass dies. Let’s look into it!

High Mass Stars: Over roughly 20 times the mass of the Sun

When a star with over 20 times the mass of the Sun dies, the process is very similar to what we just saw: Hydrogen runs out, core shrinks, Helium undergoes fusion and its products form Carbon, Oxygen, Neon, Silicon, and Iron. Iron can’t fuse, so the star stops outputting energy, which prompts gravity to make the star collapse on itself.

The big difference, however, is what comes next: on these stars, there is so, so much mass that when gravity compresses everything, electrons are pushed into Neutrons, which all get together. Now, if this were a high mass star, we would have a big explosion that we call a Supernova and a remnant Neutron Star. However, when this remnant star exceeds 3 times the mass of our own Sun (this, by the way, is INCREDIBLY DENSE – remember, all this mass is condensed into the size of Manhattan!), not even the degrading pressure of the neutrons are able to fight Gravity, so all this mass collapses even further into one single point of infinite density. This new object is so dense that not even light can escape its gravitational pull. This new object, ladies and gentlemen, is called a Black Hole.

Black Hole. In this depiction, you can even see Hawking's Radiation

Black Holes are the most massive objects of the universe and are hard to see. We have, however, been able to take a picture of one! Last year, a team of scientists, using a virtual telescope the size of Earth – which is done by combining many telescopes around the globe – were able to process data acquired into a final image of a Black Hole in a galaxy far away from ours. This was a huge scientific breakthrough, as it once and for all truly proved the existence of such object, which was for a long time a puzzling aspect of Astronomy, as scientists didn’t know for sure if Black Holes actually existed or were just a very intricate theoretical but practically impossible object.

The first image of a Black Hole. You are looking at an actual Black Hole. How crazy is that?

Well everyone, this is it for today’s post! These are, overall, the ways and fates a star can have when it dies. The next – and last – post of this series will be about some other curiosities and aspects of stars. I expect to use a lot of formulae on it as I will take the opportunity of writing that post to also study for Astronomy. Can’t miss this opportunity, sorry lol

Alrighty guys! I will see you all in the next post! Stay safe! Peace!

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