It’s Thread Thursday and this one was inspired by the recent #BlackInChemWeek

Stars are big fiery balls of amazing chemistry! They’re nature’s nuclear fusion reactors and of course I’m here to tell you all about that!

Cue the thread!

What makes a star a star is the point at which its core gets hot and dense enough to be able to combine/fuse hydrogen (H) atoms into helium (He). The reaction releases energy in the form of light and heat, which is what keeps stars hot and shiny!

How? The sum of the masses of individual H atoms is more than the mass of 1 He atom. So when H atoms fuse to make He, the extra mass from the individual H atoms is converted into energy. You know that famous equation, E=mc^2? Yeah this is the kind of thing it’s used for.

Energy (E) = mass (m) x the speed of light (c) squared . So you can calculate how much energy is released based on the difference in mass. Anyway we’re not here to do math but you can look up the proton-proton chain reaction if you want more details of this specific reaction.

When stars start fusing hydrogen, they’re said to be on the “Main Sequence”. So right now our Sun is on the Main Sequence and will stay there for the majority of it’s life until fusing hydrogen can no longer be its main source of energy

But this is when the fun starts!! At least for astronomers! The energy being released from fusion reactions keeps the star’s pressure high enough to prevent it from collapsing under its own gravity

So a lot of the fun things that happen in different stages of a star’s life are governed by this battle of pressure and gravity

So now that the star can’t generate energy by fusing hydrogen, it’s left with a core full of helium that starts to contract because gravity is winning the pressure-gravity battle. But as the helium atoms get closer together, the core gets denser and hotter until..

..the core gets hot enough to fuse helium atoms into carbon and oxygen! Remember when I told you most of the elements in your body are made in stars? Yeah this is where all the Carbon in your protein molecules came from. And oh yeah, the oxygen you’re breathing right now too!

Stars up to 8x the mass of the sun sadly end their life once they can no longer fuse helium. But their death is a beautiful planetary nebula - the outer layers of the star being shed and exposing the carbon and oxygen core (a white dwarf)

Since the white dwarf isn’t generating energy anymore, there’s no heat or light coming from it and it just slowly cools down as time goes on, eventually becoming a cold, dense ball filled with memories of what was once an amazing star

But the big boys?!?! Ohhh the stars that are more than 8x the Sun’s mass - they go out with a BANG! Literally.

If you get this punny GIF reference, you’re a real one

But we have some more stages to go before we get to the explosions. So once these big guys can’t fuse helium anymore, they go through the same core contraction until the core gets hot enough to fuse Carbon and Oxygen!! Oh yeaahhh we’re back in business baybeeee!

Carbon fuses with He to make Oxygen (O) and O fuses with itself to make Silicon (Si)

Once you get to Si, you just keep fusing with He all the way to iron (Fe)! So Si has an atomic mass of 32, He’s is 4 and Fe’s is 56! So just gotta keep adding 4 to 32 until you get to 56!!

Sounds like a lot but this stage is actually super short compared to the lifetime of a star. Stars big enough to fuse elements heavier than Helium live for millions of years. Here’s an example to compare the length of the different fusion stages for a star 25x the mass of the Sun

Hydrogen Helium - 7 million years
Helium Carbon - 700k years
Carbon Oxygen 600 years
Oxygen Silicon - 6 months!
Si Fe - only 1 day!!!

This is a fairly simplified version of all the possible reactions in stars but otherwise we’d be here all day

Because Iron is the most stable element, unlike the elements before, fusing it actually takes energy rather than releases energy. So at this point the star is stuck because how you gon ask me for energy when I need energy?!

Then BOOM!! SUPERNOVA!! EXPLOSION SO PRETTY AND BRIGHT WOW!!

Honestly, supernovae are amazing. They are bright enough to outshine their ENTIRE GALAXY, they’re gorgeous and they nourish the universe with all the yummy elements made in the star throughout its life! Behold

After the explosion, an inert Fe core is left behind. So the core starts to contract again but it’s so heavy that the Fe atoms are pushed ridiculously close together. Almost as close as people on the London Underground (pre-corona)

But the neutrons know they don’t like neutrons from the other Fe atoms in their personal space so to keep their distance and exert an outward pressure that keeps the core from collapsing completely. Unless..

Unless that core is more than 3x the mass of the Sun then the neutrons don’t have the luxury of social distancing anymore. And now there’s no force in the universe strong enough to hold up the core

Gravity wins and that’s when the core collapses into *whispers* a black hole

Not such a bad ending after all, right?

Mannn I freaking love stars!! Like how do you get to be a shiny ball of fabulousness throughout your life, have a beautiful death AND turn into arguably the most mysterious object in the universe?!

Imagine someone reading your eulogy like “After the most breathtaking explosion, she become a true enigma, a black hole so full of splendour”

Yes I made that rhyme on purpose because I’m low key a poet too

Anyways I hope you enjoyed this thread and I’m so excited right now thinking about redoing it but focusing more on the physics behind how these reactions & the gravity/pressure battle cause stars to become red giants, supernovae & other interesting late-stage phenomena. Yes?