Going Nuclear: How Stars Die

“After all, the universe required ten billion years of evolution before life was even possible; the evolution of the stars and the evolving of new chemical elements in the nuclear furnaces of the stars were indispensable prerequisites for the generation of life.” -John Polkinghorne

There are close to a whopping 10²⁸ atoms in your body. And while just over half of them are hydrogen atoms, all the rest of them — from Lithium to Uranium — were made inside of stars, and ejected back out into the Universe, where, billions of years later, they made you.

(Image credit: Ed Uthman.)\

In fact, a great many of these atoms didn’t come from just anywhere, they came from a supernova! Last week, I told you what makes a supernova so super, and now it’s time to tell you the story of just how this happens.

Our story begins all the way back when the earliest elements of the Universe — Hydrogen and Helium — were drawn together into massive clumps by the irresistible force of gravity, and formed the first stars.

(Image credit: NASA/JPL-Caltech/L. Allen and the IRAC GTO Team.)

Whenever you form stars in this Universe, they come in a huge, wide range of masses. A few percent of the stars that we form are similar to our Sun, a G-type star, but the vast majority of stars are lower in mass, cooler, and redder in color. Somewhere between 90-95% of stars come in the two coolest, reddest varieties: K- and M-class stars.

Regardless of what type of star you are, you get your energy from the same source: nuclear power! Overall, stars take the nucleus of four hydrogen atoms (which are just protons) and fuse them together to produce a helium nucleus, which contains two protons and two neutrons.

Now, I can hear some of you out there objecting to this. And it’s a very clever objection.

(Image credit: annehchem.)

Neutrons are more massive than protons, so how can you turn four protons into two protons and two neutrons?

Except you aren’t making two free protons and two free neutrons, you are making one bound helium nucleus. And if you put four protons on one side of a scale, and one helium nucleus on the other side, you would find that the protons are heavier than the helium nucleus by a little bit: about 0.7%.

So as time goes on in your star, so long as the temperature in the core is hot enough, and there’s enough hydrogen in there to fuse into helium, you keep on burning through your nuclear fuel. And that 0.7% mass difference between the hydrogen you started with and the helium you wind up with? That gets released as energy, from your old friend, E = mc².

(Image credit: retrieved from planetfacts.org.)

And all the stars, from the ultra-massive but equally rare O-stars (less than 0.1% of all stars!) to the numerous, tiny M-stars, burn their hydrogen into helium for their fuel.

But they don’t all do it at the same rate. The oldest M-star in the Universe has still not finished converting all of its hydrogen into helium, but an O-star can burn through all of the hydrogen in its core in under one million years. With the exception of M-stars, which will never be hot enough to reach the next stage, all the other star types — including our own Sun a few billion years in the future — will expand into a red giant, burning hydrogen in a shell around its helium core.

After a little while, the temperatures, pressures and densities inside the helium core become high enough that the helium atoms begin to fuse, turning every three helium atoms into a carbon atom.

(Image credit: Prof. Chang at National Tsing Hua University.)

Since carbon is more stable than helium, even more energy is given off via E = mc². If you’re a K-star, this is the end of the line for you. When you run out of helium fuel in your new core, you blow off your outer layers into a planetary nebula, while your core contracts over time, producing a white dwarf star that’s comparable in mass to the Sun, but comparable in physical size to the Earth.

(Image credit: NASA, ESA and the Hubble Heritage Team STScI/AURA.)

But the more massive stars continue to burn the heavier atoms into things like Oxygen and Neon, where the innermost layers fuse atoms into progressively heavier elements. But it’s very hard to get elements that have high atomic numbers on the periodic table. Even a bright, massive, blue A-star will only fuse atoms up to Silicon and Sulfur, the fourteenth and sixteenth elements. (And only about 1% of stars are massive enough to be A-stars!) You can even be as heavy as one of the dimmer B-stars, and your fate will still be to blow off your outer layers in a planetary nebula, leaving behind an Earth-sized white dwarf that’s comparable in mass, more-or-less, to the Sun.

But about one out of every 800 stars we make in this Universe is massive enough to go up beyond the element silicon, and will fuse elements all the way up to the heaviest ones we can make in stars: iron, nickel and cobalt.

(Image credit: Demetris Nicolaides.)

It’s kind of a misnomer to say that this is an “old” massive star; it may be less than 1% the current age of our Sun, which is still burning hydrogen in its core! But when the cores of these massive supergiant stars get large enough, the (mostly) iron atoms in there run into trouble.

(Image credit: David Darling.)

Up until now, we’ve been fusing lighter elements into heavier ones, releasing energy with each successive step. But now, there’s a tremendous pressure on the iron atoms at the core, but there’s nowhere for them to go to give off more energy. Unless, of course, they implode.

And for stars more massive than about eight times our Sun, that’s exactly what happens. There’s a mass limit to the core — about 1.38 times the mass of our Sun — and when we reach or exceed that, the iron atoms at the core, themselves, collapse. The destruction of this huge number of atoms — something like 10⁵⁶ iron atoms — releases a terrible amount of energy all at once! While the core collapses down to either a neutron star (something about the mass of the Sun but the size of a moderate asteroid) or a black hole, the outer layers get a rush of energy, the likes of which haven’t been around since the earliest seconds of the Big Bang.

(Image credit: NASA, ESA, P. Challis, and R. Kirshner.)

This release of energy not only blasts the outer layers across space for light-years, it makes possible the creation of all of the known elements of the periodic table. It doesn’t merely make elements up to Uranium, either, but Plutonium, Curium and even higher mass, shorter-lived elements. The only reason Uranium and Plutonium are the heaviest naturally occurring elements on Earth is because the heavier atoms have had enough time that every single one has radioactively-decayed away.

(Image credit: X-ray: NASA/CXC/Caltech/S.Kulkarni et al.; Optical: NASA/STScI/UIUC/Y.H.Chu & R.Williams et al.; IR: NASA/JPL-Caltech/R.Gehrz et al..)

So when we see a star go supernova, we are witnessing the formation of all the elements we find on Earth that are heavier than iron. And this is the only place in the Universe where it happens!

But just because you weren’t born as one of those (lucky?) stars, those one-in-800 stars that have enough mass to form a type II supernova doesn’t mean you’ll never go supernova. On the contrary, stars that have burned their fuel and contracted to become white dwarf stars get a second chance!

(Image credit: NASA and the former Constellation team.)

White dwarf stars can either accrete material from a companion star, as shown above, or could merge with another white dwarf star, as shown below.

In either case, once the total mass pressing down on the white dwarf’s atoms becomes more than the limit that the atoms themselves can withstand, the core of this dead star begins to collapse. Only, this time, the atoms in the core aren’t iron; they’re mostly carbon. It takes less than a second for the temperatures to rise past the ignition point for carbon, and as soon as it does, a runaway fusion reaction occurs!

(This amazing simulation courtesy of Jordan et al., 2007.)

This is the other main type of supernova, a type Ia supernova. In this case, the runaway fusion reaction destroys the entire white dwarf, leaving no neutron star, black hole, or anything else behind!

And that’s exactly what this star in the pinwheel galaxy did 21 million years ago!

(Image credit: Rose City Astronomers.)

If you’ve got clear skies in the early part of the night in the coming nights, now’s the time to go and look for it.

And this process — how stars die — is how all the elements in the Universe that aren’t hydrogen and helium are made. Not only that, but all the elements heavier than iron, including silver, gold, iodine, mercury, tin, lead, and uranium, had to have come from a supernova. As Carl Sagan said,

We are star stuff which has taken its destiny into its own hands.

Not just starstuff, in fact, but supernova stuff! This is the story of us all, simultaneously awe-inspiring and humbling to think of the generations of stars that lived, died, and recycled their elements, so that billions of years later, there was an Earth. This story is happening right now, in countless distant galaxies across the Universe. Don’t miss your chance to glimpse the closest one in a generation.

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