The year is 5.4 billion. Earth is lifeless. What was once a pale blue dot is now a desolate brown rock.
The Sun has inflated to around 1.5 times its original size and shines brighter than ever before.
But something new is about to happen.
In just a moment, our star will fuse the last of the hydrogen in its core,
triggering a cataclysmic sequence of events.
What's about to happen? Grab your popcorn and pull up a seat. It's time to watch the death of the Sun.
Like living things, stars have a life cycle. They're born inside a cloud of gas and dust.
They live their lives fusing hydrogen to helium in their cores,
shining bright and stable for as long as that hydrogen lasts.
Then they die, each going out in its own beautiful way. How a star dies depends on its mass.
The bigger they are, the shorter their lives and the more sublime their deaths.
The most massive stars live only for hundreds of thousands to millions of years, and die in spectacular explosions called "supernovae".
That is not the fate of our sun. The Sun is now 4.6 billion years old; it's middle-aged.
Based on its mass, it's expected to have a lifespan of around 10 billion years,
and here by "life", I mean the period of core hydrogen fusion when the steady flow of energy from the core
provides consistent support against the gravitational collapse.
Well, relatively consistent. Over the course of its life, the Sun's core is heating,
causing the Sun to grow and brighten with disastrous consequences for the habitability of the Earth.
In fact, we looked at that particular catastrophe recently. It's a cheery episode; check it out.
But what are the prospects for our little planet after the Sun runs out of fuel? Let's watch and see.
As the last traces of hydrogen are slammed together into helium-4 nuclei, fusion will cease.
That helium is useless as a fuel, at least for now, because helium fusion requires temperatures of around 100 million kelvin.
At the cores last gasp, it'll be less than 20 million K, so with no outward flow of energy to fight against gravity,
that core begins to collapse, dragging the surrounding layer of hydrogen with it.
Soon, a thin shell of fresh hydrogen fuel around the dead core becomes hot enough to ignite.
Hydrogen shell burning begins.
This is not the new lease on life that we might hope for. In fact, it's catastrophic.
That burning shell is so thin that radiation escapes outwards without providing pressure to resist collapse.
Instead, the helium produced in the shell is deposited on the dead core,
causing the inner regions to collapse further.
Density rises and shell fusion spikes.
That incredible outer flow of energy does counteract gravity for the outer layers of the Sun, and those layers expand.
Over the next half billion years, the Sun doubles in size as a sub giant.
In the following half billion years, the process accelerates, and the Sun inflates to around the size of Venus's orbit,
shining with a few thousand times its current luminosity.
But that energy output is spread over an increasing surface area, and so the surface cools down.
It shifts from its current yellow-green hot through orange and finally to a deep red.
The Sun is now a red giant, and it is vast on the Earth's daytime sky.
While the star's outer layers of hydrogen are expanding and cooling, the core continues to collapse until it hits a quantum mechanical limit.
The electrons are packed so close together that they become degenerate. That means that all energy states are occupied.
They run up against the Pauli exclusion principle, the rule that says that fermions, like electrons, can't occupy the same quantum state as each other.
Be grateful for Pauli exclusion; it'll save the Sun from collapsing into a black hole, and it's also currently stopping you from falling through the floor.
The core continues to shrink slowly even as it gains mass from the shell.
It heats up to that critical 100 million kelvin temperature, then in an instant it ignites in the helium flash.
The once inert helium fuses into carbon via the triple-alpha process.
The core is resurrected with a vast new supply of fuel.
It expands again, and the corresponding lower density allows helium fusion to slow down.
The entire star dims and shrinks to around 10 times the current radius, but this new lease on life is short.
The helium burning will convert that core into carbon and oxygen in only 100 million years or so,
after which the final dance with death begins.
Finally, surrounding an inert carbon–oxygen core are two fusing shells: helium fusion inside hydrogen fusion.
The energy output is insane, and the Sun swells to a red giant once more.
This time its outer layers will reach Earth's orbit and probably then some.
So is that it? Earth gets gobbled? Actually, opinion differs on this.
See, the red giant Sun is so large and luminous that it only has a tenuous hold on its outer layers.
It's been blasting its own mass into the solar system and great winds through both red giant phases.
As it loses mass, its gravitational hold weakens, and so Earth's orbit expands,
perhaps eventually taking it out beyond Mars' orbit—if the Sun doesn't overtake it first!
But it's a race; by now, the red giant Sun fills most of the sky, and it's getting larger.
Earth is wreathed in the searing stellar winds.
On the one hand these diminish the Sun's hold on us, expanding our orbit.
But on the other hand, drag from these winds may also slow our retreat.
Earth's own gravitational pull creates a tidal bulge in the Sun that may increase its gravitational hold on us;
or the Sun may just expand too quickly and too far for us to escape.
Theoretical models are still not entirely settled, but Earth's prospects look bleak.
It may well end up in orbit inside the Sun.
There, it will revolve through the tenuous searing plasma,
as the crust melts and the entire mantle begins to vaporize.
The final red giant phase is brief—only a few tens of millions of years—but it's spectacular.
Explosions of runaway fusion in the double-burning shell blast away the outer layers,
perhaps creating a beautiful planetary nebula.
The naked core of 100 thousand kelvin to generate carbon and oxygen has collapsed to the size of the Earth.
The Sun has become a white dwarf.
This ember will continue to glow for many many billions of years before fading to black.
And the Earth: perhaps it just managed to escape the expanding Sun,
and is now a lonely desolate world orbiting out in the old location of the asteroid belt;
or perhaps it didn't quite make it. In that case, the entire planet may have vaporized.
It's also possible that the iron core survives and will continue to orbit the white dwarf Sun until the end of everything.
And what about us?
It's wildly speculative and optimistic to even imagine that our species—
or its evolutionary or silicon descendants—will still exist through all of this.
But if we—or if they—do, from where do we watch this dramatic death scene?
If we're still solar system bound then its outer reaches may harbor us for a little while.
The habitable zone—the region with the right solar flux for liquid water—
will expand beyond Neptune's orbit in the first red giant phase.
Now that's Neptune's current orbit; all the planets' orbits will have expanded,
and to the moons of Neptune, Uranus, and even Saturn may provide brief refuge.
During the helium burning phase, the habitable zone contracts again for 100 million years.
Jupiter's moons may be a final temperate vantage point to watch the Sun's inevitable death,
and to look to the greater galaxy for a new home across far reaches of Space Time.
As always, a big thank you to all of our very generous Patreon supporters.
Now, usually, we do a special shout out to one of our big Patreon supporters,
but an anonymous supporter asked to donate their shout out to Elon Musk,
so this shout out's for you, Elon. Thank you for launching a gigantic rocket and landing most of it again,
and sending your car into space to give us all hope that maybe, one day, we too will ride on rockets and red sports cars,
and perhaps safer vessels to the planets and even to the stars. Keep it up, big guy.
Last week, we asked the question, "what is energy?", and I guess it's a provocative question, because you guys had a lot to say.
"Pfhorrest" asked whether the combination of two particles has a different mass when those particles are close together vs. when they're far apart due to the potential energy between them being different.
Well, actually, yes, that's right. The property of mass is just a measure of the energy bound into the system,
whether the system be an atomic nucleus or a planet, and whatever that type of energy is.
For example, a system of two protons is more massive if the protons are closer together than far apart,
because close together they have a lot of potential energy—a lot of potential to repel each other.
After a certain point—after a threshold—they come into range of each other's strong nuclear force,
and the direction of the force turns around, so the protons fall together, and they lose an enormous amount of potential energy when that happens.
The resulting combined nucleus is lighter than the sum of the masses of the two distant protons.
You know what, let me ask you a question in return.
Does this mean that the combined system of the Earth plus you gains mass every time you jump because you gained potential energy?
"william41017" asks, succinctly, "potential relative to what?"
Well that's a great question because potential energy is relative.
It represents the energy exchange that would result from a particle or system moving between two points under the action of a conservative force.
It tells you the kinetic energy that will be gained or lost in motion between two points due to the work done by that force.
Alternatively, it's the work that will be needed to be done against that force in order to execute that motion.
But potential energy doesn't have an absolute zero point; it's relative. That said, there are conventions for those zero points.
For a falling objec, we tend to take the ground as the zero point, but we could equally make it the center of the Earth or a point infinitely far away.
The difference in potential energy due to any fall would still be the same.
A few of you criticized our use of an inferior siege engine in our animation of the catapult.
I suppose you'd have preferred some sort of device that can launch a 90 kg stone over 300 m.
You asked for it. Next episode is going to be all trebuchets.