The Star at the End of Time (Script) (Patreon)

If we, or any conscious being is around to witness the very distant future our galaxy, what will they see? How long will life persist as the stars begin to die? 

For the sake of argument, let’s say that humanity survives the several ends of the world that await us.  We somehow persist the gradual heating of our Sun and the evaporation of our oceans. Our descendants cling to existence through countless generations we watch the Andromeda galaxy merge with the Milky Way, forming a vast elliptical galaxy. We seek refuge in the outer solar system as the Sun finally expands into a red giant – twice! And finally our heirs or successors find new homes among the stars after the Sun’s final death and transformation into a dim white dwarf.

We covered all of these catastrophes in past episodes. But what’s next? How long can life survive into the far future? An absolute requirement for the continued existence of life is energy – or more accurately a persistent energy gradient, as we’ve also discussed recently. For life to stave off rising entropy and decay, energy must flow. And the deepest wells of accessible energy in this universe are stars. When the last star blinks out, life must soon follow. To know the future of life we must understand the lifecycles of the longest-lived stars in the universe. That would be the red dwarf. Don’t be scornful of this little star – they have a very VERY bright future, and may even spawn a renaissance of life, trillions of years from now.

So let’s talk stellar astrophysics. Stars generate energy fusing hydrogen into helium in their cores. The Sun burns through 600 billion kilograms every second, generates 4x10^26 Watts or around the energy equivalent of 20 million times Earth’s nuclear arsenal every second. This rate will only increase as the core’s temperature increases, and so the Sun will burn through the hydrogen supply in its core in 5 billion years. Because the rate of fusion depends very sensitively on temperature, more massive stars, with their hotter cores, burn through their fuel much MUCH more quickly. The most massive stars live only a few million years.

And the relationship goes both ways. Stars less massive than the Sun burn through their fuel much more slowly.  This is all astro-101, so let’s get a little crunchy and figure out just how long red dwarfs live. We observe that a red dwarf with 10% of the Sun’s mass is about 1000 times fainter than the Sun. That means it’s burning through its fuel 1000 times less quickly. But it also has less fuel to burn, right?

Actually, wrong. Stars like our Sun can only burn the hydrogen in their cores. The layer above the Sun’s core is what we call radiative – all of the energy travels in the form of photons bouncing their way upwards. Closer to the surface, the Sun becomes convective – energy is transported in giant convection flows, rising to the surface and sinking again. That radiation zone isolates the Sun’s core, preventing new material from reaching those depths. As a result, the Sun will only have access to 10% of its mass for fusion fuel.

But red dwarfs are entirely convective.  Rivers of plasma flow from the core to the surface, carrying both energy AND the helium produced in the fusion reactions. That helium gets mixed through the star, while new hydrogen is brought to the core for fusion. Over the course of its long live a red dwarf will convert all of its hydrogen to helium. A red dwarf with 10% the sun’s mass has just as much fuel to burn as the sun does, yet it burns 1000 times slower. That means it should live 1000 times longer – so 10 trillion years instead of 10 billion.

That 10 trillion years assumes our red dwarf keeps burning at the same old rate. It doesn’t. Just like the Sun, the cores of red dwarf stars shrink and heat up over time. The heating core causes red dwarf fusion rates to increase by a factor of 10 or more, particularly towards the ends of their lives. That shortens their lifespans, but we’re still talking trillions of years. An interesting thing about red dwarfs is that they don’t expand as they brighten, unlike more massive stars. If you increase the energy output but keep the size the same then you necessarily increase the surface temperature of the star.

This is because the light produced by stars comes from the heat-glow of their surfaces. This is thermal or blackbody radiation, and it obeys a couple of very strict laws. First, the hotter something is, the more thermal photons it produces. So increasing the surface temperature allows the red dwarf to shed all of those excess photons produced by its rising fusion rate.  And rule two: the hotter something is the more energetic its individual thermal photons. The blackbody spectrum of a hot object emits relatively more photons at short, energetic wavelengths than a cooler objects. For most of its life, the spectrum of a red dwarf peaks at infrared wavelengths. To us they appear red because they’re producing more red light than yellow, green, blue, etc.

But as these stars heat up their spectrum shifts. First they shine white as their blackbody spectrum spans the visible range, just like our Sun. In the final few billion years of their lives some red dwarfs may even become hotter than our Sun, developing a faint blue tinge.  Finally, with the last hydrogen fuel spent, the entire star will be composed of helium and will quietly contract into a helium white dwarf, supported by quantum-mechanical degeneracy pressure. It will slowly radiate away its internal heat for another several billion years before turning black.

So what does all this mean for the future of our galaxy, and for any life that exists then? Long before the first red dwarfs approach the ends of their lives there will be no other living stars left in the galaxy. Many new Sun-like stars will be born in the Milky Way-Andromeda collision, 2 and a half billion years from now. They will all have expired, leaving white dwarfs, and those white dwarfs will have faded long before the first red dwarf passes away. At that point the night sky will be dark, and only a powerful telescope could reveal the trillion faint, red dots scattered across the sky. As these brighten, the most massive will shine brighter than the current Sun. Individual points of white light will appear in the night sky, shining for up to a few billion years at a time before winking out.

That dark future is inevitable, but for several trillion years red dwarfs will be the last warm places in the universe. That’s an awfully long time. These will surely be the places our own star-faring descendants will wait out eternity. But what about new life? We mow we know that red dwarfs DO have planetary systems – just look at Trappist-1, with its 7 terrestrial world111s, two of which are at the right distance from the star to have liquid water. We don’t know yet whether life can evolve around red dwarf stars – they’re violently active when they’re young, but perhaps ancient red dwarfs will have the stability needed for new life to take hold.

This may be especially true right near the end. Red dwarfs in the middle range of mass - around 15% the Sun’s mass - are predicted to enter a period of relative constant brightness right at the ends of their lives. This period could last for up to 5 billion years, during which the star will shine almost as bright as the Sun, and quite a bit hotter. Those stars will have long-frozen worlds in the outer parts of their solar systems. Those planets will thaw as their star brightens, and may enjoy billions of years of stable warmth. So could life begin from scratch in a trillion years time right as the red dwarfs begin to die? It’s very possible that most of the life in the universe is yet to evolve. Perhaps the descendants of humanity – or of some other pre-merger species from the old Milky Way – will be there to witness this.  One last, long renaissance of life, as we huddle in the warmth of last stars to burn in the darkening end of spacetime.