The Star at the End of Time

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Matt O'Dowd (1:03)

If our descendants or any conscious being is around to witness the very distant future of 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 world that await us. We somehow persist through the gradual heating of our sun and the evaporation of our oceans. Our descendants cling to existence through the countless generations as 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 the universe are stars. When the last star blinks out, life must soon follow.

To know the future of life, we must understand the life cycles of the longest lived stars in the universe. That would be the red dwarf. And don't be scornful of this little star. They have very, very bright futures 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 of hydrogen every second, generating 4 by 10 to the power of 26 watts, or around the energy equivalent of 20 million times the Earth's entire nuclear arsenal every second. This rate will only increase as the core's temperature increases, and 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 the lifespan of red dwarf stars, also known as m dwarfs. We observe that a red dwarf with 10% of the sun's mass is about a thousand times fainter than the sun. That means it's burning through its fuel a thousand 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 life, a red dwarf will convert all of its hydrogen to helium. A red dwarf with 10% of its energy will convert all of its hydrogen to helium.