End Of Days

In Sun by Brian Koberlein8 Comments

The Sun is getting hotter. Not significantly on the scale of human lifetimes, and not even enough to account for global warming. But on a geological scale it’s happening, and it has dire consequences for life on Earth. 

The Sun's changing luminosity, radius and temperature.

The Sun’s changing luminosity, radius and temperature.

The Sun gets hotter over time because of the gradual transformation of elements over time in the Sun’s core. As hydrogen is fused into helium, the core becomes more dense, and thus gravity squeezes it a bit more and the core gets hotter. As the core temperature continues to rise, hydrogen fusion (p-p chain) becomes more efficient, and a secondary fusion reaction known as the CNO cycle also starts to kick in. This heats the core even further. As a result the outer layer of the Sun swells, making the Sun slightly larger. But it also gets brighter, so the end result is that the Sun produces more energy as it ages.

For most of its lifetime this increase is pretty gradual. Over a hundred million years the luminosity of the Sun will increase by about 1%. But over billions of years this is significant. When life first appeared on Earth about 3.5 billion years ago, the Sun was about 75% as bright as it is now. In a couple billion years it will be about 20% brighter than it is now.

The changing habitable zone of the Sun.

The changing habitable zone of the Sun.

Because of Sun’s increase in energy over time, the habitable zone of the solar system is gradually thinning and getting further from the Sun. A few billion years ago both Earth and Mars were in a reasonable zone of habitability, but now only Earth remains. In another billion years or so even the Earth will leave the habitable zone and will likely be too hot to sustain life. This has interesting consequences for the possibility of life on other worlds. While many young stars may have planets conducive to life, as the stars age the range changes. The universe may be littered with planets that once had life, but are now barren. Planets where life survives long enough to develop civilization and technology may be rare, even if life is common throughout the universe.

But of course the days are numbered even for civilizations like ours. Either we become extinct like so many species before us, or we adapt and change, eventually leaving our Sun for greener pastures. To do otherwise is to face our end of days.


  1. Question: why is the habitable zone getting thinner? Also, in the graph, there is a sudden change at around 3.5By. Is that when the CNO cycle kicked in? In any case, the lower boundary seems to gradually increase, but the outer boundary (too cold) sees significant regressions. That doesn’t make sense, if the sun is getting hotter. What do I miss?

    1. I’m not entirely sure if that is the reasons but the amount of energy any object receives by the sun decreases by the square comparing to its distance. So while the sun is getting hotter – the inner edge of the habitable zone should be pushed outwards by a greater distance then the outer edge. And that should lead to a thinner habitable zone. I can’t think of a reason though why it would vanish to 0.

  2. That the inner edge of the Habitable Zone should expand as the Sun gradually warms up is obvious, but why does the outer edge of the Zone decrease ? Surely the whole Zone should just move outwards. How can the Sun get hotter and colder at the same time ?

  3. I’m not a physicist but my basic understanding is that Gravity is a product of mass. The Sun is converting hydrogen to helium which is a more dense element but the overall mass of the sun doesn’t change. Density doesn’t effect Gravity does it? So I don’t see why an increase in density would cause gravity to increase.

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