Fears about how much time humanity has left on Earth have been heightened by concerns about climate change but research into the realms beyond our planet offers a more definitive end date: the death of the Sun.
The Sun is about 4.6 billion years old, according to estimates based on the age of other objects in the Solar System that formed at around the same time. To put that in perspective, if the age of the Earth (more than 4.5 billion years) were compressed into a 24-hour day, “humans would occupy only the last second, at most”, says Live Science.
And, based on observations of other stars, astronomers predict that the Sun will reach the end of its life in about another ten billion years.
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But how will that happen and will humans be around to see it all?
When will the Sun start to die?
Scientists believe the Sun probably has enough hydrogen fuelling nuclear fusion in its core to allow it to continue shining as it has done for about another five billion years.
Eventually, almost all of the hydrogen in the Sun’s core will have fused into helium. At that point, the Sun won’t be able to generate as much energy, and will start to collapse under its own weight.
What will happen then?
When the Sun starts to die, it will get bigger and slightly colder, turning into what astronomers call a red giant.
“A shell around the core will quickly heat up and start fusing hydrogen of its own, which is the beginning of the red giant phase,” Michael Kirk, a research scientist at Nasa’s Goddard Space Flight Center, told VOA’s Science World.
“As this phase progresses, the core will continue to heat and eventually begin to fuse helium into carbon producing a significant amount of heat and puffing up the outer layers even more.”
Scientific research also suggests the red giant will continue to expand and grow larger until it engulfs Mercury and Venus, and possibly Earth too.
How long will the process take?
Scientists believe the entire process that includes the initial collapse, hydrogen shell burning, and the red giant phase will take between one billion and 1.5 billion years.
After the Sun has exhausted its available fuel (hydrogen and helium) in its core and in a shell around the core, it will collapse in on itself. The outer layers will “bounce off of the denser core and expand into a planetary nebula”, according to Nasa’s Kirk.
A study published in the journal Nature Astronomy suggests this planetary nebula will become brightly illuminated for about 10,000 years, by what would then be the Sun’s remaining tiny compacted core, known as a white dwarf.
When the Sun is a white dwarf, most of the solar system will still be around. Mercury, Venus and possibly Earth will be gone, but Mars, Jupiter, Saturn, Uranus and Neptune will survive and continue to orbit the Sun.
As a white dwarf, the Sun will no longer produce energy through nuclear fusion, but it will contain tremendous amounts of stored heat, in a very small volume, say scientists. Indeed, most of the mass of the Sun will be confined to a volume not much larger than the Earth.
As such, it will cool slowly. It will take many more billions of years for it to cool from an initial temperature of hundreds of thousands of degrees to its present-day temperature and below. But in the end, the remnant of the Sun will slowly fade from sight, becoming a so-called brown dwarf - a cooling, dead remnant of a star.
What will happen to humans in this time?
Scientists believe that - climate change aside - humanity has about one billion years left, as the Sun is increasing in brightness by about 10% every billion years.
The habitable zone, where liquid water and therefore humans can exist on a planet’s surface, is currently between about 0.95 and 1.37 times the radius of the Earth’s orbit.
That zone will continue to move outward as the Sun increases its brightness. By the time the Sun is ready to become a red giant, “Mars will have been inside the zone for quite some time”, says Live Science.
Meanwhile, Earth “will be baking and turning into a steam bath of a planet, with its oceans evaporating and breaking down into hydrogen and oxygen”, the site adds.
Unsurprisingly, this will make Earth uninhabitable for humans.
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