How Do We Know When the Sun Will Die?

How Do We Know When the Sun Will Die?
Illustration: Benjamin Currie

Consider the Sun: hot, massive, and the reason all of this (gestures wildly) is possible. Our local star fuels all life as we know it, providing the energy that sustains everything from the smallest photosynthesizing microorganisms to the largest animals on land and in the seas.

But someday — far, far into the future — the Sun will die. Things won’t just go dark, though. Rather, they will go very, very bright. Hot, too, unbearably so. The Sun will become unrecognizable, if there’s anyone still around to see it.

“One of the most basic questions that any conscious human has is: how did we get here, what’s the point, what does it all mean? The questions of our origins and of our future,” said Jackie Faherty, an astrophysicist at the American Museum of Natural History, in a phone call. “If you want to understand the habitable zone of our Sun, you need to know how long it’s going to be there, and how it evolves, and how it changes. It all comes down to that basic story.”

Which brings us to today’s puzzle: How much time does our life-giving Sun have left, and how do we know?

“Once you realise it’s a ball of gas, you know it’s not some infinite machine,” Faherty said. “You just have to figure out when it’s going to run out.” Calculating that timeline is a relatively simple equation, built on some complex maths and smaller realisations.

To know how much time the Sun has left — and, spoiler, it’s about 5 billion years — you need to know how old it already is. Stars don’t die unexpectedly, so knowing a star’s age is an important indicator of how fast it’s going up. In the 19th century, in the context of a feud about how old Earth was, Charles Darwin and Lord Kelvin, the astrophysicist, debated the Sun’s age. Darwin’s estimate ended up being closer; nuclear energy had not yet been discovered, and Kelvin worked under the assumption that the Sun was burning coal. It threw off his numbers a bit.

Our baseline for the Sun’s age is derived from the earliest rocks that still travel through the solar system, which are basically the rejectamenta that never got made into a planet or moon during the coalescing of the solar system. Those rocks consistently give us an age of 4.6 billion years, and scientists have been able to date them with precision using a number of techniques.

It’s also important to know the Sun’s brightness, because that tells us how energetic the star is. We’ve known how bright the Sun is ever since we’ve known how far we are from it, a measurement called the astronomical unit, or AU. (“Everything revolves around distance,” Faherty explained.) The measurement was painstakingly calculated using the parallax effect and the 1769 transit of Venus across the Sun; the famous Captain Cook even logged some observations in Tahiti.

One astronomical unit is now fixed at 149,597,870 km and is a vital measurement for discussing distances within and around our solar system. With that measurement, astronomers were able to determine the Sun’s luminosity, or brightness — before that, they weren’t sure whether the star was extremely close and incredibly dim or extremely distant and incredibly bright.

As it turns out, the Sun is bang average when it comes to stars. That was clearly displayed with one of the more important graphics in astronomical history, the Hertzsprung-Russell diagram, which mapped the brightness and colour of stars. The two astronomers for which it’s named alluded to the idea that the stars burn hydrogen in some way, and that burning is related to the star’s temperature and interior physics.

Things really came into focus when Cecilia Payne, then an astrophysics doctoral student at Harvard, scribed her thesis on the idea that the stars were mostly composed of hydrogen and helium. At the time, Russell (of diagram fame) and one of Payne’s supervisors called the numbers “impossible,” and Payne ended up discounting the idea in the thesis. But she was proven spot-on, and it was only through her work that the Hertzsprung-Russell diagram could truly be levied as a tool in astrophysics, to understand a star’s class; that is, what its physics are and what its fate will be. It’s only by putting our Sun in that stellar line-up that we get a sense of what kind of star it is and how brightly it shines amongst its peers.

“Observing other stars has allowed us to have a comprehensive theory of stellar evolution. In particular, a crucial role was related to stellar clusters (stars which are at the same distance, same composition, and only differ by mass). There it was possible to understand that stellar evolution is strictly dependent on stellar mass,” said Gianluca Pizzone, an astronomer at the International Astronomy Union, in an email.

Because we know the rate of the Sun’s nuclear fusion, we know the rate at which it is burning away its nuclear fuel. Albert Zijlstra, an astrophysicist at the University of Manchester, explained that that rate is extremely slow. “The Sun is not a bomb, it’s an extremely poor nuclear fusion reactor,” he said in a video call. “Per kilogram, it produces less energy than you do. It’s taking its time.” Easy does it, Sun. No rush.

But these ideas come together now. Knowing how old the Sun is and the rate that its fusion is occurring means that astrophysicists know how much it’s already burned. The Sun’s been burning for about 5 billion years and will burn for about 5 billion more. This is where things get interesting: “You’d expect nuclear fusion to slow down [over time] because there is less hydrogen. But that’s not possible — it’s the heat that keeps the Sun stable. The hydrogen is running out a bit, and the whole Sun convects a little bit, increasing the temperature,” Zijlstra said. (This is already happening, but there’s plenty more hydrogen to go.) But eventually, the hydrogen will run out, and the Sun will collapse inward — gravity always wins.

Our Sun isn’t big enough to produce a supernova, a gargantuan stellar explosion. Larger stars leave behind neutron stars or black holes; the Sun’s ending will be dramatic in a different way. As it burns through hydrogen, the Sun gets smaller and the layers outside of the star’s core get hotter. Fusion starts happening in a shall outside the core. The Sun becomes a red giant, a much more spread-out star that burns with less energy than before. The path to red giant takes a while, but once it becomes one, the demise is swift.

“At this time, it’d be a very bad time to move to Mercury,” Zijlstra said. “Eventually you find yourself inside the Sun.” The new, bloated Sun has claimed its first victim.

The Sun will continue to swell and destabilize. Venus gets swallowed up, too. (There’s some debate as to whether the fully inflated red giant Sun will reach Earth or not, but suffice to say things will be crispy here; at the very least, the oceans will boil away and Earth will resemble today’s Venus.) Eventually, the Sun is so diffuse that it begins to evaporate.

Just 100,000 years after becoming a red giant, it loses half its mass. At this point, the Sun is in its endgame. It’s a white dwarf, a dense stellar remnant about the size of our planet. It’s depleted of its nuclear energy at this point, and will slowly cool into a solid ball of carbon — basically a floating diamond in space.

And around that compact dwarf, the cloud of material the Sun ejected may fluoresce, a dazzling planetary nebula. But this isn’t for sure, said Zijlstra, who in 2019 co-authored a paper in Nature Astronomy on the likelihood of our Sun lighting up a nebula. For such a nebula to happen, the Sun will need to be hot enough while the cloud is still near it, and even then the cosmic light show would be a blink of an eye in stellar time: about 10,000 years. Pizzone said that nebula could look something like the halo of Messier 57, the Ring Nebula.

It’s worth keeping all this in perspective. That blink-of-an-eye nebula at our star’s terminus would last about twice as long as written human history. Well before life on Earth came about, the primordial planet was as inhospitable as it will be again. In other words, we’re not just in the right place — we’re at the right time.

“It’s really important to realise that we are very lucky to live right now, when there is this very delicate balance with the Sun’s energy output (and our Moon’s stabilizing orbit) in the present day,” said Adam Kowalski, a stellar astrophysicist at the National Solar Observatory, in an email to Gizmodo. “We don’t want to screw this balance up because so far, we’ve not found any planet around a different star that we know has this delicate balance.”

Needless to say, we’ve found ways to muck things up. This decade will define the trajectory of climate change patterns in the century to come and beyond. In an evolutionary sense, “we have only been here for a sneeze in the lifetime of the solar system,” Faherty said. “You shouldn’t think that the Earth’s going to get swallowed by the Sun and that’s how we’ll go … I’d be more concerned about our own influence changing things before we can even get to that phase.”

So, we know how and when the Sun will die and take Earth’s habitability with it. Whether any intelligent life will still be here 5 billion years from now to go down with the ship, however, is impossible to know.