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The end of the solar system

OUR star is not destined to explode as a supernova, hurling its planets into space. It’s just not massive enough. But when it finally burns through its supply of hydrogen some 6 billion years from now, the great sphere of hot plasma at the centre of our solar system will grow so spectacularly bloated and bright that it will transform our cosmic neighbourhood forever.

Like most stars, the sun is a main sequence star: in its core, nuclear fusion generates energy by converting hydrogen to helium. Once all the hydrogen there has been consumed, a layer of hydrogen around the core will ignite, and the extra heat produced will overcome the gravity that was keeping the sun from ballooning.

The result is a red giant: a swollen sun, thousands of times more luminous than it is now, whose outer layers will engulf the innermost planets. At full splendour, its radius will extend a little further than Earth’s current orbit.

And yet our little blue marble may yet escape. As the sun swells, it will lose up to a third of its mass to a great outward wind of charged particles. With that will go some of its gravitational pull, allowing the comets, asteroids and planets held in its sway to migrate to wider orbits.

For the innermost planets, it’s a race against time. “Mercury, Venus and Earth effectively will each try to outrun the sun as it becomes larger,” says of the University of Warwick in Coventry, UK. Mercury and then Venus will almost certainly lose, each being engulfed in the sun’s inflated atmosphere and torn apart by tidal forces.

The fate of Earth is less certain. As the planet drifts away, it will be hauled back in by tides from the sun’s outer layers. “The case is too close to call,” Veras says. Still, any life clinging on would be in trouble: the very tides tugging Earth inwards will cook its interior, giving rise to volcanic eruptions worldwide (see “The end of life on Earth“).

All the planets beyond Earth should survive, but their atmospheres will be transformed or boiled off. Our supercharged sun will even cause havoc in the asteroid belt, says Veras. When sunlight strikes asteroids they spin faster and faster, and many will centrifuge themselves into smithereens. The Oort cloud, a vast population of icy objects loosely bound at the farthest margins of the solar system, will quietly drift away into interstellar space.

There is a silver lining: the puffy old sun will be so luminous that the chilly outer regions of the solar system, including the Kuiper belt where Pluto resides, may become hospitable to life. But the opportunity will be fleeting.

After 800 million years as an inflated red giant, the sun will shrink to roughly 11 times its current size, then briefly swell again. Finally, its atmosphere will blow away to leave a glowing core: a white dwarf. The stellar embers will cool and eventually crystallise, leaving the Kuiper belt once again out in the cold. Joshua Sokol

The end of the Milky Way

FOR a tiny smear of light drifting in a sea of darkness, the Milky Way seems stable enough, and indeed it has been around almost as long as the universe itself. But just as gravity created the galaxy we call home, so it has sealed its fate: a slow-dance death spiral with the nearby Andromeda galaxy.

Andromeda, also known as the spiral galaxy M31, is heading straight for us at about 110 kilometres per second. The good news is that, being more than 2.5 million light years away, it won’t collide with the Milky Way for another 4 billion years.

Astronomers have known about Andromeda’s approach for the best part of a century, but measurements of its trajectory weren’t precise enough to tell whether our galaxy would get winged or truly clobbered. That debate is now settled. “Our measurement implies that the encounter will be a head-on collision,” says at Johns Hopkins University in Baltimore, Maryland, who has tracked Andromeda’s motion in 3D using data from the Hubble telescope.

The collision itself will play out over 2.5 billion years. Andromeda will at first loom ever brighter in the night sky. Then, as hundreds of billions of stars, vast gas clouds and swathes of dark matter from the two galaxies swirl and smash, new star-forming regions will ignite, each lasting for millennia.

The galaxies will pass through each other a number of times as they merge into a new mega galaxy, sometimes called Milkomeda. But stars and planets are unlikely to crash into one another, says Sohn. The average distance between stars in the Milky Way is 4 light years, which leaves plenty of space for Andromeda’s stars and planets to pass through unscathed. Believe it or not, the initial collision is likely to leave our solar system alone – although near misses could distort gravity, disrupting planetary orbits, says Sohn.

When the churning is done, Milkomeda will probably settle down as an elliptical galaxy – a giant ball of diffuse light in the night sky. The galactic merger will be complete, leaving a slightly larger smear of light in the endless dark. MacGregor Campbell

The end of the universe

SLURP, crunch, rip or freeze? How the grand finale of everything pans out depends on the enigmatic nature of dark energy. Or perhaps not…

Big freeze
In the 1990s, observations of distant supernovae indicated that the universe’s expansion has been gathering pace over the past few billion years. “Dark energy” is held responsible for that, but no one knows what it is. If it is unchanging, as most cosmologists assume, the cosmic ballooning will continue unabated and the universe will eventually become so thinly stretched that no galaxy, star or even particle is in contact with or even in sight of another. No new stars will form, and existing ones will burn out. As its temperature drops ever closer to absolute zero, this flaccid universe will go out in a cold, dark whimper.

Big rip
Dark energy has made its presence felt only in the past few billion years, so it might be growing stronger over time. If so, the universe is bound for a more dramatic fate than a freeze. A surging dark energy would slowly tear apart galaxies and stars, and eventually space-time itself. Recent calculations indicate that the earliest this “big rip” could happen is 2.8 billion years from now, well before our sun is due to burn out. Most cosmologists think the solar system is safe, however. A big rip, if it ever happens, is most likely to be tens of billions of years in the future.

Big crunch
If dark energy should for any reason weaken – perhaps even turn negative – gravity would finally prevail over its repellent phantom nemesis. The universe would crank into reverse gear, and begin shrinking again, right down to the same sort of pinprick of infinite density in which it started. The big bang universe will be bookended with a big crunch. Although that would be bad news for anything in the cosmos, it might not be bad for the cosmos itself: some models suggest it could rebound in a “big bounce” that would create another universe, starting the cycle all over again.

Big slurp
There’s a disturbing, and disturbingly possible, alternative endgame. The Higgs boson is the particle that gives other fundamental particles mass, and so is in some sense a guarantor of the universe’s stability. But the Higgs boson discovered at CERN in 2012 is strangely light, suggesting that the universe it builds is an unstable “false vacuum” state, teetering on the brink of ruin. A quantum fluctuation could at any moment conjure up a bubble of true vacuum. In that case the universe would eat itself from the inside out at the speed of light – faster than we would ever know. Daniel Cossins

Read more about how everything you care about will end – and what comes after

This article appeared in print under the headlines “The end of the solar system”, “The end of the Milky Way” and “The end of the universe”