Death from the Skies!: These Are the Ways the World Will End... - Philip Plait (2008)
Chapter 7. The Death of the Sun
THE PLANET IS FAIR-SIZED, CLEARLY BIG ENOUGH TO sustain a healthy atmosphere, though none is currently present. Given its distance from its parent star, it could easily have held liquid water on its surface too, once, in the far distant past. The outlines of continents are visible on its surface, though difficult to make out because of the lack of contrast. Were those deep, broad basins once ocean floors?
It’s hard to tell now. The planet may have once been green, or even blue, but now it’s all browns and grays and blacks. If any liquid water—or even water vapor—once existed there, it’s long gone, evaporated a billion years before. Without an atmosphere there can be no liquid water.
The planet’s star begins to peek over the planet’s horizon. Swollen, distorted, nebulous, and very, very red, the star rises ponderously. It almost appears flat, it’s so large. But after a few minutes the gently curved nature of the limb becomes more obvious, clarifying just how big the star is. An hour later it still hasn’t fully risen, less than half of it showing above the horizon. It looms menacingly there, glaring like a bloody half-closed eye.
Finally, once the bottom limb clears the horizon, the cause of the planet’s utter stillness and sterility is obvious. The star hangs over the landscape eating up a full 30 degrees of sky, as big as a dinner plate held at arm’s length. The glowering eye of the star bears down on the planet’s surface, which begins to heat up with the day. By midafternoon, the temperature is above the melting point of rock, and the surface of the dead planet begins to glow a soft red and liquefy once again. Mountains continue their slump, and continental shelves flow slowly, blurring into the dry ocean basins.
Finally, after hours of unleashing its crippling heat, the star sets, though its distended red glow lingers for hours. The rock begins to cool a bit, and by midnight is starting to resolidify. As the sky finally turns black, low wisps of rock vapor are illuminated from below by the still-molten lava shining through cracks in the ragged surface.
In a few hours, the cycle will start again. Every day, the star is marginally bigger, marginally brighter, marginally radiating more heat on the distressed planet. In a few more millennia the rocks will heat so much during the day that there won’t be time for them to become solid again during the ever-briefer respites of night. The entire planet will become molten, erasing any possible hope of discovering its past history.
It’s a shame. The planet’s past is a rich one indeed, in its full and lively role as the third planet from the Sun. But the Sun has since started its long descent into death, and the past of Planet Earth will be lost forever.
When you look at the Sun, it appears constant, stable, unchanging. But this is an illusion. Deep in its heart, an epic battle has been ongoing for billions of years, and will continue for billions more: the struggle between gravity and pressure. This war is fought with the weapons of contraction and expansion; more than anything else, the life of the Sun is defined by the balance between these two ancient foes.
Right now they are in rough equilibrium. The Sun’s gravity is able to hold it together, counteracting its internal pressure that is trying to blow it up like a bomb. This uneasy balance has existed for eons, and will be maintained for a long time to come.
But not forever.
There is an old phrase, very old: This, too, shall pass. We use the Sun to measure the length of our days and years. These time scales are natural to us. But on much, much longer time scales the Sun itself will prove to be a clock that is running down.
The constancy of the Sun is an illusion. Like any other star, the Sun was born, and it will live out its life.
And someday, it will die.
SWIFTLY FLOW THE EONS
The Sun’s inevitable death isn’t very pleasant to think about. But this eventuality will happen. An asteroid impact, a nearby supernova, a gamma-ray burst—these might happen. But they might not. The death of the Sun, however, is a rock-solid inevitability, and every second of every day brings us a tiny fraction closer to it. And one way or another, the end of the Sun also means the end of the Earth as we know it. It’s even possible that it might literally mean the end of the Earth in point of fact; the planet itself may not survive the demise of its parent star. It certainly won’t come out unscathed.
As described in chapter 3, the Sun cannot go supernova. While it is massive enough to fuse hydrogen into helium in its core as it does now, and will go on to fuse helium into carbon and oxygen, it just doesn’t have what it takes to continue the fusion chain from there. It will never make iron in its core, and so it won’t undergo the core collapse needed to power a supernova explosion.
So it will go out with a whimper and not a bang. But a whimper on the scale of a star is still a colossal event on a human scale.
Chapter 3 also gives a brief overview of what will happen to a star when it runs out of hydrogen in its core and begins to fuse helium. Some details not central to the discussion of how a star becomes a supernova were left out, but these details become critical when talking about the aging Sun. Perhaps you’ve heard that the Sun will one day turn into a red giant and engulf the inner planets . . . but saying that is like saying, “Star Wars is a movie about a kid who finds out he’s cooler than he thought and winds up saving the day.” The fun is in the details!
The Sun’s timeline is defined by the laws of physics, and these are laws that will not be defied. They play out over much longer spans of time than we’ve covered so far, billions of years. Time keeps going whether we want it to or not, and we will see these stages of the Sun’s life . . . and we’ll see its inevitable death.
THE SUN AS A NORMAL STAR
Age of the Sun: 4.6 billion years (Now + 0 years)
Right now, we can consider the Sun to be roughly middle-aged: it’s about 4.6 billion years old, and will live as a normal star for perhaps another 5 or 6 billion years.74 It’s currently steadily fusing hydrogen into helium in its core. That helium, over time, settles into the center of the Sun. The conditions there make hell look like the Antarctic: the temperature is 27 million degrees Fahrenheit, and the pressure is something like 250 billion times the atmospheric pressure at the surface of the Earth.
But even at these extreme conditions helium won’t fuse into carbon and oxygen. The inert helium builds up in the very center of the core. It may seem odd, but the matter in the core still behaves like a gas, and obeys the same physical laws as a normal gas. As helium accumulates, the density of the core increases, and that means the temperature increases as well—a compressed gas heats up. This heat must go somewhere, so it radiates away into the upper layers and eventually out of the Sun as light.
This has been an ongoing process ever since fusion first ignited in the Sun’s core 4.6 billion years ago. This means that for all that time, very slowly, the Sun’s core has been heating up as the helium in the core accumulates and compresses. This energy propagates through the Sun and is emitted out through the surface, so as the core heats up the Sun itself has been getting more luminous. It’s something like 40 percent brighter now than it was at the onset of nuclear fusion all those eons ago . . . and it will continue to brighten as more helium is dumped in its heart.
THE BRIGHTENING OF THE SUN
Age of the Sun: 5.7-8.1 billion years (Now + 1.1-3.5 billion years)
The Sun’s increasing brightness is a problem. The Earth is the temperature it is today because it intercepts a small amount of the Sun’s emitted energy. But extra energy from the Sun means extra heat, which in turn will warm up the Earth. Because of the current global warming concerns, scientists have extensively studied the effects of temperature increase on the Earth’s environment. If the Earth’s overall temperature were to increase even as little as 10 degrees Fahrenheit, the polar ice caps would melt, causing an enormous environmental catastrophe.
The details of the Earth’s temperature dependence on the Sun’s energy output are complicated, but the overall effect is that as the Sun brightens, the Earth warms.75 In general, this brightening happens slowly enough that life on Earth can adapt to the change. But when the Sun is about 10 percent brighter than it is today, the Earth’s temperature will have risen those critical 10 degrees. The environmental impact will be profound. When the ice caps melt, the coastal regions of every continent will flood. The equatorial regions will become too hot for comfortable living, though areas like Greenland and even Antarctica will become warm.76
But polar warming probably won’t balance the loss of habitability of the lower latitudes, because the Earth’s air will dry out. When molecules of air heat up, they move around more quickly. Lighter molecules jostle around faster than heavier ones, and as the air heats up they can actually move rapidly enough to escape the Earth altogether! That’s why the Earth’s atmosphere has almost no hydrogen and helium; they are so light that they were lost to space billions of years ago. Heavier molecules like water, N2, and O2 tend to stick around.
But as the air gets hotter because of the brighter Sun, even heavier molecules can be lost to space. Eventually, the atmosphere will be too warm to retain water vapor. It will escape into space, drying out the air and leaving the continents of the planet as parched deserts. This will have obvious repercussions on terrestrial life.
Assuming a steady increase in the Sun’s luminosity with time, that will occur in about 1.1 billion years.
That’s a long time, and, in an odd way, it’s a little bit reassuring! It doesn’t take us off the hook for any other current environmental factors that contribute to any change in the Earth’s temperature, but if you take the long view it does take the pressure off.
But that time will inevitably come. And things get worse after that.
The Sun will continue to brighten; after another 2.4 billion years (3.5 billion years from now) its brightness will have increased by 40 percent over today. The Earth’s temperature will rise so much that the oceans will totally evaporate. The planet’s atmosphere will still be too warm to hold on to all that water vapor, which will escape into space. The entire surface of Earth will be bone dry.77 Sediment at the bottom of the oceans will be exposed to the heat from the Sun. As the ocean floors bake, carbon dioxide locked into the sediment will be released. Atmospheric carbon dioxide is a greenhouse gas; it lets heat from the Sun in but doesn’t let it out. The Earth will heat even more, and the amount of CO2 released will create a thick soupy atmosphere. It’s entirely likely that in a few billion years, Earth will look very much as Venus does today: tremendously hot, and blanketed in a dense atmosphere composed almost entirely of carbon dioxide.
However, even that thick air will be lost to space over millions and billions of years. By the time the Sun’s evolution brings it to the next chapter in its life, kicking it into overdrive, the Earth will be barren rock, devoid of any trace of atmosphere. It will be utterly lifeless.78
For those of you clinging to hope,‡ there is some life that might survive this stage of the Earth’s distant future. Deep in a gold mine in South Africa, scientists found a colony of microbes that live off chemicals found there. The chemicals themselves are created by the natural radioactivity of the rocks, so these bacteria don’t need any sunlight to live, which in turn means they can survive very deep underground. So, while life on the surface of the Earth will all die off over the next 3.5 billion years, life itself will continue somewhere in the Earth. That’s cold comfort, perhaps . . . but it must be said: this, too, shall pass. After another 2 billion years, the Sun will start to really put the hurt on Earth.
THE SUN AS A SUBGIANT
Age of the Sun: 10.9—11.6 billion years (Now + 6.3—7.0 billion years)
So in this distant future, the Earth is dead, cooked to sterility by the ever-brightening Sun. However, while the story of the Earth is pretty much over at this point, the Sun’s biography is just starting to heat up.
Because hotter it will most certainly get. Eventually, roughly 11 billion years after its birth, and 6.3 billion years from now, there won’t be any more hydrogen left in the core of the Sun to fuse. The core will become entirely helium, but it still won’t be hot enough to fuse into carbon and oxygen. Sitting on top of the helium core is roughly half the Sun’s mass, pushing down on it, squeezing it, and the only thing able to support the core is its own internal pressure. As the Sun’s mass bears down, the helium core will shrink even further.79 As before, it responds by heating up. And up, and up. Although there is no hydrogen left in the core, the surrounding layers are lousy with the stuff—it’s just that, until now, the pressure and temperature outside the core weren’t high enough to cause the hydrogen there to fuse.
But at some point, the contracting core will reach a high enough temperature that the hydrogen in a thin shell surrounding it will fuse. This will add to the heat being generated inside the Sun, so the outer layers will respond by expanding. When this occurs, 6.3 billion years hence, the Sun’s diameter will increase by about 50 percent, and its brightness will more than double. Astronomers call stars like these subgiants. They’re bigger and brighter than before, but as the name implies, there’s more to come.
The Sun will be a subgiant for about 700 million years. Over that time, its brightness will stay relatively constant, but its size will increase, from 1.5 times its current size to about 2.3 times its present diameter.
The color of the Sun will shift as well. As described in chapter 3, there is more energy being radiated, but a whole lot more surface area for it to be radiated from. Each square inch of the Sun actually emits less energy than before; it’s just that there are more of them now. The surface of the Sun cools a bit, dropping a few hundred degrees, and the color becomes more orange than it is today.
The overall effect on the Earth at this point is minor. Fried from the increased energy over billions of years, the slight cooling of the Sun now hardly even makes a dent in the Earth. Life (except maybe for those subterranean bugs) is long gone.
Still, time marches on.
THE SUN AS A RED GIANT
Age of the Sun: 11.6—12.233 billion years
(Now + 7.0—7.633 billion years)
While the Sun is a subgiant, the core is still contracting and heating up. In the meantime, the hydrogen fusion occurring in the thin shell around the core is adding helium to the core as well. After the Sun has been a subgiant for about 700 million years, when it is about 11.6 billion years old, the mass of helium in the core will reach a critical point: it will become degenerate. This bizarre state is ruled by the laws of quantum mechanics and occurs when matter is compressed into incredibly dense states. Once matter is degenerate, it no longer behaves like a normal gas. For one thing, if you add mass to it, it responds by shrinking, the opposite of what you’d expect.80 Also, the added mass does not increase the pressure inside the degenerate gas (this is very important later) as you might expect. Instead, just the temperature goes up.
The core will continue to contract as more matter is dumped on it by the hydrogen fusion. The temperature continues to rise, but not the pressure. As before, this added heat gets dumped into the outer layers, but the difference now is that the core is degenerate and heating up a lot, and it’s doing it relatively rapidly.
When the core becomes degenerate, the Sun will be about 2.3 times as wide as it is now. But as the degenerate core heats up, the outer layers will respond once again to this added heat by expanding. When this process is done, the Sun will bloat to an incredible 100 to 150 times its size today—about 100 million miles in diameter. The temperature will drop by half, and its luminosity—the energy it emits per second—will increase to a fierce 2,400 times its present rate. For the next 600 million years, the Sun will glow like a ruddy, fiery beacon. It is a red giant.
The view from the Earth will be awesome. Right now, you can easily cover the Sun with your outstretched thumb. As the Sun evolves during its time as a red giant, it will eventually span a third of the sky. It’s difficult to appreciate how big that is. Go grab a yardstick. Put your left hand on one end, and your right hand at the 24-inch mark. Now extend your arms all the way out. When the Sun is at its greatest extent as a red giant, it will just fit between your two hands. Its growth would be imperceptible on a yearly basis, but over time it will grow to that immense size, appearing to loom over you in the sky.
Of course, you’d be fried long before then.
As the Sun expands into a red giant, several curious things happen. For one, its spin will slow almost to a standstill. When it expands, the spin slows in the same way a skater can slow her spin by throwing her arms wide. The amount the spin changes is more or less proportional, so if the Sun expands by a factor of 100, its spin will slow by 100. It takes a month or so to spin once now, so when it’s a red giant it will take 3,000 days to rotate: more than eight years.
The Sun also becomes more luminous, as we’ve seen, but another critical change with its increased size is that its surface gravity gets lower. The gravity felt on the surface of an object depends on the mass of that object and its radius. When the Sun expands into a red giant, the mass stays the same but the radius increases by 100 times. This means the surface gravity will drop by a factor of 10,000 times. Currently, the surface gravity of the Sun is about 28 times that of the Earth (so that, for example, I would weigh well over two tons on the surface of the Sun81).
But when the Sun bloats up into a red giant, its surface gravity will drop to less than 1 percent of the Earth’s gravity. Any particle on the Sun’s swollen surface will only be very tenuously held on by gravity.
At the same time, the Sun’s luminosity increases by 2,400 times. Any square inch of Sun will be blasting out 2,400 times as much radiation as it did before the Sun swelled up. This light has momentum that it can transfer to a particle on the surface, giving it an upward kick. To a random atom of hydrogen on the surface of the red-giant Sun, it will be as if someone had shut off the gravity at the same time he had turned on a huge fan from below: particles on the surface will be literally lifted off and blown away.
This stream of particles, called the stellar wind, is similar to the solar wind, but far denser. In fact, the red-giant Sun will shed something like one ten-millionth of its mass every year, far, far more than it does now through the solar wind.82
This mass loss is so great that during the time it takes to swell out to red-giant status, the Sun will lose a significant fraction of its mass. Since its gravity depends on its mass, its gravity will also drop. The planets, feeling a lower gravity, will migrate outward; their orbits will become bigger as the Sun loses its grip on them.
It’s a race! Will the Sun increase in size quickly enough to engulf the inner planets, or will they migrate away from the Sun in time to escape its fiery maw?
For Mercury, the outcome is clear: doom. At 36 million miles from the Sun now, it is too far behind the other planets even when the starting gun goes off. After a few million years, the Sun will catch up and expand right past the planet. Mercury will literally be inside the Sun.
What happens to it then? Interestingly, the outer envelope of a red giant is almost a vacuum. The mass of the Sun is still roughly the same, but the volume increases hugely; when it becomes a red giant the Sun will have a million times the volume it does now, so its average density will drop by that amount. In reality the density in the outer layers is even less than that, because a lot of the mass of the star is stored in the core. In the end, the density is less than one one-thousandth of the density of the Earth’s atmosphere, almost a laboratory vacuum.
But there is matter there, thin as it may be. Mercury orbits the Sun once every eighty-eight days, so as far as Mercury is concerned it’ll be sweeping through stationary material. As it plows through this matter, what is essentially air resistance will slow its orbital motion in the same way a parachute slows down a skydiver. In just a few years, Mercury will slow so much that it will spiral into the center of the Sun, where the increasing density of matter will accelerate the tiny planet’s orbital decay. If it doesn’t vaporize first, Mercury will fall into the center of the Sun, where it will most certainly meet its doom.
Of course, if drag on the Sun’s matter slows Mercury down, the reverse is true as well: Mercury will accelerate the particles in the Sun’s outer layers. As Mercury slowly spirals into the Sun, it will speed up the Sun’s spin. It won’t be by much, just a percent or two. By the time the plunge into the heart of the star is over, the only indication that the solar system ever had a planet called Mercury will be a very slight increase in the Sun’s spin.
What of Venus? As it happens, our knowledge of how the Sun will expand into a red giant is still a bit too uncertain to know if Venus will evade getting eaten or not. Some models show it escaping, while others show it suffering the same fate as its little brother. Even if it does manage to stay outside the Sun’s greedily expanding surface, Venus is doomed. From just a few million miles away, the Sun will fill Venus’s sky. The surface of Venus is hot to start with—900 degrees Fahrenheit, thanks to its runaway greenhouse effect—but when the Sun looms so terribly over the Venusian surface, the temperature will scream upward to nearly that of the Sun itself. Venus’s crust will melt and its atmosphere will be blown away.
The Earth may fare somewhat better. Some studies show the Earth’s orbit expanding more quickly than the Sun, while others show us being consumed by the ever-growing star. Astronomers are still arguing over the details, which are important in this game of catch-me-if-you-can.83 Depending on the details of how the Sun expands and how much mass it loses, the Earth will end up being about 1.4 times farther from the Sun than it is now. The Earth is currently 93 million miles from the Sun, so when the Sun stops expanding that distance will increase to 130 million miles.84
Even if we escape being engulfed, don’t breathe a sigh of relief just yet: remember, the red-giant Sun is huge. It will fill a large fraction of the Earth’s sky, radiating down on it at a temperature of over 5,000 degrees Fahrenheit.The Earth’s surface temperature at that point will be about 2,500 degrees, hot enough to melt nearly every metal and rock on its surface. Even before the Sun swelled up the Earth was quite dead, its oceans having boiled off and the atmosphere ripped away. But during the Sun’s red-giant phase, the crust of the Earth will melt as well, and that, pretty much, will be that. While it’s not literally the end of the world, it’s certainly the end of the world as we know it.
We still have room for one more “however,” however. While the Earth will be totally stewed, it’s not the only usable planet in the solar system. Mars too will move out from the Sun, but, unfortunately, will also be too hot for life. Even Jupiter’s moons will warm up too much to sustain us (the average temperature will be something over 500 degrees, hotter than your kitchen oven when you bake cookies). Jupiter’s moon Europa is an icy body, and thought to have liquid water under the surface. When the Sun expands into a red giant, Europa might entirely vaporize.
It’s possible that no existing place in the solar system will be cool enough to support life as we know it. Even the distant moons of Uranus and Neptune will be too warm. You’d have to be about 4.5 billion miles away from the Sun to get temperatures near where they are on Earth today. Of all the places in the solar system, in six billion years only the (currently) icy bodies slowly orbiting the Sun well past Pluto’s orbit may be cool enough for us. They would melt all the way through, becoming essentially giant drops of water a hundred or so miles across, with a red, swollen Sun glaring down on them. It’s known that currently these objects are loaded with organic chemicals. When those icy bodies warm up, all sorts of interesting things could happen to those chemicals. The bodies will stay liquid for hundreds of millions of years while the Sun remains a red giant, which begs the question:
What life might evolve under those circumstances?
ASIDE: DAVID AND GOLIATH
At this point in the life of the solar system, things don’t look so good for the home team. The Sun is a swollen, distended blob, it’s eaten one planet, fried three others, vaporized a retinue of moons, and generally made things uncomfortable for almost everyone else.
But what are you gonna do?
Actually, that’s an excellent question. So far, this story has unfolded in this manner because we’ve let it. That is, if we sit back and watch, this is the way it will play out.
But it doesn’t have to be that way.
For example, it will take several hundred million years for the Sun to go from subgiant to giant. During that time, the temperature on Earth will be unbearable. And once the Sun does go all the way to giant, even Mars won’t look so good. But a billion years is a long, long time, and during that time Mars may be the place to be.
It’s smaller than the Earth, and has very little atmosphere. We can’t do much about its small size, but we can bring it air . . . by dropping bombs on it. Bombs in the form of comets.
Comets are large chunks of rock and ice, and some, in the distant outer solar system, are quite large, hundreds of miles across. They move so slowly in that far realm that it wouldn’t take much of a kick to drop a few into the inner solar system. Attaching a small rocket to one would do the trick. Letting smaller pieces hit Mars, one at a time, could easily bring enough water to fill ponds, lakes, and eventually oceans. Careful manipulation of its atmosphere, using genetically engineered plants and chemical processing, could encourage the development of breathable air. Some people estimate it might only take a century or two.
This type of practice, making a planet more Earthlike, is called terraforming. It’s a staple of science fiction, but it’s based on fact; the physics, chemistry, biology, and other fields of science involved are generally well understood. The devil’s in the details of course, but we have plenty of time to work them out. I’m guessing that in that dim future, billions of years down the road, a century or two here and there will hardly matter.
In fact, we’ll have the technology to start work like this on Mars much sooner than six billion years from now; realistically it could start early in the next century. Which raises the question: in six billion years, won’t we have terraformed all the planets? Perhaps. With an ever-burgeoning population, future humans will look across the gulf of space at all that real estate with envious eyes, and slowly and surely, as H. G. Wells once wrote, they will draw their plans against them.
Still—and stop me if you’ve heard this before—there’s no place like home. The Earth is a pretty good place, and we’ve spent a lot of time evolving here to make ourselves fit in. Is there no hope for our home planet?
Actually, yes, there is, and the solution is simple: we just need to move it farther from the Sun.
How hard can that be?
Okay, in practice, pretty hard. The main problem is that the Earth is a big, massive object, so moving it takes a lot of energy.85 To move the Earth out to where the temperature will be more hospitable (around Saturn’s current orbit) takes roughly the same amount of energy that the entire Sun currently emits in a solid year. That’s the equivalent of exploding 200 quadrillion one-megaton nuclear bombs.
There might be some environmental effects from that.
There are alternatives. We could strap a few million rockets nose-down onto the Earth and fire them off, but it’s hard to know where we’d get enough fuel for them. Plus, the Earth’s rotation and revolution around the Sun would complicate things (we can assume, however, that by the time we need to do this our technology will be up to the task).
But there’s a better way, the environmental impact of which—if we’re careful—is essentially zero.
When we send probes to the outer planets, we can give them a boost in speed by “borrowing” (really, stealing) energy from the orbital motions of other planets they pass on the way. This is the so-called slingshot effect. If we want the probe to speed up, we send it on a path so that it comes in from “behind” a planet, catching up to it as the planet moves in its orbit around the Sun. As the probe passes the planet, it picks up some energy from the planet’s orbital motion, which increases the probe’s speed. The planet loses the same amount of energy, and slows down a bit in its orbit. Since a planet is typically a lot more massive than a space probe, it slows down very little, an immeasurable amount really, while the probe can gain quite a bit of speed. This means we can send probes to the outer planets without having to carry vast amounts of fuel.
Taking energy away from a planet will drop the planet ever so slightly closer to the Sun. But, if we do this in reverse—send the probe in “ahead” of the planet—then the probe loses energy, giving it up to the planet. The probe slows and drops closer to the Sun (useful for getting probes to the inner planets, such as Mercury) while the planet gains energy, moving outward from the Sun.
If we want to move the Earth farther out from the ever-increasingly sizzling Sun, this is an excellent way to do it. Instead of space probes, we can use asteroids, which are much more massive. That means the energy exchange is greater, requiring fewer slingshots. Moving asteroids isn’t all that difficult; in that case strapping a rocket onto one would work pretty well. By aiming it just so, the asteroid could give some of its orbital energy to the Earth, moving the Earth just a teeny bit outward from the Sun. Lather, rinse, repeat . . . a million times.
This scenario has been studied by the astronomers Donald Korycansky, Greg Laughlin, and Fred Adams, and they found that by using a large but typical asteroid, such a maneuver could feasibly move the Earth slowly out to a safe distance from the Sun.
Here’s how you do it. Start with a large rock about 60 miles across that is well out in the suburbs of the solar system. Change its orbit using a rocket or some other method so that it drops into the inner solar system. Aim it (here a rocket would be useful for fine-tuning) so that it passes in front of the Earth, missing us by about 6,000 miles.86 The exact amount of energy transfer depends on a lot of factors, such as the angle of the incoming rock, how close it passes, and so on, but in general a single passage of a rock this size would add about ten miles to the Earth’s orbital radius.
That’s not much, of course, but at first it doesn’t need to be much. Small steps are okay; we have plenty of time!
From this point we have two options. We could, for example, wait a few thousand years, find a second asteroid, and have another pass. But this is wasteful; for one thing there aren’t enough asteroids of this size in the solar system to do the trick. We’ll run out while the Earth is still too close to the Sun.
A second option is better: instead of simply throwing away the first asteroid, we recycle it. A little preplanning and care can save the day. Instead of letting the asteroid go away, we time the passage so that as it heads back out into deep space, it passes by either Jupiter or Saturn. This time, though, it passes behind the planet, gaining energy. Then the orbit can be adjusted again (using the onboard rocket; if it uses solar energy we even get our fuel for free) to pass by the Earth another time. If we do this, the asteroid becomes a sort of interplanetary orbital energy messenger, taking energy from Jupiter or Saturn and delivering it to Earth.
As Earth moves out, Jupiter will move in—remember, we’re stealing its energy—but Jupiter is so much more massive than the Earth (300 times, in fact) that it migrates far less than the Earth does. Moving the Earth outward far enough to keep it temperate while the Sun is in its subgiant phase (about 50 million miles or so) will require Jupiter to move only a few million miles inward (it is currently about 400 million miles from the Sun).
This will pose a problem when the Sun evolves into a red giant, however. The Earth will have to move out past where Jupiter is now. We could still steal from Jupiter’s orbital energy to do this, but once the two planets get near each other Jupiter’s gravity starts to affect the Earth directly. 87 Any encounter like that between the largest of the solar system’s planets and us is bound to have an unfortunate outcome: the most likely scenario is that the Earth gets ejected from the solar system altogether (see chapter 5).
It’s possible we could use a second set of asteroids to move Jupiter out farther from the Sun as well by stealing energy from Saturn, Uranus, and Neptune. At this point, though, the math gets pretty complicated, and results are difficult to pin down.
However, we have a few billion years to work out exactly how we’ll play musical planets. We’ve probably figured it out well enough for now. It’s a viable system, and one our descendants may very well have to employ.
THE HELIUM FLASH AND CORE HELIUM FUSION
Age of the Sun: 12.233—12.345 billion years
(Now + 7.633—7.745 billion years)
So now the calendar reads 7,633,000,000 AD (give or take a millennium), the Sun is a huge red giant, just reaching its maximum extent to a diameter of over 100 million miles, Mercury is gone, Venus may still be around but suffering, Earth is still here but possibly orbiting much farther out than it did when the Sun was middle-aged, and Pluto is a prime condo spot (complete with planet-spanning swimming pool). A time traveler from the twenty-first century would hardly recognize her neighborhood.
But we’re not done, not by quite a bit. The Sun won’t stay a red giant forever. And, as usual, the key to what’s happening lies deep in its heart.
The core of the Sun is now pure helium, and contracting. It’s degenerate, and heating up. The hydrogen around it is fusing into helium in a thin shell, adding more ash to the core. Remember too that since the core is degenerate, its pressure doesn’t change as mass is added. The temperature keeps going up, though.
At some point, something like 600 million years after beginning its transformation into a red giant, the core reaches a temperature of 100 million degrees Fahrenheit. Then all hell breaks loose. Well, to be more accurate, all hell is released, but it doesn’t break loose.
At that temperature, helium can fuse into carbon. Now, if the core were just a normal ball of gas heated to that temperature, the helium would fuse, heat would be released, and the gas would expand, adjusting its internal pressure to accommodate the extra heat—this is essentially what the core and outer layers of the Sun have been doing for billions of years, playing temperature, gravity, and pressure against one another.
But the core isn’t normal. It’s degenerate. It can’t adjust its pressure. So as the temperature increases, it cannot increase its size to compensate. Somewhere, deep in the core, the temperature reaches that critical point, and fusion of helium into carbon begins. This releases energy, which raises the core’s temperature.
This is bad. The fusion rate for helium is ridiculously sensitive to temperature. A slight increase in temperature and the fusion rate screams up, raising the temperature even more, again increasing the fusion rate. Within literally seconds this vicious circle runs away, and the inside of the Sun’s helium core explodes like a bomb.
The energy release is difficult to exaggerate: it’s colossal, epic, titanic. In that one brief moment, called the helium flash, the core of the Sun releases as much energy as all the rest of the stars in the galaxy combined. It may actually release 100 billion times the Sun’s normal output, all in a few seconds.
You’d think this would tear the star apart in a supernova, but in fact, it doesn’t. It does a funny thing: because this is all happening deep inside the core, the matter itself absorbs all the released energy. This infusion of energy is enough to overcome the degeneracy of the core, which suddenly becomes normal matter once again. It’s under tremendous pressure, to be sure, but it’s no longer held in the sway of that weird quantum mechanical state. Once the degeneracy is released, the runaway fusion flash is dampened, and everything settles down into a nice steady state.
With that huge explosion safely absorbed, and the core back to being a regular old gas, helium fusion can proceed at a more leisurely pace. So now we have a core of helium fusing into carbon (there are also some minor avenues of fusion that are producing oxygen and neon as well), releasing heat. Outside this is still a thin shell of hydrogen fusion, and surrounding that, a hundred million miles deep, are the outer layers of the star.
Ironically, however, the amount of energy being generated in the core due to helium fusion is now less than was emitted when the core was degenerate and shrinking. This means less heat is being transported into those deep outer layers, which were before being supported by that extra heat. Once the core cools off, the outer layers respond by shrinking back down. On a relatively short time scale (about a million years), just as the red giant is reaching its maximum possible size, the legs are kicked out from under it. The Sun shrinks.
When it settles down, the Sun has become considerably less bright, emitting now only about 20 to 50 times as much energy as it did when it was young, only a few percent of what it did at its peak as a red giant. It’s still bigger than it was when it was a normal star, but far smaller than a red giant: it’s now about 10 times its original size, 8 million to 10 million miles across. It’s slightly hotter now, radiating away at about 8,000 degrees Fahrenheit, still cooler than its temperature today as well. It’s a lovely orange in color.
Since it’s smaller, the Sun’s surface gravity increases (even though it lost some mass as a red giant). Particles on the surface are held on more strongly. Moreover, the luminosity has dropped, so the particles feel less of a pressure to blow off the surface. The stellar wind decreases drastically.
So now the Sun is stable once again. It’ll remain this way, a helium-fusing giant, for over a hundred million years.
The Earth, however, is once again in trouble. After all our effort to move it a billion miles out, we suddenly find that the Sun is much smaller and giving off less energy. Temperatures plummet. Those far distant descendants of ours will have to move the Earth back toward the Sun. No problem—they can do the reverse of what they did to move it out. They have a lot less time to do it, though: they had billions of years to migrate it outward, but now they’ll have to drop it inward in only a million years or so. They can use bigger objects (Jupiter has lots of moons it doesn’t need, for example) to increase the rate of energy transfer.
Or, who knows? It’s more than seven billion years in the future. Maybe they’ll just snap their fingers and the Earth will tunnel through space-time and reappear where they need it.
Let’s hope it’s that easy. Let’s also hope they’re patient and not easily irritated, because in a few dozen million years, we’re going to start all over again.
In the Sun’s core, carbon and oxygen are building up. The core is too cool to fuse them, so they are inert, like helium was before them, accumulating like ash in a fireplace. So the scenario is familiar: the core starts to contract, and the Sun slowly starts to heat up again. Over the next 20 million years it slowly starts to brighten and swell. After having moved the Earth out and back in again, we’ll be forced to migrate our planet away from the Sun once more. The outer layers of the Sun will reach an extent of 20 million miles or so before the next catastrophe occurs.
Age of the Sun: 12.345—12.365 billion years
(Now + 7.745—7.765 billion years)
That happens when the helium in the core runs out. The carbon/ oxygen core starts to contract, just as the helium core did before it, and the results are similar: the Sun, for a second time, becomes a red giant. This time, though, the onset is much faster. Carbon and oxygen have different physical properties from helium’s, and the core contraction is more rapid. Instead of taking 600 million years to expand, it takes only 20 million.
The Sun expands drastically again, achieving a diameter of well over 150 million miles. Its sudden expansion will make our human celestial engineers pull their hair out.88 Probably, at this point, it’s a good idea to abandon the solar system and look for lodging elsewhere.
It’s all for the best, perhaps. The view from far away will be spectacular, as we’ll see in a moment.
The Sun will be more luminous in its second regime as a red giant than it was the first time. It will blast out energy at 3,000 times the rate it does now, and the stellar wind will be back with a vengeance. It lost 28 percent or so of its original mass the first go-round; this time it loses more than 60 percent of what is left. With or without our help, the planets will once again migrate outward as the Sun hemorrhages away its material, with Venus and Earth possibly moving quickly enough to avoid being consumed once again. And if they escape they’ll still get roasted once again by the swollen, luminous Sun.
This is a grueling series of events for the solar system. Yet, amazingly, things are about to get worse.
Deep in the Sun, the carbon/oxygen core gets so dense it becomes degenerate. Helium fusion starts up in a thin, slightly degenerate shell outside of it, and hydrogen fusion continues in a shell outside of that. The problem is, thin-shell helium fusion is wildly dependent on temperature, even more so than before. Any slight increase in temperature causes the fusion rate to increase madly.89 As more heat is generated, the rate goes up, which generates more heat—well, we’ve seen this before. The thin helium shell can flash again, releasing huge quantities of energy. This time, though, the outer layers of the Sun won’t have time to expand slowly and accommodate the extra energy. The rate of energy being dumped into them simply overwhelms them. The Sun convulses, literally, and ejects a vast amount of material over the course of just a few years—not millions of years, mind you, just plain old years.
After the flash of energy, the helium shell cools down for perhaps 100,000 years, but then the situation builds again. A second flash occurs, and a second envelope ejection. Then, again after 100,000 years, a third, and then a fourth, most likely final, flash and ejection. During these episodic convulsions, the Sun swells for a third time, this time expanding to as much as 200 million miles across, enough to reach the Earth’s original orbit.
Even at its increased distance, the Earth won’t fare well during these eruptions. Its surface temperature will rise to well over 2,000 degrees as the swollen Sun heats it, then drop again after each pulse fades. Also, these pulses will slam the Earth with quadrillions of tons of matter moving at several miles per second. This won’t add much to the total mass of the Earth (which is thousands of times more massive than the material accumulated), but the impact of that much material, even spread out over hundreds of millennia, will severely batter the already war-torn Earth. The total impact energy is equal to the detonation of trillions of nuclear weapons, or the same as detonating a one-megaton bomb every second for a million years.
Even in death, the scale of destruction wrought by the Sun is awesome.
Every time the Sun erupts, it loses more mass. By the fourth epic heave, the last bits of the outer envelope will be shed. The majority of the Sun’s original mass will be lost to space, revealing just the degenerate carbon/oxygen core surrounded by a thin shell of very hot helium. The core has contracted to just a few thousand miles across, about the size of the Earth (assuming our planet still exists). It will have about half the mass of the original Sun, so it is phenomenally dense. It’s also still quite hot; it will radiate at a temperature of as much as 200,000 degrees Fahrenheit, and will shine at thousands of times the luminosity of the present-day Sun.
It has become a white dwarf. To someone standing on the surface of the blasted and quite dead Earth, the Sun would only be a point of light, eye-achingly bright, brighter than the full Moon is now. But it will be only a pale, dim shadow of its former glory.
We’re pretty much at the end of the line here. No more fusion, no more source of energy. After a lifetime of over 12 billion years and a dramatic saga of expansion, contraction, and eruption the Sun is, effectively, dead.
THE SUN AS A WHITE DWARF
Age of the Sun: 12.365 billion years (Now + 7.765 billion years)
However, in death there can be beauty.
The gas ejected from the Sun in its final days will be expanding rapidly. The distribution, the overall shape, of the gas will depend on many factors. In general, the Sun will emit the gas in great spherical shells like cosmic soap bubbles, with a dense edge and more tenuous inner region. However, there can be circumstances where the gas can be shaped, molded. As it expands it might hit gas that floats between the stars (what astronomers call the interstellar medium). If the Sun was spinning rapidly enough when the gas was emitted (absorbing Mercury and Venus might just be able to speed it up, since the planets would dump their angular momentum into it), the shells might be flattened by centrifugal force, shaped more like a cheese wheel or a basketball with someone sitting on it. Other physical conditions can cause clumps in the gas, or bright regions, or rings.
The white-dwarf Sun, sitting at the center of this expanding gas, may be hot enough to flood surrounding space with ultraviolet light. This would ionize the gas, causing it to glow.
If our descendants have fled to another star, what a sight they will see! Looking back on our Sun, they may see the gas glowing like a perfectly circular thin ring. The ring will glow mostly green because of the oxygen atoms in it; other elements contribute different colors, but the green glow is generally the strongest because of the way the atoms of oxygen emit light. Through a small telescope the greenish disk will resemble the planet they once lived on; astronomers today call these objects planetary nebulae for that reason.
Planetary nebulae form when a star like the Sun dies, ejects its outer layers, and ionizes them. The gas glows, forming eerie shapes. This Hubble picture is of the famous Eskimo Nebula, which indeed looks like a parka-wearing Inuit in ground-based telescope images.
NASA, ESA, ANDREW FRUCHTER (STSCI), AND THE ERO TEAM (STSCI + ST-ECF)
Planetary nebulae are among the most beautiful objects in the sky. How will those distant humans feel, looking back at their solar system? Will they feel any better at all, knowing that civilizations across the galaxy will be able to view our Sun and see its final attempt at glory? Or is it silly to try to even guess what humans, if there are any, would be feeling more than seven billion years from now?90
M2-9 is another planetary nebula, but has a more elongated shape. This may be due to the presence of a binary companion to the dying star; if the red-giant star swallows up the companion, the wind it emits as it dies can be sculpted into odd shapes.
BRUCE BALICK (UNIVERSITY OF WASHINGTON), VINCENT ICKE (LEIDEN UNIVERSITY, NETHERLANDS), GARRELT MELLEMA (STOCKHOLM UNIVERSITY), AND NASA/ESA
Finally, a few thousand years later, the gas will have expanded and thinned, and the white dwarf cooled. There won’t be enough UV light to illuminate and fluoresce the gas, and not enough gas to absorb it anyway. The expanding material that once warmed and cheered our planet will merge with and become indistinguishable from the gas that exists between stars. The white dwarf will continue to radiate away its heat, inevitably cooling, dropping in color from white to blue to yellow to orange to red, and then it will slide to infrared, and invisibility, after a few more million years.
Whatever is left of the solar system will continue to orbit the now-black dwarf. The planets will cool along with their star, eventually freezing solid, and after a few billion more years will be as dark and cold and empty as space itself.
THE LONG DESCENT INTO NIGHT
Can planets survive such a devastating series of events? Actually, the answer is yes. Depending on what you mean by “survive.”
First, well over two hundred planets have been found orbiting other stars, and a dozen or so of these have been found orbiting red giants. These planets are probably something like ours: they formed along with their star billions of years ago, and have managed to make it through at least one episode of red-giant expansion. We don’t know if those stars swallowed up any inner planets or not, but at least some planets were outside the zone of envelopment. The planets may have started rather close to their stars, though, and migrated outward; one, for example, is not much farther away from its star than the Earth is from the Sun. The surface temperature of that planet is probably about 900 degrees Fahrenheit.91
Amazingly, evidence has recently turned up for planets orbiting white dwarfs. A strong signal of the element magnesium was found coming from a white dwarf, far more than could possibly be generated by the star itself. The amount of magnesium found indicates that quite recently an asteroid must have come too close to the star, been disrupted by the dwarf’s strong tides, and formed a disk of matter around the star. The shape of the disk itself means that the asteroid may have been in a much larger orbit, only to be dislodged by a planet or some other massive body. What all this means is that planets and even asteroids may survive not only a star’s evolution into a red giant but also all the subsequent catastrophic stages. The white dwarf itself is fairly massive, about 0.8 times the Sun’s mass. When the Sun becomes a white dwarf it will have less than half its original mass, indicating that this star must have started off more massive than the Sun, which means in turn its later stages in life would have been even more extreme than the scenario outlined above.
Yet, even there, it appears that planets were able to hold it together during those hundreds of millions of years of stellar torture. Of course, by the end of this period of repeated roasting and freezing the planets would be burned-out sterilized hulks. There must not have been any civilizations there capable of manipulating planetary orbits, or able to foresee the future well enough to plan for this eventuality.92
But we’re smart, we humans. I can hope that when the time comes—and it will—we’ll be able to do something about it. And I really do hope we—or something resembling us—will be around. The death of the Sun will be sad to behold . . . but the beauty will be breathtaking.