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.
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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.
HELIUM EXHAUSTION
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.
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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.
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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.