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.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.
CHAPTER 8
Bright Lights, Big Galaxy
YOU WALK OUTSIDE ON AN EARLY WINTER’S EVE AND cast your gaze upward. The constellations in the north and east reflect the tale of heroic Perseus, sent by King Cepheus and Queen Cassiopeia to rescue the maiden Andromeda from the sea serpent Cetus. You may chuckle, thinking that two curved lines of stars hardly bring to mind the vision of a young girl chained to a rock waiting to be eaten by a monster, but as you look carefully your eye is caught by a glimpse of something fuzzy just off Andromeda’s side. It’s hard to see, but it’s definitely there: a slightly elongated patch of light.
It seems suspended there, small and unassuming, hanging for all eternity in the sky. But, like so much about the night sky, that’s an illusion. You are seeing the great Andromeda galaxy, a massive spiral galaxy similar to our own Milky Way. And it’s headed our way.
If you could speed the clock up a few trillion times, such that millions of years appeared to elapse in mere seconds, you would see the Andromeda galaxy swell and grow before your eyes. Every passing moment would see it getting larger, until a few minutes later it looks like the whole sky is about to fall on you. You see stars around you suddenly wrenched up and away, forming a long thin line stretched into space, like a tendril reaching out to the other galaxy. The rest of the sky is oddly empty of stars, the Sun too becoming a part of a stellar stream stretching countless light-years, reaching into the space between the galaxies where stars are rare.
Suddenly, the Andromeda galaxy has flown by, shrinking in the distance somewhat, having literally passed through our galaxy like a ghost through a wall. However, over the next few accelerated minutes—actually, millions upon millions of years—you see it slow, stop, and then head back your way. Flash! It fills the sky in another pass, and then once again has moved away. But this time it doesn’t get as far. Andromeda swells one last time. Over your head you see the bright core of the galaxy merge with the core of our own. Above you hangs a vast cloudy ball, the remnants of the once mighty pair of galaxies, merged to form a single, larger galaxy.
Within minutes, the sky settles down. Everything now looks calm. You sigh with relief, glad that you have survived this cosmic encounter. What you don’t know is that a beam of matter and energy is headed your way from the heart of the new galaxy, and when it touches down on the Earth, the chaos of the merger will seem as bucolic as a peaceful springtime meadow.
THERE’S NO PLACE LIKE HOME
Have you ever heard that a galaxy is like a city? A city has a downtown section, suburbs farther out, pockets of congestion, regions where there’s not much to see, and, of course, the occasional rough neighborhood. Galaxies are like that too. They have their regions of high and low activity, places that are exciting, places that are a bit duller. We even say they have a population—but instead of people, a galaxy is populated by stars, gas, and dust.
And, like a city, of course, there are places you really don’t want to go
.
Yeah, you can see where this is headed.
If you live far from civilization, in a place with dark skies, then you have certainly stepped outside on a clear, moonless night. At first, when you do so, you may only see a few stars in the sky as your eyes slowly adjust to the gloam. But over time, as your pupils dilate and your eye automatically coats your retina with a light-sensitive protein that increases sensitivity, fainter stars will slowly become noticeable. The sky will become spangled with stars, thousands of them gently twinkling down upon you.
Along with the stars, you may see a faint glowing band across the sky. It almost looks like smoke, or a jet contrail. That swath of mist is called the Milky Way, named because it looks like a river of spilled milk across the sky. It has been known for thousands of years, since humans first noticed the night sky. In the early 1600s, Galileo turned his newly fashioned telescope to the Milky Way and was shocked to see that it was not a cloud, but was in fact made up of thousands of stars, all too faint to be seen individually.
This was the first clue that we live in a galaxy.93 A clue to its shape may come with a moment’s inspection of the brightest stars in the sky, revealing that they seem to stick near this milky band. Away from the band, stars are fainter and fewer in number.
In the eighteenth century, the great astronomers William and Caroline Herschel took this idea further: using a telescope, they counted up stars in different directions in the sky to try to determine the shape of the galaxy.
The idea goes something like this: imagine you are standing in a field. As the sky darkens, you are surrounded by fireflies. If you’re in the middle of the field you’ll see the same number of fireflies in every direction you look. But if you’re near the edge of the field, you’ll see fewer fireflies toward the edge than if you look across the field to the far side.94 The farther you can see, the more fireflies you see.
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