Death from the Skies! (28 page)

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.

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.

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, N
₂
, and O
₂
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 CO
₂
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.

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.

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.