Death from the Skies! (31 page)

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 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

 

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.

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.

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.

So it goes with stars. The Herschels reasoned that if they saw more stars in one direction than another, then that means the galaxy must be longer in that direction. They found the galaxy to be a flattened cigar shape with the Sun very near the center.