The Sun is a middle-aged, normal star. It is about halfway through a long existence as a "yellow-white" star. Some day, sunrise on Earth or Mars will look something like this [Image credit, Dirk Terrell].There are a number of astronomical events that could end civilization or even all of life on Earth. Some of them could happen at any time. This one is certain to happen, at a time about six billion years in the future.
All of the plausible means by which the uncaring Universe could wipe us out are canvassed by astronomer Philip Plait in Death From the Skies! : These are the Ways the World Will End. His thesis? The world will most definitely end. When? It could be a very long time…but it might not. How? There are a number of candidates.
Long gone are the days of a cozy little Universe benevolently designed for our comfort and edification. When it behaves itself, Earth is still a rather cozy little planet that has managed to hang on to its biosphere for about four billion years. This in spite of the steady warming of the Sun, which is 40% hotter now than it was when life began. Also in spite of an early crisis or two, when plants first sucked most of the carbon dioxide out of the atmosphere, precipitating a disastrous cooling that led to a million years or more of "snowball Earth", which ended only when heat built up under the global glacier and a lot of volcanoes popped together, putting back lots of CO2.
But Dr. Plait's interest is not in what Earth might do to itself, or even what we, its most dangerous occupants, might do to ourselves and our Earth. He sets his sights on everything else, the 99.9999999999 percent of the Universe that is not Earth (You know, I think I need another dozen nines there).
In a methodical way, the author surveys things from the smallest astronomical threats to the largest and longest-enduring:
- Asteroids: Now that we know about half of the Earth-crossing asteroids, we are likely to have a year of more to anticipate the fall of one that we find is on a collision course. We just have to decide what we'll do when that happens. This is one threat we could do something about, though it is unlikely we'll muster up the political will to do so.
- Comets: A harder issue, because the ones most likely to be a threat appear once, with no regularity. The great comet of 1996, Hale-Bopp, has a nucleus four times the diameter (probably 50 times the mass) of the asteroid or comet that did in the dinosaurs. It came rather close, which is why we could see it so well. Great, spectacular comets are close-passing comets.
- Things the Sun could do: flares and coronal mass ejections top the list. Every eleven years we pass through a risky period—three or four years—during which solar shenanigans damage a few satellites, and cause postponement of space flights so astronauts won't be fried. Even near-Earth orbit is a risky place to be when the Sun is active.
- Supernovas: There are two kinds, the largest stars at the end of their "ordinary" development, and binary giants that become gamma-ray bursters. The first kind are dangerous to earth if one goes off closer than fifty light-years. The book includes an Appendix that lists the 24 stars that will become supernovas some day, that are within 1,000 light-years. None is closer than 260 light-years. But the second kind, watch out. Because their energy is focused into a beam, one could blast life right off of Earth from a distance as great as several thousand light-years.
- Black Holes: Every galaxy has one, a million Suns' mass or more, at its center. We are comfortably far away (25,000 light years) from the Milky Way's central black hole. Some supernovas also produce black holes, with masses of three Suns to about ten. The thing to remember about "stellar" black hole: its gravity is the same as the star that created it. But it can get lots closer to you, so the close-in gravity is much more intense. Fall into one that comes dead-center , and all of us, plus Earth, will be spaghettified by the tidal forces of that intensified central gravity. But the chances of any star passing close to the Solar system are very, very small. Black holes are thought to be fewer in number, perhaps a thousandth of a percent, of all stars.
- Aliens: The history of "alien" invasions, in which peoples such as the "civilized" Europeans located new peoples, makes me pessimistic about how nice and kind any space aliens will be. Most likely they'll want to exterminate us, preferably without any communication at all. The fact that we are still here indicates there aren't any close neighbors out there.
- The death of the Sun: This event, depicted above, is discussed in step-by-step manner. It will unfold beginning several billion years in the future, by which time any intelligent folk still around might have developed a technology that can move Earth, or at least themselves, out of harm's way. The Sun probably won't swallow Earth, but will simply heat it to the melting point.
- Galactic collision: The nearest big spiral galaxy, Andromeda, will get close enough to cause trouble in a half billion years or so. Stars won't likely collide, but gas clouds will, leading to lots of new star formation and a flurry of supernovas. A lot depends on whether a near-miss by large stars changes the galactic orbit of the Solar system.
- Deep, deep time and the end of everything: Let's leave that for the kicker.
But we do have eight categories of things, some of which might happen, at almost any time, and several of which are sure to happen, just not yet.
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