Death from the Skies!: These Are the Ways the World Will End... - Philip Plait (2008)


Nearby Stars (Distance < 1,000 Light-Years) That Will Eventually Go Supernova




One of the best ways to tick off an astronomer—and it can be fun sometimes just to see how he reacts—is to mix up the terms meteor, meteoroid, and meteorite. The very best way to tick off an astronomer is to call him an astrologer.


Pronounced “Cheek-shoo-lube.”


Yes, it was the first day of the nineteenth century. Don’t make me lecture you about there not being a year 0.


It’s also illegal. A 1963 test ban treaty (see chapter 4) forbids the detonation of any nuclear weapons in space. However, one would assume that international treaties might be set aside temporarily given the total annihilation of the human species as the alternative.


Technically, it never becomes a liquid; it goes directly from a solid to a gas in a process called sublimation.


Not to worry, the Sun won’t run out of hydrogen anytime soon—5 million tons sounds like a lot, but it’s only 0.00000000000000000025 percent of the Sun’s mass. We have billions of years of fusion still ahead of us.


You might say that’s a bomb’s distinguishing feature.


You can check this yourself with a simple compass. Find a house lamp or some other appliance connected to a wall outlet. Put the compass near the wire, and turn the appliance on and off. The needle will move, influenced by the temporary magnetic field.


This process, called convection, is what causes hot air to rise and cool air to fall, and also can be seen when you heat a pot of water on a stove.


When the Sun is on the horizon, its light passes through more air than when it is overhead, dimming the sunlight considerably and making it easier to see sunspots. At this point, you might be expecting me to exhort you to never ever look at the Sun. However, surprisingly, there has never been a reported case of permanent total blindness caused by looking at the Sun. It is possible to damage your eyes looking at the Sun—for example, using cheap sunglasses that dim visible light but not ultraviolet, or looking at the Sun when your pupils have been artificially dilated with drugs—but it’s actually rather difficult to do, and in general the eye heals quickly. I don’t recommend it since damage ispossible, but it’s unlikely and certainly not worth the hysterics it garners. Having said that, I will point out that looking at the Sun through binoculars or a telescope is in fact incredibly dangerous, since they concentrate sunlight. The only 100 percent safe way of looking at the Sun with optical aids without risking boiling the fluids in your eye is to project its image on a piece of paper. There are other, more expensive methods, but this one is easiest. And nothing is more expensive than losing an eye.


This stands for roentgen equivalent man where a roentgen is a measure of an amount of radiation. A source may give off a certain number of roentgens of radiation, but the amount that gets absorbed by human tissue is measured in rems.


Going to Mars is even more difficult, since it takes months to travel there. Radiation from flares will be an even bigger priority. Because of its mass, rock makes an inconvenient shield for interplanetary travelers. NASA and other space agencies are busy trying to solve this problem so that trips to Mars can become a reality.


Not all CMEs are associated with flares. Sometimes, they happen all on their own, as more and more field lines get tangled up, resisting the expansion of the matter they constrain. Eventually, the matter breaks free and forms a CME. The magnetic reconnection associated with flares makes it easier to trigger a CME, but it’s not always necessary.


In reality, the magnetic north pole and the geographic north pole don’t coincide; because of the Earth’s ever-changing dynamo the magnetic poles wander, and anyone who needs great accuracy in finding north using a compass needs to correct for that. And things get even worse: what we call the Earth’s magnetic north pole is actually, by the way magnetic poles are defined, the south magnetic pole. It’s just by tradition that we call it the north pole. And oh—it gets worse still: the poles on a bar magnet are actually labeled for the pole they attract. So the pole labeled “N” on a bar magnet actually tries to point to another magnet’s north pole (it “seeks” the north pole), so the pole labeled “N” is actually the south pole. Confused yet? Yeah, like magnetism isn’t already hard enough to understand.


This is pretty much how a neon sign works; the ionization energy comes from electricity (when the sign is plugged in), and when the electrons recombine with their parent atoms the gas glows. The neon may be mixed with other gases to get different colors.


Oil and natural gas pipelines are conductors too. An electric flow in a pipeline can increase the corrosion rate of the metal, because the voltage change can increase the ability of moist soil to erode the metal. This won’t cause a spectacular collapse like the 1989 power grid failure, but it does reduce the operating lifetime of the pipeline, and can cost billions to retrofit.


Not to be confused with nitrous oxide, N2O, or laughing gas; nitrogen dioxide is far more serious.


If you live in the northern hemisphere, that is. For Australians, New Zealanders, and other upside-down people, reverse these directions.


Even very subtle changes in the shape of the Earth’s orbit can have an effect here, given enough time (like, millennia). Some scientists even speculate that the Sun’s magnetic field protects the Earth from an onslaught of subatomic particles that come from deep space. Called cosmic rays, they might seed cloud formation in our air and thus lower the Earth’s temperature. However, the data are very marginal for this claim. Much more study is needed to understand these effects. Cosmic rays do have deleterious influences on Earth, which we will see in subsequent chapters, but we can’t include climate change among them just yet.


I’ll note that it’s not even clear that these objects are actually warming at all; the data are sparse. In the case of Jupiter, for example, it’s not a global effect; only small sections of it are warming because of local atmospheric conditions.


Of course, the devil’s in the details. See chapter 7 for an account of what happens to the Sun next.


In reality there are many fusion processes going on, sometimes simultaneously. For example, neon fusion produces the heavier element magnesium and the lighter element oxygen. Then oxygen fusion produces silicon, sulfur, and phosphorus. However, I decided to simplify all this to avoid a profusion of fusion confusion.


The word disaster comes from the Latin for “bad star,” so in this case we can be literal.


Just to be clear, there are no stars anywhere near this close capable of blowing up.


That answer shocked me. I had to calculate it twice to make sure I didn’t make some dumb mistake. The Crab is 40 quadrillion miles away, yet it hurled so much matter outward that 200,000 pounds would hit us even from that distance! Supernovae are immense.


Note the use of the word only. Astronomy has a tendency to crush our sense of scale into dust. The mass of the Earth may seem huge to us, but it’s only about a millionth of the mass of the white dwarf.


Really, it’s true. The details aren’t important here, but we’ll cover them in the last chapter. Promise.


To this day that was the largest bomb ever exploded. Theoretically, the same design for the bomb could have been ramped up to give twice that explosive yield, but thankfully it was never tested.


One incident, however, was never clearly defined. In September 1979, what appeared to be a nuclear test well south of the Cape of Good Hope in Africa was detected, but the data were just ambiguous enough that no firm conclusion was ever made. To this day, the origin of this event is unknown.


In fact, it was worse than that. Using timing delays in signal detection actually yields two different positions for a GRB; you need at least one more satellite to distinguish between the two and determine which one is correct. It’s a bit like asking, “What’s the square root of 25?” Both +5 and −5 are correct. And even if you use multiple satellites, the direction to the burst is at best only a very rough estimate.


A spectrum is what you get when you run light through a prism or a finely etched grating. The light is separated into its individual colors, like a rainbow. When measured very carefully, a wealth of information can be obtained about the source of the light, including its temperature, chemical composition, and for some objects, like galaxies and GRBs, even their distance.


Stretch your brain back to dim memories of high school math: the area of a sphere = 4πr 2.


Pronounced “ATE a CARE in Ay,” for those practicing at home.


Actually, Eta may be a binary star, two stars orbiting each other. There is so much glare and interference from all the material surrounding the star that astronomers still aren’t 100 percent sure.


These findings are still controversial. This is a new field of science, and the models are somewhat shaky. Still, if you take away anything, just remember that a nearby gamma-ray burst is bad.


Get used to that. Your common sense is going to take a beating here.


This means that astronauts orbiting the Earth are not weightless because they are beyond Earth’s gravity; they feel weightless because they’re falling. When you are sitting in a chair, you feel gravity as pulling you down into the seat, which supports your weight. If there is nothing to support your weight, you don’t feel the force of gravity, so when you’re falling you feel weightless. This is why astronauts appear weightless in orbit (described as “free fall”). At the usual orbital height (300 miles or so) the Earth’s gravity is only about 10 percent weaker than it is on the surface. Think of it this way: if the Earth’s gravity weren’t pulling on the astronauts, they would go flying off into deep space!


That distance is about 700 million miles from the Earth, so you’ll be thrusting a long time: well over a thousand years. Better pack a lunch.


More specifically, nonrotating black holes are spherical. In reality, most black holes are created from rotating stars, and that rotation is amplified when the core of a star collapses into a black hole. Like any rapidly rotating object, black holes can bulge out at their equators from centripetal acceleration.


The event horizon is not the physical surface of a black hole. A black hole doesn’t really have a surface; as far as we can tell, the matter in the black hole has shrunk all the way down to a mathematical point with literally zero size, called the singularity. The event horizon is the point where, at some distance from the singularity, the escape velocity equals the speed of light. The matter forming the black hole basically has no size, while the event horizon can be many miles across. Like I said, black holes are weird.


The corollary to this is: if you want to age less, move around really quickly all the time so others see your clock as running more slowly. Or you can sit around reading books on black holes and other astronomical dangers, and others will see your clock as running just like theirs.


Just to be clear, mass and weight are different. Mass is a property of matter; you can think of it as how much matter there is, and we measure it in grams or kilograms. Weight is the force of gravity on that mass, and we measure it in pounds. A cannonball has the same mass whether it’s on the Earth or the Moon, but on the Moon it weighs one-sixth as much because gravity is one-sixth as strong; on the Earth 1 kilogram weighs 2.2 pounds, but on the Moon it weighs about 0.36 pound.


Actually, because of the physics of solid bodies, the truth is that the mass above you doesn’t pull on you at all, oddly enough. Newton was the first person to be able to work that out mathematically. Basically, once you are inside an object like the Sun, the only mass you need to worry about is from the stuff between you and the center.


No black hole formed in a supernova can have a mass less than about three times that of the Sun, though. The core of the exploding star has to be at least this massive or else it only forms a neutron star, not a black hole. So don’t fret: the Sun cannot turn into a black hole.


Technically, this is a misnomer. It’s not a force, but a change in a force. Unfortunately, this term stuck, and that’s what we call it.


Also, the diameter of a black hole is proportional to its mass; double the mass and the black hole’s diameter doubles as well.


Yes, seriously, although a quick search didn’t yield any mention of the term in professional physics and astronomy journals.


Some black holes have been known to generate even higher-energy gamma rays as well, but this is due to nonthermal (not heat-related) processes.


As a reminder, a spectrum is created when you break up light into its individual colors, which can tell you lots of interesting things about the object that emitted the light.


Because black holes curve space, light gets bent as it travels near one—think of it as a road going around a curve, and a car on that road having to follow the curve too. An approaching black hole might be detected through this distortion—we might see star positions apparently changing, and bigger background objects like nebulae and galaxies getting smeared out. But this distortion is small when the black hole is far away, and likely to escape our notice until it is inside our solar system. And while this might give us decades of warning, there’s not a whole lot we could do about it short of evacuating the Earth . . . which presents its own set of issues.


At lower velocities, which are in general much more likely, the same events would unfold, just more slowly.


The force of gravity drops as the square of distance and goes up with mass. When the hole is √10 or about three times farther away from the Earth as the Sun, its gravity is 022 times the Sun’s gravity.


Earthquake-induced floods are called tsunamis, which many people erroneously call tidal waves. In this case, we are literally talking about a wave caused by tides.


We ran across this before—it’s how stars make energy from fusion, via E = mc2.


You might think that the particle that fell in balances the mass from the particle that escapes, so the black hole has lost no net mass. However, because of the laws of gravity (and how weird they get inside a black hole), inside an event horizon a particle can actually have negative energy—essentially, the black hole holds on to it so tightly that the total energy of the particle is less than zero. This balances the positive energy of the particle outside the black hole, and everything remains even, except for the energy lost in separating the particles. Remember what I said earlier about common sense?


Actually, it’s not the whole truth. Black holes still may have something to say about our eventual fate; see chapters 8 and 9.


A relatively recent idea is that giant planets like Jupiter and Saturn may have formed from the direct collapse (called fragmentation) of material in the disk, rather than being built up by collision. This scenario is gaining some ground among astronomers, but the actual birth mechanism of planets is still somewhat debatable.


If you wish to view this as a cautionary tale, be my guest.


Before all this oxygen was exhaled by the new microbes, the Earth’s atmosphere contained a large amount of methane. Oxygen combines readily with methane, so most of the atmospheric methane was destroyed when oxygen became abundant. Methane is a strong greenhouse gas, so when it disappeared the Earth may have cooled significantly, gripping the planet in a global ice age. This would have aided any mass die-offs of bacteria as well.


It’s much more difficult to get meteorites from the inner planets out to Earth because they would have to fight the gravity of the Sun as well as that of their own planet. Despite that, some meteorites tentatively identified as being from Mercury have been found.


Panspermia is studied by many solid researchers with good reputations, but like any other field of science at the cutting edge of knowledge, it suffers from its share of kooks. Like UFO believers, there are people who point at everything they find as evidence of panspermia, from red-tinted rain in India to odd microbes found floating in the upper atmosphere. After looking into these cases, I have found them to exhibit the same problems as every other pseudoscientific claim: lack of solid observations, poorly controlled experiments, shoddy research, a lack of critical thinking, and a very strong tendency to jump (and leap, and catapult) to conclusions. We may yet find strong—even solid—evidence of life from space, but it will be uncovered using scientific methods: careful observations, reasoned experiments, and judicious thinking. Otherwise you just get cold fusion: a lot of pomp, but no circumstance.


As noted before, getting rocks from Earth to Mars is possible, but considerably more difficult and therefore much less likely.


This is easier if the wind is actually from a red giant; those kinds of stars emit far less UV radiation that can damage or kill the bacteria.


In fact, viruses are so simple that many scientists don’t consider them to be alive. Their lack of ability to reproduce on their own substantiates that (plus they don’t eat or excrete in any real sense either).


It’s possible that viruses were the precursors of life on Earth. They certainly have been around a long time, and coevolved with us. Even if life on Earth got its start from space viruses landing here via panspermia and kick-starting our ecosphere, such viruses would be harmless today. We’ve evolved for a long time since then, and it’s not terribly likely they will still find a lock to fit their key.


Incidentally, there is an even simpler structure than viruses, called prions. They aren’t much more than complex aggregations of proteins, and aren’t actually alive in any real sense. They can, however, mess up the structures of normal proteins in tissue, causing large holes to form in cells. This in turn produces all manner of horrifying problems, such as convulsions, dementia, and death—mad cow disease and scrapie in sheep are caused by prions. However, like viruses, they can attack only certain types of proteins, and any prions that evolved on another world are unlikely in the extreme to be able to infect terrestrial life.


I’m trying to be polite here. Cut me some slack.


The exact quotation is lost to antiquity; it may have been “Where is everybody?” which is just as pithy.


I discount UFO sightings. Despite a zillion blurry photos, obvious fakes, and shaky video, there has not been a single unequivocal piece of evidence that we have been visited by aliens, ever. Deal with it.


There is another way to be alone, as we’ll see in a moment.


This is serious: called Project Orion, it was studied in the 1960s. The acceleration isn’t smooth—getting kicked in the seat of your pants by a nuclear weapon generally isn’t—but it can build up tremendous speed. Unfortunately, the Nuclear Test Ban Treaty (chapter 4) forbids the testing of such a spaceship.


This logic means that a Star Trek—like galaxy—where there are lots of aliens at roughly the same technological level—is extremely unlikely. If life abounds in the Milky Way, civilizations are far more likely to be separated by gulfs of millions of years. Some of the aliens will be more like Q and the Organians (hugely advanced beings in the Star Trek universe), with one or two like us, and the rest not much more than extremely primitive microbes and yeasts. Another Star Trek aspect of this is the Prime Directive: the procedure to quarantine rising civilizations until they develop the capability of interstellar travel. That’s an interesting idea, but I don’t buy it: it means that every single alien species out there will obey it. It only takes one maverick to spoil the secret.


You might think that maybe they were here, 65 million years ago, and pushed the dinosaur-killer asteroid our way. But remember, they’re advanced, smart, and without pity. A rock six miles across is pretty puny. They’d have dropped something a lot bigger on us, to make sure that in another few dozen million years, those little mammals crawling around the feet of the dinosaurs wouldn’t evolve into a spacefaring threat.


In the following sections, the number of years in the future should be considered approximate, perhaps accurate to a hundred million years or so.


You might expect that the Sun’s temperature is all that affects the Earth, but its size is important too. A ball bearing as hot as the Sun, for example, wouldn’t heat the Earth at all because it’s so small. Other factors in the Earth’s temperature include its distance from the Sun, its ability to shed heat (radiating it away at night), and even how rapidly it rotates. However, all of these factors can be accommodated mathematically to produce a model of the Earth’s temperature.


If you crunch the numbers, the average temperature of the Earth today at its current distance from the Sun should be just about or below the freezing point of water. It’s warmer on Earth, on average, because we have an atmosphere. The greenhouse effect keeps us nice and toasty . . . but, of course, too much of a good thing doesn’t help.


Actually, the Earth will be drier than bone, which is roughly 15 percent water by volume.


An interesting coincidence is that life has been around on Earth for 3.5 billion years (give or take), and will continue for another 3.5 billion. We’re currently right smack in the middle of the Age of Life on Earth . . . and any problems we have now may simply be chalked up to Earth experiencing a midlife crisis. ‡My suggestion: let go.


Some studies show that the core will shrink by about 100 feet per year or so, which is not a whole lot compared with the core’s size of many hundreds of thousands of miles across.


When you take a ball of clay and throw more clay on it, it gets bigger. If you take a ball of degenerate matter and throw more on it, weirdly, it gets smaller. Quantum mechanics, it cannot be said enough, is really freaky.


Even more if my wife just made cookies.


Currently, the Sun loses only about 10−14 (one one hundred-trillionth) of its mass every year. Obviously, this is an incredibly small number.


Many older books on astronomy say that the Earth will definitely be swallowed up by the Sun when it becomes a red giant, but those works don’t account for the Sun’s mass loss through its supersolar wind and the subsequent increase in the orbital diameters of the planets.


Remember, that’s the distance from the center of the Sun. The surface will be 50 million miles or so closer.


In reality, a bigger problem might be that all the plants on Earth have evolved to make oxygen using the color of sunlight we have now. A much redder Sun may be a much larger headache for our descendants than such a trivial thing as moving the Earth.


This is actually a terrifyingly close encounter. At closest approach the asteroid would be nearly as large in the sky as the full Moon—features on the surface would be easily visible to the naked eye—and be moving so rapidly that it would cross the sky in just a few minutes.


When the Sun loses mass, Jupiter will migrate outward as well, but we’ll also be stealing its energy, which moves it inward, so it’s hard to say just where it will end up.


Yes, assuming they have any. Or hands. Or heads.


Helium fusion under these circumstances has a rate that scales as the temperature to—hold on to your hat—the 40th power. This means a teeny-tiny increase in temperature causes the rate of fusion to accelerate insanely; a 20 percent rise in temperature increases the helium fusion rate by 1,500 times!


I hate to say it, but some calculations indicate that the white-dwarf Sun won’t be bright enough to ionize the expanding gas before the material disperses into interstellar space. It’s likely that when the Sun has this final fling, it will be too dark to see.


The planet in question, orbiting the red giant HD 17092, has a mass more than four times that of Jupiter, so it’s almost certainly a gas giant with no solid surface. Therefore, to be pedantic, the temperature is 900 degrees at the top of its cloud layer.


This may seem depressing to some, but it’s a relief to me: I’m not sure I want celestial neighbors capable of engineering on that scale.


The word galaxy comes from the Greek word galaxias, meaning milk, a reference to the Milky Way Galaxy. There is some confusion over the term Milky Way; sometimes it means the galaxy itself, and sometimes the milky stream of unresolved stars you can see from your backyard. Usually it’s clear in context.


Yes, this is the same analogy used for GRBs in chapter 4. Glad you noticed! The principle is the same, so I recycled it.


There are other sources of dust as well, including supernovae, but red-giant stars near the ends of their lives are the primary source.


Like the Earth’s atmosphere, the galactic disk fades away slowly with height above (and below) the plane, so an actual thickness is hard to determine. It also depends on how you measure it; bright, massive stars tend to stick near the galactic plane, while lower-mass stars can reach great heights. So the thickness changes with what kind of star you are using to trace it.


Neutron stars can be dangerous too. Some have incredibly strong magnetic fields, quadrillions of times stronger than Earth’s, which are generated inside the star and go out through the surface. A starquake—literally, like an earthquake on the star, but measuring a terrifying 30+ on the Richter scale—can shake the magnetic field violently, creating an ultra-mega-super-duper version of a solar flare. The energy released is enormous; in December 2004 such a flare from a magnetar 50,000 light-years away hit the Earth and actually had a measurable effect on our atmosphere. Magnetars are difficult to detect and incredibly rare (only a handful exist in the Milky Way), but they may in fact be the most dangerous objects in the galaxy. They’re the mob bosses of the Milky Way.


Most of the total mass of the Universe is made up of dark matter, a name scientists have hung on a type of invisible matter about which very little is known. Its existence is inferred by its effect on the normal, visible matter in galaxies, and it makes up something like 85 percent of all matter in the Universe. More is being learned about it every day, and one of the biggest goals in modern science is to determine the nature of dark matter.


At that distance, the Sun would be totally invisible to the naked eye; you’d need a telescope to see it at all.


Pronounced “thay-ta one cee ore-ee-ON-us,” if you want to impress your friends.


Assuming the Sun’s velocity through the nebula is the same as its orbital velocity around the galaxy of 140 miles per second.


We wouldn’t actually feel it, I’ll note, since even the thickest nebula is incredibly rarefied. The Earth wouldn’t slow in its orbit or anything like that. You’d hardly notice, except for the effects outlined above.


There might be a mitigating factor: the Sun will heat up the dust surrounding it, which will in turn warm up the Earth. The exact details of this, though, are difficult to calculate, and depend on lots of niggling factors, such as the density of the cloud, its composition, and all that. Would the warm dust offset the darkening Sun enough to stop the glaciers from advancing? We simply don’t know.


More or less, that is. The planets themselves do have gravity, and they do affect one another, but only very subtly and only on very long time scales. We’ll be returning to this idea in a moment.


Provocative in the literal sense as well, since these findings have provoked a flurry of papers both supporting and attacking their conclusions. I want to stress again that this periodicity in mass extinctions has not been verified, and may in fact not be real. Time will tell as more work is done.


The size of the actual energy source was known to be small because of some complicated physics involving how rapidly the source changed brightness—the bigger it is, the slower it can vary its output. Rapid fluctuations in the energy emission from 3C273 and other quasars made it clear that the source of their prodigious energy must be on the same scale as our solar system—tiny when compared to an entire galaxy.


Some very distant quasars have SMBHs estimated to have as much as 10 billion solar masses, but these have yet to be confirmed.


Because light travels at a finite speed, we see a distant object as it appeared in the past. It takes light 8.3 minutes to get to us from the Sun, so we see it as it was 8.3 minutes ago. We see a galaxy 10 billion light-years away as it was when the Universe was very young, only a few billion years old. In effect, telescopes are time machines. In reality—and as usual when dealing with relativity, time, and space—the situation is more complicated than this, but it’s not terrible to think of distance (in light-years) as equal to time (years in the past).


There is one other spiral in the group, called M33 or the Pinwheel galaxy. Although it’s a spiral like the Milky Way and Andromeda, it has only a fraction of the mass, so it’s not a big player like us.


Actually, many of the other galaxies in the Local Group are bound to us as well, but again, they are much smaller.


To be specific, this should say “collisions between large galaxies.” Big galaxies eat small ones all the time; the Milky Way is cannibalizing two dwarf galaxies right now. Both have been torn apart by the galaxy’s gravity, and their stars are slowly becoming integrated with the original Milky Way population. This has happened many times in the past as well.


One prediction of Einstein’s relativity is that merging black holes will actually cause a ripple in the fabric of space and time, like taking a bedsheet and frantically whipping it up and down. However, the gravitational waves resulting from two SMBHs merging are probably not strong enough to have any real effect on stars and other matter around them.


On the other hand, you can argue that since the Universe is all there is, everything there is, then the explosion happened everywhere all at once, and so it was big. That’s just semantics, though.


That means 0.0000000000000000000000000000000000000000001 second old.


To paraphrase the great philosopher-scientist Nigel Tufnel from This Is Spinal Tap: “How much more north could it be? The answer is none. None more north.”


Anything’s possible.


Literally, the creation of new nuclei, new elements.


A little math: Like gravity, the brightness of a star decreases with the square of the distance to the star—double the distance to a star and it will appear one-quarter as bright. But if stars are distributed evenly throughout the Universe, you’re basically adding up all the light from stars at a given distance from you, and they form the surface of a sphere. The area of the surface of a sphere depends on the square of its radius. So brightness drops with the distance squared, and the number of stars goes up with the distance squared—canceling each other out.


I am using the word theory as a scientist means it: a set of ideas so well established by observations and physical models that it is essentially indistinguishable from fact. This is different from the colloquial use that means “guess.” To a scientist, you can bet your life on a theory. Remember, gravity is “just a theory” too.


Light reflecting off water and metal can get polarized as well. Sunglasses that are polarized can block just the type of light waves that are aligned in that way, greatly reducing the glare of light reflected off cars and puddles.


Gas does get recycled in galaxies: stars explode, other stars lose mass in a stellar wind, and so on. It’s possible in some galaxies that stellar birth may continue for as long as another trillion years, but those are the exceptions, not the rules. In a trillion years or so, star formation will effectively cease.


In fact, it’s exactly like that: gas in a dwarf circulates in precisely this manner.


In reality, the explanation of this is far more complicated and involves invoking Einstein’s theory of relativity. I’ll spare you that and just leave you with the treadmill analogy, which is close enough.


Remember, as discussed earlier, that space can expand faster than the speed of light. The distant galaxies aren’t really moving faster than light; the space in between the galaxy and us is expanding such that it appears the galaxy is moving faster than light. Think of it as the track on the treadmill stretching as you’re running on it.


I’m at a loss for a name for this galaxy . . . MilkLocalGroupeda? Androgroupyway?


There are some theories stating that, depending on what’s driving the acceleration, the Universal expansion may overwhelm gravity. Eventually, all of space will stretch, including space in between bound gravitational objects. If that’s the case, then the horizon will continue to move in while space stretches. Eventually, everything will stretch—galaxies, stars, planets, even atoms. At some point, everything will get torn asunder as space itself shreds apart. For some reason, scientists call this idea the Big Rip. This turn of events seems pretty unlikely given what we know about the Universe, but it’s something to consider.


There will still be planets; they will orbit white dwarfs and brown dwarfs, and probably many more will wander the Universe after being ejected from their home stars during planetary formation. However, planets don’t generate energy, so they aren’t of much interest to us here. They’ll be frozen solid.


Binary brown dwarfs are common: two brown dwarfs in orbit around each other. Owing to some weird effects of Einstein’s relativity, the orbits of the objects will slowly decay with time. For a typical pair, the two objects will collide after about 1019 years, which is in the Degenerate Era. A merging of two brown dwarfs this way will almost certainly create a disk of material around them in the same way as an off-center collision would. This may be a “common” event during this era.


Astronomers use the term collision to mean any encounter where two or more objects interact with each other through gravity, and not necessarily to mean direct physical contact like a car crash.


Unlike smaller black holes, which will tear a star apart because of tidal forces, a supermassive black hole’s tides are far, far smaller, so stars will get eaten whole. There will be no accretion disk, and so no light emitted by the consumption of a star. Eating gas clouds, on the other hand, still will cause the black hole cosmic indigestion.


We don’t have to wait that long to see one decay: if we collect enough together, say 1037 of them, then we should see one decay every year. This has been attempted, and still no protons have decayed while scientists watched. If the decay time is off by a little bit—say it’s 1038—then this makes the process more difficult to detect . . . but 1038 years is still small compared to the time we’re talking about in this chapter.


Or whatever non-proton-based food they eat while watching movies.


Because of the bizarre nature of degenerate matter, lower-mass objects actually increase in size when they lose mass, the opposite of what we expect. White dwarfs start out roughly the size of the Earth, but in 1039 years or more, they’ll actually expand to be as big as Jupiter.


This released energy mostly gets converted into sound (footsteps) and motion (momentum as you travel down).


Yes, I had to look up those two words.