Astrophysics for Young People in a Hurry Read online

  Hydrogen

  The Vice President

  You might recognize helium from its role in birthday parties. As a gas, helium floats nearly as well as hydrogen. But hydrogen, as I mentioned earlier, is tremendously explosive. Balloons filled with hydrogen would be a very dangerous addition to a kindergartner’s birthday party. If one happened to float into a birthday candle, there would be no one left to open the presents. So we fill our balloons with helium, then suck in the strange air, speak a few lines, and sound like Mickey Mouse.

  Helium

  Helium is also the second simplest and second most common element in the universe. Like hydrogen, helium was formed during the big bang. But stars make helium, too. There isn’t nearly as much of it around as hydrogen, but there’s still four times more of it than all other elements in the universe combined.

  The Unlucky Leftover

  With three protons, lithium is the third simplest element in the universe. Like hydrogen and helium, lithium was made in the big bang, and it also helps scientists test that theory. According to the big bang model, no more than one out of every one hundred atoms in any region of the universe should be lithium. No one has yet found a galaxy with more than this upper limit. This match between our predictions and what we see through our telescopes adds to the proof that the universe really did begin with a bang.

  Lithium

  The Life-Giving Elements

  The element carbon can be found just about everywhere. Carbon is made inside stars, churned up to their surfaces, and spat out into the galaxy. You can make more molecules out of carbon than out of just about any other element. It’s one of the major ingredients of life as we know it—from microscopic plants and bugs to majestic elephants and human pop stars. Selena Gomez is a carbon-based life form.

  Carbon

  But what about life as we don’t know it? What if there are alien life forms out there in the cosmos that are built from something other than carbon and oxygen? How about life based on the element silicon? Science fiction writers love to create stories about alien, silicon-based life forms. Exobiologists, scientists who spend their time trying to understand what life on other planets might look like, have considered this possibility, too. In the end, though, we expect most life forms will be made of carbon because there’s so much more of it in the universe than silicon.

  The Heavies

  Aluminum occupies a large portion of Earth’s crust, the thick shell that surrounds our planet’s fiery center. The ancients didn’t know about aluminum. Personally, I’m fond of this element because polished aluminum can be used to make near-perfect mirrors. Telescopes have mirrors inside them to magnify and focus light, allowing astrophysicists a better view of distant objects. Aluminum is the coating of choice for nearly all telescope mirrors today.

  Titanium

  Another heavy element, titanium, gets its name from the powerful Greek gods, the Titans. Titanium is more than twice as strong as aluminum, and is used in military aircraft, prosthetics such as artificial legs or arms, and the shafts of lacrosse sticks. This element is also a good friend of the astrophysicist.

  In most places in the cosmos, oxygen outnumbers carbon. Neither molecule likes being alone, so the carbon atoms latch onto free oxygen atoms. After all the carbon has found an oxygen atom or two, there’s still some oxygen left over to bond with other elements. When oxygen bonds with titanium, the result is titanium oxide. Astrophysicists have detected traces of titanium oxide in certain stars. Recently, one group of scientists discovered a new planet surrounded by titanium oxide. We even paint parts of our telescopes with a white paint that contains titanium oxide because it helps sharpen the light from stars and other cosmic objects.

  The Star Killer

  Iron is not the most common element in the universe, but it might be the most important. Inside massive stars, tiny elements are constantly colliding and combining. Hydrogen atoms smash into each other to make helium. Then helium, carbon, oxygen, and others fuse. Eventually, the atoms inside are large enough to form iron, which has twenty-six protons and at least as many neutrons in its nucleus. Compared to hydrogen, with just one of each, that’s enormous.

  Iron

  The protons and neutrons inside the iron atom are the least energetic of any element. Yet this translates into something quite simple and exciting. Because they’re duds, they absorb energy. Normally, if you split an atom apart, it would release energy. The same happens if you jam two atoms together to make a new one.

  But iron isn’t like the other kids.

  If you split iron atoms, they will absorb energy.

  If you combine them, they will also absorb energy.

  Stars are in the business of making energy. Our Sun, for example, is an energy factory, filling the solar system with powerful photons. But as high-mass stars start to make iron in their cores, they are nearing death. More iron means less energy. Without a source of energy, the star collapses under its own weight and explodes, outshining a billion suns for more than a week. Thanks to iron, the elements cooked in its core travel across the cosmos, providing the seeds for more stars and planets.

  The Dinosaur Destroyer

  Iridium ranks as the third heaviest element in the Periodic Table. This element is rare on Earth’s surface, but there is a thin and widespread layer of it that offers evidence about our planet’s past. Sixty-five million years ago, an asteroid the size of Mount Everest collided with Earth, vaporizing on impact, and eventually killing every land creature larger than a carry-on suitcase. So, whatever might have been your favorite theory for offing the dinosaurs, a giant killer asteroid from outer space should be at the top of your list.

  Iridium

  Though rare on Earth’s surface, iridium is common in large metallic asteroids. When that giant space rock obliterated itself upon colliding with Earth, the iridium inside burst up and out in a giant cloud. The explosion scattered iridium atoms across the planet. Today, when scientists dig down below the ground and study the surface as it was sixty-five million years ago, they find a thin layer of this element spread everywhere.

  The Gods

  Some of the elements in the Periodic Table get their names from planets and asteroids, which were named after Roman gods. In the early nineteenth century, astronomers discovered two objects orbiting the Sun between Mars and Jupiter. They dubbed the first Ceres, after the goddess of harvest, and the second Pallas, for the Roman goddess of wisdom. The first element found after Ceres was named cerium, and the first element discovered after astronomers found Pallas was named palladium—the substance that Tony Stark uses to power his Iron Man exoskeleton in the movies.†

  Palladium

  Mercury, the silvery metal that’s liquid and runny at room temperature, is named for the speedy Roman messenger god of the same name. Thorium is inspired by Thor, the hunky, lightning bolt–wielding Scandinavian god. No wonder Thor and Iron Man are such good friends. They share an elemental bond.

  Mercury

  Saturn, my favorite planet,‡ has no element named for it, but Uranus, Neptune, and Pluto, all gods of Roman myths, are famously represented. Uranium was the main ingredient in the first atomic bomb ever used in warfare. Just as Neptune comes right after Uranus in the solar system, so too does neptunium come right after uranium in the Periodic Table.

  Uranium

  The next element in the table, plutonium, is not found in nature. But scientists figured out how to make enough of it to pack into an atomic bomb, which the United States exploded over the Japanese city of Nagasaki, just three days after they dropped a uranium-fueled bomb on Hiroshima, bringing a swift end to World War II. Small quantities of certain kinds of plutonium could one day be used to fuel spacecraft that travel to the outer solar system.

  Plutonium

  So ends our cosmic journey through the Periodic Table of Chemical Elements, right to the edge of the solar system, and beyond. For reasons I have yet to understand, many people don’t like chemicals. Perhaps the names just sound dangerous. But in that case we should blame the chemists, not the chemicals themselves. Personally, I am quite comfortable with chemicals. My favorite stars, as well as my best friends, are all made of them.

  * I still am.

  † Sorry, but this is pure fiction. The real palladium couldn’t supply nearly endless stores of energy. Plutonium (see the next page) would be more feasible, but it’s also highly radioactive, so Iron Man would become horribly ill or die before saving the world.

  ‡ Actually, Earth is my favorite planet. Then Saturn.

  8.

  Why the World Is Round

  The planet Saturn pops into my mind every time I take a bite of a hamburger. There’s nothing very planetary about the food on its own. But the shape of the burger, and the top bun especially, is cosmic. It reminds me of how much the universe loves the perfectly round balls known as spheres, and how these circular objects change as they spin.

  Saturn, for example. This jumbo planet spins around much faster than Earth. Your day is twenty-four hours long because it takes a given spot on our planet, such as where you’re sitting or standing right now, twenty-four hours to complete one rotation. Earth carries anything on its equator, which is the planet’s waistline, at 1,000 miles per hour. That probably sounds fast. An airliner only travels at around 550 miles per hour. But neither of those speeds compares to Saturn. My second favorite planet completes a day, or one complete rotation, in just ten and a half hours. And Saturn is much, much larger than Earth, too. So to complete that turn in time, Saturn’s equator revolves at 22,000 miles per hour.

  If our planet spun that quickly, your school day would last about twenty minutes. But summer vacation would be shorter, too, and we wouldn’t actually be here in the first place.

  Objects that rotat
e quickly tend to flatten. Earth, for example, is not a perfect sphere. Our planet spins around an imaginary line extending from the North Pole to the South Pole. The distance from one pole to the other along this line is shorter than the distance from one side of the planet through to the other, when measured at the equator. In other words, Earth is slightly flatter at the poles. And I mean slightly: the difference is only about twenty-six miles.

  Behold Saturn, my second favorite planet! A single day on Saturn lasts only ten and a half hours.

  Why Santa Should Vacation in Ecuador

  If Earth rotated just sixteen times faster, then centrifugal forces, the same ones that push riders out to the borders of a merry-go-round or keep water in a bucket as you swing it around in a circle, would make everything at the equator weightless. Even now, at Earth’s current rotational speed, chubby Santa Claus would weigh about a pound less on the equator than he would at the North Pole, where centrifugal force has no effect. Everybody likes to feel better about themselves on vacation, so if you’re looking for Santa in the off-season, I’d start there.

  The faster an object spins, the more it flattens. Which brings me back to hamburgers. Since Saturn spins at 22,000 miles per hour, the planet is a full ten percent flatter, pole to pole, than its middle. The difference is noticeable even through a small amateur telescope. Far from a perfect sphere, Saturn is more like a burger, with a wide center and a flattened bun on top.

  Trivia

  Flattened spheres are called oblate spheroids. Earth is an oblate spheroid, and so is Saturn.

  The universe loves spheres. Apart from crystals and broken rocks, not much else in the cosmos naturally comes with sharp angles. While many objects have peculiar shapes, the list of round things is practically endless and ranges from simple soap bubbles to galaxies and beyond.

  The physical laws that guide the universe favor spheres over other shapes. Surface tension, for example. This force pulls the materials in the surface of an object closer together. Consider a soap bubble. The bubble itself is made of soap and water. Inside, a pocket of air is trapped. The surface tension of the liquid that makes a soap bubble squeezes air in all directions. It will, within moments of being formed, trap the volume of air using the least possible surface area. This makes the strongest possible bubble because the soapy film will not have to be spread any thinner than is absolutely necessary. And the shape that has the smallest surface area for an enclosed volume is a perfect sphere.

  In fact, billions of dollars could be saved annually on packaging materials if all shipping boxes and all packages of food in the supermarket were spheres. The contents of a super-jumbo box of Cheerios would fit easily into a spherical carton with just a four-and-a-half inch radius. But nobody wants to chase packaged food down the aisle after it rolls off the shelves.

  On orbiting space stations, where everything is weightless because of the lack of gravity, you can gently squirt out precise amounts of molten—or liquid—metal, and the little beads just float in midair. Once they cool, they start to harden, and surface tension forms them into absolutely perfect spheres.

  For large cosmic objects such as planets and stars, surface tension is less important. Energy and gravity work to turn these objects into spheres. Gravity does not only pull apples from trees or warp space. It tries to collapse matter in all directions, shrinking it into a smaller and smaller space. But gravity does not always win—the chemical bonds of solid objects are strong. The Himalayas, the world’s largest mountain range, grew against the force of Earth’s gravity because of the powerful rocks in our planet’s crust.

  The Himalayas, Earth’s largest mountain range, couldn’t grow any taller. Gravity would pull them down.

  Before you get excited about Earth’s mighty mountains, you should know that compared with other planets, Earth has a fairly flat surface. To teeny humans hiking the Himalayas, our mountains seem giant. To a city kid like myself, a large hill can seem enormous. You would think that when viewed from a distance, Earth would look bumpy due to all the great mountains. But Earth, as a cosmic object, is remarkably smooth. If you had a super-duper, jumbo-gigantic finger, and you dragged it across Earth’s surface (oceans and all), Earth would feel as smooth as a cue ball from a game of pool. Globes that show raised portions of Earth’s mountain ranges are total exaggerations of reality. In spite of Earth’s mountains and valleys, as well as being flattened slightly from pole to pole, when viewed from space, Earth looks like a perfect sphere.

  Sure, we have towering peaks and low valleys, but when viewed from space, our planet looks like a perfectly flat sphere.

  Earth’s mountains are also puny when compared with some other mountains in the solar system. The largest on Mars, Olympus Mons, is 65,000 feet tall and nearly 300 miles wide at its base. It makes Alaska’s Mount McKinley look like a molehill. Even Mount Everest is less than half as tall.

  Unfair, you say? How could the Martians be so lucky? The cosmic mountain-building recipe is simple: the weaker the gravity on the surface of an object, the higher its mountains can reach. Mount Everest is about as tall as a mountain on Earth can grow before the lower rock layers crumble under its weight. Any higher and gravity would pull it down.

  Mars, on the other hand, has much lower gravity than Earth. A 70-pound fourth grader would weigh only 26 pounds on Mars. Because there is less gravity, the mountains can grow taller, which is why Olympus Mons towers so high.

  The stars that decorate a clear night sky are round, too. They’re big, massive blobs of gas, formed into near-perfect spheres thanks to gravity. But if a star gets too close to another object whose gravity is strong, the other object starts to strip away some of its material. This is common with binary stars, the pairs that are bound to each other by gravity, especially when one of them is an enormous dying star called a red giant. The other star in the pair starts sucking material from the red giant, distorting it into a shape that resembles a stretched-out Hershey’s kiss.

  The spinning neutron star in this artist’s version of a binary system sucks material from its dying neighbor, a glowing red giant.

  Now we’re going to get weird.

  Imagine stuffing about a hundred million elephants into a tube of ChapStick.

  To reach this density, you must do some intense work. Inside atoms, protons and neutrons are packed into the center while the electrons orbit the outside. Between those orbiting electrons and the tightly packed center of atoms, there’s empty space. To squeeze all those elephants into a container of lip balm, you must compress all the empty space between the electrons and the center of atoms. Doing so will crush nearly all (negatively charged) electrons into (positively charged) protons, creating a ball of (neutrally charged) neutrons.

  Meet the pulsar, another of my favorite cosmic objects. It is made from gas clouds, not elephants, but it is just as dense as our pachyderms-in-the-tube example, and has crazy-high surface gravity. A mountain on a pulsar might grow no taller than the thickness of this page. But because of gravity, it would take you more energy to climb that tiny hill than a rock climber on Earth would need to scale a three-thousand-mile-high cliff.

  This neutron star, a pulsar named Vela, spins faster than the blades of a helicopter.

  We expect pulsars to be the most perfectly shaped spheres in the universe.

  Galaxies are organized into clusters, and the shape of these clusters varies. Some are raggedy. Others are stretched thin in threads. Yet others form vast sheets. But the beautiful Coma cluster of galaxies, which we met in our chapter on dark matter, forms a beautiful sphere.