Astrophysics for Young People in a Hurry Read online

  The Coma cluster is also what we call a “relaxed” system. Please resist the image of a group of galaxies kicking back and listening to smooth jazz. The “relaxing” I’m referring to here is of a different kind. It means many things, including the fact that you can guess the mass of the system by studying the speed and direction of the galaxies moving around within it. You don’t have to see every massive object, however. By tracking those galaxies, scientists can also guess how much unseen or “dark” matter is hiding in the system, changing how those galaxies move.

  It’s for these reasons that relaxed systems make excellent probes of dark matter. Allow me to make an even stronger statement: were it not for relaxed systems, we might not have figured out that dark matter is everywhere in the cosmos.

  The largest and most perfect sphere of all is the observable universe, or that part of the cosmos we can see with our telescopes.

  In every direction we look, galaxies are racing away from us. The farther away the galaxy, the faster it’s moving. There is a distance in every direction from us where objects are moving away from us as fast as the speed of light. At this distance and beyond, light that shines from objects such as stars loses all its energy before reaching us. Crossing the expanding cosmos, the light becomes stretched out and dulled. And if the light from such objects cannot reach us, then these objects are no longer observable. Since these limits extend in all directions, they form a sphere.

  The universe beyond this spherical “edge” is invisible to us and, as far as we know, unknowable. But that shouldn’t stop you from wondering what might lie out there in the great beyond.

  9.

  The Invisible Universe

  On November 11, 1572, the Danish astronomer Tycho Brahe was out for an evening stroll when he noticed a spectacular new object in the sky. Brahe, who once had part of his nose sliced off in a duel, did not use telescopes to study the stars. Neither did the other astronomers of his day. Yet Brahe had been watching the heavens long enough to know that this object was a newcomer to the night sky.

  What Brahe spotted that night was an exploding star known as a supernova.

  Most supernovas appear in distant galaxies, but when a star blows up within our own Milky Way galaxy, it is bright enough for everyone to see, even without a telescope. Indeed, the wondrous visible light of that 1572 explosion was widely reported. Another supernova event, in 1604, caused a similar sensation. Unfortunately, these were the last two supernova spectaculars hosted by our galaxy.

  Tycho’s Nose

  The famed astronomer Tycho Brahe didn’t lose his nose in just any old duel. Apparently the fight sprang from an argument about math. He also lived in a castle and kept an elk as a pet. As for the fake nose he wore for most of his life, it was rumored to be made of silver or gold, but scientists actually dug up the famous scientist’s remains a few years ago, and they found traces of brass around the bones of his nose. There are also rumors—still unconfirmed—that he may have been murdered. I assure you, friends, that the life of a modern astrophysicist is not nearly as dramatic.

  Today we rely on powerful telescopes to study exploding stars in distant reaches of the universe. Every bit of information a telescope delivers to the astrophysicist comes to Earth on a beam of light. But supernovas don’t just release visible light, the kind that’s convenient for the human eye. Some of the light they send our way is, to our eyes, invisible.

  Our modern telescopes can catch all types of light, and without them, astrophysicists would be totally unaware of some mind-blowing stuff in the universe.

  Before 1800 the word “light,” apart from its use a verb and an adjective, referred just to visible light. But early that year the English astronomer William Herschel, already well known for discovering the planet Uranus* in 1781, was exploring the relation between sunlight, color, and heat. Herschel began by placing a prism, a glass instrument that separates light into its different colors, in the path of a sunbeam. Nothing new there. Sir Isaac Newton had done that back in the 1600s, leading Newton to name the familiar seven colors of the rainbow: red, orange, yellow, green, blue, indigo, and violet.

  Newton had used the prism to separate a sunbeam into its colors, but Herschel was curious enough to wonder if the temperature of each color might be different. So he placed thermometers in various regions of the rainbow. Sure enough, he showed that different colors registered different temperatures. The red light was warmer than the violet, for example.

  Yet he also laid a thermometer outside the colors, beyond the red light. He guessed it would read no more than room temperature. But that’s not what happened. The temperature of this thermometer rose even higher than the one placed in the red. That meant the sunbeam was hiding some new type of light, in addition to the colors he’d been studying.

  Roy G. Biv

  An easy trick to remember the order of the colors is to take the first letter of each, which combined spell out the name Roy G. Biv. Of course, Mr. Biv is an imaginary character, but I suspect he would have an impressive moustache, and maybe use a walking stick.

  An invisible beam of light.

  Herschel had accidentally discovered “infra” red light, a brand-new part of what we call the electromagnetic spectrum—a larger version of the rainbow that includes both visible and invisible light. Other investigators immediately took up where Herschel left off. In 1801 a German physicist found proof of invisible light next to the violet end of the spectrum. What’s beyond violet? “Ultra” violet, better known today as UV.

  Filling out the rest of the spectrum, in order of low energy and low frequency to high energy and high frequency, we have radio waves, microwaves, infrared, Roy G. Biv, ultraviolet, X-rays, and gamma rays. While many of these forms of light were new or unfamiliar to the scientists of old, today we have learned to use and study all of them.

  Mysteriously, astrophysicists were a bit slow to build telescopes that could see all these forms of invisible light. For more than three centuries, scientists thought of a telescope as a way to strengthen our limited eyesight, like a pair of cosmic spectacles. The bigger the telescope, the more distant the objects it brings into view; the more perfectly shaped its mirrors, the sharper the image it makes. But these new forms of light required new hardware. Detecting X-rays, for example, requires super-smooth mirrors. If you’re gathering long radio waves, your detector doesn’t need to be that precise, but it should be as big as you can afford to make it.

  Supernovas send out all kinds of visible and invisible light, but no single combination of telescope and detector can see all of them at the same time. The way around that problem is simple: gather pictures from multiple telescopes and stitch them all together. Although we can’t “see” invisible light, we can assign certain colors to different types of light, and create a single image that combines the findings of all the different telescopes and detectors.

  This is precisely what I did for my friend Superman. In the comic, that is. When he visits me and my associates at the Hayden Planetarium, I explain that we hadn’t just gathered information from our telescopes. To observe the death of his home planet’s sun, we asked observatories across the world to point toward Superman’s home. Gathering all the information collected by so many telescopes and detectors and pasting them into a single, visible image is an enormous challenge. In the story, in fact, this job was too much for the planetarium’s computers. So Superman himself—whose mind is apparently a supercomputer—stitches them together to reveal a picture of his sun exploding in visible, infrared, and other forms of light.

  I know people are into the whole bulletproof, laser eyes, flight thing. But processing that much astrophysical data faster than a supercomputer?

  That is real power.

  The earliest telescopes built to look for invisible light were radio telescopes. They are an amazing type of observatory. The American engineer Karl G. Jansky built the first successful one between 1929 and 1930. It looked a bit like the moving sprinkler system on a farmerless farm. Made from a series of tall, rectangular metal frames, it turned in place like a merry-go-round, but on wheels built with spare parts from a Model T Ford, a popular car from a few years earlier. Jansky had set up the hundred-foot-long contraption to capture a wavelength of about fifteen meters.

  Karl Jansky’s telescope was compared to a merry-go-round because it rotated as it captured radio waves from the cosmos.

  At the time, scientists believed radio waves came only from local thunderstorms or other sources on Earth. Using his strange antenna, Jansky discovered that radio waves can also be traced to the center of the Milky Way galaxy. With that observation, radio astronomy was born.

  Scientists were finally watching the sky for more than visible light.

  Modern radio telescopes are sometimes downright humongous. MK 1, which began its working life in 1957, is the planet’s first genuinely gigantic radio telescope—a single, steerable, 250-foot-wide, solid-steel dish at the Jodrell Bank Observatory near Manchester, England. The world’s largest radio telescope, completed in 2016, is called the Five-hundred-meter Aperture Spherical radio Telescope, or “FAST” for short. At a cost of one hundred and eighty million dollars, it was built by China in Guizhou Province, and is larger in area than thirty football fields.

  The 250-foot-wide MK 1 telescope, located in England, began searching for radio waves in 1957.

  If aliens ever give us a call, the Chinese will be the first to know.

  Searching for microwaves, we’ve got the sixty-six antennas of ALMA, the Atacama Large Millimeter Array, in the remote Andes Mountains of northern Chile, in South America. ALMA allows astrophysicists to follow cosmic action that can’t be seen with other telescopes. We can watch giant gas clouds transform into the nurseries from which stars are born.

  ALMA is located, by intention, in the most arid landscape on Earth—three miles above sea level and well above the wettest clouds. The water vapor in Earth’s atmosphere chews up the microwave signals that ALMA and other detectors try to catch. Astrophysicists want these signals to reach our telescopes with as little interference as possible. So if you want clean observations of cosmic objects, you must minimize the amount of water vapor between your telescope and the universe, just as ALMA has done.

  In the remote Andes Mountains, the sixty-six antennas of ALMA act as one giant telescope, allowing scientists to study how stars are born.

  Generally, dry skies far from major cities are a good place to observe the universe. It’s the main reason my favorite summer vacation destination as a kid, Camp Uraniborg, was based in the desert.

  We’ve covered long radio waves and microwaves. At the ultrashort-wavelength end of the spectrum you find the high-frequency, high-energy gamma rays. Discovered in 1900, they were not detected from space until a new kind of telescope was placed aboard NASA’s Explorer XI satellite in 1961.

  Anybody who reads too many comic books knows that gamma rays are bad for you. A gamma ray experiment gone wrong is the supposed reason that scientist Bruce Banner transforms into the green, muscular, rage-filled Hulk of the Avengers movies. But gamma rays are also hard to trap. They pass right through ordinary lenses and mirrors. So instead of catching them directly, the guts of Explorer XI’s telescope included a device that captured evidence of gamma rays as they raced through.

  Two years later, the United States launched a new series of satellites, the Velas, to scan for bursts of gamma rays. The United States was worried that the Soviet Union was testing dangerous new nuclear weapons. Such tests would release gamma rays, so the United States dispatched the satellites to search for evidence. The Velas indeed found bursts of gamma rays, almost daily. But Russia wasn’t to blame. The gamma ray signals were coming from explosions across the universe.

  My Least Favorite Superhero

  No, gamma rays will not transform you into a giant green monster. But that’s not what bothers me about the Hulk, scientifically. When Bruce Banner, a man of average size, turns into the Hulk, he becomes nine feet tall and weighs hundreds of pounds. Maybe more. Banner gains mass—a violation of the laws of physics. You can’t just conjure mass out of thin air. I suppose he could be transforming energy into all this new matter in his body, but if he did that, then he might knock out all the power in the surrounding city.

  Today, telescopes search for light in every invisible part of the spectrum. We can now observe low-frequency radio waves a dozen meters long, crest to crest. We can study high-frequency gamma rays no longer than a quadrillionth of a meter—an unimaginably small distance from the peak of one wave to the next.

  For the astrophysicist, these telescopes are tools for answering all kinds of questions. Curious how much gas lurks among the stars in galaxies? Radio telescopes can tell you. Interested in the cosmic background and the big bang? Microwave telescopes are critical. Want to peek deep inside galactic gas clouds, to study how stars are born? Infrared telescopes will help. How about examining black holes? Ultraviolet and X-ray telescopes are best. Want to watch the high-energy explosion of a giant star? Catch the drama via gamma ray telescopes.

  In the days of Tycho Brahe, there were so many discoveries yet to be made. But I much prefer being a skywatcher today, and not only because this is a slightly more civilized time, and no one has attempted to chop off my nose. This is an amazing time to be an astrophysicist because we know that some of the most exciting action in the universe is invisible.

  And we can see all of it.†

  * Stop laughing. Really.

  † Except for dark matter. But we’re getting there.

  10.

  Around Our Solar Neighborhood

  An alien staring back at our solar system might conclude it looks empty. The Sun, all planets, and their moons occupy a tiny fraction of the solar system. But our solar system isn’t empty. Not even close. The space between the planets contains all manner of chunky rocks, pebbles, ice balls, dust, streams of charged particles, and far-flung probes.

  Our solar system is so not-empty that Earth, as it races through its orbit, plows through hundreds of tons of meteors per day—most of them no larger than a grain of sand. Nearly all of them burn in Earth’s upper atmosphere, the layer of air that surrounds our planet. These meteors slam into the atmosphere with so much energy that they vaporize on contact. That is a good thing. Without this protective blanket of air, our ancestors might have been destroyed by space rocks long before we could evolve into our current Instagram-posting selves.

  Larger, golf-ball-size meteors often shatter into many smaller pieces before they vaporize. Still larger meteors are singed on their surface as they crash through the atmosphere, but otherwise make it all the way to the ground intact. Early in our planet’s history, so much junk rained down that the energy from the impacts melted our crust, the planet’s hard outer layer.

  Voyager 1 and Voyager 2

  Launched in 1977, these spacecraft have been racing through space ever since. In 2012, Voyager 2 became the first human-made craft to leave the solar system. Voyager 1, seen here, isn’t far behind. You can track their progress here: https://voyager.jpl.nasa.gov/mission/status/.

  One substantial hunk of space junk led to the formation of the Moon. The evidence indicates that an object the size of Mars careened off our young planet. The glancing collision sent chunks of dust and rock into orbit around Earth. This debris gradually bunched together into our lovely, low-density Moon.

  Earth was not the only object bombarded by space rocks. The many craters on the surfaces of the Moon and Mercury are evidence of past crashes. Space is filled with rocks of all sizes that were ejected from the surface of Mars, the Moon, and Earth when high-speed objects struck the surface. About a thousand tons of Martian rocks rain down on Earth each year. Perhaps the same amount reaches Earth from the Moon. So maybe we didn’t have to send astronauts to the Moon to retrieve Moon rocks. Plenty come to us.

  Cashing In on Space Rocks

  Most meteorites splash down into the oceans because water covers 72 percent of our planet’s surface. But collecting meteorites is a passionate and sometimes expensive habit. One meteorite hunter once called them “money from the sky,” and the right space rock certainly can earn you some cash. In 2012, someone sold a chunk of rock from the Moon for $330,000.

  Most of the solar system’s asteroids live in the main asteroid belt, a roughly flat zone between the orbits of Mars and Jupiter. Shaped more like a flattened donut than an actual belt, this area is often drawn as a region of cluttered, meandering rocks. Any one of a group of these asteroids, perhaps a few thousand, could one day crash into Earth. Most will hit our planet within a hundred million years. The ones larger than about a kilometer across will collide with enough energy to put most of Earth’s land species at risk of extinction.

  That would be bad.

  Comets also pose a risk to life on Earth. The most famous of them all, Halley’s comet, can be seen streaking through the night sky roughly every 75 years. This giant hunk of ice and rock is older than Earth itself and last appeared in 1986. If it were to strike our planet, it would do so with the force of ten million volcanic eruptions.

  That would be bad, too.

  But Halley won’t be back again until 2061, and it won’t pass close enough to end civilization. If you’re around then, and not too busy preparing for your trip to a Moon hotel or repairing your household robot, I suggest you find a decent telescope.