Astrophysics for People in a Hurry

The smallest shape to contain anything is always a sphere, which also evenly distributes the container material—for instance, a soap bubble—so it’s evenly strong everywhere. Very small liquid objects become spherical due to surface tension, where the surface material tends to stick to itself. Very large objects, like planets and stars, tend to become spherical due to gravity, which pulls at everything until all the surface is roughly the same distance from the object’s center. Despite its tall mountains, Earth from a distance is as smooth and spherical as a cue ball. If pairs of stars get too close, one will pull gas from the other and they’ll look a bit like a dumbbell; a very large star next to a small one will look like a Hershey’s kiss.

Our Milky Way galaxy started as a huge blob of gas; as gravity contracted it, the galaxy spun faster and faster until it flattened and became thin like a tortilla. Earth spins on its axis too, but it’s fairly solid and rotates only at about 1,000 miles an hour, and it’s wider at the equator than through the poles by only 26 miles. Saturn’s equator spins at 22,000 miles an hour, and the gaseous planet is 10% wider at the equator.

The most perfectly shaped spheres in the universe are pulsars, which are the spinning remnants of large stars whose gravity has crushed the remains into a perfectly smooth ball of neutrons. It’s so dense that a Chapstick casing would have to contain a billion elephants to weigh as much.

Some clusters of galaxies haven’t yet had time to coalesce into spherical shapes. Instead, they’re ragged collections of filaments or sheets. Other clusters have become spherical, their galaxies traveling this way and that, so they haven’t settled into a spinning disk.

The biggest sphere of all is the universe, which expands in all directions, so the farthest objects hurtle away faster than their light can reach us. These outer regions are beyond our ability to sense them.

In 1800, William Herschel used a prism to split sunlight, and he tested each color to see which was warmest. As a control, he left a thermometer at a spot just below the red color. This one turned out to be the warmest. Herschel had discovered an invisible form of light, infrared, that transmits heat. The following year, another scientist, Johann Ritter, put a light-sensitive chemical just above the violet coming from a prism; it recorded another invisible light form, ultraviolet.

Since then, people have discovered and used other types of radiant energy, so “in order of low-energy and low-frequency to high-energy and high-frequency, we have: radio waves, microwaves, infrared, ROYGBIV [visible light], ultraviolet, X-rays, and gamma rays” (151).

Astronomers have constructed telescopes that detect all types of photons. X-rays are very high-frequency, and the waves of their vibrations are very short, so detectors must be small, super-precise, and smooth. Radio antennas, at the other end of the spectrum, stretch very wide to catch the long waves of radio photons; they don’t need to be perfectly polished.

The first radio telescope, finished in 1930, was 100 feet wide. It was used to study sources of noise that might interfere with human-made radio transmissions; it also found radio noise coming from the central core of the Milky Way galaxy. The largest single radio telescope, built in China in 2016, is 500 meters, or 1,640 feet, wide. Even larger is the very large array in New Mexico that combines 27 reflectors across 22 miles to make a supersized radio telescope. Largest of all is a group of 10 dishes, each 82 feet wide, spread out from Hawaii to the Caribbean: Their combined signals effectively create a telescope 5,000 miles wide.

ALMA is a large array of telescopes, 66 in all, located high in the Chilean desert. It studies microwaves just a few millimeters wide that come from interstellar gas clouds and other objects.

Gamma rays are a tiny fraction of a millimeter in wavelength; they go right through telescopes. To detect them, scientists use scintillators that emit particles when struck by gamma rays. These were first used to monitor atomic test-ban treaties—nuclear bombs give off gamma rays—but the detectors also found high-energy photons coming from faraway cosmic explosions and nearby thunderstorms.

The best place for any telescope is out in space, away from the Earth’s atmosphere, which absorbs photons. All types of detectors now lie well beyond Earth and send back reports of signals from the entire electromagnetic spectrum, which is data scientists use to better understand the universe. 

The solar system is so vast that the Sun, planets, and moons take up only one trillionth of its volume. The system also contains rocks, dust, charged particles, and a number of human spaceships. Early in its history, the solar system was awash in debris that crashed into the planets. Earth took a hit from a Mars-sized object; the splash ejected the material that formed the Moon. Today, Earth still “plows through hundreds of tons of meteors per day” (166); most of them are the size of sand grains that quickly burn up in the atmosphere.

Between Mars and Jupiter is the asteroid belt, a region of orbiting rock whose total mass is only 1% of 1% of the Earth’s mass. Some asteroids have been pulled from their orbits and now cross Earth’s path, and they will eventually crash into Earth sometime during the next 100 million years. The largest of these will cause a lot of damage to Earth’s surface and threaten life here.

Just beyond Neptune and stretching twice its distance from the Sun is the Kuiper Belt, an asteroid-belt-like region that contains Pluto and millions of icy rocks. Some of these become comets that visit the inner solar system; a few of them will someday crash into Earth. Farther still is the Oort Cloud, which contains more comet material from which emerge long-period comets that take much more than a human lifetime to orbit the Sun. Jupiter is so large it deflects most of these objects and prevents them from reaching Earth.

The solar system’s moons are often more interesting than their parent planets. Our Moon happens to be just the right size to eclipse the Sun; no other moons in our system can do that. The Moon also is tidally locked to Earth: One side of it always faces us.

Jupiter’s nearest moon, Io, is so stressed by the gravity of the giant planet and other moons that its interior is heated to melting, and it has the most active volcanoes of anywhere in the solar system. Another Jupiter moon, Europa, is also stressed hot enough to have an ocean of water beneath its surface. Pluto and its moon Charon have a mutual tidal lock: They each present the same face to the other at all times.

Just as every planet is named for a Roman god, nearly every moon is named for a Greek deity. The exception is Uranus, whose discoverer, William Herschel, convinced others to name the moons after characters from Shakespeare and Alexander Pope. Thus, among Uranus’s moons are Miranda, Ariel, and Puck.

The Sun gives off 100 million tons of matter per second. Some of this strikes the planets, whose magnetic fields deflect the material. On Earth this deflection causes the northern and southern lights. These auroras also occur on Jupiter and Saturn.

The author was honored when an asteroid, 13123-Tyson, was named for him. It’s a fairly ordinary asteroid, and Tyson is relieved it’s not headed for Earth. 

From the ground, Earth contains endless things of interest—flowers, creatures, geologic features—but from an orbiting spaceship, none of these things are visible, and the major cities can barely be detected. From Uranus, three billion miles away, Earth would be visible as a dot next to a pale, tiny sun.

Using superb optics, aliens might first notice that the Earth is blue, which suggests the presence of liquid water. Between that and the white of the polar ice caps shifting in size as our seasons change and the daily shift between the blue of the huge Pacific Ocean and the brown of the continents as the planet rotates, it would be possible for them to learn a great deal about our planet, its weather, and how livable it is.

However, Earth is so small and the Sun so bright that even noticing these small details would be difficult for aliens. NASA’s Kepler telescope looks for slight, periodic changes in the brightness of nearby stars since this hints at planets that pass in front of the star as they orbit around it. Only a few faraway planetary systems are lined up so we can see this, but Kepler has found hundreds of planets in this way. From the evidence, we’ve learned those planets’ sizes, distance from their sun, and lengths of their year.

Earth is noisy in the radio and microwave spectrums: We manufacture energies from “not only traditional radio itself, but also broadcast television, mobile phones, microwave ovens, garage-door openers, car-door unlockers, commercial radar, military radar, and communications satellites” (185). Most rocky planets don’t do that, so to outside observers it’s a sign of something interesting.

If a faraway planet crosses in front of its sun, cosmochemists can use a prism to split the light passing through the planet’s atmosphere and discern the signature of its various chemicals, including those that indicate the presence of life. For aliens, signs of life on Earth might include methane, most of which comes from life forms, and oxygen, which binds quickly to all sorts of chemicals unless replenished by life.

Humans may someday wish to visit planets that can sustain life. For this reason, searching out Earth-like planets—and there may be as many as 40 billion in our galaxy—is a worthwhile activity. 

Observing the skies, the author sometimes forgets that poverty and oppression still exist on our planet. He believes, though, that the study of the universe is an internationally important effort that helps bind us together and thereby impels us to address the cruelty and prejudice that prevents everyone from contributing to such common efforts.

Humans are part of a “cosmic chain” that extends backward and forward through enormous swaths of time and dimension. We’re infinitesimally tiny compared to the universe as a whole, but we’re also gigantic compared to the microscopic life that lives inside us. For some people, the enormity of the cosmos makes them feel insignificant; for others, it simply amazes them.

Humans are proud of their intelligence, yet their brains are very similar to those of chimpanzees, who are completely unable to do calculus or other advanced forms of thought. It wouldn’t take much, then, to vastly outdo us in intellect. Humans aren’t beyond nature; they’re “neither above nor below, but within” (201).

There are more stars than grains of sand on a beach. The history of the universe comes to us from light that’s traveled billions of years to get here. The collisions of asteroids onto planets sometimes splashes up material that lands on other planets; thus, life on Earth may have come from an early, wet version of Mars.

People thought their planet was unique, but then they learned there are many planets. They thought the Sun was unique, but now we know it’s one of trillions. We thought our galaxy was the only one, but instead there are 100 billion. We believed we were part of the only universe, but it’s possible that it, too, is but one of countless many.

In short, all this information generates a “cosmic perspective,” available to all, that’s humble, spiritual without being doctrinaire, open-minded, respectful of our tiny planetary oasis, receptive to beauty as well as science, and aware of the material kinship we share with the rest of the universe.