Astrophysics for People in a Hurry (Astrophysics for People in a Hurry Series)

by Neil deGrasse Tyson

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So prevalent is this tendency that often we assume something is spherical in a mental experiment just to glean basic insight even when we know that the object is decidedly non-spherical.

Spheres in nature are made by forces, such as surface tension, that want to make objects smaller in all directions.

For large cosmic objects, energy and gravity conspire to turn objects into spheres. Gravity is the force that serves to collapse matter in all directions, but gravity does not always win—chemical bonds of solid objects are strong.

So, contrary to what it looks like to teeny humans crawling on its surface, Earth, as a cosmic object, is remarkably smooth.

in spite of Earth’s mountains and valleys, as well as being slightly flattened from pole to pole, when viewed from space, Earth is indistinguishable from a perfect sphere.

The cosmic mountain-building recipe is simple: the weaker the gravity on the surface of an object, the higher its mountains can reach.

If a solid object has a low enough surface gravity, the chemical bonds in its rocks will resist the force of their own weight. When this happens, almost any shape is possible. Two famous celestial non-spheres are Phobos and Deimos,

In space, surface tension always forces a small blob of liquid to form a sphere. Whenever you see a small solid object that is suspiciously spherical, you can assume it formed in a molten state. If the blob has very high mass, then it could be composed of almost anything and gravity will ensure that it forms a sphere.

The stars of the Milky Way galaxy trace a big, flat circle. With a diameter-to-thickness ratio of one hundred to one, our galaxy is flatter than the flattest flapjacks ever made.

the mid-plane and is responsible for all subsequent generations of stars, including the Sun. The current Milky Way, which is neither collapsing nor expanding, is a gravitationally mature system where one can think of the orbiting stars above and below the disk as the skeletal remains of the original spherical gas cloud.

This general flattening of objects that rotate is why Earth’s pole-to-pole diameter is smaller than its diameter at the equator. Not by much: three-tenths of one percent—about twenty-six miles.

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

The sphere to end all spheres—the largest and most perfect of them all—is the entire observable universe. In every direction we look, galaxies recede from us at speeds proportional to their distance. As we saw in the first few chapters, this is the famous signature of an expanding universe, discovered by Edwin Hubble in 1929. When you combine Einstein’s relativity and the velocity of light and the expanding universe and the spatial dilution of mass and energy as a consequence of that expansion, there is a distance in every direction from us where the recession velocity for a galaxy equals the speed of light. At this distance and beyond, light from all luminous objects loses all its energy before reaching us. The universe beyond this spherical “edge” is thus rendered invisible and, as far as we know, unknowable.

Herschel inadvertently discovered “infra” red light, a brand-new part of the spectrum found just “below” red,

In 1801 the German physicist and pharmacist Johann Wilhelm Ritter found yet another band of invisible light.

Sure enough, the pile in the unlit patch darkened more than the pile in the violet patch. What’s beyond violet? “Ultra” violet, better known today as UV.

Filling out the entire electromagnetic spectrum, in order of low-energy and low-frequency to high-energy and high-frequency, we have: radio waves, microwaves, infrared, ROYGBIV, ultraviolet, X-rays, and gamma rays.

We can now observe phenomena ranging from low-frequency radio waves a dozen meters long, crest to crest, to high-frequency gamma rays no longer than a quadrillionth of a meter. That rich palette of light supplies no end of astrophysical discoveries: Curious how much gas lurks among the stars in galaxies? Radio telescopes do that best. There is no knowledge of the cosmic background, and no real understanding of the big bang, without microwave telescopes. Want to peek at stellar nurseries deep inside galactic gas clouds? Pay attention to what infrared telescopes do. How about emissions from the vicinity of ordinary black holes and supermassive black holes in the center of a galaxy? Ultraviolet and X-ray telescopes do that best. Want to watch the high-energy explosion of a giant star, whose mass is as great as forty suns? Catch the drama via gamma ray telescopes.

From a distance, our solar system looks empty. If you enclosed it within a sphere—one large enough to contain the orbit of Neptune, the outermost planet†—then the volume occupied by the Sun, all planets, and their moons would take up a little more than one-trillionth the enclosed space.

Interplanetary space is so not-empty that Earth, during its 30 kilometer-per-second orbital journey, plows through hundreds of tons of meteors per day—most of them no larger than a grain of sand.

One substantial hunk of junk led to the formation of the Moon. The unexpected scarcity of iron and other higher-mass elements in the Moon, derived from lunar samples returned by Apollo astronauts, indicates that the Moon most likely burst forth from Earth’s iron-poor crust and mantle after a glancing collision with a wayward Mars-sized protoplanet.

Most of the solar system’s asteroids live and work in the main asteroid belt, a roughly flat zone between the orbits of Mars and Jupiter.

A simple calculation reveals that most of them will hit Earth within a hundred million years.

The Kuiper belt is a comet-strewn swath of circular real estate that begins just beyond the orbit of Neptune, includes Pluto, and extends perhaps as far again from Neptune as Neptune is from the Sun.

a spherical reservoir of comets called the Oort cloud,

Unlike Kuiper belt comets, Oort cloud comets can rain down on the inner solar system from any angle and from any direction.

If we had eyes that could see magnetic fields, Jupiter would look five times larger than the full Moon in the sky.

Earth’s Moon is about 1/400th the diameter of the Sun, but it is also 1/400th as far from us, making the Sun and the Moon the same size in the sky—a coincidence not shared by any other planet–moon combination in the solar system, allowing for uniquely photogenic total solar eclipses.

The Sun loses material from its surface at a rate of more than a million tons per second. We call this the “solar wind,” which takes the form of high-energy charged particles. Traveling up to a thousand miles per second, these particles stream through space and are deflected by planetary magnetic fields. The particles spiral down toward the north and south magnetic poles, forcing collisions with gas molecules and leaving the atmosphere aglow with colorful aurora.

Newton’s formula specifically prescribes that, while the gravity of a planet gets weaker and weaker the farther from it you travel, there is no distance where the force of gravity reaches zero. The planet Jupiter, with its mighty gravitational field, bats out of harm’s way many comets that would otherwise wreak havoc on the inner solar system. Jupiter acts as a gravitational shield for Earth, a burly big brother, allowing long (hundred-million-year) stretches of relative peace and quiet on Earth. Without Jupiter’s protection, complex life would have a hard time becoming interestingly complex, always living at risk of extinction from a devastating impact.

The nearest exoplanet—the nearest planet in orbit around a star that is not the Sun—can be found in our neighbor star system Alpha Centauri, about four light-years from us and visible mostly from our southern hemisphere.

The method works because every element, every molecule—no matter where it exists in the universe—absorbs, emits, reflects, and scatters light in a unique way. And as already discussed, pass that light through a spectrometer, and you’ll find features that can rightly be called chemical fingerprints. The most visible fingerprints are made by the chemicals most excited by the pressure and temperature of their environment. Planetary atmospheres are rich with such features. And if a planet is teeming with flora and fauna, its atmosphere will be rich with biomarkers—spectral evidence of life. Whether biogenic (produced by any or all life-forms), anthropogenic (produced by the widespread species Homo sapiens), or technogenic (produced only by technology), such rampant evidence will be hard to conceal.

Another readily detected biomarker is Earth’s sustained level of the molecule methane, two-thirds of which is produced by human-related activities such as fuel oil production, rice cultivation, sewage, and the burps and farts of domestic livestock. Natural sources, comprising the remaining third, include decomposing vegetation in wetlands and termite effluences.