On being rarefied

Natural History, Feb, 1998 by Neil de Grasse Tyson

The astronomically common yet deadly gas cyanogen was discovered in comet spectra in 1881 by Sir William Huggins. When it was later announced that Earth would pass through the tail of Halley's comet during the comet's 1910 visit to the inner solar system, gullible people were sold anti-comet pills by pharmaceutical charlatans, even though the total quantity of comet tail that Earth swept up amounted to no more than an ounce or two. One of life's important lessons: knowledge of rarefied cosmic phenomena can save you money.

The core of the Sun, where all the star's thermonuclear energy is generated, is not a place to find low-density material. But the core is a mere 1 percent of the Sun's volume. The average density of the entire Sun is only one-fourth that of Earth and only 40 percent greater than that of ordinary water. In other words, a spoonful of Sun would sink in your bathtub, but it wouldn't sink fast. In five billion years, however, the Sun's core will have fused nearly all its hydrogen into helium and will shortly thereafter begin to fuse helium into carbon. Meanwhile, the luminosity of the Sun will increase a thousand-fold while its surface temperature drops to half of what it is today. We know from the laws of physics that the only way an object can increase its luminosity while simultaneously getting cooler is for it to get bigger. The Sun as we know it will ultimately expand to a bulbous ball of rarefied gas that will completely fill, and extend beyond, the volume of Earth's orbit. The average density of the Sun will fall to less than one ten-billionth of its current density. (Of course, Earth's oceans and atmosphere will have evaporated into space, and life will have vaporized; but let's ignore these complications.) The Sun's outer atmosphere, rarefied though it will be, will manage to impede the motion of Earth in its orbit and force us on a relentless spiral inward toward thermonuclear oblivion. Knowledge of rarefied phenomena can also give you nightmares.

Going beyond our solar system, we venture into interstellar space. Humans have launched four spacecraft with enough speed to escape the Sun's gravity: Pioneer 10 and 11, and Voyager 1 and 2. The fastest among them, Voyager 2, will exceed the distance of the nearest star to the Sun in about 25,000 years. Yes, interstellar space is vast and empty. It's where you might expect to find only one atom or molecule for every two cubic centimeters. Like the remarkably visible rarefied comet tails in interplanetary space, gas clouds in interstellar space (with at least 100 times the ambient density) can readily reveal themselves in the presence of nearby luminous stars. When the light from these colorful nebulosities was first analyzed, their spectra once again revealed unfamiliar color patterns. The hypothetical element "nebulium" was proposed. A century ago there was clearly no unclaimed spot in the Periodic Table of Elements that could possibly be occupied by nebulium. As laboratory vacuum techniques improved, and as unfamiliar spectral features became routinely identified with familiar elements, suspicions grew - and were later confirmed - that nebulium was ordinary oxygen in an extraordinary state. What state was that? The atoms were each stripped of two electrons and were in the near-perfect vacuum of interstellar space.