The Cosmic Connection
by Carl Sagan
From earliest times, human beings have pondered their place in the universe. They have wondered whether they are in some sense connected with the awesome and immense cosmos in which the Earth is imbedded.
Many thousands of years ago a pseudoscience called astrology was invented. The positions of the planets at the birth of a child were supposed to play a major role in determining his or her future. The planets, moving points of light, were thought, in some mysterious sense, to be gods. In his vanity, Man imagined the universe designed for his benefit and organized for his use.
Perhaps the planets were identified with gods because their motions seemed irregular. The word "planet" is Greek for wanderer. The unpredictable behavior of the gods in many legends may have corresponded well with the apparently unpredictable motions of the planets. The argument may have been: Gods don't follow rules; planets don't follow rules; planets are gods.
When the ancient priestly astrological caste discovered that the motions of the planets were not irregular but predictable, they seem to have kept this information to themselves. No use unnecessarily worrying the populace, undermining religious belief, and eroding the supports of political power. Moreover, the Sun was the source of life. The Moon, through the tides, dominated agriculture-especially in river basins like the Indus, the Nile the Yangtze, and the Tigris-Euphrates. How reasonable that these lesser lights, the planets, should have subtler but no less definite influence on human life!
The search for a connection, a hooking-up between people and the universe, has not diminished since the dawn of astrology. The same human needs exist despite the advances of science.
We now know that the planets are worlds more or less like our own. We know that their light and gravity have negligible influence on a newborn babe. We know that there are enormous numbers of other objects-asteroids, comets, pulsars, quasars, exploding galaxies, black holes, and the rest-objects not known to the ancient speculators who invented astrology. The universe is immensely grander than they could have imagined.
Astrology has not attempted to keep pace with the times. Even the calculations of planetary motions and positions performed by most astrologers are usually inaccurate.
No study shows a statistically significant success rate in predicting through their horoscopes the future or the personality traits of newborn children. There is no field of radioastrology or X-ray astrology or gamma-ray astrology, taking account of the energetic new astronomical sources discovered in recent years.
Nevertheless, astrology remains immensely popular everywhere. There are at least ten times more astrologers than astronomers. A large number, perhaps a majority, of newspapers in the United States have daily columns on astrology.
Many bright and socially committed young people have more than a passing interest in astrology. It satisfies an almost unspoken need to feel a significance for human beings in a vast and awesome cosmos, to believe that we are in some way hooked up with the universe-an ideal of many drug and religious experiences, the samadhi of some Eastern religions.
The great insights of modern astronomy have shown that, in some senses quite different from those imagined by the earlier astrologers, we are connected up with the universe.
The first scientists and philosophers-Aristotle, for example - imagined that the heavens were made of a different sort of material then the Earth, a special kind of celestial stuff, pure and undefiled. We now know that this is not the case. Pieces of the asteroid belt called meteorites; samples of the Moon returned by Apollo astronauts and Soviet unmanned spacecraft; the solar wind, which expands outward past our planet from the Sun; and the cosmic rays, which are probably generated from exploding stars and their remnants-all show the presence of the same atoms we know here on Earth. Astronomical spectroscopy is able to determine the chemical composition of collections of stars billions of light-years away. The entire universe is made of familiar stuff. The same atoms and molecules occur at enormous distances from Earth as occur here within our Solar System.
These studies have yielded a remarkable conclusion. Not only is the universe made everywhere of the same atoms, but the atoms, roughly speaking, are present everywhere in approximately the same proportions.
Almost all the stuff of the stars and the interstellar matter between the stars is hydrogen and helium, the two simplest atoms. All other atoms are impurities, trace constituents. This is also true for the massive outer planets of our Solar System, like Jupiter. But it is not true for the comparatively tiny hunks of rock and metal in the inner part of the Solar System, like our planet Earth. This is because the small terrestrial planets have gravities too weak to hold their original hydrogen and helium atmospheres, which have slowly leaked away to space.
The next most abundant atoms in the universe turn out to be oxygen, carbon, nitrogen, and neon. These are atoms everyone has heard of. Why are the cosmically most abundant elements those that are reasonably common on Earth-rather than, say, yttrium or praseodymium?
The theory of the evolution of stars is sufficiently advanced that astronomers are able to understand the various kinds of stars and their relations-how a star is born from the interstellar gas and dust, how it shines and evolves by thermonuclear reactions in its hot interior, and how it dies. These thermonuclear reactions are of the same sort as the reactions that underlie thermonuclear weapons (hydrogen bombs): The conversion of four atoms of hydrogen into one of helium.
But in the later stages of stellar evolution, higher temperatures are reached in the insides of stars, and elements heavier than helium are generated by thermonuclear processes. Nuclear astrophysics indicates that the most abundant atoms produced in such hot red giant stars are precisely the most abundant atoms on Earth and elsewhere in the universe. The heavy atoms generated in the insides of red giants are spewed out into the interstellar medium, by slow leakage from the star's atmosphere like our own solar wind, or by mighty stellar explosions, some of which can make a star a billion times brighter than our Sun.
Recent infrared spectroscopy of hot stars has discovered that they are blowing off silicates into space-rock powder spewed out into the interstellar medium. Carbon stars probably expel graphite particles into surrounding cosmic space. Other stars shed ice. In their early histories, stars like the Sun probably propelled large quantities of organic compounds into interstellar space; indeed, simple organic molecules are found by radio astronomical methods to be filling the space between the stars. The brightest planetary nebula known (a planetary nebula is an expanding cloud usually surrounding an exploding star called a nova ) seems to contain particles of magnesium carbonate: Dolomite, the stuff of the European mountains of the same name, expelled by a star into interstellar space.
These heavy atoms-carbon, nitrogen, oxygen, silicon, and the rest-then float about in the interstellar medium until, at some later time, a local gravitational condensation occurs and a new sun and new planets are formed. This second-generation solar system is enriched in heavy elements.
The fate of individual human beings may not now be connected in a deep way with the rest of the universe, but the matter out of which each of us is made is intimately tied to processes that occurred immense intervals of time and enormous distances in space a way from us. Our Sun is a second or third-generation star. All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are made of star stuff.
Our atomic and molecular connection with the rest of the universe is a real and unfanciful cosmic hookup. As we explore our surroundings by telescope and space vehicle, other hookups may emerge. There may be a network of intercommunicating extraterrestrial civilizations to which we may link up tomorrow, for all we know. The undelivered promise of astrology-that the stars impel our individual characters - will not be satisfied by modern astronomy. But the deep human need to seek and understand our connection with the universe is a goal well within our grasp.
