Category Dreams of Other Worlds

In August 1929, the New York Times science section ran an article titled “The Star Stuff that is Man.” Astronomer Harlow Shapley had been popularizing the point that humans are the mere by­products of stars. In a radio talk series a few years earlier produced by the Harvard College Observatory, Shapley pointed out that “we are made out of the same materials that constitute the stars.”26 In the Times article, Shapley similarly observed: “We are made of the same stuff as the stars, so when we study astronomy we are in a way only investigating our remote ancestry and our place in the universe of star stuff. Our very bodies consist of the same chemical

elements found in the most distant nebulae.”27 At the time, some were disconcerted to think that humans might be little more than the product of fission and fusion occurring in stars.28

Cosmologists have good evidence that all the hydrogen and most of the helium in the universe have existed since close to the time of the big bang. Hydrogen, the simplest element, is truly primor­dial, while most of the helium was formed soon after in a process known as primordial nucleosynthesis or big bang nucleosynthe­sis. “The primordial nuclei of the matter constituting the universe were formed in the first three minutes,” explains CERN theoretical physicist Luis Alvarez Gaume. “The cosmic oven produced a num­ber of nuclei, made up of about 75% hydrogen and 24% helium. Small amounts of deuterium, tritium, lithium and beryllium were also produced, but hardly any of the other atoms that make up our bodies and the matter around us: carbon, nitrogen, oxygen, sili­con, phosphorus, calcium, magnesium, iron, etc. All of these were formed in the cosmic ovens of subsequent generations of stars. As Carl Sagan put it, we are just stardust, remains of dead stars.”29

The Oxford English Dictionary lists a definition for the term stardust as “fine particles supposed to fall upon the Earth from space; ‘cosmic dust.’” The OED likewise cites one published com­mentary from 1879 that claimed “the very star-dust which falls from outer space forms an appreciable part” of the mud accumu­lated on the ocean floor.30 In fact, the estimate for interplanetary dust particles swept up each year by the surface of the Earth as it churns through space is a substantial 10,000 tons, but that’s a tiny fraction of the material added to the seafloor. The most pristine reservoirs of stardust or interplanetary dust are comets. Having formed early in the life of our Solar System approximately 10 mil­lion years after the Sun’s protoplanetary disc stabilized, comets spend most of their time in the cold extreme outskirts of the Solar System. As a result, comet nuclei are largely preserved from heat­ing, melting, and collisions with other planetary bodies. The ice composition and dust grains of comets reveal elements present in the Solar System’s primordial past.

Nuclear fusion reactions in stars create heavier elements by pro­cessing or fusing together lighter atomic nuclei. The base of the fusion “pyramid” is the fusion of hydrogen into helium, which oc­curs in the Sun and low-mass stars. Stars approximately the mass of our Sun can also synthesize helium, carbon, and oxygen, but thereafter, they never reach a dense enough or hot enough state to fuse yet heavier elements. Massive stars have the ability to fuse their hydrogen to produce helium, carbon, and oxygen, but then progress in successive stages of evolution to neon, magnesium, sili­con, sulfur, nickel, and iron. Iron is the most stable element, so typ­ically the fusion chain stops there. Still heavier elements are forged two ways: by the slow capture of neutrons in the atmospheres of massive stars and more rapidly by stellar explosions known as novae and supernovae.31 Since rarer, larger mass stars are required to generate the heaviest elements, these elements are cosmically scarce compared to the light elements hydrogen and helium.

The first generation of stars probably formed approximately 200-300 million years after the big bang.32 According to cur­rent theoretical ideas, none of those stars still exist and certainly none have yet been found. They have since exploded in novae or massive supernovae, collapsed into black holes, or have oth­erwise expended their fuel and burned out. The oldest stars cur­rently are generations removed from the first stars that shone in the universe.33 Astronomers estimate that approximately twenty – five novae occur each year in an average galaxy generating inter­stellar dust enriched with heavier metals. From the clouds of gas, dust, carbon, silicon, and metals produced by repeated generations of novae and supernovae, new stars and their attendant planets are born.

A nova is a sudden brightening of a star believed to occur when the outer layers of a star are pulled by gravity onto the surface of a companion white dwarf. Pressure due to the mass that accumu­lates on the white dwarf causes nuclear fusion of hydrogen into he­lium at its surface that in turn blows material off the surface of the white dwarf. One type of supernova is an extreme form of a nova, where mass acquired from a young companion star causes a white dwarf to begin carbon fusion and explosively detonate. Another type, a core collapse supernova, occurs when a single massive star exhausts its nuclear fuel and suffers a core collapse followed by explosive detonation. In comparison to a nova, a supernova will produce a million times the energy and can for weeks shine as brightly as all the stars of an entire galaxy. In the case of the Milky Way, that’s equivalent to about 400 billion Suns. A supernova is a prodigious alchemical event; one that detonated in a nearby gal­axy in 1987 generated enough stardust to build 200,000 Earths.34

The Milky Way formed approximately 10 billion years ago, but our Sun is only 4.5 billion years old. The Sun and its neighbor­ing stars were likely born in a nearby region of dense, coalescing interstellar gas and dust. Isotopic studies of meteorite samples in­dicate that our Sun formed from the detritus of a massive super­nova explosion in nearby space roughly 1 million years prior to the formation of the protoplanetary disc that became our Solar System.35 We know this simply by the presence of metals that our Sun could not have produced, such as the iron that makes up the Earth’s core. Other than hydrogen, which is primordial and dates back to cosmic genesis, almost all the other atoms in the universe have been recycled, silent witnesses to amazing trips through fiery cauldrons and into frigid space, some experiencing multiple adven­tures. Unfortunately, no atom bears the imprint of its particular passages through the core of a star; astronomers can only describe the origins statistically.

To judge the scientific contributions of Hipparcos, we start by rec­ognizing that measuring the positions of stars is both fundamental and unglamorous. It’s fundamental because it’s the key to measur­ing the physical properties of celestial objects. Positions are the keys to the trigonometric determination of distance, and distance is needed to calculate the size, mass, and intrinsic brightness of any planet, star, or galaxy.28 Without distances we’re stuck with the appearance of stars in the sky, and a star that’s far away and lumi­nous can appear to be the same brightness as a star that’s nearby and dim. That ambiguity is fatal to any reliable understanding of the denizens of the night sky. It’s unglamorous because measuring a position is the simplest and most obvious way to characterize a star: there’s no image, just two angles to identify a unique spot on the sky, with no units. Needless to say, the people who do such prosaic work don’t always get their due.

It was not always that way. On the spinning Earth, the measure­ment of star positions is critical for keeping time and navigating. Early cultures noticed and tracked star positions as if their lives depended on it, which they did! In the third century BC, Timo – charis and Aristillus produced the first star catalog in the Western world while working for the Great Library at Alexandria. About a century later, Hipparchus extended their work, generating a cata­log with 850 star positions. He also divided the stars into intervals of logarithmic brightness that form the basis for a system that as­tronomers still use. This was a natural way to classify brightness since the eye has a nonlinear or logarithmic response to light. Ptol­emy increased the catalog to 1,022 stars.29 These star catalogs are among the most impressive intellectual achievements of antiquity; later generations of admiring astronomers called Ptolemy’s stellar compendium Almagest, which means “greatest” in Arabic.30

As in many other aspects of astronomy, the torch for mapping stars was then taken up by Arabs for a millennium. Around AD 964, the Persian Abd al-Rahman al-Sufi wrote his Book of Fixed Stars, which depicted the constellations in glorious, natural color. Al-Sufi was the first to catalog the Large Magellanic Cloud and the Andromeda Nebula, two distant star systems whose true na­ture would not be fully understood until the 1930s. The pinnacle of pre-telescopic observations was reached by Tycho Brahe in the sixteenth century. Through relentless attention to detail and the control of systematic errors, he improved on the positional er­rors of earlier catalogs by a factor of fifty. His reputation didn’t suffer from doing these mundane measurements; Brahe was cel­ebrated in his lifetime and is considered the greatest observer before Galileo.

The big prize in astrometry was its use to measure the distance to a star. Stars are so far away that the apparent seasonal shift of a nearby star with respect to more distant stars—the effect called parallax—was not observable for the first two centuries of use of the telescope. Friedrich Bessel won the race to detect parallax by showing that 61 Cygni, one of the closest stars, was nearly 10 light – years away, or a staggering 60 trillion miles.31 Bessel didn’t have a university education, but his meticulous calculations elevated him to fame as one of the most noted scientists and mathematicians of the nineteenth century. The parallax shift is extremely subtle, and far more difficult to detect than the large-scale migration of constellations through the night sky as the Earth spins on its axis and orbits the Sun (figure 8.4). Almost all stars have parallax shifts over the course of a year of about one second of arc or less, and most stars visible to the naked eye have parallax shifts smaller than 0.1 second of arc. For comparison, each of the letters on an eye chart that defines 20/20 vision spans an angle of five minutes of arc, a 3,000 times larger angle.

Thereafter astrometry lapsed into the status of a worthy but dull aspect of astronomy. In part, this was because it was so chal­lenging to measure parallax; in the half century after Bessel’s mea­surement, new star distances were only added at the rate of about one per year. Through the twentieth century, photographic plates made it easier to capture and measure star positions, and Her – schel’s project to map the Milky Way galaxy was carried out by researchers in Europe and the United States. But the air blurs out the light of all stars to about 1 arc second in diameter (1/3600 of a degree), larger than the size of the angle that had to be measured to detect parallax. Refraction and telescope flexure also complicate a parallax measurement. Astronomers were bumping up against the limitations of the atmosphere and the only solution was to go into the pristine environment of space.