The history of the attempts by astronomers to measure the distance of a star is presented. The achievements of astronomers Tycho Brahe, James Bradley, William Herschel, Friedrich Bessel and F.G. Struve in detecting and measuring parallax are described.
ASTRONOMER STOILED FOR CENTURIES TRYING TO DETERMINE HOW FAR AWAY THE STARS TRULY ARE.
The measurement of a star's distance is rooted in an everyday phenomenon called parallax, the apparent shift in an object's position when viewed alternately from different vantages. This is one basis upon which our eyes gauge distance within our immediate surroundings. Distance and parallax go hand in hand: the farther an object, the smaller its parallax shift.
Since the advent of Nicholas Copernicus's Sun-centered conception of the universe in the mid-1500s, astronomers had hoped that the Earth's wide-swinging orbital motion would reveal the parallaxes--and thus the distances--of stars. In this way, they might transform the night-sky illusion of a starry celestial vault into a full-blown, three-dimensional cosmos.
Tycho Brahe, an irascible, red-bearded Danish nobleman, was the first to take up the search for the elusive parallax in the late 1500s. Kidnapped as an infant by a jealous uncle, Brahe was raised in privilege a short distance from the home of his parents. His intellect was as sharp as the sword that lopped off his nose in an ill-considered duel--purportedly over mathematics. Brahe attended some of the finest universities in Europe and garnered such acclaim as an astronomer that the Danish king, fearful of losing this rising star, awarded him an entire island upon which to erect an observatory. Brahe's feudal estate, complete with turreted mansion, jail, and jester, became the world's premier astronomical research institute.
Crosschecking Brahe's star catalog against those of more modern vintage, we know that he determined the places of stars to within 1 arcminute (1/60[degrees]) of their actual positions. This is an astonishing achievement, considering that he worked without the benefit of a telescope. However, even Brahe's sharp eye and advanced instruments proved unequal to the task of detecting stellar parallax.
In 1669 British physicist Robert Hooke attempted to bring technical sophistication to the enterprise in the form of the recently invented telescope. The ever-perceptive Samuel Pepys wrote of the dyspeptic Hooke that he "is the most, and promises the least, of any man in the world that I ever saw." Dogged by recurrent nightmares and all manner of illness, both real and imagined, Hooke nevertheless compiled an enviable record of innovation in the sciences. With naive determination, he vowed to solve the stellar-parallax problem. To do so, he secured a fixed, vertical telescope within a hole he had sawed through the roof of his London apartment and began to observe the 2nd-magnitude star Gamma Draconis, which happened to wheel almost precisely overhead each day.
Following a method devised by Galileo, Hooke spent four months searching for a slight alteration in the position of this "overhead" star as the Earth circled the Sun. Ultimately, Hooke was defeated by the poor state of technology, as well as his own impatience. He nevertheless convinced himself that he had observed the parallax of Gamma Draconis, though nobody took his claim seriously. (His parallax value implied that the star was 0.1 light-year away.) Hooke's "Archimedean Engine," as he had dubbed his telescope, was much too blunt an instrument to detect the subtle shifting of the stars.
Watching More Carefully
Patience was the watchword of James Bradley, who was as unlike his restless predecessor Hooke as one could be. From his vicarage at Bridstow, England, Bradley tended his flock of parishioners during the day. At night, however, while his congregation slept, pulpit gave way to telescope, vestments to warm coat, and prayer to observation. When Bradley raised his eyes to the heavens after dark, it was more often not as a servant of God but as an agent of science.
Bradley was a detail man, known to test an instrument "almost to destruction before he would rely on it." In the 1720s, Bradley installed his own, much-improved version of Hooke's vertical telescope in his aunt's house and, reclining underneath the eyepiece in the coal cellar, studied Gamma Draconis all over again. To Bradley's delight, the star displayed an unmistakable shift as the Earth progressed around the Sun, just as Galileo had predicted. But his excitement soon evaporated. The timing of Gamma Draconis's movement was entirely wrong: the star appeared to inch northward when it should have shifted southward, and southward when it should have shifted northward.
Bradley pondered the problem for three years before he stumbled upon the solution during a pleasure cruise on the Thames River. Observing the windblown pennant atop the boat's mast, he realized that the pennant's orientation was determined not just by the wind but by the forward velocity of the boat. Likewise, he reasoned, the perceived position of a star is influenced by the Earth's velocity through space, a phenomenon termed the aberration of light. In the wobble of Gamma Draconis, Bradley had gathered the first definitive proof, not of stellar parallax, but of the Earth's orbital movement around the Sun. Bradley went on to become England's Astronomer Royal, with an arsenal of telescopes at his disposal. Even though he spent the remainder of his long career measuring the precise positions of stars, their parallaxes remained beyond his grasp.
The mammoth ambition of William Herschel looms large in astronomical lore. Had People magazine existed in the 18th century, Herschel would undoubtedly have been one of "England's Most Fascinating People." Everyone from the head of the local ladies' club to King George III was clamoring to meet this immigrant from Hanover, Germany, who spoke impeccable English, composed symphonies, constructed telescopes, and, with his discovery of the planet Uranus, single-handedly changed the popular conception of the universe. Here was a scientific Olympian who excelled in every arena of observational astronomy.
So when Herschel announced that he would attack the stellar-parallax problem, there appeared little doubt that he would succeed. Unlike his contemporaries, who would have been gratified by the detection of even a single star's parallax, Herschel aspired to become astronomy's wholesale purveyor of stellar distances. And he believed he had the means to do it: Galileo's double-star method. Given a bright star, presumed to lie relatively close to the solar system, in a chance alignment with a faint, faraway star, the bright star's parallactic wobble should stand out against its immobile "neighbor." But after decades of finding and monitoring hundreds of bright-and-dim star couplings, Herschel succeeded only in overthrowing his own assumptions about the nature of double stars: most of these systems are not random alignments, as Galileo had supposed, but actual stellar pairs bound to one another by mutual gravitation. As such, they could not be used to reveal parallaxes in the way Galileo --and Herschel--had intended.
Trying to measure a star's parallax with an 18th-century-era telescope was like trying to measure a microbe with a ruler. The largest instruments, typified by Herschel's massive, wood-framed reflectors, resembled cosmic siege engines; they were designed to peer deep into space, not to measure precise star positions. Even the sleek refractors of the day, all polished brass and wood, were insufficiently rigid and too coarsely calibrated to record a parallax. The telescope, that remarkable instrument that had once opened up the heavens to investigation, had become the main hindrance to progress in stellar astronomy.
The essential leap in telescopic sophistication came in the 1820s with the finely crafted instruments of Joseph Fraunhofer. Having risen from circumstances worthy of a Charles Dickens tale, Fraunhofer was a most unlikely hero in the hunt for stellar parallax. Orphaned at age 11, he eked out an existence as a virtual slave to a Munich glass cutter, who permitted the young Fraunhofer neither an education nor a candle by which to read at night. His life turned around when the building in which he worked collapsed, trapping him within the rubble. His dramatic rescue brought him to the attention of the future king of Bavaria, who saw to it that the young apprentice was properly educated. Fraunhofer took up the study of optics and, through tireless experimentation, became the world's foremost telescope maker. Never before had telescopes of such exquisite precision and clarity been raised to the heavens.
Two of Fraunhofer's state-of-the-art instruments found their way into the able hands of German-born astronomers Friedrich W. Bessel and F. G. Wilhelm Struve. The mathematically gifted Bessel had abandoned a lucrative business career to pursue his passion for astronomy. Entirely self-taught, he burst onto the astronomical scene with an incisive theoretical analysis of the orbit of Halley's Comet. Having established his observing credentials while serving as a low-paid assistant to a wealthy amateur astronomer, the 26-year-old Bessel was tapped by the king of Prussia to direct the new national observatory in Konigsberg. In April 1812, Napoleon's ill-fated Grande Armee marched past Bessel's half-completed observatory on its way to Russia. The French emperor is reported to have wondered aloud how the Prussians could afford such extravagance amid the turmoil of the times.
Like many of Fraunhofer's other creations, the instrument that Bessel cast his eyes upon in Konigsberg in March 1829 was almost painfully beautiful: copper-shaded, mahogany-veneer tube; burnished knobs, gears, and wheels; and wooden equatorial mount that descended to Earth through a complex of gracefully splayed struts and stout beams. It was a working piece of sculpture balanced proudly atop its pedestal. Fraunhofer himself didn't live to see completion of the instrument--he died of pneumonia in 1826 at age 39--but had instructed his assistants to follow Bessel's exacting specifications to the letter.
Struve's path to the stars had been no less eventful than Bessel's and Fraunhofer's. As a youth, he was kidnapped by recruiting agents for Napoleon's army but escaped by jumping from a second-story window. Struve's parents dispatched their son to faraway Dorpat University, in the present-day Baltic Sea republic of Estonia, where he would be safe--and where he discovered the pleasures of astronomy. By the time he turned 21, Struve was appointed chief astronomer at Dorpat Observatory. In 1824, he persuaded university officials to purchase a 9 1/2-inch Fraunhofer refractor, the largest ever built. Thus began Struve's rise in the astronomical world, where he became the acknowledged expert in the study of double stars.
The Race Is On
In 1835 Struve turned the Great Refractor, as it would become known, toward brilliant Vega. He hoped to detect Vega's parallax by monitoring its position against that of a dim comparison star. In 1837 he published a preliminary value based on only 16 measurements: a parallax of 1/8 arcsecond, which implied a distance of 26 light-years.
When Bessel learned of Struve's intention, he cleared his own observing calendar and pointed Konigsberg's 6-inch Fraunhofer refractor at 61 Cygni, a star that over the course of a century had moved about 10 arcminutes. The fleetness of the so-called Flying Star suggested that it was close by.
Meanwhile, Scotsman Thomas Henderson, having returned from a miserable year in the wilds of South Africa, struggled to wring an honest parallax of Alpha Centauri from data acquired with a hand-me-down telescope he knew to be flawed.
Struve's parallax ambitions were hampered by the Czar's directive that he establish and equip a new Imperial observatory in Pulkovo. Bessel, on the other hand, worked virtually without interruption. By October 1838, Bessel had accumulated hundreds of position measurements of 61 Cygni. The wobble in the star's placement was subtle but unmistakable: as the Earth headed toward one side of its orbit, 61 Cygni appeared to stray in the opposite direction; months later, when the planet had swung to the other side of the Sun, the star obediently retreated. Bessel knew he had reached the end of his quest.
The extraordinary number and precision of Bessel's observations left little doubt among astronomers that the first stellar parallax had indeed been recorded. In 1842, John Herschel (William Herschel's son) laid his own opinion before the Royal Astronomical Society: "[T]he results have been placed before you:--oculis subjecta fidelibus. If all this does not carry conviction along with it, it seems difficult to say what ought to do so." A faint glimmer of light near the wing of the Swan had become the first milepost in the seemingly endless ocean of space.
The parallax angle of 61 Cygni proved to be exceedingly small: about one-third (0.314) arcsecond, equivalent to the apparent size of a Manhattan taxicab as viewed from Mexico City. The star's computed distance came out to about 660,000 astronomical units, that is, 660,000 times the Sun-Earth distance, or more than 99 trillion kilometers (60 trillion miles), or 10.3 light-years.
The remoteness of 61 Cygni was not unexpected; astronomers had come to realize that if the stars were closer, their parallaxes would not have eluded detection for so long. Nevertheless, the huge number--real, this time, not conjectured--stirred the imagination, as scientist and nonscientist alike tried to comprehend in their own minds the sheer vastness of the universe. Far-flung as it was, 61 Cygni's apparent movement across the sky of 5.2 arcseconds per year translated into an actual velocity through space of almost 275,000 km per hour. The Flying Star had lived up to its billing.
Crossing the Line
Trailing Bessel's triumphant announcement by only two months, Henderson published his parallax for Alpha Centauri: just more than 1 arcsecond, which placed the star less than 200,000 a.u. away, or about one-third as far as 61 Cygni. Henderson had completed his observations while at the Cape of Good Hope in 1833, long before Bessel had even started. However, Henderson put off the dirty work of data analysis, apparently suspicious of his own measurements with the Cape's second-rate telescope. Evidently, with Bessel's success now fresh in astronomers' minds, Henderson felt that his less-than-rigorous Alpha Centauri parallax might receive more support.
In late 1839, Struve published a parallax measurement for Vega: about one-quarter (0.261) arcsecond, placing the night sky's fifth brightest star some 800,000 a.u. (12.5 light-years) from Earth. This parallax was about twice as large as a preliminary value he had circulated in 1837, casting doubt on the whole enterprise. Although several revisionist astronomers have sought to crown Struve the winner of the parallax race with his 1837 announcement, Struve himself never claimed that honor. In an 1848 paper he acknowledged that Bessel had preceded him in obtaining the first definitive parallax of a star.
Modern measurements have reinforced Bessel's reputation as a meticulous observer. The true parallax of 61 Cygni is 0.287 arcsecond, within 10 percent of Bessel's value, situating the star about 11 light-years from Earth. Henderson and Struve were much farther off. Alpha Centauri is about 25 percent more distant than Henderson had thought, and Vega almost twice as far as Struve's final estimate (but close to his first!).
Within the span of a year, a trio of astronomers had vaulted the gulf of space and planted three flags in the previously uncharted territory of the cosmic third dimension. Although a modest claim, to be sure, on what appeared to be a virtually unlimited universe, nonetheless it offered to astronomers the same sense of excitement that Earthly explorers feel when they set foot on a long-awaited shore. That the journey had taken centuries made the hard-won success all the more sweet. In John Herschel's words, it was "the greatest and most glorious triumph which practical astronomy has ever witnessed."
Struve eventually left Dorpat to become director of Pulkovo Observatory, which he turned into the most advanced astronomical facility in the world. Amid his constant frenzy of work and travel, Struve managed to produce 18 children in two marriages, out of which grew an astronomical dynasty that spanned four generations.
In 1834, a year after he left South Africa, Henderson became Astronomer Royal of Scotland and Regius Professor at the University of Edinburgh. Over the next decade he secured the accurate positions of some 60,000 stars in the northern sky, in addition to reducing the data he had acquired at the Cape.
Bessel remained at Konigsberg for the rest of his life, having turned down the prestigious directorship of the Berlin Observatory. But in June 1842 Bessel finally embarked for England, fulfilling a longtime dream to travel abroad. He was feted in Manchester at the Congress of the British Association for the Advancement of Science. He then traveled to Edinburgh to meet fellow parallax explorer Henderson, who reportedly described the meeting with Bessel as one of the highlights of his life. Two years later, in 1844, Henderson died of heart disease at age 46.
Bessel would outlive Henderson by only two years--years that were marked by debilitating illness. Before his death in 1846, Bessel made another profound discovery--and a prediction. The motion of Sirius, according to Bessel's own observations, was distinctly erratic, as though the star were alternately being pushed and pulled by some hidden hand. Sirius, he surmised, has an unseen partner of considerable mass, and the pair twirl about each other like dancers in a gravitational embrace.
The "dark" star did not reveal itself in the telescope, but Bessel was certain of its existence. Not until 1862 did American telescope maker Alvan Graham Clark spy Sirius's elusive companion. It was a star of exquisite strangeness, even by modern standards, with a Sun's mass compressed into an Earth's volume: a white-dwarf star. To an astronomer with Bessel's vision, even the invisible is rendered plain.
Today astronomers are able to measure a million stellar parallaxes in the time it took Bessel to measure just one. And with one position-measuring satellite--Hipparcos--having been lofted into space, and four others poised to follow before this decade's end, astronomers expect to accumulate roughly a billion parallaxes of stars as far as 100,000 light-years away.
In this age of technological wonders, where computer screens glow and satellites soar and the accumulated knowledge of centuries can be accessed at a finger's touch, it is easy to forget how we got to where we are. Yet as the Chinese proverb wisely counsels: When you drink from the well, remember the people who dug the well.
ALAN W. HIRSHFELD, an astronomer at the University of Massachusetts, Dartmouth, provides a detailed account of these people and events in his recently published book Parallax (W. H. Freeman and Co.).