Part II
“I thought of a maze of mazes,
of a sinuous, ever growing labyrinth
which would take in both past and future
and would somehow involve the stars.”
Jorge Louis Borges (1962)1
New Stars
For at least two thousand years, astronomers, hunters, mariners, and anyone else familiar with the brightest stars must have been amazed by a nova, or “new star,” that would appear suddenly at a point in the sky where no star had been previously seen. For a few days or weeks the nova might be one of the brightest stars in the dark night sky. But then it would begin to fade away, and in about a month it would disappear back into invisibility, often without a trace.
Sometimes the new star would become so bright that it was easily visible even in full daylight. The emperor’s astronomers in the Sung dynasty of China recorded one of them on July 4, 1054, near the constellation now known as Taurus, the Bull. The Chinese chronicles indicate that the new star became as bright as Venus, could be seen during the daytime for three weeks, and remained visible in the night sky for 22 months. These temporary residents were called guest stars or visiting stars. They would appear suddenly and then depart abruptly, like uninvited guests.
More than four centuries passed before the Oriental records again noted the unheralded appearance of brilliant guest stars, and this time they shook the very foundations of European thought. As Aristotle taught, the stellar Heavens were supposed to be eternal, pure, changeless, incorruptible and perfect, quite unlike anything on Earth. Yet, in a span of just 32 years two new stars appeared, remained fixed in the Heavens for about a year each, and then disappeared from view. Tycho Brahe witnessed the first visitor in 1572; it appeared in the constellation Cassiopeia with a light that was brighter than the planet Venus. Then Johannes Kepler spied another one as it lit up the Heavens in 1604.2
The bright new stars of 1572 and 1604 could be observed by anyone in the Earth’s Northern Hemisphere. The English poet John Donne most likely referred to the 1604 nova with:
“Who vagrant transitory Comets sees,
Wonders, because they are rare; but a new star,
Whose motion with the firmament agrees,
Is miracle, for there, no new things are.”3
Once thought to be eternal, some stars arose in the darkness and then disappeared back into it, with an apparent beginning and ending. Where had these strange and unexpected stars come from, and why did they suddenly vanish from sight after shining so brightly for months? Had they always been there and suddenly became visible, and how far away were they located? As John Donne had pointed out, comets could suddenly appear out of nowhere, remain visible for a few months, and disappear. But comets moved across the celestial background, and the novae remained fixed in the distant realm of the stars.
The incorruptibility of the Heavens was at first defended by assigning the novae to a region between the Earth and the Moon, where change was permissible. But Tycho Brahe’s meticulous observations of the nova of 1572 and Galileo Galilei’s and Johannes Kepler’s observations of the nova of 1604 placed them far beyond the Moon and within the supposedly eternal and changeless celestial realm.4
The die had been cast, and there was no getting around it. At least one star residing within the celestial Heavens did come and go. Although uncommon, such a change set the stage for the ensuing realization that all stars have a history. Like humans, the stars are born, live and die. But since only a few of the thousands of the stars visible with the unaided eye had been observed to change, their lifetimes just had to be exceedingly long, much greater than those of human beings.
William Herschel’s telescopic discoveries in the 18th century indicated that stars congregate into clusters, and that there are nebulous regions in the night sky that contain no stars. This indicated to him that stars and star clusters formed in the past, are changing, and will eventually perish over vast spaces and enormous times.
William Herschel, Musician Turned Astronomer
William Herschel, one of England’s most famous astronomers, started life in Germany, never received formal training in astronomy or physics, and earned his living as a musician for the first half of his life. He was born in Hanover on November 15, 1738, and named Friedrich Wilhelm Herschel. At the age of 14 he joined his father Isaac and older brother Jacob as an oboist in the regimental band of the Hanoverian Foot-Guards and accompanied the regiment to England in 1755, as a precaution against the threat of a French invasion. The following year they returned home, where the French defeated the Foot-Guards. Wilhelm and Jacob fled to England in the autumn of 1757.
Wilhelm arrived in London at the age of 19, nearly penniless, but soon found work as a free-lance musician. After nearly ten years of this wandering life, William, as he had become known, settled down as the organist of the Octagon Chapel in Bath, a fashionable health resort for the aristocracy, and brought his sister Caroline to Bath, to keep house, launch a brief musical career, and help with his astronomical observations.5
For the next decade, Herschel became a musician by day and an astronomer by night. He was totally self-educated in astronomy, by reading books and learning to make excellent telescopes. Leisure, comfort, meals, and sleep were sacrificed to grinding and polishing metal mirrors of ever-increasing size that would collect more light. With unrivaled skill, William was able to build telescopes with the largest known mirrors, which could resolve greater detail on bright cosmic objects or bring fainter ones into view. He could then see what no one else had ever seen before, either with unaided eyes or with any other person’s telescope during his lifetime.
At a time when most astronomers were preoccupied with the planets, which moved against a backdrop of fixed stars, Herschel wanted to find out more about the stars themselves. In the fall of 1779, at 41 years of age, he therefore embarked on a systematic review of the stars using a telescope of his own making.
Herschel was looking for two stars; one bright and the other dim, but so close together that they appeared to be single even with the best telescope other than his own. He found hundreds of these “double” stars; even the North Star turned out to be one. At the time, no one knew how far away the stars were, and Herschel hoped that the two stars could be used to infer the distance of the brightest one by an annual parallax change in the apparent positions of the two stars. The technique was flawed because it assumed the fainter member of the pair would be much farther away than the brighter one. When Herschel returned to some of these newfound double stars later in his career, he discovered that some pairs had changed angular separation from each other, which meant that they were close companions joined together in orbital motion around a shared center and not widely separated in distance from the Earth.
After about a year and a half of this work, William discovered, on March 13, 1781, a totally unexpected, non-stellar object, which he initially thought was a comet.6 But he was using very high telescopic power, and saw right away that this object had an observable disk, unlike any star. “The goodness of my telescope,” he recalled, “was such that I perceived its visible planetary disk as soon as I looked at it.”7 Several months of observation by Herschel and others showed that the moving object is a major planet that orbits the Sun beyond Saturn, at about twice Saturn’s distance.
Herschel proposed that the new planet be named Georgium sidus, Latin for “George’s star,” in honor of King George III, England’s reigning monarch.8 After considerable dispute, the new planet eventually became known as Uranus, after Urania a Greek goddess and muse of astronomy, which is the name used today.
It was the first planet to be discovered since the dawn of history, and it made Herschel famous. By the end of 1781 he had been elected a Fellow of the Royal Society, was awarded one of its gold medals, and would soon be supported by the King.
Just eight years after Herschel’s discovery of the unknown planet, a newly discovered, heavy element was designated uranium in honor of the finding of the new world. Then in a little more than a quarter century, the young English poet John Keats likened his own discovery of Homer’s poetry to Herschel’s sighting of a new planet and to an explorer finding new lands or seas:
“Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez when with wondering eyes
He stared at the Pacific — and all his men
Looked at each other with a wild surmise —
Silent upon a peak in Darien.”9
At almost exactly midway in his 83-year lifetime, William became Astronomer to the King with a royal pension, which permitted him to give up music as a career and devote full time to astronomy, except when called upon to demonstrate his telescopes to the Royal Family and guests.
For the rest of his life, Herschel continued with what he did best — constructing telescopes with larger mirrors and greater light gathering power than ever obtained before, and using them to survey the Heavens.
By late in 1783, Herschel had built a telescope that would be his main observing instrument. It had a mirror of 18.7-inches (0.47-meters) diameter and a 20-foot (6.1-meter) focal length. He used it to count the number of stars in various directions and thereby establish the “Construction of the Heavens.”
With extraordinary perseverance and zeal, Herschel would sit at his telescope from sunset to sunrise, always in the open air, often in freezing temperature, and in great physical discomfort. Night after night and year after year he continued to sweep the sky in an organized and methodical way, often with the help of his faithful sister Caroline.
It took them two decades, from 1783 to 1802, to complete a survey of the entire sky visible from their home, which resulted in the discovery of over 2,500 nebulae and star clusters, most of them never seen before.10
William Herschel’s reports of vast numbers of previously unknown nebulae and star clusters suggested that they change as the result of relentless gravity, a universal agent of transformation.
The Stars Gather Together
Most scientists of William Herschel’s time agreed with the ancient Greek concept of immortal, unchanging stellar Heavens. To avoid the inevitable gravitational disruption of the stars, Herschel adopted Isaac Newton’s proposal that the known Universe began with a near-uniform distribution of stars, which “the great Author” would preserve, but his observations also suggested that the stars are not regularly spread through the space they reside in. After over two decades of surveying the “Construction of the Heavens,” Herschel discovered more than a thousand star clusters, which indicated to him that gravity had begun to draw some stars together.
The French astronomer Charles Messier had already prepared a list of 103 bright star clusters and nebulae in order to avoid confusion in his hunts for comets.11 Published in final form in 1781, the year that Uranus was discovered, the Messier list includes some of the most widely studied objects in the Universe, all designated by the letter “M” followed by the number in the Messier catalogue. Herschel naturally looked at the bright Messier objects using his superior telescope. It had twenty five times the light collecting area and five times the resolving power of the one Messier used, which meant that Herschel could detect much fainter and finer details.
All but one of Messier’s twenty-nine star clusters were listed by Messier as “round nebulae,” which might be confused with a comet without a tail, but they were not yet resolved into stars. Herschel was able to detect stars in some of them, and coined the term globular star cluster for these objects. Prominent examples include M 3, M 5 and M 80, which have all been well studied by subsequent generations of astronomers.
Herschel discovered that the stars are compressed together near the centers of the brightest globular star clusters, and that they are more sparsely congregated further out. Individual stars could only be picked out near a cluster’s outer boundary, which gradually thinned out to the emptiness of surrounding space (Figs. 7.1, 7.2). So how, he wondered, did the globular star clusters get this way?
To Herschel, their spatial distribution portrayed a Natural History of the Heavens in which star systems have a beginning, development, and end. Any dense irregularity in the original distribution of stars would, as the result of its greater gravitational pull than normal, attract adjacent stars into a common center. As time went on, the central stars would be drawn toward each other by their mutual gravitational attraction and move ever closer together, so they evolved from widely dispersed conditions to the tightly packed globular star clusters. This would account for their round shapes, as well as their dense centers and more rarefied peripheries. It would also explain the surrounding empty space from which the stars had apparently been swept up and pulled out of.
Moreover, Herschel proposed, star clusters might clock the growth of the Universe. Each cluster would be as old as the time it took for it to achieve its present shape. Dispersed star clusters would be relatively young in comparison to older, compact ones, and the star clusters could collectively serve as “laboratories of the Universe.” They resembled cosmic fossils with their life history imprinted into their shape, and signaled the eventual decay of the known stellar Universe, the Milky Way.
Figure 7.1 Globular star cluster Hundreds of thousands of stars are held together by their mutual gravitational attraction in M 80, a globular star cluster whose age is about 14 billion years. It is about as old as the observable Universe and significantly older than our Sun whose age is about 4.6 billion years. (A Hubble Space Telescope image courtesy of NASA/AURA/STScI/Hubble Heritage Team.)
An Enchanted, Luxuriant Garden in the Sky
According to Herschel, the nebulae and star clusters display the history of a heavenly garden, a life cycle of stellar systems in which gravity acts as a universal agent of change. He therefore wrote in 1789:
“[The Heavens] now are seen to resemble a luxuriant garden, which contains the greatest variety of productions, in different flourishing beds; and one advantage we may at least reap from it is that we can, as it were, extend the range of our experience [of them] to an immense duration.”12
Figure 7.2 Stellar beauties A Hubble Space Telescope image of stars in the globular star cluster NGC 6397, which has an estimate age of about 12 billion years. (Courtesy of NASA/ESA/Harvey Richer, U. British Columbia.)
In this interpretation, star systems were like plants at various stages of growth and decay, and you could “judge the relative age, maturity or climax from the disposition of their component parts.” The stable, clockwork Universe of Newton was thus being replaced by a Cosmos that is always changing and has a history. The exact time scale remained unknown, but cosmic change would have to happen over enormous times, owing to the vast spatial scale of the stellar Universe.
Erasmus Darwin, an English poet, physician, and naturalist, celebrated Herschel’s vision of stellar change in a popular poem entitled The Botanic Garden, published in 1791. It described an enchanted garden that included the wonders of stars and planets that exploded into being once God, the Creator, started it all, including:
“So late described by Herschel’s piercing sight,
Hang the bright squadrons of the twinkling night.
— Roll on, ye stars! Exult in youthful prime,
Mark with bright curves the steps of time.”13
There would come a time, Darwin proclaimed, when the light of stars would be extinguished into the dark, for:
“Flowers of the sky! Ye to age must yield,
Frail as your silken sisters of the field!
Star after star from Heaven’s high arch shall rush,
… to one dark center fall,
And death and night and chaos mingle all!”14
Then in November 1790 Herschel came across “a most singular phenomenon,” a bright star enveloped by a faint luminous nebula (Fig. 7.3). The “nebulosity about the star,” he noted, “is not of a starry nature,” and its self-luminous material was “more fit to produce a star by its condensation than to depend on a star for its existence.”15
Figure 7.3 Planetary nebula When a Sun-like star uses up its nuclear fuel, the star’s center collapses into an Earth-sized white dwarf star while its outer gas layers are ejected into space. (A Hubble Space Telescope image of NGC 6751 courtesy NASA/STScI/AURA/Hubble Heritage Team.)
Near the end of his career, Herschel arranged his observations of nebulae and star clusters into a proposed sequence of advancing age. It was supposed to demonstrate how gravity dominates stellar life and sculpts cosmic objects into different configurations and shapes or forms. The process begins with faint, milky nebulosity covering considerable areas of the sky. Ongoing gravitational attraction draws these diffuse clouds into smaller, concentrated patches, which grow more regular in shape and brighten in the center. As gravity continues to work its magic, these in turn become smaller and more compact stellar systems, and the aging process ends with the magnificent globular star clusters.
It was, Herschel thought, like observing babies, children, and adults to see how human life unfolds. Furthermore, when most of the stars have ripened into globular form, the Milky Way must break up and cease to be a stratum of stars. After all, it was already starting to fragment under the destructive power of gravity.
And if the Milky Way was falling apart, it must have had a beginning, or in Herschel’s words: “Since the breaking up of the parts of the Milky Way affords a proof that it cannot last forever, it equally bears witness that its past duration cannot be admitted to be infinite.”16 He associated this starting point with its Divine Creation.
Creation of the Stellar Universe
Like many educated people in his day, Herschel believed the first stars were fashioned by a Divine Creator, whose power and glory can be seen in the heavenly, star-clad night sky. To him, it provided evidence for, and even proves: “the existence of a first cause, the infinite author of all dependent things.”17
William nevertheless spoke so little of his faith that after his death his son John wanted it to be “distinctly understood that my father … was a sincere believer in, and worshipper of, a benevolent, intelligent and super-intending Deity, whose glory he conceived himself to be legitimately forwarding by investigating the magnificent structure of the Universe.”18
This structure was a testimony to the Creator of the Heavens, as in Joseph Addison’s Hymn, written a quarter century before Herschel’s birth:
“The Spacious Firmament on high,
With all the blue, Ethereal Sky,
And spangled heavens, a shining frame,
Their great Original proclaim.”19
Herschel’s contemporary, the composer Joseph Haydn, similarly celebrated the stars in his The Creation. And about a quarter century after that, Friedrich Schiller’s beautiful Ode to Joy was set to music in Ludwig van Beethoven’s Ninth Symphony to proclaim our Creator, who above the stars must dwell.20