I have in my hand a little book titled The Starry Messenger (Sidereus Nuncius in its original Latin), written by the Italian mathematician and scientist Galileo Galilei in 1610. There were 550 books in the first printing of Messenger. One hundred and fifty still remain. A few years ago, Christie’s valued each first edition at between $600,000 and $800,000. My paperback copy was printed in 1989 for about $12.

Although the history of science has not awarded Messenger the same laurels as Newton’s Principia or Darwin’s On the Origin of Species, I regard it as one of the most consequential volumes of science ever published. In this little book, Galileo reports what he saw after turning his new telescope toward the heavens: strong evidence that the heavenly bodies are made of ordinary material, like the winter ice at Lute Island. The result caused a revolution in thinking about the separation between heaven and earth, a mind-bending expansion of the territory of the material world, and a sharp challenge to the Absolutes. The materiality of the stars, combined with the law of the conservation of energy, decrees that the stars are doomed to extinction. The stars in the sky, the most striking icons of immortality and permanence, will one day expire and die.

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Galileo was born in Pisa and grew up in Florence. From 1592, he taught mathematics at the University of Padua. Unable to discharge his financial responsibilities on his academic salary alone—he had to pay the dowries of his sisters in addition to supporting his three children by a mistress—he took in boarders and sold scientific instruments. In the late 1580s, he performed his famous experiments with motion and falling bodies. In 1609, at the age of 45, he heard about a new magnifying device just invented in the Netherlands. Without ever seeing that marvel, he quickly designed and built a telescope himself, several times more powerful than the Dutch model. He seems to have been the first human being to point such a thing at the night sky. (The telescopes in Holland were called “spyglasses,” leading one to speculate on their uses.)

Galileo ground and polished his own lenses. His first instruments magnified objects a dozen or so times. He was eventually able to build telescopes that magnified a thousand times and made objects appear 30 times closer than they actually were. You can see Galileo’s surviving telescopes in the rarely visited Museo Galileo, in Florence. His first one was 36.5 inches long and 1.5 inches wide, a tube made of wood and leather with a convex lens at one end and a concave eyepiece on the other. I recently looked out of a replica. First of all, I was surprised at how small the field of view was, appearing as a dime-sized circle of light at arm’s length at the end of a long tube. And dim. However, after squinting for a while, I could indeed make out the faint images in that dime of dim light. And when I trained the primitive telescope on a building a hundred yards away, I could see details in the bricks not visible to my naked eye.

It’s hard to imagine the thrill and surprise Galileo must have felt when he first looked up with his new instrument and gazed upon the “heavenly bodies”— described for centuries as the revolving spheres of the moon, sun, and planets. Beyond were the revolving crystalline spheres holding the stars, and finally the outermost sphere, the Primum Mobile, spun by the finger of God. All of it supposedly constructed out of aether, Aristotle’s fifth element, unblemished and perfect in substance and form, what Milton described in Paradise Lost as the “ethereal quintessence of Heaven.” And all of it at one with the divine sensorium of God. What Galileo actually saw through his little tube were craters on the moon and dark acne on the sun.

It was that materiality, that humbling of the so-called heavenly bodies, that struck at the absolute nature of the stars.

A few centuries earlier, Saint Thomas Aquinas had successfully married Aristotelian cosmology with Christian doctrine, including the ethereal nature of the heavenly bodies and the notion that the earth stood motionless at the center of the cosmos. (With one Aristotelian idea Aquinas took exception: the lifetime of the universe, infinite according to Aristotle, finite according to Christianity.) Galileo’s findings of imperfections in the heavenly bodies severely challenged the Church. But the telescope itself was also a challenge. Galileo’s 3-foot tube was one of the first instruments that amplified the human senses, that showed a world not apparent to the natural eyes and ears. Nothing like this instrument had ever been seen before. Many people were skeptical, questioning the legitimacy of the device and thus the validity of its findings. Some regarded the strange tube as magical, not of this world, as if a cell phone were presented to someone in the year 1800. Galileo himself, although a scientist, did not understand exactly how the thing worked.

We should recall that belief in magic, sorcery, and witchcraft was widespread in Europe in the 16th and 17th centuries. In just those two centuries, 40,000 suspected witches, most of them women, were burned at the stake, hung from the gallows, or forced to put their heads on the chopping block. In 1597, King James VI of Scotland (who in 1603 became James I of England) complained about the “fearefull abounding at this time [and] in this Countrey, of these detestable slaves of the Divel, the Witches or enchaunters.” It was believed that sorcerers could cast spells by damaging a strand of hair or a fingernail of an intended victim. Was the Italian mathematician’s device a bit of sorcery?

Others regarded Galileo’s telescopic findings with suspicion not because they reeked of black magic or contradicted theological doctrine but because they challenged personal worldviews and philosophical commitments. Cesare Cremonini, professor of Aristotelian philosophy at the University of Padua and a colleague of Galileo, denounced Galileo’s claims of craters on the moon and spots on the sun but refused to look out of the tube. Cremonini was later quoted as saying, “I do not wish to approve of claims about which I do not have any knowledge, and about things which I have not seen ... and then to observe through those glasses gives me a headache. Enough! I do not want to hear anything more about this.” Another contemporary of Galileo, Giulio Libri, professor of Aristotelian philosophy at Pisa, also refused to peer through the tube. Galileo replied to these rejections in a letter to fellow scientist Johannes Kepler:

My dear Kepler, I wish that we might laugh at the remarkable stupidity of the common herd. What do you have to say about the principal philosophers of this academy who are filled with the stubbornness of an ass and do not want to look at either the planets, the moon or the telescope, even though I have freely and deliberately offered them the opportunity a thousand times? Truly, just as the ass stops its ears, so do these philosophers shut their eyes to the light of truth.

Galileo’s little book is addressed to the Most Serene Cosimo II De’ Medici, Fourth Grand Duke of Tuscany. The title page reads: “SIDEREAL MESSENGER, unfolding great and very wonderful sights and displaying to the gaze of everyone, but especially philosophers and astronomers, the things that were observed by GALILEO GALILEI, Florentine patrician and public mathematician of the University of Padua, with the help of the spyglass lately devised by him ...” In his book, Galileo exhibits his own pen-and-ink drawings of the moon seen through his telescope, showing dark and light areas, valleys and hills, craters, ridges, mountains. He even estimates the height of the lunar mountains by the length of their shadows.

When he peered at the dividing line between light and dark on the moon, the so-called terminator, it was not a smooth curve as would be expected on the perfect sphere of theological belief, but a jagged and irregular line. “Anyone will then understand,” Galileo writes, “with the certainty of the senses that the Moon is by no means endowed with a smooth and polished surface, but is rough and uneven and, just as the face of the Earth itself, crowded everywhere with vast prominences, deep chasms, and convolutions.” Also reported were the sightings of moons around Jupiter, lending credibility to the notion that the other planets were similar to the earth. In other words, the earth was no longer special. All of which supported the proposal of Copernicus, 67 years earlier, that the sun, rather than the earth, is the center of the planetary system. These were quite a few new ideas to pack into such a little book. And with no apologies to Aristotle or the Church.

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Within a couple of months of the publication of Sidereus Nuncius, Galileo became famous throughout Europe—in part because the telescope had military and commercial value as well as scientific. (From “the highest bell towers of Venice,” Galileo wrote to a friend, you can “observe sea sails and vessels so far away that, coming under full sail to port, 2 hours and more were required before they could be seen without my spyglass.”) Word of the invention traveled by letter and mouth.

Galileo’s announcement of dark spots on the sun was an even greater challenge to the divine perfection of the heavens. We now know that “sunspots” are caused by temporary concentrations of magnetic energy in the outer layers of the sun. Being temporary, sunspots come and go. In 1611, Christoph Scheiner, a leading Jesuit mathematician in Swabia (southwest Germany), procured one of the new gadgets himself and confirmed Galileo’s sightings of moving dark spots in front of the sun. However, Scheiner began with the unquestioned Aristotelian premise that the sun was perfect and unblemished, and he went from there to proposing various precarious arguments as to why the phenomenon was caused by other planets or moons orbiting the sun rather than the sun itself.

The stars could no longer be considered perfect things, composed of some eternal and indestructible substance unlike anything on earth.

As stated on the title page of his book, Galileo was a mathematician. Mathematics was generally regarded as existing in an abstract and logical world. Mathematics helped scholars calculate and predict the “real world,” but it was distinct from that world. In particular, anti-theological models of the system of heavenly bodies were taken as merely calculational devices, describing appearances as opposed to reality. Thus, the earth-centered planetary system of Aristotle and Ptolemy and the sun-centered system of Copernicus could be placed on equal footing as calculational methods, as they both gave fairly accurate accounts of the positions of the planets. But the former accorded with theological and philosophical belief and was thus deemed to reflect reality.

When Galileo’s observations became known, churchmen reacted with skepticism. On March 19, 1611, Cardinal Robert Bellarmine, head of the Collegio Romano, wrote to his fellow Jesuit mathematicians:

I know that your Reverences have heard about the new astronomical observations by an eminent mathematician ... This I wish to know because I hear different opinions, and you Reverend Fathers, being skilled in the mathematical sciences, can easily tell me if these new discoveries are well founded, or if they are apparent and not real.

Although the Church mathematicians argued about the details of Galileo’s findings, they unanimously agreed that the sightings were real. Nevertheless, Galileo’s telescopic findings and his support of the heliocentric model of Copernicus were considered an unpardonable attack on theological belief. For that offense, Galileo, a pious Roman Catholic who had once seriously considered the priesthood, was eventually tried by the Inquisition, forced to recant most of his astronomical claims, and spent the later years of his life under house arrest.

I want to focus now not on the displacement of earth as the center of the cosmos but on the newly conceived materiality of the heavens. Because it was that materiality, that humbling of the so-called heavenly bodies, that struck at the absolute nature of the stars. The demotion started with the observed craters and ruts on the moon. After 1610, dozens of thinkers and writers began to view the moon and planets as places of soil, air, and water, fit for human-like, if strange, habitation. In 1630, Johannes Kepler, the same fellow to whom Galileo wrote about the “stupidity of the common herd,” finished work on a highly popular fantasy titled Somnium (Dream), in which a boy and his mother travel through space to the moon, called Levania. Everything in Levania is more extreme than on earth. On Levania, the mountains rise far higher than on earth, and the valleys plummet much lower. In the hot zone of Levania dwell living creatures, who are monstrously large and live only a single day. These animals, which swim, fly, and crawl, do not live long enough to build towns or governments, but they are able to find sustenance for life. Because Kepler was a distinguished scientist, his novel was taken seriously by the educated world and was read in the 17th, 18th, and even 19th centuries.

There were many other such fantasies. In “The Elephant Moon” (1670) by the poet Samuel Butler, self-satisfied gentlemen scientists, while observing the moon through a telescope, sight a battle of armies under way, during which a lunar elephant leaps from one line of soldiers to the other in mere seconds (possibly liberated by the reduced gravity of the moon). In 1698, the Dutch mathematician and scientist Christiaan Huygens wrote a book titled The Celestial Worlds Discovered, or Conjectures Concerning the Inhabitants, Plants, and Productions of the Worlds in the Planets. These books and poems were written for the general public. They give some sense of how people in the 17th century came to view the planets as ordinary material. Elephants do not rampage across divine spheres of ethereal quintessence.

But it was for the nature of stars that Galileo’s findings had perhaps their most profound impact. The idea that stars might be suns had been proposed by the Italian philosopher and writer Giordano Bruno. In his On the Infinite Universe and Worlds, published in 1584, Bruno wrote that “there can be an infinite number of other worlds [earths] with similar conditions, infinite suns or flames with similar nature ...” (For his astronomical proposals as well as his denial of other Catholic beliefs, Bruno was burned at the stake in 1600.) By the early 17th century, various thinkers entertained the idea that stars might be suns. Thus, when Galileo reported blemishes on the sun, his findings had dramatic implications for all of the stars. The stars could no longer be considered perfect things, composed of some eternal and indestructible substance unlike anything on earth. The sun and the moon looked like other material stuff on earth. In the 1800s, astronomers began analyzing the chemical composition of stars by splitting their light into different wavelengths with prisms. Different colors could be associated with different chemical elements emitting the light. And stars were found to contain hydrogen and helium and oxygen and silicon and many of the other common terrestrial elements. Stars were simply material—atoms.

Once Galileo and others had declared the stars to be mere material, their millennia were numbered— because all material things are subject to the law of the conservation of energy. This law is a paradigm of all laws of nature, both in its grand sweep of applicability and in its quantitative and logical formulation. Essentially, the law says that energy cannot be created or destroyed. Energy can change from one form to another, as when the chemical energy of a match turns into the heat and light of its flame. But the total energy in a closed and self-contained system remains constant.

A star is like a giant match. It has a finite amount of energy stored within it—in the star, nuclear rather than chemical energy. That nuclear energy is released when atoms fuse together to make heavier atoms. But the supply of nuclear energy in a star is limited, just like the supply of chemical energy in a match. As the star “burns” its nuclear fuel, the energy is released into space, mostly in the form of light. If we imagine putting our star into a giant box, the total energy in that box remains constant, but the energy is gradually shifted from the star to the light in the box and the increased thermal and chemical energy of all things absorbing that light.

Of course, stars are not contained in giant boxes. But the principles remain. Stars, being physical material according to Bruno and Galileo and subsequent scientists, have a limited amount of energy. Stars radiate energy into space, thus depleting their finite supply of nuclear energy. Eventually that precious stellar commodity will be spent, at which point the stars will burn out and go dark. As will our sun, in about 5 billion years. In something like 1,000 billion years, all of the stars in the sky will have gone cold.

At that point, the night sky will be completely dark. And the day sky will also be completely dark. The myriad stars in the sky, once thought to be the final resting place of dead pharaohs, once thought to be the embodiment of constancy and immortality and other dispositions of the Absolutes, will eventually be cold floating embers in space.





Alan Lightman, physicist and novelist, is a professor of humanities at MIT. His books include Einstein’s Dreams, an international bestseller; The Accidental Universe, chosen by BrainPickings as one of the best science books of 2014; and Screening Room, chosen by the Washington Post as one of the best books of 2015.