Chapter 1 ¥ Among the Stars
The intellect is empty if the body has never knocked about, if the nose has never quivered along the spice route. Both must change and become flexible, forget their opinions and expand the spectrum of their tastes as far as the stars. ¥ Michel Serres, The Five Senses, 1985
Yes, the stars!
The sensory spread that's laid on for us every day of our lives went onto the fire around fourteen billion years ago and has been simmering around the stars ever since. Our universe is a stew of matter and energy, and some of the molecules that we smell and taste today bubbled up in it very early on, long before the simplest form of life.
It may sound crazy to sniff and slurp through airless interstellar space, but generations of astronomers have opened the heavens for us to imagine just that. So: you're standing somewhere under the open sky, on a clear night, away from city lights. After you let your eyes adapt to the darkness, you can make out hazy patches here and there, perhaps under Orion's belt in the winter, or the band of the Milky Way in Sagittarius in summer. Zoom your mind's eye in on those indistinct patches, and borrow from the telescopic images you've seen of nebulas in deep space: dramatic swathes and swirls of light set in star-studded blackness, sometimes backlighting darker swirls. These are immense clouds of stardust, diffuse matter that has been driven out of stars as they burned, burned out, collapsed, and exploded. The bright clouds glow with energy; the dark ones coldly absorb it.
Now release a super-volatile emanation of yourself. You're a space-time traveler, an assistant to the Chef of the cosmos, disembodied except for chemical senses sensitive enough to sample-and robust enough to withstand-its primordial flavors. Fly light-years into the stew, plunge into those dusty clouds, and open up.
You taste mineral saltiness, and bitterness, and sharp acids, and even sweetness. You feel and smell the irritating pungency of ammonia cleaner, and the stink that it dispels. You catch the heady smells of solvents, of alcohols, of campstove fuel. Vinegar. Eggs. A hint of fruit!
By earthly standards that doesn't sound like an especially delicious composition. But it's intriguing. What are those familiar molecules doing out there? And why just those? To start so way out and way back helps stretch both our understanding and our sense of wonder. It shows that the smells and tastes to come, the various earthly creatures that produce their own, and the perfumers and cooks who modify and multiply them, are all participants in the original, ongoing project of the cosmos: the unfolding of matter's possibilities.
This chapter is about the initial stages of that unfolding, the fires of the stars and their flavorful ashes.
Recipe for the universe:
mix matter and energy, and cook
How did volatile molecules that we smell every day come to exist both here and in outer space? It's quite a story, one that emerges from the collective observations and thinking of thousands of scientists from many countries over many decades. It involves the birth of the cosmos as a whole and the origins and evolution of life on Earth. And at the heart of this nondenominational, transcultural creation story is a cosmic version of cooking.
Consider making caramel on your stovetop. You start with a single ingredient, white crystals of table sugar, which taste simply sweet and have no aroma. Put the sugar in a pot, apply heat energy, and stir. After a few minutes, you've turned the solid crystals into a colorless liquid. Still no aroma. Keep heating, and that liquid turns pale yellow-and begins to smell. It gets light brown, then progressively darker and stronger smelling. In the end you've made a dark syrup that's sweet but also sour and bitter, and richly aromatic. From one substance you've made many: from simplicity, complexity.
A similar process cooked up the entire universe as we know it. The original recipe from the Chef of the cosmos goes something like this. Mix a dozen kinds of elementary particles together with four fundamental forces, and set aside. After a few hundred million years, the particles have combined to form atoms, a hundred different kinds. After another long stretch, many of those atoms have combined to form molecules-and the mix begins to smell. Some of the molecules combine to form particles of dust, and the dust clumps up to form planets. At least one planet, our own, produces increasingly complex molecules, then collectives of molecules that somehow come alive-and these generate a vast bouquet of new volatiles for the Chef to savor, caramel included. So: from a handful of elementary particles the Chef has made countless kinds of molecules, with countless qualities.
This primordial cooking underlies all of our experience, mundane and miraculous. To understand why volatile molecules exist at all for us to smell, and why they exist where they do, let's start in the pristine cosmic kitchen as the Chef gets things going. No smells yet, but just wait.
Cooking up stars
However the known universe came into being, most astrophysicists agree that it did so around fourteen billion years ago in an explosive flash at an unimaginably high temperature. From the moment of this "Big Bang" the universe expanded outward. As it expanded it cooled down, and the kinds of matter and energy that we know on Earth began to appear. In the first fraction of a second emerged packets of electromagnetic energy called photons, which we know as light and heat and radio waves. Along with photons appeared three kinds of raw matter, the subatomic particles that combine to make atoms: protons and neutrons that form the central nucleus of the atom, and electrons that orbit around the nucleus. It's the different numbers of subatomic particles in atoms that give us the hundred-odd different elements with their different qualities: hydrogen, carbon, oxygen, and so on. One solitary proton forms the simple nucleus of atoms of hydrogen, so hydrogen was the first element to be born, followed by nuclei of helium and a bit of lithium.
After only a matter of minutes, the continuing expansion of the universe cooled and slowed everything down to the point that the protons and neutrons no longer had enough energy to fuse together to make heavier atomic nuclei. The evolution of matter paused, for some hundreds of millions of years.
But during that long hiatus, one of the universe's fundamental forces worked inexorably to reenergize matter. Gravity is a force that acts between any two bodies of matter, tiny or huge, and pulls them toward each other. In the newborn three-element universe, neighboring atoms gradually felt each other's gravitational pull. They gathered into clusters, clusters into more crowded clusters, all the while moving faster and faster, bouncing off each other with more and more force, releasing more and more heat energy as they did.
As the universe as a whole continued to expand and cool off, gravity created hot pockets of densely crowded atoms, some of them so dense and hot that they began to emit enough energy to glow. This was the first generation of stars.
Cooking up chemical elements in stars
The material richness of our world is a reflection of its chemical complexity, its countless combinations of the hundred-odd chemical elements. The first stars had just three elements to work with. They generated nearly all the rest by becoming fantastic self-adjusting, self-destroying, billion-degree ovens.
Imagine a member of that first generation of stars. As gravity causes its matter to crowd together and collide with ever increasing force, its temperature and energy increase. At a few million degrees, the conditions are right for two hydrogen nuclei to fuse into a single helium nucleus. This reaction releases energy-which jolts the nuclei into moving fast enough to resist the gravitational force. Fusion and gravity balance each other, and the star can burn with a steady flame like this for billions of years, using hydrogen nuclei as fuel and producing helium nuclei as the residue. When it has consumed most of its hydrogen fuel, the fusion reaction slows down, gravity begins to dominate again, the largely helium core of the star begins to contract, the temperature rises-until the helium nuclei can become the new fuel, fuse to form yet larger nuclei, and again balance gravity so that steady burning can continue. Now we have oxygen and carbon: two of the primary chemical players in the saga of life and the osmocosm.
Then the cycle of contraction, temperature rise, and new fusion repeats again and again, at ever escalating temperatures. The star takes on an onionlike structure, with portions of the newly formed elements surviving in the outer, cooler layers. After twenty-five new elements have been cooked up, gravity finally prevails over fusion and forces the star into a final burst of creativity-and generosity. It contracts the star's core to such an extreme density and temperature that the core explodes and becomes what's called a supernova. The energies released are so extreme that they trigger the formation of ninety-odd more elements. And the explosion blasts these and the first twenty-six into interstellar space, the space between stars.
The supernova thus serves up its creations to the calmer cosmos at large. And it's here that the elements can manifest their individual qualities, explore their affinities for each other, join up, and initiate the next stage in the unfolding of matter's possibilities-the stage in which the first molecules of smell emerge.
Cooking up molecules between stars
Molecules are the stuff of our world, the substance of nearly everything that we see and touch, taste and smell. They're simply combinations of elements, two or more atoms that have joined with each other in a specific arrangement. Given a hundred elements to work with, the number of possible arrangements is-well, astronomical. It's with the birth of molecules that the cosmos attained entirely new levels of complexity.
Molecules are products of the electromagnetic force, the attraction between particles of opposite electrical charge. The nucleus of an atom carries a positive electrical charge thanks to its protons. Electrons that orbit the nucleus carry a negative electrical charge, and it's the attractive force between positive protons and negative electrons that keeps the electrons in orbit. Molecules are the structures that result when the nuclei of different atoms share orbiting electrons with each other. That electron sharing is the bond that holds them together in a stable structure. Some molecules consist of just two or three atoms-for example, carbon monoxide, CO, and water, H2O-while DNA molecules include many thousands. Most volatile molecules have between a few and a few dozen.
The electromagnetic force isn't strong enough to withstand the energies at work in a star. It readily forms molecules at the moderate temperatures that we experience in everyday life, between fire and ice, where atoms are moving slowly enough that they can bump into each other and bond without immediately getting knocked apart again. In the near motionless cold of deep space, and with atoms few and far between, it can take many years for those atoms to encounter and react with each other. More favorable interstellar regions are "giant molecular clouds," the smudgy chiaroscuro patches familiar from telescopic images of the constellations Orion and Sagittarius. These are remnants of supernovas and aged stars that gravity has slowly pulled together, along with new stars that are beginning to burn nearby. They harbor regions denser with atoms, and temperatures around those of our kitchens and ovens. As their name indicates, these clouds are where astrochemists have had the best luck at detecting cosmic molecules.
Molecules in open space exist because their atoms happened to bump into each other and stick. The most abundant atoms in space include hydrogen (abbreviated H), oxygen (O), and carbon (C), whose particular electron-sharing tendencies naturally lead to the formation of small molecules like oxygen gas (O2), water (H2O), and carbon monoxide and dioxide (CO and CO2). Carbon atoms also readily bond with each other to form long chainlike molecules, as well as six-cornered ring molecules. The chains and rings readily nestle together with others of their kind, and can aggregate to form ever larger masses: cosmic soot. The dark swirls in the molecular clouds are a mixture of carbon soot and similar aggregates of primordial minerals. These various particles make up what's called interstellar dust.
The individual grains of interstellar dust are microscopically tiny, but their influence on the development of the cosmos is huge. They provide a solid surface to which free-floating atoms and small molecules can stick. They thus act as gathering places that encourage chemical activity, new reactions, larger molecules. On them the material world became increasingly diverse, complex, capable of further development. And to the nose of the cosmic Chef, it became aromatic-billions of years before our own sun began to shine.
In 2020, the roster of known interstellar molecules numbered more than two hundred. Here I'll note only the few dozen molecules that we also sense in everyday life, along with the everyday materials that they dominate and so "smell like" to us.
Detecting the Smells of Space
At last, smells! But how can we possibly know-not imagine, but know-what molecules are so far out there in space?
By the telltale traces they leave in the energy that the cosmos constantly rains down on our planet. Astrochemists are connoisseurs of electromagnetic radiation, and in particular the visible light, infrared light, and radio waves that originate in stars and galaxies and reach us across the vastness of space. In one year these forms of radiation travel staggering distances, so when we see stars and galaxies, we are seeing deep into both space and time, into the past history of the cosmos.
Astrochemists collect the faintest light and radio emissions with telescopes that are far more efficient and sensitive than our eyes or radios. They then pass them through the electronic equivalent of a prism that separates them into their component colors, or frequencies. The pattern of frequencies-the spectrum-is a kind of fingerprint that makes it possible to identify the kind of matter that emitted it, and the kind of matter that might have absorbed some of it on its long journey to Earth.
Stars give off mainly high-energy electromagnetic radiation-as we know from our sun's blinding visible light and the UV that can burn us. The space between the stars is cold, so most atoms and molecules there don't have enough energy to radiate. Instead, they tend to absorb radiation from the stars. When they do, this produces a dark absorption line in the spectrum coming toward us from the stars behind them. The cold matter may then re-radiate some of the energy it absorbed in a lower-energy form, often in the infrared and radio parts of the electromagnetic spectrum.
Chapter 1 ¥ Among the Stars
The intellect is empty if the body has never knocked about, if the nose has never quivered along the spice route. Both must change and become flexible, forget their opinions and expand the spectrum of their tastes as far as the stars. ¥ Michel Serres, The Five Senses, 1985
Yes, the stars!
The sensory spread that's laid on for us every day of our lives went onto the fire around fourteen billion years ago and has been simmering around the stars ever since. Our universe is a stew of matter and energy, and some of the molecules that we smell and taste today bubbled up in it very early on, long before the simplest form of life.
It may sound crazy to sniff and slurp through airless interstellar space, but generations of astronomers have opened the heavens for us to imagine just that. So: you're standing somewhere under the open sky, on a clear night, away from city lights. After you let your eyes adapt to the darkness, you can make out hazy patches here and there, perhaps under Orion's belt in the winter, or the band of the Milky Way in Sagittarius in summer. Zoom your mind's eye in on those indistinct patches, and borrow from the telescopic images you've seen of nebulas in deep space: dramatic swathes and swirls of light set in star-studded blackness, sometimes backlighting darker swirls. These are immense clouds of stardust, diffuse matter that has been driven out of stars as they burned, burned out, collapsed, and exploded. The bright clouds glow with energy; the dark ones coldly absorb it.
Now release a super-volatile emanation of yourself. You're a space-time traveler, an assistant to the Chef of the cosmos, disembodied except for chemical senses sensitive enough to sample-and robust enough to withstand-its primordial flavors. Fly light-years into the stew, plunge into those dusty clouds, and open up.
You taste mineral saltiness, and bitterness, and sharp acids, and even sweetness. You feel and smell the irritating pungency of ammonia cleaner, and the stink that it dispels. You catch the heady smells of solvents, of alcohols, of campstove fuel. Vinegar. Eggs. A hint of fruit!
By earthly standards that doesn't sound like an especially delicious composition. But it's intriguing. What are those familiar molecules doing out there? And why just those? To start so way out and way back helps stretch both our understanding and our sense of wonder. It shows that the smells and tastes to come, the various earthly creatures that produce their own, and the perfumers and cooks who modify and multiply them, are all participants in the original, ongoing project of the cosmos: the unfolding of matter's possibilities.
This chapter is about the initial stages of that unfolding, the fires of the stars and their flavorful ashes.
Recipe for the universe:
mix matter and energy, and cook
How did volatile molecules that we smell every day come to exist both here and in outer space? It's quite a story, one that emerges from the collective observations and thinking of thousands of scientists from many countries over many decades. It involves the birth of the cosmos as a whole and the origins and evolution of life on Earth. And at the heart of this nondenominational, transcultural creation story is a cosmic version of cooking.
Consider making caramel on your stovetop. You start with a single ingredient, white crystals of table sugar, which taste simply sweet and have no aroma. Put the sugar in a pot, apply heat energy, and stir. After a few minutes, you've turned the solid crystals into a colorless liquid. Still no aroma. Keep heating, and that liquid turns pale yellow-and begins to smell. It gets light brown, then progressively darker and stronger smelling. In the end you've made a dark syrup that's sweet but also sour and bitter, and richly aromatic. From one substance you've made many: from simplicity, complexity.
A similar process cooked up the entire universe as we know it. The original recipe from the Chef of the cosmos goes something like this. Mix a dozen kinds of elementary particles together with four fundamental forces, and set aside. After a few hundred million years, the particles have combined to form atoms, a hundred different kinds. After another long stretch, many of those atoms have combined to form molecules-and the mix begins to smell. Some of the molecules combine to form particles of dust, and the dust clumps up to form planets. At least one planet, our own, produces increasingly complex molecules, then collectives of molecules that somehow come alive-and these generate a vast bouquet of new volatiles for the Chef to savor, caramel included. So: from a handful of elementary particles the Chef has made countless kinds of molecules, with countless qualities.
This primordial cooking underlies all of our experience, mundane and miraculous. To understand why volatile molecules exist at all for us to smell, and why they exist where they do, let's start in the pristine cosmic kitchen as the Chef gets things going. No smells yet, but just wait.
Cooking up stars
However the known universe came into being, most astrophysicists agree that it did so around fourteen billion years ago in an explosive flash at an unimaginably high temperature. From the moment of this "Big Bang" the universe expanded outward. As it expanded it cooled down, and the kinds of matter and energy that we know on Earth began to appear. In the first fraction of a second emerged packets of electromagnetic energy called photons, which we know as light and heat and radio waves. Along with photons appeared three kinds of raw matter, the subatomic particles that combine to make atoms: protons and neutrons that form the central nucleus of the atom, and electrons that orbit around the nucleus. It's the different numbers of subatomic particles in atoms that give us the hundred-odd different elements with their different qualities: hydrogen, carbon, oxygen, and so on. One solitary proton forms the simple nucleus of atoms of hydrogen, so hydrogen was the first element to be born, followed by nuclei of helium and a bit of lithium.
After only a matter of minutes, the continuing expansion of the universe cooled and slowed everything down to the point that the protons and neutrons no longer had enough energy to fuse together to make heavier atomic nuclei. The evolution of matter paused, for some hundreds of millions of years.
But during that long hiatus, one of the universe's fundamental forces worked inexorably to reenergize matter. Gravity is a force that acts between any two bodies of matter, tiny or huge, and pulls them toward each other. In the newborn three-element universe, neighboring atoms gradually felt each other's gravitational pull. They gathered into clusters, clusters into more crowded clusters, all the while moving faster and faster, bouncing off each other with more and more force, releasing more and more heat energy as they did.
As the universe as a whole continued to expand and cool off, gravity created hot pockets of densely crowded atoms, some of them so dense and hot that they began to emit enough energy to glow. This was the first generation of stars.
Cooking up chemical elements in stars
The material richness of our world is a reflection of its chemical complexity, its countless combinations of the hundred-odd chemical elements. The first stars had just three elements to work with. They generated nearly all the rest by becoming fantastic self-adjusting, self-destroying, billion-degree ovens.
Imagine a member of that first generation of stars. As gravity causes its matter to crowd together and collide with ever increasing force, its temperature and energy increase. At a few million degrees, the conditions are right for two hydrogen nuclei to fuse into a single helium nucleus. This reaction releases energy-which jolts the nuclei into moving fast enough to resist the gravitational force. Fusion and gravity balance each other, and the star can burn with a steady flame like this for billions of years, using hydrogen nuclei as fuel and producing helium nuclei as the residue. When it has consumed most of its hydrogen fuel, the fusion reaction slows down, gravity begins to dominate again, the largely helium core of the star begins to contract, the temperature rises-until the helium nuclei can become the new fuel, fuse to form yet larger nuclei, and again balance gravity so that steady burning can continue. Now we have oxygen and carbon: two of the primary chemical players in the saga of life and the osmocosm.
Then the cycle of contraction, temperature rise, and new fusion repeats again and again, at ever escalating temperatures. The star takes on an onionlike structure, with portions of the newly formed elements surviving in the outer, cooler layers. After twenty-five new elements have been cooked up, gravity finally prevails over fusion and forces the star into a final burst of creativity-and generosity. It contracts the star's core to such an extreme density and temperature that the core explodes and becomes what's called a supernova. The energies released are so extreme that they trigger the formation of ninety-odd more elements. And the explosion blasts these and the first twenty-six into interstellar space, the space between stars.
The supernova thus serves up its creations to the calmer cosmos at large. And it's here that the elements can manifest their individual qualities, explore their affinities for each other, join up, and initiate the next stage in the unfolding of matter's possibilities-the stage in which the first molecules of smell emerge.
Cooking up molecules between stars
Molecules are the stuff of our world, the substance of nearly everything that we see and touch, taste and smell. They're simply combinations of elements, two or more atoms that have joined with each other in a specific arrangement. Given a hundred elements to work with, the number of possible arrangements is-well, astronomical. It's with the birth of molecules that the cosmos attained entirely new levels of complexity.
Molecules are products of the electromagnetic force, the attraction between particles of opposite electrical charge. The nucleus of an atom carries a positive electrical charge thanks to its protons. Electrons that orbit the nucleus carry a negative electrical charge, and it's the attractive force between positive protons and negative electrons that keeps the electrons in orbit. Molecules are the structures that result when the nuclei of different atoms share orbiting electrons with each other. That electron sharing is the bond that holds them together in a stable structure. Some molecules consist of just two or three atoms-for example, carbon monoxide, CO, and water, H2O-while DNA molecules include many thousands. Most volatile molecules have between a few and a few dozen.
The electromagnetic force isn't strong enough to withstand the energies at work in a star. It readily forms molecules at the moderate temperatures that we experience in everyday life, between fire and ice, where atoms are moving slowly enough that they can bump into each other and bond without immediately getting knocked apart again. In the near motionless cold of deep space, and with atoms few and far between, it can take many years for those atoms to encounter and react with each other. More favorable interstellar regions are "giant molecular clouds," the smudgy chiaroscuro patches familiar from telescopic images of the constellations Orion and Sagittarius. These are remnants of supernovas and aged stars that gravity has slowly pulled together, along with new stars that are beginning to burn nearby. They harbor regions denser with atoms, and temperatures around those of our kitchens and ovens. As their name indicates, these clouds are where astrochemists have had the best luck at detecting cosmic molecules.
Molecules in open space exist because their atoms happened to bump into each other and stick. The most abundant atoms in space include hydrogen (abbreviated H), oxygen (O), and carbon (C), whose particular electron-sharing tendencies naturally lead to the formation of small molecules like oxygen gas (O2), water (H2O), and carbon monoxide and dioxide (CO and CO2). Carbon atoms also readily bond with each other to form long chainlike molecules, as well as six-cornered ring molecules. The chains and rings readily nestle together with others of their kind, and can aggregate to form ever larger masses: cosmic soot. The dark swirls in the molecular clouds are a mixture of carbon soot and similar aggregates of primordial minerals. These various particles make up what's called interstellar dust.
The individual grains of interstellar dust are microscopically tiny, but their influence on the development of the cosmos is huge. They provide a solid surface to which free-floating atoms and small molecules can stick. They thus act as gathering places that encourage chemical activity, new reactions, larger molecules. On them the material world became increasingly diverse, complex, capable of further development. And to the nose of the cosmic Chef, it became aromatic-billions of years before our own sun began to shine.
In 2020, the roster of known interstellar molecules numbered more than two hundred. Here I'll note only the few dozen molecules that we also sense in everyday life, along with the everyday materials that they dominate and so "smell like" to us.
Detecting the Smells of Space
At last, smells! But how can we possibly know-not imagine, but know-what molecules are so far out there in space?
By the telltale traces they leave in the energy that the cosmos constantly rains down on our planet. Astrochemists are connoisseurs of electromagnetic radiation, and in particular the visible light, infrared light, and radio waves that originate in stars and galaxies and reach us across the vastness of space. In one year these forms of radiation travel staggering distances, so when we see stars and galaxies, we are seeing deep into both space and time, into the past history of the cosmos.
Astrochemists collect the faintest light and radio emissions with telescopes that are far more efficient and sensitive than our eyes or radios. They then pass them through the electronic equivalent of a prism that separates them into their component colors, or frequencies. The pattern of frequencies-the spectrum-is a kind of fingerprint that makes it possible to identify the kind of matter that emitted it, and the kind of matter that might have absorbed some of it on its long journey to Earth.
Stars give off mainly high-energy electromagnetic radiation-as we know from our sun's blinding visible light and the UV that can burn us. The space between the stars is cold, so most atoms and molecules there don't have enough energy to radiate. Instead, they tend to absorb radiation from the stars. When they do, this produces a dark absorption line in the spectrum coming toward us from the stars behind them. The cold matter may then re-radiate some of the energy it absorbed in a lower-energy form, often in the infrared and radio parts of the electromagnetic spectrum.
