Less than two centuries ago, what the stars were made of was the subject of myth and mystery. Some believed it would never be possible to find out the truth. But the development of spectral analysis and atomic physics in the 19th and 20th centuries has enabled revelations of stellar composition and cosmic abundance, of which earlier astronomers could only dream.
As surprising as it may seem, the chemicals we are most familiar with on Earth—oxygen in the air, carbon in living things, silicon in rocks—represent only a tiny fraction of the substances in the Universe as a whole. Across the entire cosmic landscape, it is hydrogen that turns out to be by far the most abundant chemical element. By mass, hydrogen accounts for 74 percent of all the atoms in the Universe; another 24 percent are helium atoms, and only the remaining two percent is made up of all the other chemical elements. In order to find out how astronomers have come by these figures, and how they have discovered the internal composition of stars, it is helpful to first delve a little into what an atom is like.
When, in 1869, Russian chemist Dmitri Mendeleev famously organized the chemical elements into the periodic table, which charts their similarities and differences, no one knew what gave the elements their chemical identities. That knowledge had to wait until after 1911, the year in which Ernest Rutherford and his team of physicists at Cambridge University’s Cavendish Laboratory unraveled the structure of atoms.
“It is sound judgment to hope that in the not too distant future we shall be competent to understand so simple a thing as a star.”
ARTHUR EDDINGTON 20TH CENTURY ASTROPHYSICIST
Rutherford discovered that atoms have a highly dense, positively charged nucleus. He likened the nucleus within the atom to a gnat in London’s Albert Hall, although most people today refer to the analogy as “the fly in the cathedral.” It conveys the idea that most of an atom is empty space, with the majority of its mass concentrated into a tiny central region. The key to an atom’s chemical identity is the composition of this nucleus. Inside it are minuscule subatomic particles called protons. Each one measures just one billionth of a micrometer across and carries an electrical charge. The number of protons determines how the atom behaves chemically. In the case of hydrogen, the nucleus contains a single proton, making it the simplest kind of atomic nucleus; oxygen has eight protons; carbon has 16; and so on across the periodic table.
The fact that the protons are all positively charged means they try to repel each other. To stop atoms spontaneously flying apart, nature has conspired to include other particles in the nucleus as well, to act like glue. Called neutrons, these subatomic particles are similar to protons but carry no electrical charge. They cannot exist for long outside an atomic nucleus—they decay in around 15 minutes if forced into isolation. Inside the nucleus, however, they are stable and they hold the protons together. The more protons in a nucleus, the more neutrons there need to be. For example, the second simplest atom, helium, contains two protons and two neutrons; lithium contains three protons and four neutrons; and so on. Unlike the number of protons, which defines its chemical properties, an element can contain different numbers of neutrons and still be the same element. These variants are known as “isotopes” and can sometimes be radioactive (see How Old is the Universe?). Normally hydrogen has only a single proton and no neutron, but a version of hydrogen exists that also has a neutron; this “heavy” hydrogen is an isotope known as “deuterium.”
Being composed of differing numbers of protons and neutrons, it is obvious that different atoms will have different masses. The lightest element is hydrogen, and the heaviest naturally occurring element on Earth is uranium, with 92 protons and 146 neutrons. This difference in the mass of elements is the reason for the Earth’s particular chemical identity: our smallish planet does not generate enough gravity to hold onto the lightest elements, such as hydrogen and helium. Mercury, Venus, Mars, and the moons that dot the Solar System, are also too small to retain hydrogen. Only when a planet reaches the mass of Jupiter or Saturn, approximately 300 times the mass of Earth, does its gravity become strong enough to retain every element.
Although the neutrons hold each atomic nucleus together, there is one thing they cannot do: cancel out the electrical charge of the protons. Every atomic nucleus is naturally positively charged. This is usually balanced by even tinier particles called electrons, which are negatively charged and encircle the nucleus. If there are ten protons in a nucleus, then ten electrons will orbit that nucleus, keeping the atom electrically neutral. To return to our analogy, the electrons will be microscopic specks spread throughout the whole cathedral, whizzing around the fly.
Electrons in atoms can absorb energy if the atom is bathed in radiation, and be boosted to a higher-energy orbit within the atom; when this takes place the atom is said to be excited. Very quickly the electron will drop back to a lower orbit and the atom returns to a more stable state, radiating energy as it does so. Electrons can even be temporarily stripped away from the atom completely if the absorbed energy is sufficient, leaving the atom positively charged. This is called ionization; the atom soon attracts another negatively charged electron to fill the gap, and radiates energy as the electron is captured into orbit. Strange as it may sound at first, this behavior of atomic electrons has given astronomers a perfect tool for investigating the composition of celestial objects.
In 1830, French philosopher Auguste Comte published his Cours de philosophie positive. In it was a remarkable claim that, at the time, must have seemed entirely reasonable. Comte stated that mankind would never know anything about the Sun’s composition because it was too far away to analyze. By extension, the same went for the other celestial objects. The only exceptions to this rule were the occasional meteorites that fell from the sky, which could be collected for analysis. These were often made of iron, which at the time was taken to mean that perhaps metals were abundant throughout space.
Just three years after the Comte’s death in 1857, German scientists Gustav Kirchhoff and Robert Bunsen discovered a way to make the analysis of distant celestial objects commonplace. It must have seemed like a dream come true and opened up the Universe for study in a way no one could have previously imagined. The pair was investigating the puzzling phenomenon that when sunlight is passed through a prism and split into its constituent colors, known as a spectrum, a pattern of vertical dark lines appears along the spectrum. It looks rather like a barcode superimposed on the otherwise continuous sweep of rainbow colors.
Around the same time, chemists were noticing that when they burned a pure sample of a chemical element it turned the flame a characteristic color. For example, sodium burns orange and lithium burns red. When this light is passed through a prism it resolves into a pattern of bright vertical lines, the brightest of which is the color seen during the flame test; indeed for some elements, the there is only a single vertical line of color. Wondering what the connection was between the dark lines in a continuous spectrum and the bright lines from flame tests, Kirchhoff began investigating using Bunsen’s newly devised burner. He burned chemicals, he split light through prisms, he investigated sunlight further, and eventually it dawned on him that elements only ever absorb the same wavelengths of light as they give out. In effect, they store the absorbed light energy for a short time and then re-emit it back into their surroundings. Sodium, for example, is only capable of giving out and absorbing yellow light with a wavelength of 589 nanometers; for lithium it is red light with a wavelength of 670 nanometers. With the hindsight of our knowledge of atomic structure and electron energy levels that was developed in the early 20th century, this all makes sense. But Kirchhoff made his analyses with no idea why elements showed characteristic spectra.
SPECTRAL ANALYSIS: THE PATTERN OF SPECTRAL LINES BETRAYS A CELESTIAL OBJECT’S CHEMICAL COMPOSITION
All elements have their specific wavelengths, like fingerprints, and whether they emit or absorb light depends entirely upon their physical condition. An incandescent vapor, such as one created when burning an element in a flame test, gives out that element’s characteristic light. But a cooler vapor with a source of light behind it absorbs its characteristic wavelengths, creating dark lines in the light’s spectrum. Astronomers swiftly understood that the Sun’s surface was just such a light source and that it was surrounded by an atmosphere of cooler gases. Kirchhoff’s work meant that they could deduce the chemical composition of the Sun and of other unreachable celestial objects. All they had to do was collect sufficient light to pass through a prism or some other device capable of splitting the light into a spectrum, and then measure the wavelengths of the dark lines. The chemical composition of the Universe opened up before their very eyes.
Early spectral analysis made it look as if the stars were largely composed of metallic elements, mirroring the meteorites that fell to Earth. But the presence, or not, of a specific dark absorption line was later understood to be dependent on the ionization of the atoms, that is whether one or more electrons had been stripped from the atom. It meant that not every element present would necessarily show up in an absorption spectrum. After decades of laboratory work to understand the effects of ionization, correction factors have allowed us to arrive at the correct cosmic abundance of elements: 74 percent hydrogen, 24 percent helium, and two percent all the other chemical elements.
This does not mean, however, that these proportions have been fixed since the moment of the Big Bang. The stars themselves are chemical factories, converting the Universe’s vast stocks of interstellar hydrogen into the heavier chemical elements. This fearsome activity, known as “nuclear fusion,” takes place at the core of each and every star, where the temperatures reach millions of degrees. It was in the early years of the 20th century that French physicist Jean-Baptiste Perrin suggested that the nuclear fusion of hydrogen was at work in the Sun, but it wasn’t until 1938 that scientists understood the details of the mechanism.
In the Sun’s core, where the temperature reaches 15 million degrees and the gas is squeezed to about 150 times the density of water, nuclei of hydrogen are being fused into nuclei of helium, releasing vast amounts of energy. Four hydrogen nuclei, that is four protons, are converted into a single helium nucleus—two protons and two neutrons bound tightly together. Even at the density of the Sun’s core, it is unlikely that four nuclei will collide with one another simultaneously; instead, a helium nucleus is built up in a sequence of collisions, during which two out of four protons convert to neutrons. Energy is released at all stages as mass is lost, thus causing the star to radiate heat and light.
About 4 million tons of matter are transformed into energy every second inside the Sun—enough mass to build more than ten Empire State Buildings. This energy pushes its way outward, taking hundreds of thousands of years to reach the surface, where the temperature is a mere 6000 degrees and the density is only a tenth of a percent of the density of Earth’s air at sea level. From here, the energy shoots off into space as electromagnetic radiation. If it happens to be heading for Earth, its journey will end just eight minutes later in our skies. It is remarkable to think that the warm sunlight on your face was generated in the heart of the Sun, hundreds of thousands of years ago.
Investigating the way in which stars build other, heavier elements through nuclear fusion and how that alters a star’s composition is not easy, since those changes are locked into the core of the star. Only in the star’s death throes does it effectively dissect itself for the whole Universe to see—and on Earth there are plenty of astronomers poised with their telescopes.
Broadly speaking, stars die in one of two ways, depending on how much mass they contain. The first scenario applies to the low-mass stars, which contain less than eight times the Sun’s mass. As with all stars, energy is generated only in the core and as more and more of the hydrogen in the core is converted into helium, so the process falters. Although the star’s surrounding layers contain more than half the mass of the star, this matter cannot penetrate the core to replenish the waning stocks, and so, as hydrogen fusion in the core slows down, the outpouring of energy dwindles and the star begins to contract. This squeezing drives up the temperature and ignites hydrogen fusion in a shell around the now largely inert core. This shell releases a new rush of energy that pushes the upper layers of the star outward, bloating it from a diameter of roughly a million kilometers to hundreds of millions of kilometers. The outer layers cool and the star becomes a “red giant.” When this happens to the Sun (in five billion years’ time) it will engulf Mercury, Venus and Earth.
THE INTERIOR OF A RED GIANT STAR
Inside the red giant star, the core continues to shrink and heat up. When its temperature reaches 100 million degrees and its density approaches 1000 times that of water, the helium begins to fuse into carbon, and into oxygen if the temperature is high enough. This activity lasts for between 10 million to 100 million years and then the helium runs out and the star contracts again. The shell of hydrogen fusion continues around the core, creating enough energy to lift the outer layers still further, blowing them off into space to create a nebula of glowing gases. William Herschel, the 18th century astronomer who had recently discovered Uranus, thought these nebulae looked rather similar, so he misnamed them “planetary” nebulae and the name has stuck.
The more layers the dying star sheds, the closer to the core astronomers can peer. Spectroscopic studies of planetary nebulae do indeed show enrichments of heavy elements, but still most of these remain locked in the stellar core which sits at the heart of the nebula. As the surrounding layers lift off into space, the core can eventually be seen as a sphere of compressed gas, about the size of the Earth but 200,000 times more dense, with a temperature around 500,000 degrees. These are “white dwarf” stars that astronomers use to age the Milky Way (see How Old is the Universe?). They are composed of almost pure carbon and oxygen, with small quantities of the other chemical elements.
By contrast, the second scenario applies to high-mass stars, with more than eight times the mass of the Sun. The greater mass generates even higher temperatures in the core following the cessation of hydrogen fusion. These temperatures allow the star to build ever-heavier chemical elements through fusion, and spur the star on to expand to gargantuan proportions. If one of these red “supergiants” were to magically replace the Sun, it would reach to the planet Jupiter. These stars suffer a more spectacular fate than their low-mass cousins.
The evolution of a red supergiant follows the same pattern as a red giant up until the helium fusion stage. Then, because of the extra mass pushing down on the core, it experiences further bouts of reignition, allowing it to fuse heavier and heavier chemical elements. The star develops an onion-like structure of layers around the core, each with different nuclear fusion reactions taking place: from the outside layers to the inner layers, there is fusion of hydrogen to helium, helium to carbon and oxygen, carbon and oxygen to neon and magnesium, and neon and magnesium to silicon and sulfur. At the very center of a mature red supergiant, silicon and sulfur fuse into iron and nickel; and this is the death knell for the star.
All other fusion processes in the star have released energy, but to fuse iron and nickel requires energy to be put into the reaction. Since the energy generated inside the star flows outward toward the less dense regions—like water running downhill—there is no available energy in the core to fuse the iron and nickel, and the generation of further energy stops. According to computer simulations, it takes just 24 hours to build up an iron-nickel core containing almost one and a half times the mass of the Sun. Having achieved this mass (the Chandrasekhar limit, see How Big is the Universe?), gravity overwhelms the forces holding the atoms apart, and the core rapidly collapses into a ball of tightly packed neutrons just 10 to 20 kilometers (6 to 12 miles) across. When it does this, the upper layers come crashing down on the core, initiating a tremendous explosion called a type II supernova. The energy released in the supernova drives a final giant burst of nuclear fusion that builds all the chemical elements heavier than iron and nickel, including radioactive elements. In the explosion, these newly minted elements are scattered throughout space.
Astronomers measuring supernova remnants today find some that are still expanding into space at a few thousand kilometers per second. The gases glow with their characteristic colors even after the initial energy of the explosion has been dissipated, because the radioactive elements in their midst decay, providing more energy to keep the gases excited. Eventually, the debris from both planetary nebulae and supernova remnants merge into the general reservoir of interstellar gas and dust, where they are subsumed into newborn stars. Overall, this grand process of stellar life and death has the effect of enriching the chemical composition of the Universe with heavier elements.
So the answer to the question “What are stars made from?” is that stars are made from the debris of the former celestial generations, and in this way each new generation contains a greater proportion of heavy elements than the last. Even though these account for only two percent of atoms across the Universe, they have a profound effect because they allow planets like the Earth to form, and life to emerge.