A History of Hydrogen Trivia Quiz

Answer: A proton and an electron

A neutral atom has a nucleus -- a compact conglomeration of positively charged protons and electrically neutral neutrons -- at its center, with negatively charged electrons inhabiting a comparatively large volume around. (For the atom to be neutral, the number of protons should equal the number of electrons so that their electrical charges will cancel out.) The simplicity of hydrogen makes it a great test case for physics theories: quantum mechanics was developed in part to calculate the behavior of a hydrogen atom.

An element is defined by the number of protons in its nucleus: one for hydrogen, two for helium, three for lithium, and so on down the Periodic Table. The number of neutrons can vary, though, and nuclei with the same number of protons and different numbers of neutrons are called isotopes. The most common hydrogen isotope has no neutrons at all -- the nucleus is just a bare proton. A hydrogen isotope called deuterium has one neutron for its one proton; when deuterium substitutes for regular hydrogen in a water molecule, it's called heavy water. And the heaviest kind of hydrogen, with two neutrons and one proton, is called tritium; this stuff is radioactive, and is found in common items (exit signs) as well as classified ones (hydrogen bombs).

Answer: Proton

A proton is made up of quarks -- two up quarks and one down quark, to be precise -- held together by the strong force. (If you really zoom in, all that strong-force action means that there are also gluons and quark-antiquark pairs inside a proton, but we can neglect those for now.) The quarks are fundamental particles, so zooming in on them doesn't change the picture.

At the beginning of the universe -- the Big Bang -- everything was dense and hot, so hot that you wouldn't be able to bind quarks together in a proton. As soon as you got them close together, their thermal energy would bounce them apart! The universe was thus filled with something called a quark-gluon plasma, a soup of unbound fundamental particles. This plasma would only have lasted about a millionth of a second before it cooled down enough that quarks began condensing out into protons and neutrons.

Answer: There must be slightly more matter than anti-matter

Science-fiction fans know that matter and anti-matter cancel each other out: when a matter particle and its anti-matter partner meet, they annihilate each other, leaving only energy behind. (How much energy? E = mc^2 -- the annihilation energy is equal to the sum of mass energies of the two particles.) But this is a process that can also run backward: where there's energy, which is everywhere, you also have matter-antimatter pairs popping spontaneously into existence. It can just be hard to tell the difference, since most of the time the pairs annihilate each other immediately and all is as it was.

But in this kind of mechanism for the creation of matter -- which is the only mechanism we've seen -- there's perfect symmetry between matter and antimatter. If you make an antiquark, you also make a quark. If you make an electron, you also make an antielectron (or positron). And this is a problem, because there doesn't seem to be much antimatter in our universe: virtually everything we see is matter. In fact, if there had been exactly equal proportions at or near the beginning, you wouldn't expect there to be much of a universe at all, just a quick, continuous two-step of creation and annihilation. So there must have been something to break the symmetry and give matter the edge over antimatter, either in the Big Bang itself or in the physics that happened right after.

This problem is called baryogenesis, because baryons -- of which protons and neutrons are the most famous examples -- could not have been formed without an excess of matter. And, without protons, you can have neither hydrogen nor any other element. We don't yet understand how matter prevailed, but I for one am very glad it did, although I suppose anti-CellarDoor might feel differently...

The wrong answers are very wrong, by the way: as far as we can tell (and scientists have been looking!) they do not apply to the universe we live in.

Answer: Deuterium

The identity of an element is fixed by its atomic number, the count of protons in its nucleus. For hydrogen, this number is always one! But each nucleus can come in several different isotopes, characterized by different numbers of neutrons. Hydrogen has three isotopes. Protium, which is just a bare proton, is by far the most common. Deuterium, which is a proton and a neutron, is rare but stable; heavy water is water in which the two hydrogen atoms are the deuterium isotope. Tritium, which is a proton bound with two neutrons, is radioactive with a half-life of only about twelve and a half years, so any primordial tritium is long, long gone.

Big Bang nucleosynthesis is the name for the primordial production of nuclei, from deuterium to helium to lithium all the way up to beryllium (although the relevant beryllium isotope is unstable, so that's long gone, too). It started something like ten or twenty seconds after the birth of the universe, and continued for about twenty minutes -- after which the universe had cooled too much for these nuclear reactions to continue. Much later, nucleosynthesis would start again in the hot hearts of stars, but this could never compete with the Big Bang for scale. Most of the helium in the universe was made in that wild twenty minutes, and astronomers' observations of the relative amounts of helium and hydrogen constitute a major piece of supporting evidence for the Big Bang theory.

Answer: Cosmic microwave background radiation

Neutral atoms will begin to form when the thermal energy drops below the ionization energy -- that is, when the energy that the particles have from heat is lower than the energy needed to knock a bound electron out of its atom. Before things have cooled to that point, any new atom will immediately be shaken apart! This exciting time in the history of the universe -- the first time that chemical reactions could occur -- is called recombination, even though it represents the very first combination of electrons and nuclei.

Before recombination, the universe was opaque: light couldn't travel very far without scattering from free electrons. Once a significant amount of the electrons were bound in hydrogen atoms, though, light decoupled from matter -- from that moment forward, light and matter mostly went their own independent ways. Astronomers can see light from this "surface of last scattering"; it falls in the microwave part of the electromagnetic spectrum and it's fainter than closer, more recent sources of light, so it's called the cosmic microwave background radiation or CMB. CMB light is the very oldest light we'll ever see, and it's a powerful tool for examining the formation and structure of the universe.

Answer: Main sequence

In the early 1900s, when astronomers were first beginning to piece together the inner workings of the stars, they plotted stellar characteristics on a Hertzsprung-Russell diagram: brightness (or absolute magnitude, in astronomy terms) on the vertical axis, and color index on the horizontal axis. Brighter stars appear near the top, and bluer (or hotter) stars appear near the left. Early astronomers immediately noticed a dense, continuous band of stars stretching from the upper left to the lower right, and called it the main sequence.

Much later, as physicists began to understand nuclear fusion reactions, it was realized that main-sequence stars were all powered the same way, by hydrogen fusion in their cores. Stars in other parts of the diagram are powered in different ways; for example, the supergiants on the horizontal branch fuse helium in their cores and hydrogen in an outer layer. The diagram tells us that most visible stars fuse hydrogen, including our sun. Nearly all life on earth ultimately relies on sunlight, which means hydrogen is at the bottom of it all.

Answer: Proton-proton chain

The proton-proton chain, or pp chain, is the dominant mode of hydrogen fusion in stars up to around one and a quarter times the mass of the sun. Its name simply comes from its ingredients: the very first step is the fusion of one hydrogen nucleus (i.e. a proton) with another! The process is quite slow, which is good news for humanity as it means that the sun will be burning through its hydrogen fuel for quite a while yet. The bottleneck is that first step: two protons fuse into helium-2, which must then undergo radioactive decay to form deuterium (or hydrogen-2). That decay process is rare, though: it's much more common for helium-2 to simply split into two protons again, leaving the cycle right where it started.

Working out the p-p chain was a tough problem for astrophysicists, not least because it's rather hard to get a look at the core of the sun where all these nuclear reactions are taking place! The light that we see comes from the sun's outer layers, which absorbs the heat of the fusion reactions by convection. However, ghostly particles called neutrinos are produced at various stages of the process, and they so rarely interact with matter that they pass right through the outer layers of the sun. Detecting solar neutrinos is tough, but we've seen enough of them to confirm most of our understanding of the p-p chain.

In heavier stars, fusion usually proceeds via the CNO cycle, in which an initial carbon nucleus catalyzes the fusion process.

Answer: The supermassive black hole at the center of the Milky Way

The supermassive black hole at the galaxy's center has never been observed directly, but it is the best explanation for the intense radio emissions observed in the region. The very nature of black holes means that outside observers can't see inside them: anything inside the event horizon, even light, is trapped by the strong gravitational field and cannot escape.

We've seen in previous questions that hydrogen is the fuel for fusion reactions in the sun's core, but hydrogen is present in all layers of the sun. In fact, hydrogen accounts for 91% of all atoms in the sun, and 71% of the sun's mass. (Almost all the rest is helium, although there are also small amounts of heavier elements like oxygen and carbon.) Hydrogen is also by far the predominant element in the atmosphere of Jupiter and other gas giants.

Even the space between stars is filled with hydrogen. The material outside of solar systems -- gas and dust -- is called the interstellar medium. The first man-made object to leave the solar system, entering the interstellar medium, was the Voyager 1 probe in August of 2012. Amazingly, that probe was still sending data back to Earth, 35 years after its 1977 launch.

Answer: A spectrum

A spectrum contains all kinds of useful information. The shape of the spectrum of sunlight, for example, tells us the temperature of the sun. Meanwhile, spectral lines (that is, features corresponding to a narrow wavelength range and therefore a narrow energy range) show the signatures of various elements.

Every type of atom has a large number of possible energy levels -- different configurations of electrons that have different characteristic energies. If an atom absorbs light at a given energy, it will "jump" to a configuration with that much more energy than the starting state. The spectrum will show an absorption line, a narrow region "missing" light. Meanwhile, an atom that's already in a high-energy configuration might relax to a lower-energy configuration, emitting a photon to carry the excess energy away. The spectrum will show an emission line, an excess of light at that energy. The pattern -- that is, the locations and intensities of the lines -- is different for every element. Absorption lines in the spectrum of sunlight tell us about the composition of the solar atmosphere, which all sunlight must pass through on the way to us!

Answer: Redshift

A longer wavelength means lower-energy light: the signal is shifted toward the red end of the spectrum. By contrast, if the source is moving toward us, the wavelength gets shorter and the change is called a blueshift. The Doppler effect can also be observed in sound; the sirens of an ambulance sound higher-pitched as it drives toward you, and lower-pitched as it's driving away.

Since the universe is expanding, faraway galaxies are all moving away from us and their light is redshifted. The further away they are, the faster they're moving, and the higher the redshift, so astronomers use redshift as a proxy for the amount of time that the light has been traveling since it was first emitted. A narrow, distinctive spectral feature like the 21-cm line can be picked out by radioastronomers even with a substantial redshift. Close to home, astronomers have used this line to measure the rotation speeds of the arms of our galaxy. Farther away, astronomers plan to use this line to look far back into the history of hydrogen, into the time between the moment of the first formation of neutral hydrogen, and the moment when light from the very first stars ionized some of that hydrogen again.