* The Earth is an exception, because our primordial hydrogen, only weakly bound by our planet’s comparatively feeble gravitational attraction, has by now largely escaped to space. Jupiter, with its more massive gravity, has retained at least much of its original complement of the lightest element.
We have examined our Sun, the nearest star, in various wavelengths from radio waves to ordinary visible light to X-rays, all of which arise only from its outermost layers. It is not exactly a red-hot stone, as Anaxagoras thought, but rather a great ball of hydrogen and helium gas, glowing because of its high temperatures, in the same way that a poker glows when it is brought to red heat. Anaxagoras was at least partly right. Violent solar storms produce brilliant flares that disrupt radio communications on Earth; and immense arching plumes of hot gas, guided by the Sun’s magnetic field, the solar prominences, which dwarf the Earth. The sunspots, sometimes visible to the naked eye at sunset, are cooler regions of enhanced magnetic field strength. All this incessant, roiling, turbulent activity is in the comparatively cool visible surface. We see only to temperatures of about 6,000 degrees. But the hidden interior of the Sun, where sunlight is being generated, is at 40 million degrees.
Stars and their accompanying planets are born in the gravitational collapse of a cloud of interstellar gas and dust. The collision of the gas molecules in the interior of the cloud heats it, eventually to the point where hydrogen begins to fuse into helium: four hydrogen nuclei combine to form a helium nucleus, with an attendant release of a gamma-ray photon. Suffering alternate absorption and emission by the overlying matter, gradually working its way toward the surface of the star, losing energy at every step, the photon’s epic journey takes a million years until, as visible light, it reaches the surface and is radiated to space. The star has turned on. The gravitational collapse of the prestellar cloud has been halted. The weight of the outer layers of the star is now supported by the high temperatures and pressures generated in the interior nuclear reactions. The Sun has been in such a stable situation for the past five billion years. Thermonuclear reactions like those in a hydrogen bomb are powering the Sun in a contained and continuous explosion, converting some four hundred million tons (4 x 1014 grams) of hydrogen into helium every second. When we look up at night and view the stars, everything we see is shining because of distant nuclear fusion.
In the direction of the star Deneb, in the constellation of Cygnus the Swan, is an enormous glowing superbubble of extremely hot gas, probably produced by supernova explosions, the deaths of stars, near the center of the bubble. At the periphery, interstellar matter is compressed by the supernova shock wave, triggering new generations of cloud collapse and star formation. In this sense, stars have parents; and, as is sometimes also true for humans, a parent may die in the birth of the child.
Stars like the Sun are born in batches, in great compressed cloud complexes such as the Orion Nebula. Seen from the outside, such clouds seem dark and gloomy. But inside, they are brilliantly illuminated by the hot newborn stars. Later, the stars wander out of their nursery to seek their fortunes in the Milky Way, stellar adolescents still surrounded by tufts of glowing nebulosity, residues still gravitationally attached of their amniotic gas. The Pleiades are a nearby example. As in the families of humans, the maturing stars journey far from home, and the siblings see little of each other. Somewhere in the Galaxy there are stars – perhaps dozens of them – that are the brothers and sisters of the Sun, formed from the same cloud complex, some 5 billion years ago. But we do not know which stars they are. They may, for all we know, be on the other side of the Milky Way.
The conversion of hydrogen into helium in the center of the Sun not only accounts for the Sun’s brightness in photons of visible light; it also produces a radiance of a more mysterious and ghostly kind: The Sun glows faintly in neutrinos, which, like photons, weigh nothing and travel at the speed of light. But neutrinos are not photons. They are not a kind of light. Neutrinos, like protons, electrons and neutrons, carry an intrinsic angular momentum, or spin, while photons have no spin at all. Matter is transparent to neutrinos, which pass almost effortlessly through the Earth and through the Sun. Only a tiny fraction of them is stopped by the intervening matter. As I look up at the Sun for a second, a billion neutrinos pass through my eyeball. Of course, they are not stopped at the retina as ordinary photons are but continue unmolested through the back of my head. The curious part is that if at night I look down at the ground, toward the place where the Sun would be (if the Earth were not in the way), almost exactly the same number of solar neutrinos pass through my eyeball, pouring through an interposed Earth which is as transparent to neutrinos as a pane of clear glass is to visible light.
If our knowledge of the solar interior is as complete as we think, and if we also understand the nuclear physics that makes neutrinos, then we should be able to calculate with fair accuracy how many solar neutrinos we should receive in a given area – such as my eyeball – in a given unit of time, such as a second. Experimental confirmation of the calculation is much more difficult. Since neutrinos pass directly through the Earth, we cannot catch a given one. But for a vast number of neutrinos, a small fraction will interact with matter and in the appropriate circumstances might be detected. Neutrinos can on rare occasion convert chlorine atoms into argon atoms, with the same total number of protons and neutrons. To detect the predicted solar neutrino flux, you need an immense amount of chlorine, so American physicists have poured a huge quantity of cleaning fluid into the Homestake Mine in Lead, South Dakota. The chlorine is microchemically swept for the newly produced argon. The more argon found, the more neutrinos inferred. These experiments imply that the Sun is dimmer in neutrinos than the calculations predict.
There is a real and unsolved mystery here. The low solar neutrino flux probably does not put our view of stellar nucleosynthesis in jeopardy, but it surely means something important. Proposed explanations range from the hypothesis that neutrinos fall to pieces during their passage between the Sun and the Earth to the idea that the nuclear fires in the solar interior are temporarily banked, sunlight being generated in our time partly by slow gravitational contraction. But neutrino astronomy is very new. For the moment we stand amazed at having created a tool that can peer directly into the blazing heart of the Sun. As the sensitivity of the neutrino telescope improves, it may become possible to probe nuclear fusion in the deep interiors of the nearby stars.
But hydrogen fusion cannot continue forever: in the Sun or any other star, there is only so much hydrogen fuel in its hot interior. The fate of a star, the end of its life cycle, depends very much on its initial mass. If, after whatever matter it has lost to space, a star retains two or three times the mass of the Sun, it ends its life cycle in a startlingly different mode than the Sun. But the Sun’s fate is spectacular enough. When the central hydrogen has all reacted to form helium, five or six billion years from now, the zone of hydrogen fusion will slowly migrate outward, an expanding shell of thermonuclear reactions, until it reaches the place where the temperatures are less than about ten million degrees. Then hydrogen fusion will shut itself off. Meanwhile the self-gravity of the Sun will force a renewed contraction of its helium-rich core and a further increase in its interior temperatures and pressures. The helium nuclei will be jammed together still more tightly, so much so that they begin to stick together, the hooks of their short-range nuclear forces becoming engaged despite the mutual electrical repulsion. The ash will become fuel, and the Sun will be triggered into a second round of fusion reactions.
This process will generate the elements carbon and oxygen and provide additional energy for the Sun to continue shining for a limited time. A star is a phoenix, destined to rise for a time from its own ashes.* Under the combined influence of hydrogen fusion in a thin shell far from the solar interior and the high temperature helium fusion in the core, the Sun will undergo a major change: its exterior will expand and cool. The Sun will become a red giant star, its visible surface so far from its interior that the gravity at its surface grows feeble, its atmosphere expanding into space in a kind of stellar gale. When the Sun, ruddy and bloated, becomes a red giant, it will envelop and devour the planets Mercury and Venus – and probably the Earth as well. The inner solar system will then reside within the Sun.