ABOUT 31 AUGUST; ABOUT 665,000 METERS UP THE LINE
For the first early humans, the ones who perceived the world as a place of objects and causes and effects, the Sun was simply a bare fact of life. It was the reason for and ruler of the day, making its way across the sky, reaching its apogee, and then heading to its resting place at night only to emerge again to rule another day, in endless succession. Regarding it, there was nothing much to think about. It was the source of light and heat, brighter and hotter on some days and in some times than others. In the regions where humans first appeared, it was overwhelming in its seemingly omnipotent strength. Who can say how early it became the master of human dream worlds and trances, personified and given godlike attributes? Its power was too obvious to deny. And when, after millennia of evolving and traveling and learning and surviving, some of the humans settled in definite places, its power became even greater. In its absence, nothing could grow, but if it were too mercilessly persistent in driving away the rain, nothing could grow then, either. It had to be begged and urged and placated. Humans had to plead with it to be moderate and generous, so that they and their children could go on. They would even murder on its behalf, if that’s what it required of them. In every place it was held in awe.
But no one knew what it actually was, and what our true relationship to it was, until almost the last moment of our imaginary year or the last lengths of our imaginary line. There were those, such as observers in Greece before the Common Era, who conceived of the Sun as a huge flaming ball, but few took much stock in their opinions. The Sun’s true nature was more exotic and overwhelming than the ones who built temples to it could ever have dreamed. Its power dwarfed their tiny imaginations. And yet, in the Sun’s own realm and scale of being it was in many ways unremarkable. The humans who worshiped it were made of the very substances of which it was composed. And it had been “waiting” for them to appear for more centuries than they had the capacity to understand. More than the first ones who were awed by it realized, it truly was indispensable. But, like them, it also had a birth, and it will ultimately have a death.
Current Evidence About the Origin of the Solar System
The dominant idea in discussions of our solar system’s origins is the nebular hypothesis. While this concept goes back to the 18th century, recent research has given it much greater depth. As we saw in the very basic, very elementary description of star formation in The First Stars, nebula, clouds of gas and dust, called in this case the interstellar medium, begin falling in on themselves due to localized gravitational instabilities. In the case of our Sun, this process took place in a specific kind of star-forming environment within the physical context of our particular galaxy. In a 2010 paper, theoretical astrophysicist Fred Adams summarized the birth environments that were possible at the time of our Sun’s emergence. Adams first points out that stars generally do not form in isolation within a galaxy, but rather as members of a group or cluster. Our Sun seems to have formed as part of such a group, so we are interested in knowing how big such a group might have been, and what physical properties the cloud of matter out of which our group of stars formed might have possessed. The way in which star clusters are generated affects the natures of the solar systems that emerge out of the individual nebular disks.1 Although stars can form in groups ranging from 1,000 to 1,000,000,2 radiation levels indicate the Sun must have been born in a cluster of perhaps as many as 4,000 others. Such clusters are relatively common.3 Technically known as open clusters, these gravitationally bound systems last 100-500 million years.4
The solar nebula must have extended at least out to Neptune’s orbit to facilitate planet formation, a distance of about 30 astronomical units. (One AU=the average distance between the Sun and the Earth.) There is evidence that the nebular cloud may even have reached out to the zone of small rocky bodies known as the Kuiper Belt. The presence of these bodies begins dropping off at about 50 AU (about 7.5 billion kilometers), which indicates where the outer limit of the nebular cloud might have been.5
The element iron-60 (60Fe) is a short-lived radioactive isotope that is exclusively the product of supernova nucleosynthesis and not something that can be produced in the protoplanetary disk of an emerging solar system. Its presence in our solar system suggests strongly that the interstellar medium that gave rise to our solar system was enriched by more than one supernova event.6 The presence of short-lived radio isotopes implies there was only a short time between their production and their incorporation into the new solar system. They must have been produced locally, near the time and location of the solar birth.7 Other evidence points to the possibility of some internal enrichment being present as well.8 There seems to be a growing consensus among astrophysicists and astronomers that supernovae contributed significantly to the formation of our Sun. Additional confirmation of this supernova enrichment appears to have been found in the Orgueil meteorite, which contains chromium-54 (54Cr), an isotope that can only be produced by a supernova.9 And an analysis of several different meteorites, including those from Mars and the moon, indicates the presence of different isotopes of titanium, 46Ti, which is created in Type II supernovae, the explosion of a massive star, and 50Ti, which is created in Type Ia supernovae, the type that occurs when a white dwarf star explodes.10 Indeed, there is evidence from meteorite analysis of the presence of material from several dozen stellar sources, and, interestingly, organic materials as well. The conclusion is unavoidable: our Sun gained its elements from a variety of different supernova events. 11
The line between being close enough to a supernova and too close is a fine one. A solar nebula has to be close enough to a supernova event to be enriched by its radio isotopes, but not close enough to be destroyed by it. The circumstances would have had to have been just right for our solar system’s nebula to receive the supernova enrichment it did and survive.12 In this respect, we were fortunate.
Current research indicates that the Sun seems to have emerged and begun radiating energy around 4.6 billion years before the present. Again, the meteorites that formed at the time of the solar nebula’s emergence, known as chondrites, are the basis of this estimate, one which is regularly being made more precise. As evidence of this increasing precision, a revision in the method used to calculate the abundance of radioactive isotopes in meteorites has revealed that the age of the solar system is currently thought to be 4,567,000,000 years.13
The minimum quantity of solid material out of which a solar system forms its planets is known as the Minimum Mass Solar Nebula. In our solar system, it is estimated that the MMSN was between 0.035 and 0.1 solar masses14, again revealing the dominance of the Sun over its subjects. The Sun created a localized bending of spacetime, causing the lesser bodies that spun off from it to come under its gravitational sway. Their counter-gravity on it is utterly negligible.
Basic Features of the Sun
The Sun, despite the extravagant conjectures of humans over the centuries, turned out to simply be the local star, similar in its essential nature to most of the countless thousands of bright pinpoints that humans see in the night sky. Its proximity to us is its only truly unique feature. Our Sun is an above average-sized, rather ordinary hydrogen-burning main sequence star, one with a lifespan of about 10 billion years. It is a Population I star, a member of the most recent group, the stars with the highest metallicity.
Everything about the Sun is overwhelming in comparison to the Earth. Its diameter of 1,391,000 kilometers (a little over 864,000 miles), is about 109 times the diameter of the Earth. This means the Sun’s size relative to our planet is such that the Earth would fit into it about 1,300,000 times over (as we noted in an earlier chapter). Even though the density of the Sun’s matter is only about a quarter of the Earth’s per cubic meter, the Sun’s immense size in comparison to our world makes it 333,000 times more massive. In fact, the Sun’s mass is so huge that it comprises over 99.9% of the total mass of the solar system it dominates.15 Of the 50 nearest stars, the Sun ranks fourth in size. The Sun’s mass is considered to be large, but unremarkable. The Sun is a single star, rather than a binary. About one-third of solar-type stars are singles, so this is not statistically unusual. It is high in metallicity, although perhaps not as much as scientists once believed (see below).16 Recent findings indicate that stars the size of the Sun form about 12% of the time. Our Sun has giant planets, which are thought to be present in about 20% of solar systems, although this figure could be as high as 50%.17 It also, of course, has terrestrial planets, but these are thought to form rather readily. When we consider all of the Sun’s characteristics, we find that solar systems like ours form perhaps less than 1% of the time in the Milky Way galaxy. However, that low percentage doesn’t mean our Sun is rare; in a galaxy of this size, there could easily be a billion just like it. (And in truth we don’t yet know the exact probability of a solar system like ours forming.)18
The Sun produces all of the energy it sends to the rest of the solar system in its inner core. At the center of the Sun, the temperature of 15,600,000 degrees K and pressures approximately 233 billion times those found in the atmosphere of our planet make the Sun a gigantic fusion reactor.19 Half the Sun’s mass is in its core, but the volume of the core is only a small fraction of the Sun’s total volume. The Sun avoids gravitational collapse in this situation because, as we noted in the chapter on stars, the outward pressure of the energy it is pouring out counterbalances the gigantic gravitational pressures acting on it.20
The Sun’s interior is a plasma. Hydrogen breaks down under the intense heat and pressure there, and a mass of protons, electrons, and other ionized particles swirls about in the inner core. The protons have a natural electrical repulsion to each other, but there are enough of them moving fast enough that a certain number, governed by quantum randomness, succeed in tunneling through the electrical barrier between them. Two protons, interacting via the weak force, combine to form a deuteron, the nucleus of 2H. One of the protons loses its charge, becoming a neutron. The proton’s positive electrical charge is carried away by the ejection of a positron (an anti-electron) and an electron neutrino. The positron often strikes an electron, and in the mutual annihilation that occurs, two gamma rays are produced. Then the deuteron combines with another proton to form the nucleus of a light isotope of helium, 3He. The helium nucleus needs less energy to maintain itself than the deuteron and the proton do separately, and the excess energy is emitted as a photon, also in the form of gamma-ray radiation. Then two light helium nuclei combine to form the nucleus of 4He, which consists of two protons and two neutrons. No energy is released in this final step. However, two protons are released, perhaps ultimately to be the start of another chain of energy production. In sum, four protons have combined to make a helium nucleus, gamma-rays, and an electron neutrino. There is a slight loss of mass in the process of converting hydrogen to helium, and the photons are the result of this loss. It is the photons in the form of gamma ray radiation that will have to make the journey from the center of the Sun to the surface. The gamma rays, ordinarily deadly to life, lose energy as their photons are forced upward, absorbed, and re-emitted. At the surface, the energy that started as a gamma ray will take the form of optical photons. It is this process, repeated countless times in our star’s superheated depths, that causes the Sun to shine.21
Surrounding the inner core is a region known as the radiative zone, which comprises the largest part of the Sun’s radius. Its temperature is estimated to be about 9 million degrees K in its deepest levels, and although it has a high opacity, photons from the core are conveyed by the temperature gradients that run through the radiative zone (near its outer layer the temperature is about 7 million degrees K cooler than the region nearest the core). These photons move in random ways toward the next layer of the Sun.22 That next layer is the convection zone, which runs almost to the surface. In the process of convection, the Sun’s plasma flows and circulates somewhat like a liquid (although it is not in such a state). Analyzing this flow is extremely challenging, inasmuch as the temperature drops from 2 million degrees K to 7,000 degrees K from the bottom to the top of the convection zone and the pressure is reduced by a factor of 100. The turbulence patterns of the solar plasma in these conditions are very complicated and not yet fully understood, although major advances are being made.23 Within a few thousand kilometers of the Sun’s apparent surface, enormous cell-like structures or “granules” form. In fact, the apparent surface of the Sun is granulated, covered in a tile of convective cells, many of which are larger than Brazil.24
There is no perfectly defined surface of the Sun per se. There is simply a limit to its gas field, in a manner analogous to a cloud. The Sun’s outer layer, which is what is visible to us in normal circumstances, is known as the photosphere. It is constantly being roiled by disturbances caused by energy eruptions near its (apparent) surface. The density of the Sun and the opaqueness of the photosphere increase with depth. It radiates a white light, although because of our planet’s atmosphere the light most often appears to be yellowish to human eyes.25 The photons we see as light have bounced around a great deal on their “drunkard’s walk” journey from the core. The time it takes a photon to travel from the center of the Sun to the surface is a matter about which there is some difference of opinion. An estimate made in 1992 put the figure at 170,000 years26, while some researchers say that it can take as long as a million years, given the fact that photons can be absorbed, emitted, or scattered in such a random way.27 There is as yet no consensus on this matter. The temperature at the Sun’s surface is about 5,780 degrees K—a drop of more than 99.9% from the 15.6 million degrees of the inner core.28
A layer of gas, mostly hydrogen and helium, just a few thousand kilometers thick, known as the chromosphere envelops the photosphere. It is usually visible only during eclipses, but specialized equipment allows astronomers to see it at other times. Its temperature averages around 10,000 degrees K. It is a region where curtains of fiery energy known as prominences can be found, and where spikes of energy called spicules shoot into the solar atmosphere. Prominences can reach 15,000 degrees K.29 The emissions of energy from the chromosphere can be so vast in size that they would easily engulf the entire Earth were our planet unfortunate enough to be in range of them. There is a narrow transition zone, and then the Sun’s corona begins. This is a region where the hot gases coming off the Sun and intense magnetic activity have interacted to form a part of the solar atmosphere with a temperature in excess of 1 million degrees K, and perhaps as high as 5 million degrees K. Its actual operation is still not completely understood.30 The corona produces the solar “wind” as its contents boil off into the surrounding space at speeds up to 800 kilometers per second. This solar “wind” can affect the Earth’s magnetic field (which shields us from it) and disrupt human electronic communications.31
Scientists estimate that the Sun puts out an inconceivable 3.8 decillion (3.8 x 1033) ergs, or 380 septillion (3.8 x 1026) watts of power per second. In one one-thousandth of a second the Sun generates enough energy to supply the Earth’s current energy needs for 5,000 years.32 In order to generate this energy output, it consumes 600,000,000 tons of hydrogen every second of its existence.33 (Some sources put the consumption rate at 700 million tons per second.) As we saw, this hydrogen is being converted to helium, but as we also noted above, the conversion is not 100% efficient. It is the difference between the amount of hydrogen burned and the amount of helium produced, less than 1%, that is emanated as the Sun’s heat and light. (Yes, the heat and light on which this planet utterly depend are the byproducts of this process.)
The Sun emits every kind of energy in the electromagnetic spectrum: gamma rays, X-rays, ultraviolet light, infrared light, microwaves, radio waves, and, of course, the visible light spectrum.34 It is the infrared rays that convey heat to our planet. It is the ultraviolet rays that inflict sunburn damage and which can affect eyesight in humans. It is the visible light spectrum that human eyes have evolved to see (hence the name), and it is the visible light spectrum that powers photosynthesis. The activity (or lack of activity) of sunspots, regions of the photosphere cooler than their surrounding areas, can indicate changes in the Sun’s magnetic field strength, changes which can affect the Earth’s climate. Sunspot activity seems to follow an 11-year pattern.35
It is the Sun, therefore, that makes the Earth an open energy system, one which is locally resistant to the effects of entropy, one that allows temporary structures, such as humans, to emerge and flourish. Every aspect of the planet’s surface is affected by this torrent of energy.
The Earth’s orbit around the Sun is elliptical, as is the case with all the planets in our system. The Sun is closest to us in January, the Earth’s perihelion. It is farthest from us in July, a point known as the Earth’s aphelion. By NASA’s calculation, the Earth is 147.5 million kilometers (about 91,652,000 miles) from the Sun in January, and 152.6 million kilometers (about 94,821,000 miles) away in July. The variation of a little more than 3% in the minimum and maximum distances is not considered significant.36 (We see, therefore, that it is the tilt of the Earth, not its distance from the Sun, that determines the seasons on our planet.) The Earth’s average distance from the Sun is 149.6 million kilometers (92,957,000 miles).37 In terms of how long it takes the Sun’s light to reach the Earth, we would say that the Sun is eight light-minutes distant from us, meaning we see the Sun as it was 8 minutes ago.
Tireless research has been done on the chemical composition of the Sun. Findings published in 2009 indicate that the Sun is 71.54% hydrogen, 27.03% helium, and 1.42% metals (in rounded figures). This is a lower metallicity than had been thought to be the case. Careful analysis of the Sun’s photosphere and evidence obtained from meteorites, formed at the time of the Sun’s emergence, indicate the presence in the solar system outside of the Earth of every naturally occurring element in the periodic table.38
General Features of the Solar System
As seen from above, all of the planets in the solar system rotate counter-clockwise around the Sun. All of the orbits are slightly elliptical, not all of them have the same inclination, in other words, not all are on the exact same plane, and the orbital speeds of the planets vary greatly.39 The four terrestrial planets that emerged through the accretion of heavy materials during the several million year-long period when the solar nebula was coalescing form what is known as the Inner Solar System. Closest to the Sun is Mercury, a scorched, lifeless, airless, desolate wasteland.40 Next in distance lies Venus, similar in size to the Earth, but enshrouded in perpetual cloud cover that has raised its surface temperature to a maximum of over 800 degrees F. Its atmosphere is largely carbon dioxide, its surface reflects extensive volcanic activity, and it has only trace amounts of water.41 After the Earth, a planet in the most habitable zone of the solar system, is Mars, a world smaller than Earth which seems to have had abundant liquid water at some time in its past, and perhaps some sort of life forms, but which is now apparently (but not certainly) barren. Mars is cold and wind-swept, with a thin atmosphere that supports some clouds. Its surface indicates that there was once significant geological activity on the planet, and dust storms are frequent.42
Between Mars and Jupiter lies the asteroid belt, a region populated by tens of thousands of orbiting pieces of rock too small to be called planets. Although some have speculated that the asteroids are the remnants of a shattered planet, the current hypothesis is that they are pieces that never came together to form a planet, perhaps because of the disruptive effect of Jupiter’s gravity.43 It is this same Jovian gravity that can elongate an asteroid’s orbit sufficiently to hurtle the asteroid toward the Earth.
The Outer Solar System consists of two planets called gas giants, Jupiter and Saturn, with their multitude of moons, two planets called ice giants, Uranus and Neptune, the dwarf planets (such as Pluto and Eris), that reside in the Kuiper Belt, and dwarf planets and other objects beyond the Kuiper Belt such as the dwarf planet Sedna.44 Finally, there is a collection of comets known as the Oort Cloud that extends to about 60,000 AU (0.3 parsecs, almost a light year); this structure probably formed after the Sun had left its birth environment.45 The Oort Cloud may possess billions of icy bodies, and some sources put its farther reaches well beyond the postulated 60,000 AU limit.
The Fate of the Sun
As we have seen, the Sun will probably last another 5 billion years, but well before that increases in the heat transmitted by it will make the Earth uninhabitable. The Sun will enter a red giant phase, and engulf the entire Inner Solar System, but it does not possess sufficient mass to go supernova. It will eventually collapse into a white dwarf, and ultimately into a cold, dead, black dwarf. The solar system it once ruled will be utterly cold and illuminated only by the light of other, more distant stars. The solar system’s intelligent inhabitants, if any survive to witness the Sun’ death, will have to abandon the dead solar system if they wish to perpetuate themselves elsewhere.
Out of the maelstrom of the solar system’s creation our tiny planet coalesced. The interstellar medium out of which that system was composed contained within it every element out of which the Sun, the Earth, and the life forms that evolved on the Earth would be made. Those life forms are utterly and completely dependent on the energy washing over this planet from the star around which the Earth orbits, a star which in comparison to the Earth is immense. It was our local star that made life on the Earth possible. It is that same star that nurtures and fuels that life—and which will eventually annihilate it. No one knows the ultimate fate of the humans who evolved on this minor terrestrial planet, but we now know, without any doubt, what our origin was.
We are the descendants of the Sun.