THE SUN
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’s 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.
1. Fred
C. Adams, “The Birth Environment of the Solar System” in The Annual Review of Astronomy and Astrophysics, 2010, p. 46.
2. Adams,
p. 54
3. Adams,
pp. 76-81
4. Adams,
p. 54
5. Adams,
p. 52
6. N. Grevesse, M. Asplund, , A. J. Sauval, and
P. Scott, “The chemical composition of the Sun” in Annual Review of Astronomy and Astrophysics, 2009
7. Adams, pp. 53-54
8. Adams, p. 74
12. Adams 71-72
13. Science News, via Wired
14. Adams, p. 51
15. Cambridge
16. Adams, p. 50
17. Adams, p. 51
18. Adams, pp. 76-81
19. Cambridge, 56
20. Cambridge, p. 76
21. Cambridge, pp. 57-61; http://cde.nwc.edu/SCI2108/course_documents/the_sun/thermonuclear_fusion/thermonuclear_fusion.htm
22. Zirker, pp. 40-43
23. Zirker, pp. 61-71
24. Zirker, pp. 57-59; p. 99
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