The sun is as essential for life in general as it is for individual humans. Its energy drives the climate and it nourishes life itself through photosynthesis, converting inorganic substances into organic molecules needed by animals and humans alike for their energy supply. Any change in solar activity produces changes on our planet that can be very drastic indeed. Cold and warm periods modify climatic zones, arable land is converted to desert, and agriculture becomes possible in zones formerly covered by seemingly-perpetual ice. In the extreme, as on Mars, life would become completely impossible if the planet were to leave the habitable zone surrounding the sun.
Despite its all-encompassing importance for us, the sun has yielded surprisingly few of its secrets to science so far. Drastic conditions near the sun are the biggest obstacle to detailed scientific research. What we see as the solar surface is a relatively thin layer. What is below is shielded from direct observation, so scientists must resort to indirect methods to learn anything at all. The following chapters summarize what has been learned so far, and what requires further research.
Prepare to have your mind blown. Some of the following concepts and accompanying numbers are all but incomprehensible to the human mind. One example: Our sun, a comparatively small ‘main sequence star,’ is nevertheless so big that 1,300,000 Earths would fit inside it.
The Color of the Sun
The color of the sun depends on how you look at it. If you look out through a window you will see a yellow or orange disk in the sky, depending on the time of the day. The sun however does not emit more yellow or red light than other colors. Its light should appear white to the human eye. That is how you would see the sun if you were in space. Earth’s atmosphere acts like a prism, deflecting some parts of the solar spectrum more strongly than others. The shorter wavelengths (blue to violet) get deflected more strongly and reach the eye indirectly through light scattering—this is the origin of our blue sky. That leaves the longer wavelengths (yellow to red) for direct impact to the eye, yielding the typical yellow-orange color. When the sun sets in the evening, your viewing angle means the light travels slightly farther through the atmosphere than during the day. This enhances the effect and provides lovely red sunsets. The impact of light scattering varies by atmospheric composition, so any planet with a different atmosphere may well have a different color for its sky.
Nomenclature classifies the sun as a yellow dwarf of spectral class ‘G2V.’ The yellow designation originates from the solar spectral color of about 5,800 degrees Kelvin, a color that you may know as ‘warm white’.
Our Mother Star and Its Environment
If Earth feels big, then the sun will put things into perspective. It weighs about 2 billion billion billion tons, which makes it about 700 times as heavy as all the planets of the solar system combined. In other words, the sun is 330,000 times as heavy as Earth and contains 99.86 percent of the total mass in the solar system. Yes, that leaves just 0.14 percent mass for everything else in the solar system!
The sun’s diameter is 1.39 million kilometers, which is about 109 times Earth’s diameter. Earth would fit into the sun about 1.3 million times, yet the solar surface is only about 12,000 times as large as that of Earth. All that mass makes for high gravity. At the surface the solar constant of gravity is 274 meters per second squared, which compares to 9.81 m/s2 on Earth. A human weighing 70 kg would be crushed under his own weight of 2 tons at the solar surface.
The average density of the sun is 1.4 grams per cubic centimeter. That points to a very different composition from our planet. Nearly three quarters (73 percent) of the solar mass consists of hydrogen. One quarter is helium. The remainder is split between heavier elements including carbon, neon, and nitrogen.
Its metallicity, the proportion of heavier elements, is relatively high, putting the sun into the group of Population I stars. Population III stars are the original stars, which formed right after the Big Bang. They did not contain any heavier elements at all and have long since burned out. Their remains formed Population II stars that are mostly more than 6 billion years old by now and tend to be located at the rims of the galaxies. The conversion of the burnt-out remains of Population II stars into Population I stars like the sun is an ongoing process.
Earth is on average 150 million kilometers away from the sun. This distance is the definition of one astronomical unit (AU). It takes light from the sun eight minutes and twenty seconds to reach earth—a distance of just 1.58 × 10-5 (0.0000158) light-years.
In contrast, the distance to the center of the Milky Way is 27,200 light-years, or 2.5544 × 1017 (255,440,000,000,000,000) kilometers! The entire solar system orbits that galactic center at the incredible speed of 220 kilometers per second. Despite the speed, one full turn takes 230 million years.
The galactic neighborhood of the sun is unrelated—the sun is wandering in the wild, so to speak. Its walking speed relative to surrounding stars is 20 kilometers per second. The walk has taken the sun into a somewhat more dense area called the Local Fluff, or Local Interstellar Cloud, including stars like Altair, Vega, Arcturus (Alpha Boötis), Fomalhaut (Alpha Pisces Austrini), and Alpha Centauri. The Local Fluff is like an oasis of matter within the relatively-dust-free Local Bubble of 300 light-years diameter. The Local Fluff may have been blown into existence by one or more supernovae. Possible origins are the supernova-remains of Geminga, or several such occurrences in the Pleiades group. The Local Bubble spread in the Gould Belt, a kindergarten of stars with many young stars between 20 and 60 million years old, all unrelated to the much older sun.
Looking at the Milky Way from afar, the sun is located on the inner rim of the Orion arm. The neighboring Perseus arm is about 6,500 light-years away. As the Milky Way travels through space, our sun invariably travels along. Where is it heading? This is exactly the question that an international team of astronomers has been working on as they study our ‘local neighborhood’—the 1,400 galaxies that are contained in a radius of 100 million light-years around the Milky Way. Scientists have analyzed and compared the movements of these galaxies across the last 13 billion years. They didn’t find anything that should alarm us but they did uncover a few interesting trends.
The gravitational center of mass is the Virgo cluster, 50 million light-years away from us. The mass of its 600 trillion suns is drawing all matter inward from its surroundings. Over 1,000 entire galaxies have been trapped this way, and all others that are closer than 40 million light-years cannot avoid following their fate. The Milky Way and the Andromeda galaxy are both outside this zone. Our luck is limited however, because these galaxies—with 2 trillion suns each—are due to collide in about 5 billion years, eventually forming a single ellipsoid galaxy. Research has also found two overriding patterns. One half of the surveyed zone, including our Milky Way, is moving toward the same flat plane. And in our local neighborhood of space, those 1,400 galaxies flutter like leaves in the wind while moving toward a distant and much larger gravitational attractor.
How the Sun Works
The sun does not have a surface, in the true sense of the word, as it is a huge ball of gas. To qualify the term ‘gas’ more precisely, we should be talking about ions, atoms that have been stripped of their electrons. This state of aggregation is called plasma. The solar radius is measured from the center of its core to the outer limit of the photosphere. The photosphere is a relatively thin layer in which the sun converts some of its energy to emit it as visible light. It is hot, about 6,000 degrees Kelvin. The particle density is very low. The ‘air’ has about one third of a percent of the particle density of the Earth atmosphere at sea level.
It took a very long time before mankind found out how the sun generates energy. In ancient times the sun was pictured as a huge burning ball. William Thomson, later known as Lord Kelvin of Kelvin temperature scale fame, used thermodynamics in the 19th century to explain the situation. Assuming that the sun is a ball of gas and plasma that is contracting very slowly and cooling down in the process, one can calculate
the radiation intensity in terms of converting gravitational energy. However, in this framework, it turns out that the sun could not be older than 20 million years—while Charles Darwin had already assigned sediments an age of more than 300 million years. In 1904, after the discovery of nuclear fission, Ernest Rutherford suggested fission as an explanation. It took until the 1930s for physicists to understand that the conditions inside the sun would also enable the fusion of hydrogen nuclei.
Radioactivity, part of school curricula today, was hard to accept at the time as it embodied the medieval alchemistic concept of converting one element into another. Nuclear fusion of protons is like using hydrogen nuclei as Lego blocks to construct a helium kernel, with enough excess energy emitted to drive a power plant on a solar scale. However, a pair of hydrogen nuclei are both positively charged and repel each other, thus requiring extremely high temperature and pressure to push them close enough for nuclear fusion to occur. On Earth this is an elusive phenomenon that physicists have only been able to trigger for very short periods of time, despite the highly attractive promise of clean nuclear energy from a working fusion reactor. In the kernel of the sun all the necessary conditions are met, since the molecular cloud had pulled together sufficiently about five billion years ago.
Ever since ignition at that time, the reaction has been running smoothly and that is not set to change for the foreseeable future. That is, until the sun runs out of fuel. So far the sun has converted an amount of hydrogen around 14,000 times the mass of Earth into helium, liberating 90 times the mass of Earth as pure energy. Every single second, our star supplies more energy than one would obtain by running all the nuclear power stations that were online in 2011 for 750,000 years. Every square meter on Earth receives 1.36 kilowatts—to obtain the same effect it would be necessary to set up an electrical heater over each square meter of the Earth.
The Solar Core
The core of the sun occupies the inner quarter of its diameter. It has a density of 150 grams per cubic centimeter—more than thirteen times higher than the density of lead with its 11.3 grams per cubic centimeter. A sugar cube of core solar matter would weigh 150 grams and a 10-liter bucket (1.5 metric tons) would need a crane to hoist.
The temperature at the innermost area of the sun is around 15.6 million Kelvin, with pressure at 200 billion atmospheres. Compression of the pyramid of Cheops into the head of a pin would require a similar pressure. The solar core rotates somewhat faster than the exterior regions and the area capable of sustaining nuclear fusion defines its extent.
Approximately 99 percent of the solar energy is derived from the fusion of two hydrogen protons to a deuterium core in a first step. Adding a third proton yields a light helium nucleus that in turn has four possible ways to add a fourth proton and thus become a ‘real’ helium nucleus—also known as an alpha particle.
A second process, the Bethe-Weizsäcker cycle—also known as the CNO cycle—requires carbon, nitrogen, and oxygen cores as catalysts to fuse 4 protons into a single helium nucleus. This process yields another 0.8 percent of solar energy. Either way, helium is always the result.
The solar core hosts 1038 such reactions per second, that is a 1 followed by 38 zeroes, or 100,000,000,000,000,000,000,000,000,000,000,000,000 reactions every second! The sun is a busy alchemist. It converts 600 million tons of hydrogen into helium every second. During the conversion 0.7% of the mass is converted to pure energy, reducing the solar mass by 4 billion metric tons every second. The incredible amount of fuel still present in the sun prevents us from noticing this consumption as shrinkage of the sun.
In spite of all these huge numbers, the amount of energy generated per unit volume is surprisingly low. A hot plate would generate more heat: 276 watts per cubic meter puts the sun in a league with the metabolism of a reptile. Packing 1028 crocodiles in a sphere of the diameter of the sun and shooting it into the sky would generate a comparable amount of heat.
Of course, Earth has far too few crocodiles and the huge herd would need to be fed somehow… This highly absurd comparison is only to point out the size of the sun, rather than its energy density, as the driving factor in its energy production.
The sun has solved a problem that hopeful designers of fusion power stations on Earth are still busy working on—how to keep the reaction stable. Inside the sun things are simple. If the reaction rate happens to rise, the energy surplus causes the core to expand. That reduces the concentration of protons, which in turn slows down the reaction rate—and less energy is produced, meaning the core will shrink in short order. The sun is self-stabilizing in a way that human engineers have yet to master.
The Outer Layers
Should the fusion chain-reaction in the core ever come to a standstill—which is physically impossible at this point—we would not notice for 10,000 years, the minimum time it takes for energy from the core to reach the solar surface.
Initially it needs to traverse the part of the sun called the radiative zone. The majority of the energy here travels in the form of soft x-ray photons. These photons will be deflected by particles in the plasma rather quickly, time and again, driving the overall length of their trajectory to more than ten thousand light-years.
On the outer rim of the radiative zone, at around 70 percent of the solar radius, density has dropped to 0.2 grams per cubic centimeter, which is about one fifth of the density of water. From here outward, heat is primarily transmitted by convection, much like what happens when heating a pot of water. Hot material rises from below, cools down, and sinks back to the bottom. This layer of the sun is called the convective zone.
The tachocline is the thin transition layer between the radiative and the convective zone. The solar core rotates below it much like a solid body would. The outside of the sun rotates at speeds that vary with latitude. While the rotational period at the solar equator is 25.4 days it reaches 36 days near the solar North Pole. The tachocline sandwiches between these two layers. It induces strong magnetic fluctuations through the relative movement of two independent fields, like a dynamo. It is believed that this makes the tachocline the source for the strong magnetic field of the sun.
Inside the convective zone the temperature keeps dropping. An increasing share of atoms own their electrons so they are not ionized. Its outer border is the photosphere, several hundred kilometers thick and about 5,500 degrees.
At this layer the density of solar matter has dropped sufficiently for emitted photons to mostly penetrate the photosphere on a direct path that leads some to hit Earth about 8 minutes later. The ease of photon transition is mainly due to the scarcity of negatively-charged hydrogen ions. These would love to eject an electron the moment they can catch a photon to start the helium-building process. Not surprisingly, the light we see from the sun is created in the reverse process of electrons reacting with protons to form negatively-charged hydrogen ions.
Hot Atmosphere
Above the photosphere there are various layers of the solar atmosphere. The coolest area is located directly above the photosphere. In its coldest areas temperatures reach 3,900 degrees, which is sufficiently cool for simple molecules to form. While temperature drops with altitude in our atmosphere on Earth, things are different for the sun. After this minimum-temperature layer comes the chromosphere, a layer of about 2,000 kilometers where the temperature rises to about 20,000 degrees. And there is more: In the following transition layer, about 200 kilometers thick, temperatures rise rapidly up to about one million degrees.
This is the beginning of the solar corona, which you can see very nicely with the naked eye during a solar eclipse. It reaches far out into space and gradually transitions into solar winds. Its hottest spots are hotter, at up to 20 million degrees, than the solar core itself.
Particle density however is rather low. Even the lower layers of the atmosphere don’t have more than about 107 particles per cubic meter. This is about one billionth of the normal pressure of the Earth atmosphere.
Sunspots and Solar Tsunamis
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br /> Back in 1610, Galileo Galilei observed odd spots on the surface of the sun with a telescope. These spots moved with the rotation of the sun, and quickly led him to conclude that the sun had to be a sphere. The incongruous rotation of the inner core and the outer solar layers generates the spots, which are about 1,500 degrees cooler than their surroundings.
In physics we have learned that electrical charge in motion creates (induces) a magnetic field. The differing rotational speeds ensure that these magnetic fields change all the time, distorting and even changing their directions every 11 years. Try twisting a square rubber band a few times about its axis and you will quickly get something incredibly squiggly. Magnetic fields inside the sun suffer a similar external influence, and their twisting and turning impedes the flow of gas from the inside out, and the flow of heat, too. This makes the solar surface cool down where magnetic fields point away from the sun.
The cooler plasma affects the sunspot shape. Sunspots usually appear in pairs, since the magnetic field must turn back into the sun somewhere to close the loop, and a sunspot forms there, too. Often there are groups of spots that merge and grow into surface areas larger than an equatorial slice of Earth. Half of the sunspots disappear within two days, but some sunspots can survive for several months. Only ten percent of sunspots get older than 11 days. The number of emerging sunspots changes in the 11-year cycle, which may be part of a larger cycle of several hundred years. The current estimate is about 400 years. Times of lower sunspot activity also see the sun radiating up to one tenth of a percent less energy. Such change could trigger a minor ‘ice age’ on Earth.
Silent Sun: Hard Science Fiction Page 26
