Flares are another type of solar activity that impacts our planet. They are created when neighboring field lines of opposite polarity suddenly combine. This is similar to sticking two permanent magnets together and sliding them against each other. At first you will notice an opposing force. Slide far enough and suddenly things snap into a new stable position. This sudden realignment is what happens to the magnetic field lines in the sun, especially in times of high sunspot activity.
The snap realignment ejects large amounts of solar matter into space with speeds of up to 1,000 kilometers per second. The outbreak, also called a protuberance or coronal mass ejection, can interfere with artificial satellites and create more plentiful polar lights on Earth.
Protuberances can also follow magnetic fields that form a loop or an arc outside the solar surface. Such protuberances can remain stable for several months.
Hearing the Sun?
When water boils in a pot you can hear the activity quite well. The sound reaching your ear is created by pressure variations of the currents (called convection) that reach the surface. The same happens on the sun, the sun being a much larger pot, of course. The currents are slower relative to the size of the container, and the resulting sound is in the very low frequency band between two and seven millihertz. Humans can’t hear sound below about 16 hertz.
Solar Problems
The sun doesn’t just keep us alive, it also helps science with difficult questions. Once the solar function had been understood well enough, scientists were interested in verifying certain aspects of their new theories. One project was proposed to investigate the neutrino, a particle that rarely interacts with other matter, and is electrically neutral.
It turned out that Earth only received about a third of the neutrino count that was anticipated. So were the theories wrong? Eventually the neutrino revealed itself to be more flexible than expected. On their way to Earth some of these particles changed form and morphed into tau or muon neutrinos from the original electron neutrino. It turns out detectors for electron neutrinos simply missed the other types.
The high temperatures in the corona present a second issue—to date, no widely accepted model is able to explain them. Initially it was believed that sound, gravitational, or magnetic waves out of the convective zone were responsible for heating the corona, much like a microwave heats up soup. Closer inspection revealed that such radiation would dissipate in the solar material well before reaching the corona. Currently, flares are suspected to contribute to the heating of the corona.
A third problem is related to the construction described in Silent Sun. Theoretically, based on current models of stellar development, the sun would have produced much less energy in its youth, between 3.8 and 2.5 billion years ago. The assumption is that energy output was about 25 percent lower than today for a much cooler surface temperature on Earth that would not have been favorable to the development of life. In spite of much less solar energy, the Earth at the time was about as warm as today—paleontologists know that from their discoveries. The sun would not have been strong enough to warm the oceans above the freezing point of water, quite unfavorable conditions for the initial development of life on Earth. Models could be false, of course. Yet they have been confirmed many times for other stars.
Or was it the fault of the atmosphere on Earth? Certain types of gas could have helped heat up the planet. Global warming to the rescue? Which theory fits history best is hard to determine. Mankind only started recording the atmospheric composition very recently. Ice drilling in the Antarctic does not yet yield information for the relevant time. A team from the University of Washington published an article in Nature with a clever idea to obtain more information. The scientists investigated a ubiquitous and age-old phenomenon—drops of rain. Rain falling millions of years ago left traces that can be investigated today.
In Earth’s atmosphere, raindrops have a maximum size. That is because aerodynamics shreds larger drops once surface tension and internal hydrostatic forces are overcome. These effects have not changed over time—drops of rain have a maximum diameter of 6.8 millimeters to this day. The energy they impart to the ground on impact depends on their size and terminal speed. The latter is a measure for the density of the atmosphere, and that is where it got interesting for the scientists. Depending on the energy dissipated on impact, and the characteristics of the ground, raindrops have left traces of varying sizes. Those traces can be measured. Combined with information about the material carrying the traces, one can work out the speed of the drops and the density of the atmosphere from there.
The scientists were able to show that atmospheric pressure 2.7 billion years ago was not significantly different from the conditions today. While that doesn’t solve the riddles posed by the young sun, it does exclude a few potential solutions, particularly the involvement of some greenhouse gases. Their presence would imply higher atmospheric density. This leaves only the particularly efficient and simple greenhouse gases like methane, ethane, or carbonyl sulfide that could have been liberated by volcanic eruptions.
Theoretically... it could also have been a gigantic construction encouraging higher solar output, like I have described in Silent Sun.
The Birth of the Sun
The very first stars, which were formed after the Big Bang, constitute the so-called ‘Population III stars.’ They didn’t contain any of the heavier elements and all have been extinct for a very long time. Their remains were the basis of Population II stars, all older than 6 billion years, and usually found in the rims of galaxies. And their remains have turned into Population I stars like our sun.
Apparently a gigantic star had to be extinguished in a supernova event to form our solar system.
The oldest traces of matter that have been found in extra-terrestrial matter are 4.582 billion years old. That must be the age of our solar system. Despite no living witnesses being left from that time, scientists have been able to come up with a surprising number of details about the early history of the solar system. Three sources of information that supplement each other are the basis of most of these findings:
Today’s properties of the solar system. There are a number of things that have been noted since the first scientific observations of the night sky. Planets all orbit the sun in the same direction and pretty much in the same plane. In effect, the solar system forms a disk. Despite the majority of the mass being in the sun, the majority of the momentum is stored in the planets. Inner planets are terrestrial, outer ones gas giants. All these facts, and a few more, indicate that the solar system was formed out of a rotating cloud of gas. This cloud began to contract under its own weight for a certain reason—more on this later. Approximately 99 percent of its mass formed the center, the sun, while the rest was spread out as the planets and other bodies in the solar system.
Existing celestial objects that display properties similar to the molecular cloud that eventually formed the sun. The Orion Nebula, visible to the naked eye as part of the sword of Orion, is the most salient example. It is an area of around 30 light-years in diameter that contains a cluster of stars that are only about a million years old. The gas cloud forming the sun was probably quite a bit larger at 65 light-years. The Hubble telescope has identified proto-planetary disks in the Orion Nebula. These are a few hundred astronomical units large and relatively cool—just as science pictures the young solar system before nuclear fusion was ignited in the sun. In that arena the sun was a so-called T-Tauri star, an object that radiates energy liberated by gravitational contraction.
Petrified witnesses—the material forming meteorites, especially its molecular composition. Analysis of meteoric fragments has already served to determine the approximate age of the solar system. And these messengers from outer space offer yet more fascinating clues. An article published in the Annals of the National Academy of Sciences investigates the distribution of certain heavy radioactive isotopes including Niobium-92, Iodine-129, Samarium-146, and Hafnium-182, which have half-lives in the ran
ge of millions of years. These must have been present in the early days of the solar system, in larger quantity, of course. Finding out how these elements might have formed provides another piece of the puzzle. The authors of this paper manage to show that a core collapse (hydrodynamic) supernova explains the distribution of nuclides, while other supernova types do not. In this type of supernova a star collapses under its own weight because the fuel in its core has been exhausted, eliminating the opposing pressure from inside. This process requires a star of 8 to 25 times the solar mass. The specific distribution of the nuclides also tells us that a minimum of 10 million years have passed between the supernova event and the formation of the solar system. The scientists deduced that, for the cloud to have existed so long, it must have been surprisingly massive. Cosmology does actually suppose that approximately 1,000 to 10,000 other stars with a total mass of 3,000 solar masses were formed in the same period of time and in an area of approximately 20 light-years diameter around the solar position at the time. That wild group of new stars then disappeared about 100 to 500 million years later.
The process leading to the formation of a star is more or less the same in all cases. The result however depends on the starting conditions. Around 4.6 billion years ago there was a huge molecular cloud—an aggregation of interplanetary gas in non-ionized form—in the area that would form our sun.
This original cloud must have been spread over many light-years, and it probably gave birth to many other stars besides our sun. It consisted mainly of hydrogen, helium, and some traces of heavier elements. However it bore no resemblance to clouds as we know them on Earth. Its density was very low at around 10,000 atoms per cubic centimeter—our planetary atmosphere is many trillion times more dense. And this cloud was incredibly cold, between minus 250 and 260 degrees Celsius.
The enormous size of the cloud meant there was still an incredible amount of matter assembled in this part of space—several thousand solar masses according to scientific estimates. Matter was not distributed evenly within the cloud. Inhomogeneities—essentially, lumps of slightly higher density—ended up attracting more matter through gravitation, a process that might have been accelerated by the shockwave of a nearby supernova. One of these huge lumps would be the birthplace of our sun.
The individual particles inside the lump moved in a random way, but the cloud had a distinct rotation overall at that time. Just like ice skaters speed up their pirouettes by pulling in their arms, the gravitational shrinkage accelerated the rotation of the cloud. This in turn favored the transition from a cloud toward a disk. Particles in the plane of rotation weren’t just attracted by gravitation, but also accelerated outward by centrifugal forces. Particles above and below the plane of rotation had a higher force of attraction toward the disk, eventually flattening out the disk. At this point the disk had an extension of about 200 astronomical units—200 times the distance between Earth and the sun.
The particles kept accelerating as the disk began to collapse. Their kinetic energy, or energy of movement, increased accordingly and turned into heat. This also increased the likelihood of collisions between particles. The center of the disk had more collisions and heated up faster than the rim, reaching up to ten million Kelvin in the core, which contained 99.8 percent of the total mass. Eventually heat and density were sufficient to trigger the fusion reaction. Hydrogen nuclei combined into helium nuclei and released energy. The proto-star, which we know today as the sun, had self-ignited. It is our fortune that its mass was sufficient for ignition and, at the same time, small enough to avoid becoming a short-lived giant star.
The heat generated by nuclear fusion slowed the contraction of the disk. After about 50 million years, equilibrium would have been reached between the gravitational attraction of the star and the opposing pressure of radiation outward from the core. A stable star had been formed.
It is likely that our sun had a sibling when it came into existence—astronomers have been assuming this for some time. The unknown twin even has a name, Nemesis, as its gravity has been held responsible for sending a giant meteorite on a collision course with Earth, eliminating dinosaurs.
Nemesis has yet to be discovered. This is strange, as evidence is accumulating that stars are generally born in pairs. A paper in the Notices of the Royal Astronomic Society argues the case based upon observation of several very young stellar systems in the Perseus Molecular Cloud, 600 light-years away from Earth. The stellar distribution found there can only be explained using a mathematical model based on star births in pairs.
The twins start out at a good distance from each other, about 500 astronomical units, or about 17 times the distance from the sun to Neptune. It takes a few million years for the system to decide on their fate—either the twins move closer or they go their separate ways.
The latter seem to have occurred in our solar system, like it does in about 60 percent of systems. Nemesis at some point must have gone on a journey to mingle with the other stars of the universe so that we can no longer identify it. The theory must be tested on further molecular clouds, of course.
The Future of the Sun
Like the rest of the universe, our central star is aging. However its radiation intensity does not diminish. It is in fact increasing. This phenomenon will lead to an average temperature of 30 degrees Celsius on Earth in about 900 million years, independent of global warming. A construction like the one described in Silent Sun could be useful in a few hundred million years to tone down the solar activity.
A billion years later the oceans will be boiling, once the average temperature exceeds 100 degrees.
The beginning of the end comes in 4.8 billion years—hydrogen in the solar core will be exhausted. From then on, nuclear fusion will primarily take place in the outer layers of the sun.
The core temperature will drop for lack of energy release, and it will begin to shrink as a consequence of the missing radiation pressure. In the outer layers the temperature will rise, and the rate of reaction will rise alongside, while the sun will bloat and its spectrum will shift toward the red.
Things won’t become critical until the sun has reached an age of about 11 billion years. At that point, the brightness and diameter of the sun will rise relatively rapidly to reach 2300 times today’s value. Mercury and Venus will be part of the sun, now a red giant. It is unclear how strongly Earth will be affected. Life, of course, will have become impossible. As a red giant, the sun will lose mass far more quickly than today, and planets will loosen their orbits accordingly because the sun’s gravitational attraction will be diminishing.
The core temperature will drop continuously while the core is being compressed under the weight of the sun. The outside universe won’t notice much until core density has reached values that permit nuclear fusion of helium to carbon. That process will ignite with a gigantic flash of light, ten billion times the usual intensity. This will alert all our extraterrestrial neighbors, as the brightness will—for a few seconds—equal the combined total brightness of a tenth of all the stars in the Milky Way. Following ignition the red giant will shrink to about one tenth today’s size, while shining fifty times brighter than today as it continues to burn the helium in its core.
This source of energy will in turn be consumed, initiating a new round in the stellar aging process quite similar to the hydrogen-fusion cycle. The sun will grow into a giant once again, and will alternate in unstable phases, growing and shrinking several times, more or less within the limits of Earth’s current orbit. Earth itself will probably avoid being swallowed due to its orbit having loosened in the previous cycle.
At the end of this phase, the sun will be 12.45 billion years old, and the remainder of the outer shell will have been lost. The remaining core, a white dwarf, will start out at around 100,000 degrees and will be about the size of Earth. It will consist mainly of incredibly dense carbon and oxygen (this material in the size of a sugar cube would weigh a metric ton) and will no longer generate any energy internally. The sun will
start cooling rapidly at this stage, but the cooling will slow down so that the remaining heat will radiate for several billion years before the sun will burn out completely as a black dwarf.
It is pretty certain the sun will no longer exist in about ten billion years. Astronomers are, of course, thinking even further. They wonder what will become of our star after that. Nine out of ten stars leave a planetary nebula as they change from red giant to white dwarf. Such massive, luminous clouds of interstellar gas and dust can contain up to half the mass of the erstwhile star and remain visible for about 10,000 years while the core is hot enough to illuminate it from the inside. So far the sun has been thought to be too lightweight to leave such a memorial in space, twice the solar mass being the currently accepted tipping point.
However, in a very recent paper published in 2018 in Nature Astronomy, scientists from the University of Manchester revisit this question. They used a new model based on the known data from many stars. This model shows the core heating up three times faster than previously believed after shedding the nebula. This compensation mechanism for the reduced mass of the core puts the sun just within the limits for creating a nebula. If so, our extremely distant descendants can expect to admire a beautiful planetary nebula when they come to pay burnt-out Earth a visit.
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Silent Sun: Hard Science Fiction Page 27