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Understanding the Sun

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Introduction

By far, the Sun is the most massive body in our solar system. The mass of all the planets combined is only about 0.2% of the Sun’s mass. The Sun is also the only object whose internal temperature is high enough to produce nuclear reactions. If Jupiter had been 100 times more massive, or 1/10 of the mass of the Sun, ours would have been a binary star system. While gas giant planets such as Jupiter do emit more energy than they receive from the Sun, only the Sun owes its internal pressure to nuclear fusion.

Nuclear fusion generates all the power emitted by our star. This energy heats up the gas to very high temperatures. The Sun shines because it is made of incandescent gas, with a surface temperature of about 5,800 K. Because of its high temperature, the Sun emits light in a wide spectrum of wavelengths, with a peak in what we consider the ‘visible’ part of the spectrum.

The fact that our eyes are sensitive to light of wavelengths corresponding to the Sun’s peak emission is no coincidence, of course. Most of the other light from our Sun fortunately does not reach the ground, since our atmosphere absorbs it. If ult raviolet and X-ray radiation reached the Earth’s surface, they would be devastating to on our planet.

The portion of the light that we receive from the Sun powers all atmospheric phenomena, and ultimately life itself. Far from having a uniform surface and from emitting a constant amount of energy per unit time, the Sun is very dynamic and displays activity cycles. The best known is the eleven-year cycle, during which the number of sunspots and other disturbances of the solar atmosphere greatly change in number and intensity.

The eleven-year cycle is intimately connected with the intensity of the solar wind, a stream of charged particles emitted by our star that continuously collides with the Earth’s magnetosphere. At times, solar eruptions give rise to ejections of gas t hat stream out of the Sun and reach the Earth. The strong flow of particles thus generated can be quite dangerous for the network of communication satellites orbiting our planet.

The Sun through History

The Sun has been an essential part of human culture and mythology since prehistoric times. The obvious reason is that the Sun’s position in the sky is linked with the seasonal changes on Earth, and seasons have had a great importance both for agricultural and pre-agricultural societies. This point is clearly illustrated by the tremendous effort that ancient people put into building structures like Stonehenge at a time when no technology other than ropes was available to transport boulders weighing several tons. It is now believed that the orientation of the temple/observatory at Stonehenge and other such monuments was chosen so as to mark the Sun’s solstices, and to celebrate the change of the seasons.

In classical Greece, and throughout the Renaissance, the Sun was believed to be made of ‘ethereal’ matter, i.e. perfect and devoid of any blemishes. The same substance was believed to make up all planets and the Moon as well, and the uneven tint of the Moon was explained away by our satellite’s vicinity to the Earth. The Earth, contrary to celestial objects, was supposed to be made of corruptible elements.

Given this premise, Galileo’s detailed telescopic observation of the Sun in 1610 caused quite a stir. Galileo showed that the Sun has spots on its surface and rotates with a period of about 27 days. Although Chinese astronomers had already observed sunspots with the naked eye, this fact was not known in the West. Galileo’s observation, together with the others he made of the solar system, were instrumental in the acceptance of the modern view of the universe, where the same physics applies to the Sun as to any other object, and laboratory experiments on Earth can have universal application.

In the 19th century another debate ensued centering on the reach of scientific knowledge, and once again the Sun was the protagonist. The French philosopher Auguste Comte claimed that, given that we cannot access stars and other astronomical bodies directly, there could be no chance of humanity ever being able to know what exactly they are made of. As it often turns out in the history of science, one should never say never.

Around the same period that Comte made his sweeping statement, it was discovered that different elements, when in gaseous form, absorb light passing through them in a very particular way: only light of particular wavelengths get absorbed, and such wavelengths depend on the element making up the gas. Armed with this knowledge, based on Earth lab experiments, Kirchhoff and Bunsen showed in 1859 that the atmosphere of the Sun was made of hydrogen as well as other known elements.

In fact, the analysis of the solar spectrum soon led to the discovery of helium. Nowadays taking the spectrum of astronomical objects is an essential step in determining their nature. As discovered by the young astronomer Cecilia Payne in 1925, the compo sition of the Sun is very close to the average in the rest of the Universe, and very different from the Earth’s. Hydrogen makes up 70.5% of the Sun’s mass, followed by 27.5% helium and only 2% of all the other elements. The composition is almost constant th roughout the Sun, although the percentage of helium is higher in the Sun’s core, where helium is being formed by nuclear fusion.

The Sun’s Atmosphere

The atmosphere of the Sun and most of its interior are made mostly of hydrogen and helium. In the atmosphere, helium constitutes 73% of the mass while helium constitutes 25%, leaving only 2% for other elements. For the Sun as a whole (both atmosphere and interior), hydrogen averages at 70.5%, helium at 27.5%, and all other elements at 2%. The Sun is completely in a gaseous phase. The gas, made of the elements mentioned above, is either neutral or ionized depending on the atmospheric parameters at different locations. In an ionized gas, also called ‘plasma,’ some or all electrons orbiting the nuclei are stripped from the atoms, due either to violent collisions with other atoms or the absorption of light of sufficient energy. The higher the temperature of the gas the more favorable the conditions for the formation of a plasma.

The boundary between the atmosphere and the interior of the Sun is a region about 1,000 km thick, called the photosphere. Given that the radius of the Sun is 696,000 km, the photosphere is a relatively thin layer. Most of the light that we receive from the Sun comes from this boundary, which we customarily associate with the Sun’s ‘disk.’ The existence of the photosphere is due to a drop in the ‘opacity’ of the gas in that region.

Opacity is an important concept, deserving a more detailed description. A gas is called opaque when propagating photons can only travel short distances before being deflected. The net effect of these many scatterings is a modification and randomization of the average wavelength of light in direct correspondence to the temperature of the gas. The light, in other words, gets ‘thermalized’ by its interaction with the gas.

Transparent gas represents the opposite situation: scattering and absorption of light happens seldom, allowing light to cover large distances without being deflected. While the interior of the Sun is opaque, its atmosphere is largely transparent. In fact, the transition between opaque and transparent layers is what defines the ‘surface’ of the Sun. The capacity of the gas to scatter light drastically diminishes at the base of the photosphere. One can make a loose comparison between the surface of the Sun and the surface of a cloud on Earth, where the ‘border’ of the cloud is defined by the opacity of the water droplets.

The spectrum of the Sun resembles pretty closely that of a black body at a temperature of 5,800 K. That is the temperature of the gas at the base of the photosphere. The light coming from the interior of the Sun is scattered many times below the photosphere, but from the base of the photosphere upward it is almost free to travel without deflection, keeping its spectrum almost unchanged.

The farther up one goes in the thin layer of the photosphere, the colder the gas becomes. The temperature actually drops to about 4,200 K. As light passes through the transparent and colder gas of the upper photosphere, dark lines appear in the solar spectrum in the foreground of an otherwise featureless black body spectrum. This phenomenon was first observed by Fraunhofer in the early 19th century.

The dark lines correspond to the specific wavelengths at which the various elements absorb light passing through the gas. The fact that one sees dark lines is correlated with the lower temperature of the gas in the upper photosphere, as compared to its base: if the temperature were increasing with height one would see bright lines superposed on the black body spectrum, as Kirchhoff and Bunsen showed in their laboratory.

The photosphere is far from being a homogeneous surface. It shows what is called ‘granulation.’ The granules are regions 1,500 km wide, on average. At the center of a granule the temperature of the photosphere is few hundreds Kelvin degrees higher than at its edge. The surface of the Sun appears coarse-grained because it is the outer edge of a vast convective region in the Sun’s interior.

Chromosphere

The layer of the atmosphere adjacent to the photosphere and extending outward to the corona is called chromosphere. Its boundary is defined by an increase in the atmospheric temperature with altitude, in contrast with the decrease seen in the photosphere. In about 2,000 km, the temperature of the chromosphere increases from 4,200 K to 25,000 K. Its density, though, is only about 10-4 that of the photosphere.

Due to its low density, seen against the backdrop of the photosphere, the chromosphere is all but invisible. Hence, it was only discovered when astronomers observed the Sun during solar eclipses. During such eclipses, the disk of the Moon covers the photosphere and permits the view of the upper layers of the atmosphere of the Sun, i.e. the chromosphere and the corona.

The chromosphere owes its name to its bright red color, against the dark background of the sky during solar eclipses. Under these circumstances, its spectrum is made of several emission lines (no black body component is expected from a transparent gas). Given the temperature and the composition of the gas, much of the light comes from the red Balmer-a spectral line of the hydrogen, a fact that explains the prevalent color of the chromosphere.

Corona

Sun’s corona extends to distances comparable to our star’s radius. At wavelengths in the visible spectrum, the corona is only visible during solar eclipses. It can also be seen by using ‘coronagraphs,’ which block the sunlight from the photosphere directly within telescopes, thus simulating solar eclipses. The corona is irregularly shaped and extends farther where there are disturbances in the underlying layers of the atmosphere. The corona is very hot and extremely diluted. It can reach temperatures of few million Kelvins, and a density 10-12 that of the photosphere.

At the shorter UV and X-ray wavelengths, only accessible by telescopes orbiting above Earth’s atmosphere, the irregular shape of the corona is strongly correlated with the distribution of the Sunspots and of the solar eruptions. The corona shines brightly in the X-ray region of the spectrum, against the dark background of the photosphere: the photosphere emits as a black body at 5,800 K, which tapers off at wavelengths in the ultraviolet region of the spectrum. The transparent hot gas of the corona emits a line spectrum, just like the spectrum of fluorescent light bulbs. The emission is strong in the X-rays because of the extreme temperature of the gas. It is still not certain why the corona is so hot. It seems likely that the gas is heated up by colliding with the particle streams generated by the photosphere during solar eruptions. This would explain why the corona emits the strongest radiation in correspondence with eruptions and sunspots.

Because of its temperature, the corona is a highly ionized gas. Oxygen, for instance, is often stripped of two of its eight electrons. As a direct consequence of the ionization, the corona is electrically charged and its gas particles are deviated in their motion when subjected to the Sun’s strong magnetic field. The magnetic field is a very important component of solar atmospheric activity. The temperature of the corona is so high that the gravitational attraction of the Sun is not strong enough to keep the corona from escaping the Sun. The gas is bound to the star mainly because of the trapping action of the star’s magnetic field. Because of its temperature, the corona is a highly ionized gas. Oxygen, for instance, is often stripped of two of its eight electrons. As a direct consequence of the ionization, the corona is electrically charged and its gas particles are deviated in their motion when subjected to the Sun’s strong magnetic field. The magnetic field is a very impo!

rtant component of solar atmospheric activity. The temperature of the corona is so high that the gravitational attraction of the Sun is not strong enough to keep the corona from escaping the Sun. The gas is bound to the star mainly because of the trapping action of the star’s magnetic field.

Sunspots, Prominences and Flares

s was noticed long ago through naked eye observation made by Chinese astronomers, and then by Galileo using the telescope, the Sun is dotted by several spots. Spots are transitory phenomena that appear as darker patches in the photosphere. They are irregular and their size can easily reach more than 10,000 km in diameter. Spots are usually found in groups, and quite often the groups form pairs, oriented along the Sun’s parallels. Each spot is composed of a central dark region, called ‘umbra,’ surrounded by a lighter region, called ‘penumbra.’

The cause of the darkness is simply related to the temperature of the gas. Temperatures at the center of the umbra are usually around 3,000 K. The lower the temperature, the weaker the blackbody emission of the photosphere. As an analogy, think of what happens when one turns down the voltage applied to an incandescent bulb, thereby lowering the temperature of is filament. The lower the voltage, the dimmer and redder the black body radiation emitted by the light bulb, in complete analogy with the sunspot being just a colder region of the photosphere.

Flares and prominences are phenomena of the chromosphere and of the corona. They are associated with the sunspot groups, and they are part of the same physical phenomenon. While spots are at times detectable even without the aid of a telescope, flares and prominences are best seen either during solar eclipses or using special filters that highlight their emission in the backdrop of the emission from Sun’s photosphere. Flares are localized eruptions that can emit great amounts of energy. They appear brightest in the X-ray portion of the spectrum (at which the background of the photosphere is weaker), and are associated with the sunspots. An individual flare can emit up to 1033 ergs of energy.

Prominences assume different shapes. They typically appear as arcs of gas following the magnetic field lines associated with the sunspot groups, and they are of comparable size. Often prominences extend to well within the Sun’s corona, and sometimes some of the gas completely escapes the Sun’s gravity–a phenomenon called ‘coronal mass ejection.’ The ejected gas is highly ionized, just as are the prominences that originate it. When the ions reach the Earth they often cause damage to our telecommunication satellites.

The Solar Activity Cycle

Much solar activity observes cycles, the best known of which lasts about eleven years. As was first noticed by H. Schwabe in 1843, the average number of sunspots changes over time. Spots are nearly absent at the ‘solar minimum,’ reaching a peak at the ‘solar maximum.’ At the maximum it is not uncommon to count up to ten groups of sunspots at one time. The distribution of the spots also changes. Near the solar minima, spots are confined to a latitude of about 30-40 degrees North and South on the Sun’s surface. As the cycle progresses, the spots are gradually found closer to the Sun’s equator, and they increase in number.

An explanation for the solar cycle, and all the phenomena associated with the sunspots, was given by H. Babcock in 1960. The cycle is correlated with the distribution of the magnetic field in the outer layers of the Sun’s interior. At the solar minimum the field is roughly oriented along the meridians, and the Sun’s magnetic poles are not far from the poles of its rotation axis. Gradually, as the cycle progresses, the field lines are stretched and deformed, winding more and more around the Sun, in a pattern resembling a winding coil.

The field lines gradually become more closely oriented with the Sun’s parallels, and their distance from each other decreases. Since the distance between field lines is correlated with the strength of the magnetic field, as the cycle approaches a solar maximum the magnetic field increases in strength. As it turns out, the stretching of the field lines is due to the differential rotation of the Sun. Closer to the equator, the Sun’s outer layers rotate with a period of about 25 days, while the period gradually increases to 27 days at mid-latitudes.

Much of the Sun’s interior is plasma, i.e. charged particles. From studying the dynamics of charged fluids we know that the magnetic field lines tend to move together with the plasma. `Since the differential rotation of the Sun causes a lag in the rotation of regions farther away from the equator, one can intuitively see why the field lines get stretched. As the plasma finds itself immersed in an increasingly strong magnetic field, it becomes unstable, and it arches out of the Sun’s surface forming the sunspot groups. Pairs of spot groups correspond to places where the magnetic field arches out to the Sun’s corona, in the shape of an W .

The disruptions in the magnetic field lead to great differences in the field’s strength from place to place around the sunspots. These disruptions are responsible for the motion of vast quantities of plasma that we see as prominences. Flares are due to the collision of high energy plasma particles, where the particles are accelerated to high speeds and kinetic energy by processes similar to those used in particle accelerator laboratories on Earth. Flares occur in the vicinity of sunspot groups, where we find strong variable magnetic fields.

The mechanism that heats the corona to more than 106 K is correlated with these phenomena as well, although the detailed processes responsible for the heating are not completely understood. Because of the extreme temperature of the corona, its particles are held in the proximity of the Sun only where the magnetic field lines form loops, causing the particles to move along an arched trajectory that takes them back to the Sun’s surface. At the poles of the Sun’s magnetic field the high energy charged particles do escape the Sun, which is why the corona is absent there.

The Sun continuously emits a steady stream of charged particles. This flow does not directly reach the Earth because it is deviated by the Earth’s magnetic field. Some particles, though, are able to penetrate this barrier and hit the Earth’s atmosphere, giving rise to the phenomenon called aurora, a colorful luminescence in the night sky at high latitudes (such as Northern Canada and Scandinavia). The auroras happen preferentially close to the magnetic poles of the Earth’s magnetic field, since the charged particles tend to follow the magnetic field lines.

The eleven-year cycle has been studied in great detail. The long-term activity cycles are less known because they take place over periods of time for which we do not have a direct record. There is evidence indicating that the long-term cycles of the Sun give rise to important oscillations in the Earth’s climate. Between 1645 and 1715 no sunspots were detected at all. This sudden solar inactivity coincided with a decrease in temperature on our planet, called the ‘little ice age’. It is possible that the main ice ages are also correlated with the Sun’s activity, although this conjecture has not been sufficiently corroborated, and the Sun may be just a co-factor in this equation.

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