IndexIntroductionThe Sun through historyThe solar atmosphereChromosphereCoronaSunspots, prominences and flaresThe solar activity cycleIntroduction 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 mass of the Sun. 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 the mass of the Sun, ours would be a binary star system. Although gas giant planets like Jupiter emit more energy than they receive from the Sun, only the Sun owes its internal pressure to nuclear fusion. Say no to plagiarism. Get a tailor-made essay on "Why Violent Video Games Shouldn't Be Banned"? Get an original essay Nuclear fusion generates all the energy emitted by our star. This energy heats the gas to very high temperatures. The Sun shines because it is made up of glowing gas, with a surface temperature of about 5,800 K. Because of its high temperature, the Sun emits light in a broad spectrum of wavelengths, peaking at what we consider the ' visible' of the planet. the spectrum. The fact that our eyes are sensitive to light of wavelengths corresponding to the Sun's peak emission is obviously no coincidence. Fortunately, most of the light from our Sun does not reach the ground, as our atmosphere absorbs it. If ultraviolet rays and X-rays reached the Earth's surface, they would have devastating effects on our planet. The portion of light we receive from the Sun powers all atmospheric phenomena and, ultimately, life itself. Far from having a uniform surface and emitting a constant amount of energy per unit time, the Sun is very dynamic and exhibits cycles of activity. The best known is the eleven-year cycle, during which the number of sunspots and other disturbances of the solar atmosphere changes markedly in number and intensity. The eleven-year cycle is intimately connected with the intensity of the solar wind, a stream of energy-charged particles emitted by our star that continually collides with Earth's magnetosphere. Sometimes, solar flares give rise to expulsions of gas that escape from the Sun and reach the Earth. The strong flow of particles thus generated can be quite dangerous for the network of communications satellites that orbit 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 position of the Sun in the sky is linked to seasonal changes on Earth, and the seasons were of great importance to both agricultural and pre-agricultural societies. This point is clearly illustrated by the enormous effort ancient people put into building structures like Stonehenge at a time when no technology other than ropes was available to carry boulders weighing several tons. It is now believed that the orientation of the Stonehenge temple/observatory and other similar monuments was chosen to mark the solstices of the Sun and to celebrate the changing of the seasons. In classical Greece, and throughout the Renaissance, the Sun was believed to be made of 'ethereal' matter, that is, perfect and free of any blemish. It was believed that the same substance made up all the planets and even the Moon, and the irregular coloring of the Moon was explained by the proximity of our satellite to the Earth. The Earth, unlike celestial objects, was supposed to be composed of corruptible elements. Given this premise, Galileo's detailed telescopic observation of the Sun in 1610caused quite a stir. Galileo demonstrated 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 observations, together with others of the solar system, were decisive for the acceptance of the modern vision of the universe, where the same physics applies to the Sun as to any other object, and laboratory experiments on Earth can have application universal. In the 19th century, another debate centered on the scope of scientific knowledge arose, and once again the Sun was the protagonist. French philosopher Auguste Comte argued that since we cannot directly access stars and other astronomical bodies, there could be no chance that humanity would ever know what exactly they are made of. As often happens in the history of science, one should never say never. Around the same time that Comte made his radical statement, it was discovered that several elements, when in gaseous form, absorb light passing through them in a very particular way: only light of particular wavelengths is absorbed, and such wavelengths depend on the element constituting the gas. Armed with this knowledge, based on laboratory experiments on Earth, Kirchhoff and Bunsen demonstrated in 1859 that the Sun's atmosphere was composed of hydrogen and other known elements. In fact, the analysis of the solar spectrum soon led to the discovery of helium. . Nowadays, measuring the spectrum of astronomical objects is an essential step in determining their nature. As the young astronomer Cecilia Payne discovered in 1925, the composition of the Sun is very close to the average of the rest of the Universe and very different from that of the Earth. Hydrogen makes up 70.5% of the Sun's mass, followed by 27.5% of helium and only 2% of all other elements. The composition is nearly constant throughout the Sun, although the percentage of helium is highest in the Sun's core, where helium is formed through nuclear fusion. The Sun's Atmosphere The Sun's atmosphere and most of its interior are made up primarily of hydrogen and helium. In the atmosphere, helium makes up 73% of the mass while helium makes up 25%, leaving only 2% for the other elements. For the Sun as a whole (both atmosphere and interior), hydrogen averages 70.5%, helium 27.5%, and all other elements 2%. The Sun is completely in the gaseous phase. The gas, made up of the above-mentioned elements, is neutral or ionized depending on the atmospheric parameters in different places. In an ionized gas, also called "plasma", some or all of the electrons orbiting the nuclei are stripped from the atoms, either by violent collisions with other atoms or by the absorption of light of sufficient energy. The higher the temperature of the gas, the more favorable the conditions for plasma formation. The boundary between the atmosphere and the interior of the Sun is a region about 1,000 km thick, called the photosphere. Since the radius of the Sun is 696,000 km, the photosphere is a relatively thin layer. Most of the light we receive from the Sun comes from this boundary, which we usually associate with the "disk" of the Sun. The existence of the photosphere is due to a decrease in the "opacity" of the gas in that region. Opacity is an important concept, which deserves a more detailed description. A gas is said to be opaque when the propagating photons can only travel short distances before being deflected. The net effect of these numerous diffusions is a modification and randomization of the average wavelength of light in direct correspondence with the temperature of the gas. The light, in other words, is "thermalized" through itsinteraction with the gas. Transparent gas represents the opposite situation: diffusion and absorption of light occur rarely, allowing light to cover large distances without being deflected. While the Sun's interior 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 gas's ability to scatter light decreases dramatically at the base of the photosphere. A rough comparison can be made between the surface of the Sun and the surface of a cloud on Earth, where the "boundary" of the cloud is defined by the opacity of the cloud.the water droplets. The spectrum of the Sun closely resembles that of a black body at a temperature of 5,800 K. This is the temperature of the gas at the base of the photosphere. The light coming from the interior of the Sun is scattered several times below the photosphere, but from the base of the photosphere upwards it is almost free to travel without deflections, keeping its spectrum almost unchanged. The higher you go into the thin layer of the photosphere, the colder the gas becomes. The temperature actually drops to about 4,200 K. When light passes through the cooler, transparent gas of the upper photosphere, dark lines appear in the solar spectrum in the foreground of an otherwise featureless blackbody 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 the light passing through the gas. The fact that you see dark lines is related to the lower temperature of the gas in the upper photosphere, compared to its base: if the temperature increased with height you would see bright lines superimposed on the blackbody 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 on average 1,500 km wide. At the center of a grain the temperature of the photosphere is a few hundred degrees Kelvin higher than at its edges. The Sun's surface appears coarse-grained because it is the outer edge of a vast convective region within the Sun. Chromosphere The layer of the atmosphere adjacent to the photosphere and extending outward to the corona is called the chromosphere. Its boundary is defined by an increase in atmospheric temperature with altitude, in contrast to the observed decrease in the photosphere. In about 2,000 km, the temperature of the chromosphere increases from 4,200 K to 25,000 K. Its density, however, is only about 10-4 that of the photosphere. Due to its low density, seen against the background of the photosphere, the chromosphere is almost invisible. Therefore, it was only discovered when astronomers observed the Sun during solar eclipses. During such eclipses, the disk of the Moon covers the photosphere and allows viewing of the upper layers of the Sun's atmosphere, i.e. the chromosphere and corona. The chromosphere owes its name to its bright red color, against the dark background of the sky during solar eclipses. In these circumstances its spectrum is made up of multiple emission lines (no blackbody component is expected from a transparent gas). Given the temperature and composition of the gas, much of the light comes from Balmer red, a spectral line of hydrogen, which explains the prevailing color of the chromosphere. Corona The Sun's corona extends to distances comparable to the radius of our star. At wavelengths of the visible spectrum, the corona is visible only during solar eclipses. It can also be seen using "coronagraphs", which block sunlight from the photosphere directly into telescopes, thus simulating solar eclipses. The crown has an irregular shape and yesextends farther where disturbances are present in the underlying layers of the atmosphere. The corona is very hot and extremely diluted. It can reach temperatures of a few million Kelvin and a density of 10-12 that of the photosphere. At the shorter wavelengths of UV and X-rays, accessible only by telescopes orbiting above Earth's atmosphere, the irregular shape of the corona is strongly correlated with the distribution of sunspots and solar flares. The corona shines in the X-ray region of the spectrum, against the dark background of the photosphere: the photosphere emits as a blackbody at 5,800 K, which tapers off at wavelengths in the ultraviolet region of the spectrum. The hot transparent gas in the corona emits a striped spectrum, just like the spectrum of fluorescent light bulbs. The emission is strong in X-rays due to the extreme temperature of the gas. It is not yet clear why the corona is so hot. It seems likely that the gas will heat up as it collides with streams of particles generated by the photosphere during solar flares. This would explain why the corona emits the strongest radiation at flares and sunspots. Due to its temperature, the corona is a highly ionized gas. Oxygen, for example, is often stripped of two of its eight electrons. As a direct consequence of ionization, the corona becomes electrically charged and its gas particles are deflected in their movement when subjected to the strong magnetic field of the Sun. The magnetic field is a very important component of solar atmospheric activity. The temperature of the corona is so high that the gravitational pull of the Sun is not strong enough to prevent the corona from escaping the Sun. The gas is bound to the star mainly due to the trapping action of the star's magnetic field. Due to its temperature, the corona is a highly ionized gas. Oxygen, for example, is often stripped of two of its eight electrons. As a direct consequence of ionization, the corona becomes electrically charged and its gas particles are deflected in their movement when subjected to the strong magnetic field of the Sun. The magnetic field is a very important component of solar atmospheric activity. The temperature of the corona is so high that the gravitational pull of the Sun is not strong enough to prevent the corona from escaping the Sun. The gas is bound to the star mainly due to the trapping action of the star's magnetic field. Sunspots, Prominences, and Flares As was noticed long ago through naked-eye observation by Chinese astronomers, and later by Galileo using the telescope, the Sun is dotted with several dots. Spots are transient phenomena that appear as darker spots 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 very often the groups form pairs, oriented along the parallels of the Sun. Each spot is composed of a dark central region, called the 'umbra', surrounded by a lighter region, called the 'penumbra'. The cause of the darkness is simply related to the temperature of the gas. Temperatures at the center of the shadow are usually around 3,000 K. The lower the temperature, the weaker the blackbody emission from the photosphere. As an analogy, think about what happens when you lower the voltage applied to an incandescent light bulb, thereby lowering the temperature of its filament. The lower the voltage, the weaker and redder the blackbody radiation emitted by the light bulb, in complete analogy with sunspots which are precisely a colder region of the photosphere. Flares and prominences are phenomena of the chromosphere and corona. They are associated with sunspot groups andthey are part of the same physical phenomenon. While spots are sometimes detectable even without the aid of a telescope, flares and prominences are best seen either during solar eclipses or by using special filters that highlight their emission against the background of the solar photosphere's emission. Flares are localized eruptions that can emit large amounts of energy. They appear brightest in the X-ray portion of the spectrum (where the photosphere background is dimmer) and are associated with sunspots. An individual flare can emit up to 1033 ergs of energy. The bumps take different shapes. They typically appear as arcs of gas that follow the magnetic field lines associated with sunspot groups and are of comparable size. Often the prominences extend well into the solar corona, and sometimes some of the gas escapes the Sun's gravity entirely, a phenomenon called "coronal mass ejection." The expelled gas is highly ionized, as are the protuberances that originate it. When the ions reach Earth they often cause damage to our communications satellites. The Cycle of Solar Activity Much of solar activity observes cycles, the best known of which lasts about eleven years. As first noted by H. Schwabe in 1843, the average number of sunspots changes over time. Spots are almost absent at “solar minimum”, reaching a peak at “solar maximum”. At most it is not uncommon to count up to ten sunspot groups at the same time. The distribution of the commercials also changes. Near solar minima, the spots are confined to a latitude of about 30 to 40 degrees north and south on the surface of the sun. As the cycle progresses, the spots gradually find themselves closer to the Sun's equator and increase in number. An explanation of the solar cycle and all phenomena associated with 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 solar minimum the field is oriented approximately along the meridians, and the magnetic poles of the Sun are not far from the poles of its rotation axis. Gradually, as the cycle progresses, the field lines are stretched and deformed, wrapping further and further around the Sun, in a pattern resembling a winding coil. The field lines gradually become more closely oriented with the parallels of the Sun and their distance from each other. decreases. Since the distance between the field lines is related to the strength of the magnetic field, as the cycle approaches the solar maximum, the magnetic field increases in strength. Apparently, the stretching of the field lines is due to the differential rotation of the Sun. Closer to the equator, the outer layers of the Sun rotate with a period of about 25 days, while the period gradually increases to 27 days at mid-latitudes . Much of the interior of the Sun is plasma, that is, charged particles. From the study of the dynamics of charged fluids we know that magnetic field lines tend to move together with the plasma. “Since the differential rotation of the Sun causes a delay in the rotation of regions farther from the equator, one can intuitively see why the field lines become longer. When the plasma is immersed in an increasingly stronger magnetic field, it becomes unstable and curves out from the Sun's surface, forming sunspot groups. Pairs of sunspot groups correspond to places where the magnetic field curves toward the solar corona, in a W shape. Disturbances in the magnetic field lead to large differences in intensity. of the field from one place to another around sunspots. These disruptions are responsible for the movement of large amounts of plasma that we see as.