Sun: Location, Characteristics, Composition, Energy

The Sun (Latin: Sol) is the parent star of the solar system, around which orbit the eight main planets (including Earth), dwarf planets, their satellites, countless other minor bodies and the dust scattered throughout space, which forms the interplanetary medium. The mass of the Sun, which amounts to about 2 ×10³⁰ kg, alone represents 99.86% of the total mass of the solar system.

Orbital parameters
Classification	Yellow dwarf
Spectral class	G2 V
(at the time J2000.0)
Semi-major axis	(26±1.4) ×10³ at 7.62±0.32 kpc
Orbital period	(2,25–2,50)×10⁸ years 1 galactic year
Orbital speed	217 km/s (average)
Planetary system	Yes (Solar System)
Physical data
Equatorial diameter	1,391 ×10⁹ m
Polar diameter	1.3909×10⁹ m
Average diameter	1.39095×10⁹ m
Crushing	9×10⁻⁶
Surface	6.0877×10¹⁸ m²
Volume	1.4122×10²⁷ m³
Mass	1.9891×10³⁰ kg
Average density	1,408×10³ kg/m³
Average density of the core:	1.5×10⁵ kg/m³
Average density of the photosphere:	2×10⁻⁴ kg/m³
Average density of the chromosphere:	5×10⁻⁶ kg/m³
Average density of the crown:	10^-12 kg/m³
Acceleration of gravity on the surface	274.0 m/s²
Escape velocity	617.54 km/s
Rotation period at the equator:	27 d 6 h 36 min
Rotation period at latitude 30°:	28 d 4 h 48 min
Rotation period at 60° latitude:	30 d 19 h 12 min
Rotation period at latitude 75°:	31 d 19 h 12 min
Speed of rotation (at the equator)	1,993 mps
Inclination of the axis on the ecliptic	7,25°
Inclination of the axis on the galactic plane	67,23°
Right Ascension North Pole	286.13° (19h4m30s)
Declination	63,87° (63° 52)
Surface temperature	5777K (average)
Temperature of the crown	5×10⁶ k
Temperature of the nucleus	~1.57×10⁷ K
Brightness	3,827×10²⁶ W
Radiance	2,009×10⁷ W/(sr×m²)
Metallicity	Z = 0.0177 [Fe/H] = 0
Estimated age	4.57 billion years
Observational data
Apparent magnitude	−26.8 (average)
Apparent magnitude	−26,832
Absolute magnitude	4.83
Apparent diameter	31′ 31″ (min) 32′ 03″ (medium) 32′ 35″ (max)

The Sun is a small to medium-sized star consisting mainly of hydrogen (about 74% of its mass, 92.1% of its volume) and helium (about 24-25% of mass, 7.8% of volume), plus other heavier elements present in traces.

It is classified as a “yellow dwarf” of spectral type G2 V: “G2” indicates that the star has a surface temperature of 5 777 K (5504 °C), a characteristic that gives it an extremely intense white color and chromatically cold but often appears yellowish due to the light scattering in the Earth’s atmosphere, due to the elevation of the star on the horizon and nevertheless to the atmospheric clarity. The V (5 in Roman numerals) indicates that the Sun, like most stars, is on the main sequence, that is, in a long phase of stable equilibrium in which the star fuses, in its core, hydrogen into helium.

This process generates every second a large amount of energy (equivalent to a power of 3.9×10²⁶ W), emitted into space in the form of electromagnetic radiation (solar radiation), particle flux (solar wind) and neutrinos. Solar radiation, emitted basically as visible and infrared light, allows life on Earth by providing the energy necessary to activate the main mechanisms underlying it; in addition, the insolation of the Earth’s surface regulates climate and most meteorological phenomena.

Located within the Orion Arm, a secondary arm of the galactic spiral, the Sun orbits the center of the Milky Way at an average distance of about 26000 light-years and completes its revolution in 225-250 million years. Among the nearest stars, located within a radius of 17 light years, the Sun is the fifth brightest in intrinsic terms: its absolute magnitude, in fact, is equal to +4.83. If it were possible to observe our star from α Centauri, the nearest star system, it would appear in the constellation Cassiopeia with an apparent magnitude of 0.5.

The Sun symbol consists of a circle with a dot in the center.

Observation

The Sun is the only star whose shape can be appreciated simply by sight, thanks to its average apparent angular diameter of 32′ 03″ of arc, which varies however depending on the point where the Earth is in the course of its orbit: it reaches its maximum value (32′ 35″) when our planet is at perihelion, while the minimum value (31′ 31″) at aphelion. Similar apparent dimensions allow, after the use of special instrumentation and adequate protection, to observe the details of the surface of our star in order to reveal and study the phenomena that characterize it.

With the naked eye, it is possible to distinguish the solar disk at sunset or in the presence of fog and clouds, when the light intensity is significantly lower. These observations allow, albeit in rare circumstances, to observe particularly extensive sunspots. Using a modest telescope, equipped with an adequate filter or used to project the image of the star on a white screen, it is possible to easily observe sunspots and flares. However, due to the risks to which the retina of the eye is subjected, the observation of the Sun without the right protections is harmful to the eye: in fact, the strong radiation can cause the death of part of the cells of the retina, responsible for vision, or the degeneration of some ocular structures, like the crystalline lens.

The combination of the size and distance from the Earth of the Sun and the Moon is such that the two stars appear in the sky with approximately the same apparent diameter; This situation is at the origin of periodic occultations of the star by our only natural satellite, which are called solar eclipses; Total eclipses, in particular, allow you to visualize the solar corona and prominences.

Another observation concerns its apparent motion in the sky. This motion during the day is exploited in the scanning of the hours, with the help of tools such as sundials.

In addition, the star seems to make a journey along the zodiac belt in a year that varies from day to day. The trajectory described by the Sun, detected by determining its position at the same time every day during the year, is called an analemma and has a shape resembling the number 8, aligned according to a north-south axis. The variation of the annual solar declination in the north-south direction is about 47 ° (due to the inclination of the Earth’s axis with respect to the ecliptic of 66 ° 33 ‘, a fundamental cause of the alternation of the seasons); there is also a small east-west variation caused by the different orbital velocity of the Earth, which, in accordance with Kepler’s laws, is maximum at perihelion and minimum at aphelion.

History of observations of the Sun

First acquaintances

Man, since his origins, has made the object of attention and often veneration many natural phenomena, including the Sun. The first astronomical knowledge of prehistoric man, who considered the stars immutable dots “embedded” in the celestial sphere, consisted essentially in the prediction of the motions of the Sun, the Moon and the planets against the background of the fixed stars.

An example of this “proto astronomy” is given by the orientations of the first megalithic monuments, which took into account the position of the Sun at various times of the year: in particular, the megaliths of Nabta Playa (in Egypt) and Stonehenge (in England) had been built taking into account the position of the star during the summer solstice. Many other monuments of antiquity were built taking into account the apparent motions of the Sun: an example is the Temple of Kukulkan (better known as El Castillo) in Chichén Itzá, Mexico, which was designed to cast snake-shaped shadows during the equinoxes.

The apparent motion of the Sun against the background of the fixed stars and the horizon was used to draw up the first calendars, used to regulate agricultural practices. Compared to the fixed stars, in fact, the Sun seems to make a rotation around the Earth within a year (on the plane of the ecliptic, along the zodiacal belt); for this reason, our star, contrary to what is known today, was considered by ancient Greek astronomers as one of the planets that revolved around the Earth, which was considered at the center of the Universe; this conception takes the name of “geocentric system” or “Aristotelian-Ptolemaic system” (from the names of the Greek philosopher Aristotle, IV century BC, and the Alexandrian astronomer Claudius Ptolemy, 2nd century AD).

Development of modern scientific knowledge

One of the first “scientific explanations” of the Sun was provided by the Greek philosopher Anaxagoras. He imagined it as a large sphere of flaming metal larger than the Peloponnese and thought it impossible that it could be dragged by the chariot of the god Helios. For teaching this doctrine, considered heretical, he was accused by the authorities of impiety, imprisoned and sentenced to death (but he was later released by the intervention of Pericles).

Eratosthenes of Cyrene, probably, was the first to accurately calculate the distance of the Earth from the Sun, in the third century BC, according to Eusebius of Caesarea, he calculated the distance to our star in “σταδίων μυριάδας τετρακοσίας καὶ ὀκτωκισμυρίας” (stadìōn myrìadas tetrakosìas kài oktōkismyrìas), or 804 million stadia, equivalent to 149 million kilometers: a result surprisingly very similar to that currently accepted, from which it differs by only 1%.

Another scientist who challenged the beliefs of his time was Nicolaus Copernicus, who in the sixteenth century resumed and developed the heliocentric theory (which considered the Sun at the center of the Universe), already postulated in the second century BC by the Greek scientist Aristarchus of Samos. It is also thanks to the work of important scientists of the seventeenth century, such as Galileo Galilei, Descartes and Newton, that the heliocentric system finally came to prevail over the geocentric one. Galileo was also the pioneer of solar observation, thanks to the telescope; the Pisan scientist discovered sunspots in 1610, and refuted an alleged demonstration by Scheiner that they were objects transiting between the Earth and the Sun rather than present on the solar surface.

Isaac Newton, the father of the law of universal gravitation, observed white solar light through a prism, showing that it was composed of a large number of shades of color, while towards the end of the eighteenth century, William Herschel discovered infrared radiation, present beyond the red part of the solar spectrum.

In the nineteenth and twentieth centuries

In the nineteenth century spectroscopy achieved enormous progress: Joseph von Fraunhofer, considered the “father” of this discipline, made the first observations of the absorption lines of the solar spectrum, which are now called, in his honor, Fraunhofer lines.

In the early years of the modern scientific era, scientists wondered what caused solar energy. William Thomson, 1st Baron Kelvin, hypothesized that the Sun was a liquid body in gradual cooling, emitting its internal reserve of heat into space; the energy emission was explained by Kelvin and Hermann von Helmholtz through the theory called the Kelvin-Helmholtz mechanism, according to which the age of the Sun was 20 million years: a value much lower than the 4.6 billion years suggested for our planet by geological studies.

In 1890 Joseph Norman Lockyer, discoverer of helium in the solar spectrum, suggested that the star was formed by the progressive aggregation of rocky fragments similar to meteors.

A possible solution to the discrepancy between the Kelvin-Helmholtz and geological data came in 1904, when Ernest Rutherford suggested that the Sun’s energy could originate from an internal source of heat, generated by a mechanism of radioactive decay. However, it was Albert Einstein who provided the decisive starting point on the question, with his mass-energy relationship E=mc².

Einstein himself was able to demonstrate between 1905 and 1920 the reason for the particular orbital motion of Mercury, initially attributed to the perturbations of an innermost planet, called by astronomers Vulcan. Einstein assumed that the particular motion of the planet was not due to any planetary perturbation, but to the gravitational field of the Sun, whose enormous mass generates a curvature of space-time. The magnitude of curvature would depend on the relation:

$Magnitude of curvature$

$G$ is the constant of universal gravitation,

$M$ is the mass of the body,

$R$ indicates the deflection of the rays (measured in degrees) and

$c$ is the speed of light in a vacuum.

This curvature would therefore be responsible for the precession of the perihelion of the planet and the slight deflection that light and any other electromagnetic radiation, as a consequence of the theory of general relativity, would undergo near the gravitational field of the Sun. It has been calculated that the space-time curvature would cause a shift in the position of a star equal to 1.7 arcseconds.

In 1919 the English physicist Arthur Eddington confirmed the theory at an eclipse. The following year the English physicist hypothesized that solar energy was the result of nuclear fusion reactions, caused by the pressure and internal temperature of the Sun, which would transform hydrogen into helium and produce energy due to the difference in mass. The theory was further developed in the thirties by astrophysicists Subrahmanyan Chandrasekhar and Hans Bethe; the latter studied in detail the two main nuclear reactions that produce energy in stars, namely the proton-proton chain and the carbon-nitrogen cycle, calculating the amount of energy developed by each reaction.

In 1957, an article was published, entitled Synthesis of the Elements in Stars, in which a model consistent with the available data, and still valid today, was proposed, according to which most of the elements in the Universe were created by nuclear reactions inside stars, with the exception of hydrogen. helium and lithium, formed mostly during primordial nucleosynthesis and therefore already present in considerable quantities before the first stars formed.

Space missions

With the advent, in the early fifties, of the space age and the beginning of explorations of the solar system, there were numerous probes specially designed to study our star.

The first satellites designed to observe the Sun were NASA’s Pioneers 5, 6, 7, 8 and 9, launched between 1959 and 1968. The probes orbited the Sun at a distance slightly less than that of Earth’s orbit and made the first detailed measurements of the wind and solar magnetic field. The Pioneer 9 spacecraft operated for a long time, transmitting data until 1987.

In the seventies, the Helios 1 probe and the Skylab space station provided scientists with new and significant data on solar wind emission and the corona. Additional data were provided by NASA’s Solar Maximum Mission probe, launched in 1980, which was intended to observe ultraviolet, gamma and X-ray radiation emanating from solar flares during the period of maximum activity.

The nineties saw the launch of numerous probes, such as the Japanese Yohkoh (1991), designed to observe solar flares at X-ray wavelengths, and the Solar and Heliospheric Observatory (SOHO, 1995), the result of collaboration between ESA and NASA; The latter in particular has guaranteed since its launch a constant observation of our star in most of the wavelengths of the electromagnetic spectrum, also allowing the discovery of a large number of grazing comets.

However, these probes made detailed observations only of the equatorial regions of the Sun, since their orbits were located in the plane of the ecliptic. The Ulysses spacecraft was designed to study the polar regions, including measurements of the solar wind and magnetic field strength. Launched in 1990, Ulysses was initially directed towards Jupiter in order to take advantage of the gravitational slingshot effect of the gas giant and move away from the plane of planetary orbits. In 1998, the TRACE probe was launched, aimed at identifying the connections between the magnetic field of the star and the associated plasma structures, thanks also to the help of high-resolution images of the photosphere and the lower atmosphere of the Sun.

Unlike the photosphere, which is well studied through spectroscopy, the internal composition of the Sun is poorly understood. The Genesis mission was designed to take samples of solar wind and have a direct measurement of the composition of the matter constituting the star. In 2006, the Solar Terrestrial Relations Observatory (STEREO) mission was launched, consisting of two identical spacecraft placed in orbits that allow to obtain a stereoscopic view of the star.

On December 14, 2021, NASA’s Parker Solar Probe flies closer to the sun than it has ever done before by other spacecraft. Launched in 2018, the Parker Solar Probe wants to uncover the mysteries of the Sun by flying inside the solar corona, to capture the structure and scale of the Sun’s magnetic field.

Location within the Galaxy

The Sun orbits at a distance from the center of the Milky Way estimated at 26,000±1,400 light-years (7.62±0.32 kpc). The star is located in a peripheral region of the galaxy, more precisely within the Local Bubble, a cavity in the interstellar medium of the Gould Belt, located in the innermost edge of the Orion Arm, a secondary galactic arm placed between the Perseus Arm and the Sagittarius Arm; the two arms are separated by about 6500 light-years apart. Our star is currently in the Local Interstellar Cloud, a thickening of the interstellar medium due to the union of the Local Bubble with the adjacent Ring Bull I. Given its relative remoteness from the galactic center, other regions with high stellar density, and strong sources of radiation such as pulsars or similar objects, the Sun, and therefore the solar system, is located in what scientists call the galactic habitable zone.

The solar system takes 225–250 million years to complete one revolution around the center of the galaxy (galactic year); thus the Sun would have completed 20–25 orbits from the time of its formation and 1/1,250 of orbit from the appearance of humans on Earth. The orbital velocity of our star is about 220 km/s; At this speed, the solar system takes about 1 400 years to travel the distance of one light-year, or 8 days to travel an astronomical unit (AU). The apparent direction in which our star moves during its revolution around the center of mass of the Galaxy is called the solar apex and points towards the star Vega and the constellation Hercules, with an inclination of about 60° in the direction of the galactic center.

It is believed that the orbit of the Sun has an almost circular elliptical shape, taking into account the perturbations caused by the different distribution of masses in the arms of the galactic spiral; moreover, the Sun oscillates above and below the galactic plane on average 2.7 times each orbit, according to a trend similar to a harmonic motion. Because stellar density is quite high in and around the galactic plane, these oscillations often coincide with an increase in the rate of meteoritic impacts on Earth, sometimes responsible for catastrophic mass extinctions. This increase is due to the fact that other stars exert tidal forces on main-belt or Kuiper belt asteroids or Oort Cloud comets, which are consequently directed towards the inner solar system.

The Sun is part of a group of more than 100 million stars of spectral class G2 known within the Milky Way and exceeds in brightness as much as 85% of the stars in the Galaxy, most of which are faint red dwarfs. Among the nearest bright stars, placed within a radius of 17 light years, the Sun occupies the fifth position in terms of intrinsic brightness: its absolute magnitude, in fact, is equal to +4.83.

The Sun from α Centauri

If around the α Centauri system, the closest star system to the solar system (about 4.3 light years away), rocky planets orbited, in which intelligent life forms had developed that could observe the sky and understand its mechanisms, they would see it not very different from how we see it.

The differences would remain limited to some details: for example, the star Sirius would be found in the constellation of Orion, a few degrees from Betelgeuse, instead of in Canis Major; the constellation of Centaurus would be deprived of its brightest star, while Cassiopeia would have a bright star of magnitude 0.5 more: it is the Sun. The location of our star is easily calculated, since it would be at the antipodes of the position of α Centauri seen from Earth: it would therefore have a right ascension of 02h39m35s and a declination of +60° 50′ 00″, which would lead it to be to the left of Segin (ε Cassiopeiae); The constellation would no longer take the well-known “//” shape, but a shape similar to this: “///”.

Life cycle

The Sun is a population I (or third generation) star whose formation would have been induced by the explosion, about 5 billion years ago, of one or more supernova(s) in the vicinity of an extensive molecular cloud of the Orion Arm. It is established that, about 4.57 billion years ago, the rapid collapse of the cloud, triggered by supernovae, led to the formation of a generation of very young T Tauri stars, including the Sun, which, immediately after its formation, assumed an almost circular orbit around the center of the Milky Way, at an average distance of about 26000 to the center of the Milky Way.

The inclusions rich in calcium and aluminum, remnants of star formation, then formed a protoplanetary disk around the nascent star. This hypothesis was formulated in light of the high abundance of heavy elements, such as gold and uranium, in our planetary system. Astronomers believe that these elements were synthesized either through a series of endoergonic nuclear processes during the supernova explosion (a phenomenon called supernova nucleosynthesis), or thanks to transmutations, by means of subsequent neutron absorptions, by a massive population II (or second generation) star.

The Sun is currently on the main sequence of the Hertzsprung-Russell diagram, i.e. in a long phase of stability during which the star generates energy through the fusion, in its core, of hydrogen into helium; Nuclear fusion also causes the star to be in an equilibrium state, either hydrostatic, i.e. it does not expand (due to the radiation pressure of thermonuclear reactions) nor contract (due to the force of gravity, to which it would naturally be subject), or thermal. A G2 star like the Sun takes about 10 billion (10¹⁰) years to completely deplete the hydrogen at its core.

The Sun is about halfway through its main sequence. At the end of this period of stability, in about 5 billion years, the Sun will enter a phase of strong instability that takes the name of red giant: when the hydrogen of the core will be totally converted into helium, the immediately upper layers will undergo a collapse due to the disappearance of the radiation pressure of thermonuclear reactions. The collapse will determine a thermal increase until temperatures are reached that trigger the fusion of hydrogen in the upper layers, which will cause the expansion of the star beyond the orbit of Mercury; the expansion will cause cooling of the gas (up to 3500 K), which is why the star will have a typically intense yellow photospheric coloration.

When even the hydrogen of the upper layer of the nucleus will be totally converted into helium (within a few tens of millions of years) there will be a new collapse, which will determine an increase in the temperature of the helium nucleus up to values of 10⁸ K; at this temperature, the fusion of helium (helium flash) into carbon and oxygen will be triggered suddenly. The star will undergo a reduction in size, moving from the giant branch to the horizontal branch of the H-R diagram.

Due to the very high temperatures of the core, the fusion of helium will be exhausted in a short time (a few tens of millions of years) and the fusion products, not be usable in new thermonuclear cycles due to the small mass of the star, will accumulate inert in the core; meanwhile, when the radiation pressure that pushed outwards is lost again, a subsequent collapse will occur that will determine the triggering of the fusion of helium in the shell that surrounds the nucleus and hydrogen in the layer immediately above it. These new reactions will produce such a high amount of energy as to cause a new expansion of the star, which will thus reach dimensions close to 1 au (about 100 times the current ones), so that its atmosphere will most likely encompass Venus.

The fate of the Earth is uncertain: some astronomers believe that our planet will also be absorbed by the dying star; others instead hypothesize that the planet will be saved, since the loss of mass by our star would widen its orbit, which would consequently slide up to almost 1.7 au. However, our planet will be uninhabitable: the oceans will have evaporated due to the strong heat and much of the atmosphere will be dispersed into space by intense thermal energy, which will increase the kinetic energy of atmospheric gas molecules, allowing them to overcome the gravitational attraction of our planet. All this will happen within the next 3.5 billion years, that is, even before the Sun enters the red giant phase.

Within 7.8 billion years, once all thermonuclear processes are exhausted, the Sun will release its outermost layers, which will be wiped out in the form of a “superwind” creating a planetary nebula; the innermost parts will collapse and give rise to a white dwarf (about the size of the Earth), which will slowly cool to become, over hundreds of billions of years, a black dwarf.

This evolutionary scenario is typical of stars with a mass similar to that of the Sun, that is, they have a mass not high enough to explode as supernovae.

Morphological characteristics and rotation

The Sun is an almost perfect sphere of plasma, whose dimensions are a little larger than those of a medium-sized star, but much smaller than those of a much larger blue giant or red giant. It has an ellipticity estimated at about 9 millionths: in fact, its polar diameter differs from the equatorial one by just 10 km.

This difference exists because the rotation of the body on its axis gives rise to a force at the equator that would tend to make it assume an ellipsoidal shape: the centrifugal force. However, because the star’s rotation is very slow, the centrifugal force is 18 million times weaker than surface gravity; from this, it follows that the star does not possess a very pronounced equatorial bulge, characteristic instead of some stars, such as Achernar, which have high rotation speeds. Moreover, the tidal effects exerted by planets on the star do not significantly affect its shape.

Since it is in the plasma state and does not have, unlike a rocky planet, a solid surface, the star is subject to a differential rotation, that is, it rotates differently depending on the latitude: in fact, the star rotates faster at the equator than at the poles and the rotation period varies between 25 days at the equator and 35 at the poles. However, since the observational point of view from Earth changes as our planet makes its own motion of revolution, the apparent rotation period at the equator is 28 days. In addition, the density of the gases that make up the star decreases exponentially with increasing distance from the center.

Structure of the Sun

The Sun has a well-defined internal structure, which is not, however, directly observable due to the opacity of the inner layers of the star to electromagnetic radiation. A valid tool to determine the solar structure is provided by helioseismology, a discipline that, just like seismology, studies the different propagation of seismic waves to reveal the interior of the Earth, analyzes the different propagation of pressure waves (infrasound) that cross the interior of the Sun. The helioseismological analysis is often combined with computer simulations, which allow astrophysicists to determine the internal structure of our star with good approximation.

The radius of the Sun is the distance between its center and the limit of the photosphere, a layer above which the gases are cold or rarefied enough to allow the irradiation of a significant amount of light energy; It is therefore the layer best visible to the naked eye.

The internal structure of the Sun, like that of other stars, appears to consist of concentric envelopes; each layer has very specific characteristics and physical conditions, which differentiate it from the next.

The layers are, from the center out:

The core;
The radiative zone;
The tachocline;
The convective zone;
The photosphere, the surface of the Sun;
The atmosphere, divided into:
- Chromosphere;
- Transition zone;
- Crown.

Nucleus

The solar core represents 10% of the star by volume, over 40% by mass. This is where nuclear fusion reactions, the main source of solar energy, take place.

Astrophysicists believe that the solar core has dimensions close to 0.2 solar radii, with a density greater than 150 000 kg / m³ (150 times that of water), a temperature of about 13600000 K (for comparison, the surface temperature of the star is 2 350 times lower – 5777 K -) and pressure of almost 500 billion bar; it is the combination of similar values that favors the nuclear fusion of hydrogen into helium. The core is the only region of our star where nuclear fusion currently occurs.

These reactions release energy in the form of γ radiation, which, once emitted by the nucleus, is absorbed and re-emitted by the matter of the upper layers, helping to keep the temperature high; as it passes through the layers of the star, electromagnetic radiation loses energy, assuming longer and longer wavelengths, passing from the γ band to the X-band and ultraviolet, and then diffusing into space as visible light. Another product of nuclear reactions are neutrinos, particles that rarely interact with matter and therefore pass freely through space.

Photosphere

The photosphere is the layer of the Sun below which the star becomes opaque to visible light; it is, therefore, the first visible layer, from which the energy coming from within is free to propagate in space. It is home to phenomena such as sunspots and flares.
It is characterized by a density of 10²³ particles per cubic meter (equivalent to 1% of the density of the Earth’s atmosphere at sea level), while its thickness varies from a few tens to a few hundred kilometers.

The change in opacity with respect to the lower layers (its opacity is in fact slightly lower than that of the Earth’s atmosphere) is due to the decrease in the number of hydride ions (H⁻), which easily absorb visible light; the light we perceive is instead produced by the recombination between free electrons and hydrogen atoms to generate H⁻ ions.

Because the upper layers of the photosphere are colder than the deeper ones, the image of the Sun appears brighter in the center, and becomes gradually dimmer as you proceed toward the edge of the perimeter of the visible disk; this phenomenon is called edge obscuration and is caused by a perspective phenomenon.

The photospheric spectrum exhibits characteristics relatively similar to those of the continuous spectrum of a blackbody heated to a temperature of 5777 K and appears interspersed with the absorption lines of the tenuous stellar atmosphere. At direct observation, the photosphere has a grainy appearance, due to the presence of granulation and supergranulation.
During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond with any known element on Earth. In 1868, Norman Lockyer hypothesized that these lines were caused by a new element, which he named helium, after the Greek god of the Sun of the same name; twenty-five years later, helium was isolated on Earth.

Radiative zone

Located outside the nucleus, the radiative zone extends from about 0.2 to 0.7 solar radii; It absorbs the energy produced by the nucleus and transmits it by radiation (hence the name) to the upper layers. Pressure and temperature are still high enough to allow energy transfer to the next layer.

In this band the transfer of the energy produced in the nucleus takes place towards the upper layer, the convective zone; The radiative zone appears devoid of convective motions: in fact, while the matter becomes colder at increasing altitudes, the temperature gradient remains lower than that of the adiabatic fall rate, which facilitates the transfer of energy by radiation.

The energy is transferred to the outer layers very slowly: in fact, hydrogen and helium ions emit photons, which travel through a short distance before being reabsorbed and re-emitted by other ions.

A recent analysis of data collected by the SOHO mission suggests that the rotation speed of the radiative zone is slightly lower than that of the core.

Transition zone (Tachocline)

The transition zone between the radiative and convective portions is called tachocline and extends, according to recent helioseismological studies, starting from 0.7 solar radii. Astrophysicists believe that these dimensions play a decisive role in the genesis of the solar magnetic field, as they would intervene in the solar dynamo (mechanism thanks to which the magnetic field of our star originates) reinforcing the weak poloidal fields to create a more intense toroidal one.

Convective zone

The convective zone has a thickness of about 200000 km and is located in the outer portion of the Sun, starting from about 70% of the solar radius.

The area is characterized by temperatures and densities lower than those of the underlying layers; Consequently, energy and heat cannot be transferred by radiation, but through convective motions. The hottest and less dense matter is brought to the surface, where it gives up part of its thermal energy; Once cooled, the matter sinks to the base of the convective zone, where it again receives heat from the radiative zone. Unlike the underlying layer, therefore, in the convective zone matter is in constant motion. This constant and turbulent motion seems to be one of the fundamental causes of the solar dynamo.

The thermal columns of the convective zone leave marks on the solar photosphere that are called solar granules or supergranules.

The Sun atmosphere

The layers above the photosphere make up the solar atmosphere and are visible at all wavelengths of the electromagnetic spectrum, from radio waves to gamma rays via visible light. The layers are, in order: the chromosphere, the transition zone, the crown and the heliosphere; the latter, which can be considered the tenuous continuation of the corona, extends beyond the Kuiper Belt, to the heliopause, where it forms a strong boundary shock with the interstellar medium. The chromosphere, transition zone and corona are much hotter than the solar surface; The reason for this warming remains unknown.

Here is also the coldest layer of the Sun: it is a band called the minimum temperature region, located about 500 km above the photosphere: this area, which has a temperature of 4000 K, is cold enough to allow the existence of some molecules, such as carbon monoxide and water, whose absorption lines are clearly visible in the solar spectrum.

Chromosphere

Above the photosphere is a thin band about 2000 km thick, called chromosphere (from the Greek χρῶμα, χρώματος – chroma, chromatos -, meaning color) because of its colored flares visible just before and just after the total eclipses of the Sun. It is a thin envelope consisting of rarefied gas that appears reddish in color; In fact, the layer is transparent. The reddish coloration is due to hydrogen atoms, which at the lowest pressures of the chromosphere emit radiation of this color.

The chromosphere is affected by various emission phenomena of magnetic origin, such as spicules and solar prominences. The temperature in the chromosphere gradually increases as you move away from the star, reaching 100000 K in the outer layers.

Transition zone

Above the chromosphere is the transition zone, in which the temperature rises rapidly from about 100000 K of the outermost layers of the chromosphere, up to one million kelvins of the corona; this increase causes a phase transition of helium, which here becomes completely ionized due to high temperatures. The transition zone does not have a defined altitude limit: it forms a sort of halo around the chromosphere formations such as spicules and filaments and is in constant and chaotic motion. The transition zone is not easily visible from Earth, but it is well detectable from space through instruments sensitive to distant ultraviolet wavelengths.

Crown

The corona is the outer part of the solar atmosphere, has no defined limits and extends into space for tens of millions of kilometers in a very tenuous way. It consists of plasma at a very high temperature (over one million kelvins). Since plasma is very rarefied, the temperature is not to be understood in the conventional sense; In this case we speak of kinetic temperature.

The inner layers of the corona have a density of 10¹⁴ – 10¹⁶ particles per cubic meter (the Earth’s atmosphere at sea level has a density of 2 × 10²⁵ particles per cubic meter) and is home to numerous magnetic phenomena, such as mass ejections (CMEs) and coronal rings.

Astrophysicists have not yet been able to understand why the corona has such a high temperature; they believe that some of the heat originates from the reconnection of solar magnetic field lines (the topic is discussed more extensively in the paragraph Coronal heating problem).

Solar wind

The Sun, like other stars, emits a stream of particles from the upper atmosphere: the solar wind.

The solar wind is formed by plasma and its chemical composition is identical to that of the corona: 73% hydrogen and 25% helium, with the remaining 2% formed by trace elements. Near Earth, the speed of the solar wind varies between 200 and 900 km/s (average 450 km/s). Every second the star loses, through the solar wind, an amount of matter equal to 1.37×10⁹ kg; however, this is an insignificant loss, since in a year it corresponds to 2.18 × 10⁻¹⁴ times the total mass of the Sun.

The solar wind carries with it, due to the peculiar behavior of the magnetized plasma, the magnetic field of the Sun in interplanetary space, up to a distance of about 160 astronomical units. The solar wind moves in a radial direction relative to the Sun; Due to its rotation the field lines curve in the shape of a spiral.

Some studies hypothesize that the solar wind plays an important protective function against the planets, that is, it would “shield” cosmic rays thanks to its ionized nature.

Heliosphere

The solar wind creates a “bubble” in the interstellar medium, which is called the heliosphere. The heliosphere extends from a distance of about 20 solar radii (0.1 au) from the surface of the Sun to the most extreme regions of the solar system. Its innermost limit is defined as the region in which the flow of the solar wind becomes “superalfvénico“, i.e. exceeds the speed of the Alfvén wave; the dynamic and turbulent forces outside this limit cannot, however, influence the shape of the solar corona, since within this limit the flux travels at speeds lower than or equal to those of the Alfvén wave.

The solar wind travels continuously through the heliosphere, until it collides with the heliopause, more than 50 AU from the Sun. In December 2004, the Voyager 1 spacecraft crossed the heliopause; both Voyager probes, as they approached the heliopause boundary, recorded an increasingly high level of energetic particles.

Magnetic field of the Sun

The turbulent motion of the plasma and charged particles of the convective zone generate a powerful magnetic field, characterized by paired poles (north and south) arranged along the entire solar surface. The field reverses its direction every eleven years, at the maximum of the solar cycle. The solar magnetic field is at the origin of several phenomena that together take the name of “solar activity”; These include photospheric spots, flares (or flares ) and variations in the intensity of the solar wind, which spreads matter through the solar system.

The differential rotation of the star causes a strong deformation of the magnetic field lines, which appear tangled on themselves; on them is the plasma of solar flares, which form vast rings of glowing matter, known as coronal rings. The deformations of the field lines give rise to the dynamo and the eleven-year cycle of solar activity, during which the strength of the magnetic field undergoes variations.

The density of the solar magnetic flux is 10⁻⁴ tesla near the star.

The interaction between the solar magnetic field and the plasma of the interplanetary medium creates a diffuse heliospheric current, that is, a plane that separates regions where the magnetic field converges in different directions.

Solar cycle

The solar cycle (also called the cycle of solar magnetic activity) is the time, on average equal to eleven years, that elapses between two periods of minimum solar activity; The length of the period is not strictly regular, but can vary between ten and twelve years. It is also the main cause of the periodic variations of all solar phenomena that affect space weather.

shapes the atmosphere and the solar wind;
modulates solar irradiance;
modulates the flow of short-wavelength radiation, from ultraviolet to X-rays;
modulates the frequency of eruptive phenomena, such as flares and mass ejections;
It indirectly modulates the flow of high-energy cosmic rays penetrating the solar system.

The solar cycle is divided into two phases: a maximum phase, in which the activity of the star is more frenetic, and a minimum phase, in which the activity is less intense. Solar activity during idle often coincides with lower-than-average temperatures on Earth, while closer maximum phases tend to be related to higher-than-average temperatures.

Since magnetic fields can affect stellar winds, coming to act as “brakes” that progressively slow down the rotation of the star as it completes its evolutionary path, stars no longer young, such as the Sun precisely, make their rotation in longer times and have less intense magnetic activity. Their activity levels tend to vary cyclically and may cease completely for short periods of time. An example was the Maunder minimum, during which the Sun underwent seventy years, during the seventeenth century, of minimum activity; during this period, also known as the Little Ice Age, Europe experienced a sharp drop in temperatures.

The first solar minimums of considerable duration were discovered through the dendrochronological analysis of the annual rings of the trunks of some trees, whose thickness depends on the environmental conditions in which the plants live; The thinnest lines seemed to coincide with periods when global temperatures had been below average.

Sunspots

By observing the Sun with suitable filters, it is possible to see along its surface the characteristic photospheric spots, well-defined areas that appear darker than the rest of the photosphere due to their “lower” temperature (of the order of 4500 K). These are regions of intense magnetic activity, in which convection (visible in the rest of the surface in the form of granulation) is inhibited by the strong magnetic field, which reduces the transport of energy from the warmer inner regions to the surface. Larger sunspots can extend for thousands of miles.

The number of sunspots visible on the surface of the Sun is not constant, it varies during the solar cycle. Normally, during solar minimum the spots are absent or very small; Those that appear are usually found at high latitudes (far from the equator). As the cycle continues, advancing towards the maximum, the spots become more and more frequent and tend to move towards the equatorial areas of the star, in compliance with Spörer’s law. The spots are usually found in pairs of opposite magnetic polarity; the magnetic polarity of the spots reverses during each solar cycle, so that if in one cycle one assumes the characteristics of a magnetic north pole, at the next cycle it becomes a magnetic south.

Possibility of long-term cyclical phenomena

A recent theory states that there can be magnetic instabilities within the Sun that cause fluctuations with periods of 41 000 or 100 000 years; such fluctuations could provide an explanation for both ice ages and Milanković cycles.
However, like many theories in astrophysics, this one cannot be directly verified.

Chemical composition

The Sun, like every other celestial body in the Universe, is made up of chemical elements. Many scientists have analyzed these elements to know their abundance, their relationships with the building blocks of the planets and their distribution within the star.

The star has “inherited” its chemical composition from the interstellar medium from which it originated: hydrogen and helium, which make up a large part of it, were formed thanks to the nucleosynthesis of the Big Bang, the heavier elements were synthesized by the nucleosynthesis of the most evolved stars, which, at the end of their evolution, They spread them in the surrounding space. The composition of the nucleus is strongly altered by nuclear fusion processes, which have increased the percentage by mass of helium (34%) at the expense of hydrogen (64%).

The percentage of heavy elements, conventionally called metals, has remained almost unchanged. These, present in traces especially in the most superficial layers, are: lithium, beryllium and boron; neon, the actual quantity of which would be greater than that previously estimated by helioseismological observations; the elements of group 8 of the periodic table, to which iron, cobalt and manganese belong. Many astrophysicists have also considered the existence of mass fractionation relationships between the isotopic compositions of noble gases, such as neon and xenon, present in the solar and planetary atmospheres.

Since the inner parts of the star are radiative and not convective, the photosphere, consisting essentially of hydrogen (about 74% of its mass, 92% of its volume), helium (about 24-25% of the mass, 7% of the volume) and trace elements, has maintained and maintains a chemical composition essentially unchanged since the formation of the star, so that many tend to consider it as an example of the primordial chemical composition of the solar system.

Until 1983 it was widely believed that the star had the same composition as its atmosphere; in that year it was discovered that the fractionation of the elements in the Sun was at the origin of the distribution of the same within it. This fractionation is determined by various factors, such as gravity, which causes heavier elements (such as helium, in the absence of other heavier elements) to be arranged in the center of mass of the celestial body, while the less heavy elements (therefore hydrogen) diffuse through the outer layers of the Sun; the diffusion of helium within the Sun tends to speed up over time.

Energy production: nuclear reactions

Every second in the core of our star 600 000 000 tons of hydrogen (equivalent to 3.4×10³⁸ protons) are converted into 595 740 000 tons of helium. After this transformation, 4 260 000 tonnes of hydrogen (0.75%) appear to have been lost; in reality, this missing mass has been transformed directly into energy, that is into electromagnetic radiation, according to Albert Einstein’s mass-energy equation: E = mc².

Considering that the sun has a mass of 2×10 27 tons and assuming that the loss of mass always remains 4.26×10⁶ tons per second, it is easy to calculate that in a billion years the loss of mass will be 1.34×10²³ tons, equal to about 22 times the mass of the Earth. It sounds like a huge amount, but it represents much less than a thousandth of the mass of the sun (about 0.06 per thousand).

Hydrogen is fused in a series of reactions known as the proton-proton chain:

4 ¹H → 2 ²H + 2 e⁺ + 2 ν_e (4,0 MeV + 1,0 MeV)

2 ¹H + 2 2 H → 2 ³He + ²γ (5.5 MeV)

2 ³He → ⁴He + 2 ¹H (12.9 MeV)

The previous reactions can therefore be summarized in the formula:

4 ¹H → ⁴He + 2 e⁺ + 2 ν_e + 2 γ (26,7 MeV)

where e⁺ is a positron, γ is a photon in the frequency of gamma rays, ν_e is an electron neutrino, H and He are the isotopes of hydrogen and helium, respectively. The energy released by these reactions is expressed in millions of electron volts, and is only a fraction of the total energy released. The concomitance of a large number of these reactions, which occur continuously and non-stop until the hydrogen is exhausted, generates the energy necessary to sustain the gravitational collapse to which the star would naturally be subjected.

The energy thus generated, in 1 second is equal to 3.83×10²⁶ joules (383 YJ), equivalent to 9.15×10¹⁰ megatons of TNT: an amount of energy unthinkable to reproduce on Earth. To understand the enormity of this energy, which expressed in watt-hours (Wh) is equivalent to 106 400 000 000 TWh, the only data that can serve as a term of comparison is the world production of electricity, which in 2012 was about 22500 TWh.

At this rate of production, to equal the energy produced by the Sun in 1 according to all the electricity production plants of our planet would have to operate at full capacity for more than 4 million years (approx. 4 525 000 years).

The photons, emitted at high energy (therefore in the frequencies of γ and X-rays), are absorbed in just a few millimeters of solar plasma and then re-emitted in random directions, with lower energy; For this reason, the radiation takes a very long time to reach the surface of the star, so much so that it is estimated that a photon, to reach the photosphere, takes between 10 000 and 170 000 years. Photons, once they reach the photosphere after this “long journey”, are emitted mainly in the form of visible light, although there are emissions in all wavelengths of the electromagnetic spectrum.

Unlike photons, neutrinos released by reactions interact very weakly with matter and therefore reach the surface almost immediately. For many years, measurements of the number of neutrinos produced in the solar core gave lower results, equal to 1/3 of what was theorized. This discrepancy, known as the solar neutrino problem, has recently been understood thanks to the discovery of the effects of a phenomenon known as “neutrino oscillation”: the Sun, in fact, emits the number of neutrinos hypothesized, but the detectors could not identify 2/3 because the particles had changed flavor (the quantum number of elementary particles related to their weak interactions).

It is of fundamental importance to remember how the process of nuclear fusion inside the Sun, like all physical processes that involve a transformation, takes place in absolute compliance with the law of conservation of energy (first law of thermodynamics): nothing is created and nothing is destroyed, but everything is transformed. The nuclear fusion mechanisms that power the Sun are not fully compatible with the initial formulations of the principle of conservation of mass and energy, instead they become so thanks to Einstein’s equation. In fact, he understood and demonstrated that the principle of conservation involves both matter and energy, no longer considered as two distinct but unitary realities, since one can transform into the other according to a precise mathematical relationship; The sum of mass and energy expressed in units of mass remains constant in the universe.

Solar energy

Solar energy is the primary source of energy on Earth. The amount of light energy that arrives per unit of time on each unit of surface exposed directly to solar radiation is called the solar constant and its value is approximately 1370 W / m². Multiplying this value by the surface area of the Earth’s hemisphere exposed to the Sun yields a power greater than 50 million gigawatts (GW). However, as sunlight is attenuated as it passes through Earth’s atmosphere, the power density value at the surface of our planet drops to around 1000 W/m², achieved in clear weather when the Sun is at its zenith (i.e. its rays are perpendicular to the surface). Taking into account the fact that the Earth is a rotating spheroid, the average insolation varies according to the points on the surface and, at European latitudes, is about 200 W/m².

Solar radiation is the basis of life on our planet: it makes possible the presence of liquid water, essential for life, and allows photosynthesis by plants, which produce the oxygen necessary for most living beings. Photosynthesis uses the energy of this radiation, which is stored in chemical bonds, to synthesize organic compounds (essentially carbohydrates) from inorganic substances (CO₂ and H₂O). Human also uses the energy of the Sun, which is collected by structures, such as solar panels, used for different purposes, such as heating water or producing electricity (photovoltaic panels). In addition, the energy stored in oil and all other fossil fuels comes from that of our star, which was converted into chemical energy thanks to the photosynthesis of plants that lived millions of years ago.

Solar ultraviolet (UV) radiation has an important antiseptic function and is used for the disinfection of some objects and water thanks to the SODIS method. It is responsible for tanning and sunburn due to excessive exposure to the sun, but it also has a fundamental role in medicine: in fact, it induces the synthesis, by the skin, of vitamins of group D, essential for bone well-being.

The amount of ultraviolet that reaches the Earth’s surface is considerably lower than that recorded at the top of the atmosphere, since the ozone molecules, which form a belt (called ozone layer) in the lower part of the stratosphere, shield and reflect much of the radiation back into space. The amount of UV also varies depending on latitude and is highest at the equator and tropical regions, where insolation is greatest. This variation is responsible for different biological adaptations, such as the skin color of different human populations spread over different regions of the globe.

Alternative energy source

The amount of solar energy that arrives on the earth’s soil is enormous (about ten thousand times the energy used by humanity at the same time), but not very concentrated, therefore it is necessary to collect energy from very large areas to obtain significant quantities; It is also quite difficult to convert into easily exploitable energy, such as electricity, with acceptable efficiencies. Its exploitation for electricity production requires generally high-cost products (such as photovoltaic panels), which make solar energy more expensive than other energy sources. The development of technologies that can make the use of photovoltaics economical is a very active field of research, for the moment it has not achieved significant results.

Conversely, solar energy can be conveniently used to generate heat (solar thermal).

There are three main technologies for acquiring the Sun’s energy:

The solar thermal panel uses the sun’s rays to heat a liquid with special characteristics, contained in its interior, which transfers heat, through a heat exchanger, to the water contained in a storage tank. Temperatures are generally below 100 °C.
The concentrating solar panel uses a series of linear parabolic mirrors to concentrate the sun’s rays on a receiver tube in which a heat transfer fluid flows (a fluid capable of transporting the heat received from the Sun to the storage and exchange systems) or a series of flat mirrors that concentrate the rays at the end of a tower in which a boiler filled with salts is placed for heat Blend. In both cases, the “receiving apparatus” heats up to relatively high temperatures (400 °C ~ 600 °C) useful for both purely thermal and thermoelectric purposes.
The photovoltaic panel exploits the properties of particular semiconductor elements to produce electricity when stimulated by light radiation (photoelectric effect).

Open theoretical questions

Although it is the closest star to Earth and is the subject of countless studies by scientists, many questions about the Sun remain unresolved, such as, for example, why the solar atmosphere has a temperature of over one million kelvins while the temperature at the photosphere does not reach 6000 K. Currently astrophysicists are interested in discovering the mechanisms that regulate the sunspot cycle, the causes of solar flares and prominences, the magnetic interaction between the chromosphere and the corona and the causes of the solar wind.

Solar neutrino problem

For many years the number of solar neutrinos detected on Earth has been less (by a third to a half) than the number predicted by the Standard Solar Model; This anomalous result was called the solar neutrino problem. The theories proposed to solve the problem suggested a reconsideration of the internal temperature of the Sun, which would therefore have been lower than previously accepted to explain such a low influx of neutrinos, or claimed that neutrinos could oscillate, that is to say, that they could mutate into the undetectable tau neutrinos or muon neutrinos while covering the Sun-Earth distance.

In the eighties, neutrino detectors were built, including the Sudbury Neutrino Observatory and the Super-Kamiokande, in order to measure the flux of solar neutrinos with the greatest possible accuracy. The results revealed that neutrinos have an extremely small rest mass and can indeed oscillate. In addition, in 2001 the Sudbury Neutrino Observatory was able to detect all three types of neutrino directly, finding that the Sun’s total neutrino emission confirms the Standard Solar Model. This proportion accords with that theorized by the Mikheyev-Smirnov-Wolfenstein effect (also known as the “matter effect”), which describes the oscillation of neutrinos in matter. The problem is therefore now solved.

Coronal heating problem

The solar photosphere is known to have a temperature of about 6000 K. Above it extends the stellar atmosphere, which reaches, at the corona, a temperature of 1000 000 K; The high temperature of the corona suggests that the source of this heat is something other than the thermal conduction of the photosphere.

It is thought that the energy needed to heat the corona is provided by the turbulent movement of the plasma of the convective zone. Two mechanisms have been proposed to explain coronal heating: the first is that of the heat wave, according to which from the convective zone sound waves are produced, gravitational and magneto-dynam, which propagate outwards and disperse in the corona, giving their energy to the coronal plasma in the form of thermal energy. The other theory takes into account magnetic heat: magnetic energy is continuously produced by the motions of the convective zone and is released through magnetic reconnections in the form of large flares or similar events of lower intensity.

Nowadays it is unclear whether waves are an efficient heating mechanism; it has been found that all waves dissipate or refract before reaching the corona, with the exception of those of Alfvén which, however, do not disperse easily into the corona.

The focus of current research is on the cause and mechanism of warming. One possible solution to explain coronal warming is the continuous flares affecting the photosphere on a small scale, but this still remains an open field of research.

Weak young Sun problem

Theoretical models of the evolution of the Sun suggest that in the period between 3.8 and 2.5 billion years ago, i.e. during the Archaean eon, the Sun had only 75% of its current luminosity. Such a faint star would not have been able to keep liquid water on Earth’s surface, making it impossible for life to develop. However, geological evidence shows that the Earth maintained a relatively constant average temperature throughout its existence, indeed that the young Earth was even warmer than it is today. There is consensus among scientists that Earth’s atmosphere in its distant past was richer in greenhouse gases, such as carbon dioxide, methane and/or ammonia than it is today; these gases retained more heat to compensate for the lower amount of solar energy arriving on Earth.

Planetary system

The Sun is one of several stars to possess its own planetary system, the solar system, consisting of all bodies that are kept in orbit around the star by its gravitational pull. These are divided into: planets, dwarf planets and minor bodies.

There are eight planets in the solar system; in order of increasing distance from the star: Mercury, Venus, Earth, Mars, Jupiter, Saturn (known since antiquity), Uranus (discovered in 1781) and Neptune (discovered in 1846). The planets are divided into terrestrial or rocky and gaseous or Jovian, depending on their chemical-physical characteristics; the first, solid, dense and not very massive, are found in the innermost and warmest part of the solar system; the latter, gaseous, not very dense and extremely massive, are typical of the outermost and coldest areas of the system.

From 1930 to 2006 there were nine planets: the ninth was Pluto, discovered in 1930. In 2006 the International Astronomical Union decided to downgrade Pluto to the rank of dwarf planet, promoting the asteroid Ceres and the trans-Neptunian object Eris in this category. Recently, a new category of objects has been introduced, the plutoids, which includes the trans-Neptunian dwarf planets; as of September 2008 four objects belong to this category: in addition to the aforementioned Pluto and Eris, Haumea and Makemake; however, the number of dwarf planets is expected to increase in the coming years. All dwarf planets discovered so far are, by definition, within asteroid belts.

To the group of minor bodies belongs a vast number of objects; among them, we remember the asteroids, arranged in asteroidal belts: between Mars and Jupiter extends the main belt, composed of millions of rocky objects characterized by more or less variable orbits; beyond Neptune lies a second asteroid belt, the Kuiper belt, whose actual density is unknown. Even more externally, between 20 000 and 100000 au away from the star, lies the Oort cloud, believed to be the place of origin of comets.

All these objects make up a small part of the system: in fact, 99.86% of the mass of the solar system is made up of the Sun. Within the solar system the space between one celestial body and another is not empty: dust, gases and elementary particles constitute the interplanetary medium.

The planets, and in particular the most massive of all, Jupiter, exert gravitational influences on the center of mass of the solar system such that it does not coincide with the center of the Sun, but rather, depending on the magnitude of the interactions (which vary over time), that it most often falls outside the star. The fact that the barycenter of the system and the center of the star do not coincide is responsible for the motion of revolution that the center of mass of the star, or its core, makes around the barycenter, a motion that after a few hundred years varies assuming a direction now prograde now retrograde.

The Sun in culture

Etymology and other names

The term “Sun” derives from the Latin sol, solis, which would derive, together with the Sanskrit term सऊरयअस (sûryas, originally *svaryas, whose root svar- means to shine), from the Indo-European root: sóh₂wl̥. From the same root derives the Greek adjective σείριος (séirios; originally σϝείριος, swéirios), shining; this adjective, especially in its personified form ὁ Σείριος (ho Séirios, which means He who shines), was one of the epithets of the Sun, especially in the poetic-literary field. It should also be noted that from the same adjective derives the name of the brightest star in the night sky, Sirius (α Canis Majoris).

The prefix helium-, which indicates various aspects concerning the Sun (such as helio-graphy, helio-seismology and so on), derives from the Greek Ἥλιος (Helios), which was the name with which the Ancient Greeks currently designated the star and the deity in charge. The term ἥλιος, mainly in the Doric variant αἔλιος (āèlios, which stands for an ancient *ayelios), derives from an Indo-European root *us- elongated in *aus-, meaning to burn, to shine.

In the Far East the meaning “Sun” is given by the symbol 日 (Chinese pinyin rì), although it is also called 太阳 (tài yáng). In Vietnamese these Han words are known as nhật and thái dương respectively, while the original Vietnamese word mặt trời literally means “face of the heavens”. The Moon and the Sun are associated with Yin and Yang, respectively Yang the Sun and Yin the Moon, as dynamic opposites.

The Sun in mythology and religion

In many ancient cultures, starting from prehistoric times, the Sun was conceived as a deity or a supernatural phenomenon; the cult attributed to it was central to many civilizations, such as the Inca in South America and the Aztec civilization in Mexico.

In Egyptian religion the Sun was the most important deity; the pharaoh himself, considered a deity on earth, was considered the son of the Sun. The oldest solar deities were Wadjet, Sekhmet, Hathor, Nut, Bastet, Bat and Menhit. Hathor (later identified with Isis) begat and cared for Horus (later identified as Ra). The motions of the Sun in the sky represented, according to the conception of time, a struggle waged by the soul of Pharaoh and Osiris. The assimilation to solar worship of some local deities (Hnum-Ra, Min-Ra, Amon-Ra) reached its peak at the time of the fifth dynasty.

During the eighteenth dynasty, Pharaoh Akhenaten attempted to transform the traditional Egyptian polytheistic religion into a pseudo-monotheist, known as Atonism. All the deities, including Amun, were replaced by Aten, the solar deity who ruled over the region of Akhenaten. Unlike the other deities, Aten did not possess multiple forms: his only effigy was the solar disk. This cult did not survive long after the death of the pharaoh who introduced it and soon traditional polytheism was reaffirmed by the same priestly caste, which had long before embraced the atonistic cult.

In Greek mythology, the main solar deity was Helios, son of the Titans Hyperion and Teia. The god is normally represented driving the chariot of the sun, a quadriga pulled by horses that emit fire from the nostrils. The chariot rose every morning from the Ocean and pulled the Sun into the sky, from east to west, where the two palaces of the god were located. In more recent times, Helios was assimilated to Apollo.

The cult of the Sun as such also found fertile ground in Rome; the first attempt to introduce solar worship was by Emperor Elagabalus, priest of the Syrian solar god El-Gabal. El is the name of the main Semitic deity, while Gabal, which is linked to the concept of “mountain” (compare with the Hebrew jevul and the Arabic jebel), is its manifestation in Emesa, its main place of worship. The deity was later imported into the Roman pantheon and assimilated to the Roman sun god known as Sol Indiges in the Republican age and then Sol Invictus in the second and third centuries. Another important solar cult, of a mysterious character, was Mithraism, from Mithras, its main deity, which was imported into Rome by legions stationed in the Middle East, mainly in Syria.

However, the affirmation of the solar cult, the Sol Invictus, came with Aurelian, who proclaimed himself his supreme priest. The celebrations of the rite of the birth of the Sun (the Christmas of the infant Sun, later Dies Natalis Solis Invicti, Christmas of the unconquered Sun), took place on December 25, with particular solemnity in Syria and Egypt, provinces where this cult had been rooted for centuries. The rite provided that celebrants, retired to special sanctuaries, came out at midnight, announcing that the Virgin had given birth to the Sun, depicted in the form of an infant. The cult of Sol Invictus lasted until the advent of Christianity and its formalization as the state religion with the Edict of Thessalonica of Theodosius I, on 27 February 380.

On March 7, 321, Emperor Constantine I decreed that the seventh day of the week, the Dies Solis, should become the day of rest; The decree had not been issued in favor of any religion but was an act of regulation of weekly activities that became part of the Roman legislative body.

“Imperator Constantinus. Omnes iudices urbanaeque plebes et artium officia cunctarum venerabili Die Solis quiescant. Ruri tamen positi agrorum culturae lifree licenterque inserviant, quoniam frequenter evenit, ut non alio aptius die frumenta sulcis aut vineae scrobibus commendentur, ne occasione momenti pereat commoditas caelesti provisione concessa.

Const. A. Helpidio. *A. CCCXXI PP. V. Non. Mart. Crispus II and Constantine II Conss.”

*(Codex Justinian 3.12.2)

Some Christians took advantage of the imperial decree to transfer the meaning of the Jewish Shabbat to the Dies Solis, which, since the time of Justin (second century), began to assume among the Christian communities the name of Dies Dominica (Lord’s Day), the weekly memorial of the Resurrection of Jesus which, according to the Gospel account, took place on the first day after the Sabbath (Mt 28:1; Mk 16:1; Lk 24:1; Jn 20:1); On 3 November 383, at the behest of Theodosius, the Dies Solis was finally officially renamed Dies Dominica.

After embracing the Christian faith, in 330 the emperor made the Dies Natalis Solis Invicti coincide with the date of birth of Jesus, considered by Christians the “Sun of righteousness” prophesied by Malachi (Mal, 4:2), ormalizing the Christian celebration for the first time. Cyprian, Bishop of Carthage, wrote a century earlier: “How magnificently Providence has acted in ensuring that, on the day when the Sun was born, Christ is born!” In 337 Pope Julius I made official the liturgical date of Christmas by the Christian Church (today divided into Catholic, Orthodox and Coptic), as reported by John Chrysostom in 390: “On this day, December 25, the nativity of Christ was also definitively fixed in Rome”.

In literature and music

In culture, the Sun is mainly used as a mythological and mystical-religious reference, rather than in the literary field: unlike the stars, in fact, which are cited as nocturnal wonders by poets and writers, the Sun in literature is used above all as a reference for the alternation of day and night. However, there are strong references specifically dedicated to this star in literature, painting and even music.

One of the most famous and oldest texts of Italian literature that refers to the Sun is in Canticle of Brother Sun, also known as the Canticle of the Creatures written by St. Francis of Assisi, completed, according to legend, two years before his death, which took place in 1226. The Canticle is a praise to God, a prayer permeated by a positive vision of nature, since the image of the Creator is reflected in creation. With the birth of historiographical science, between the eighteenth and nineteenth centuries and with the romantic ideals of the “popular roots of poetry”, the work was taken into consideration by the critical and philological tradition.

Even Dante Alighieri, as a good connoisseur of astronomy, does not fail to mention the Sun in his works, using it as an astronomical reference: in the First Canto of Paradise, for example, he describes the light of the Sun, explaining that since it illuminates the hemisphere in which Purgatory is located, the city of Jerusalem, which is on the opposite side of the Earth, It is at that moment immersed in the darkness of the night. Dante thus pauses to observe the splendor of our star, imitating his guide, Beatrice.

Even in fairy tales, the figure of the Sun is occasionally used, where, however, it appears as a character in all respects; among the best known examples are, in addition to those of Phaedrus, the fables written by Jean de La Fontaine, a writer French lived in the seventeenth century, such as The Sun and the Frogs or The Sun and the Wind.

The Sun has even directly influenced some pieces of symphonic music: during Romanticism and subsequent phases, in fact, composers frequently take up “natural” themes with the intention of translating them into scores for various musical instruments. One of the best known examples is the sunset orchestrated by Ludwig Van Beethoven in the final bars of his Sixth Symphony, a piece rich in countless naturalistic references.

Another well-known example is given by Richard Strauss’ s Symphony of the Alps , in which references to the rising and setting of the Sun are explicitly present (both in the orchestration and as the title of the various sections of the symphonic poem). Other authors have described in music the various phases of the day, with a reference to the rising of the Sun, including Anton Bruckner (in the fourth symphony) and Modest Petrovič Musorgskij (in the piece entitled A Night on Bald Mountain, also taken up by Walt Disney for the finale of his famous Fantasia).

Among the various references present in the music of the twentieth century, an important Italian reference is given by the title of the famous Canzone del sole, signed by Lucio Battisti and Mogol and recorded for the first time in 1971 on a 45 rpm; This piece is also often performed by those who learn to play the guitar, as a practice.

Use of the term Sol

The term Sol is the Latin form of Sun, from which the Italian word derives; the name Sol is however also understood by the citizens of the Anglo-Saxon countries, where, however, the form Sun predominates. The term Sol is frequently used in English in science fiction (such as Star Trek), as a common name for the star where the events take place. By extension, the term Solar System is often used to define the planetary system of storytelling.

The term Sol is also used by English-speaking astronomers to refer to the length of a solar day on Mars. An Earth’s solar day is about 24 hours, while a Martian day, or sol, is 24 hours, 39 minutes, and 35,244 seconds.

Sol is also the word used for “Sun” in Portuguese, Spanish, Icelandic, Danish, Norwegian, Swedish, Catalan and Galician. The Peruvian currency is called nuevo sol (New Sun); in Persian the term Sol is used to indicate the solar year.

References (sources)

M. Gillman, H. Erenler, The galactic cycle of extinction, International Journal of Astrobiology, vol. 386, 2008;
Mark J. Reid, The distance to the center of the Galaxy, Annual review of astronomy and astrophysics, vol. 31, 1993;
Article Sole, Wikipedia;
NASA Enters the Solar Atmosphere for the First Time, Bringing New Discoveries, nasa.gov;
Studying the stars, testing relativity: Sir Arthur Eddington, ESA Space Science, 2005.