Sun, definition and study. A-Z index of Cognitio.

The Sun is the parent star of the solar system, around which the eight major planets (including Earth) orbits, planetary dwarfs, their satellites, innumerable other smaller bodies, and the diffused powder for space, which forms the interplanetary medium. 

Solar system, definition and study. A-Z Index of Cognitio.

Solar system


The mass of the Sun represents 99.9% of the total mass of the solar system. The Sun is the only star whose shape can be appreciated simply by the sight, thanks to its angular diameter, which varies depending on the point where Earth is in the course of its orbit: it reaches the maximum value when our planet is located at perihelion, while the lowest value for the Aphelion. Similar apparent dimensions allow, with the help of special instrumentation and appropriate protections, to observe the details of the surface of our star in order to reveal and study the phenomena that characterize it. With the advent in the early fifties of the space age and the beginning of explorations of the solar system, numerous probes were specially designed to study our star. The first satellites designed to observe the Sun were the Pioneers of NASA, launched between 1959 and 1968.

Pioneers, definition and study. A-Z index of Cognitio.


The probes orbited around the Sun at a distance just below Earth’s orbit and carried out the first detailed measurements of the wind and solar magnetic field. The latest Pioneer 9 probe worked for a long time, transmitting data until 1987. In 2006, the Solar Terrestrial Relations Observatory (STEREO) mission was launched, consisting of two identical orbiting naval vessels that allow a stereoscopic view of the star.

STEREO, definition and study. A-Z index of Cognitio.



The Sun is a star of the I (or third generation) population whose formation would have been caused by the explosion, about 5 billion years ago, of one or more supernovae near an extensive molecular cloud of the Orion Arm. Is a nearly perfect plasma sphere whose size is a bit bigger than a mid-sized star, but considerably smaller than a much bigger giant blue giant or giant red giant. It has a well-defined internal structure, which is, however, not directly observable due to the opacity to electromagnetic radiation of the star’s inner layers. A valid tool for determining the solar structure is provided by the 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 associated with computerized simulations, which allow astrophysics to determine with good approximation the inner structure of our star. The solar nucleus represents 10% of the star in volume, in excess of 40%. This is where nuclear fusion reactions, the main source of solar energy, take place. Located outside the nucleus, the radiative zone, it absorbs the energy produced by the nucleus and transmits it by irradiation (hence the name) to the upper layers. Pressure and temperature are still high enough to allow energy to be transferred to the next layer. The photosphere, on the other hand, is the superficial layer of the Sun below which the star becomes opaque in visible light; it is, therefore, the first visible layer, from which the energy coming from the inside is free to propagate in space.

 It is home to phenomena like sunburn and shivers. Even the Sun, like other stars, emits a stream of particles from the upper atmosphere – the solar wind. The solar wind is made up of plasma and its chemical composition is identical to that of the crown: 73% hydrogen and 25% helium, with the remaining 2% formed by trace elements. The solar cycle (also referred to as the cycle of solar magnetic activity) is the time, averaging eleven years, between two periods of minimal solar activity; the length of the period is not strictly regular but can vary between ten and twelve. It is also the main cause of the periodic variations of all solar phenomena that affect spatial weather. The Sun, like any other celestial body in the universe, consists of chemical elements. Many scientists have analyzed these elements to know their abundance, their relationship to the constituent elements of the planets, and their distribution within the star. Every second in the nucleus of our star 600 000 000 tonnes of hydrogen is converted into 595 740 000 tonnes of helium. After this transformation, 4 260 000 tonnes of hydrogen seems to have been lost; in fact, this lacking mass has turned directly into energy, that is, in electromagnetic radiation. The Sun is one of the many stars to possess its own planetary system, the solar system, consisting of all the bodies that are orbiting the star from its gravitational attraction. These are subdivided into: planets, dwarf planets, and smaller bodies.

Sun, definition and study. A-Z index of Cognitio.



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Sun, definition and study. A-Z index of Cognitio.