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Sun, a simple introduction to "our" star

Sun is similar to all the other stars in the sky; it appears larger due to its proximity, about 150 million kilometers (compared to many billions of kilometers corresponding to different light-years of other stars). It's therefore a privileged study object since it let us closely observe various phenomena. Energy produced by the Sun, thanks to internal nuclear reactions, generates a electromagnetic radiation continuous spectrum, from gamma rays to radio waves, which illuminates whole Solar System and Earth's surface, that is filtered through our atmosphere.

Sun has an high density and temperature (about 15 million degrees) inner core with a radius of about 150000 km where nuclear fusion of hydrogen (the "engine" that illuminates our star) into helium occur.

From 150000 km to 450000 km from its center (note: more of Earth-Moon distance) radiative zone extends, where temperature drops to about 4 million degrees and where energy is transmitted and absorbed by plasma that makes up this layer billions of times before it gets to upper layers. From 450000 to 700000 km there is convection zone with a temperature of about 600000° in which the energy is transmitted through a number of currents that lead to outside the material warmer and inwards the colder one. Then we find photosphere, the part of Sun that we can see and which has a temperature of about 6000°. Although it appear to be a well-defined sphere, Sun is a fluid object that is too hot to keep solid matter. For this reason, even if photosphere is considered the real solar surface, it is not correct to consider it a real limit. It's instead right to know that photosphere we see is only the Sun layer where gas becomes opaque to visible radiation.


Sun imaged in h-alpha line (left, courtesy: ESA / NASA / SOHO) and Sun layers (right, courtesy: NASA) Sun imaged in h-alpha line (left, courtesy: ESA / NASA / SOHO) and Sun layers (right, courtesy: NASA)


Above there is the chromosphere, not more opaque to visible light, ie, the low solar atmosphere with a temperature of about 500000° where many phenomena such as faculae, flares and spicules occur. The last layer is corona, a region wide and thin around the Sun, not characterized by well-defined limits. Considered the high solar atmosphere, chromosphere has an average temperature of 1-2 million degrees, it extends from the Sun for about two million kilometers and is visible from Earth only during total eclipses. In this region other important phenomena such as prominences occur.

Among solar phenomena we can list:

- Granulation: visible in photosphere, it is the most external part of convection currents. Granulation is composed of lighter parts (warmer current rising) and dark parts (cooler current downhill). Each element is large from 300 to 1000 km.

- Facole: they are chromosphere phenomena, gas areas hotter and therefore brighter than the surrounding ones that often appear where sunspots born.

- Flares: they are powerful jets of plasma, which pushed up to corona, create spectacular arched structures with a diameter of 3000-4000 km. They can be accompanied with strong emission of radiation that make up solar wind.

- Sunspots: they are photospheric phenomena that appear darker because of their lower temperature (about 4000° compared to about 6000° of surrounding areas). It's known that sunspots are home of strong magnetic fields that would increase ability of convection to disperse energy causing temperature decrease in concerned area. Within a group of spots their polarity is often opposite: so they generate magnetic loop along which material can move into immense and spectacular arcs.

- Protuberances: also called filaments when viewed in perspective from above, they are the most spectacular phenomena since widespread. They arise as a result of strong explosions that project solar material into space even at heights of 150,000 km.


Sunspot and several small protuberances imaged in h-alpha line (author's image) Sunspot and several small protuberances imaged in h-alpha line (author's image)


Sun has a very strong magnetic field that controls solar phenomena and which extends beyond orbit of outer planets o Solar System. Gases that move away from the Sun traveling along magnetic field lines of force form solar wind whose concentration varies periodically causing changes in the Earth's magnetosphere and atmospheric phenomena such as aurora.