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The Sun

The Sun

The diameter of the Sun is about 109 times that of the Earth. Most of its mass consists of hydrogen.



Sun, structure of the Sun, Solar System, Milky Way, hydrogen, helium, fusion, solar flare, sunspot, photosphere, chromosphere, corona, solar wind, granulation, solar prominence, space probe, astronomy, atomic physics, particle physics, geography, physics

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Milky Way

The Sun is the central star of the Solar System. It is located 25-28 thousand light years from the centre of the Milky Way. The Sun is now halfway through its lifespan, which lasts for about 12 billion years. When its hydrogen supply is exhausted, it will become a red giant.
The Earth is about 150 million kilometres from the Sun (1 astronomical unit), it takes 8.3 minutes for sunlight to travel this distance.

Its diameter is 109 times that of the Earth. Three quarters of the Sun's mass consists of hydrogen, which is converted into helium by fusion in the core, thus emitting energy. The internal pressure of the Sun is as large as 150 million tons per square centimetre. Since all the matter in the Sun is in plasma form, it rotates faster at its equator (about 25 days) than it does at higher latitudes (about 32 days at its poles). This causes its magnetic field lines to become twisted, leading to the formation of sunspots and solar prominences. Its atmosphere is layered (made up of the photosphere, chromosphere and corona), and it gradually merges into the interplanetary medium.
It takes about 225-250 million years for the Sun to complete one orbit around the centre of the Galaxy, with a speed of 220km/s.

Definitions of terms:

Star: A giant, hot, thus luminous sphere of gas, held together by gravity. It radiates the vast amount of energy released by the fusion of atomic nuclei in its core. Its surface temperature is several thousand degrees Celsius, its atmosphere consists mostly of hydrogen. Its matter has a spherical internal structure.

Astronomical unit: Unit of distance used in astronomy, equal to the mean Earth-Sun distance, or the length of the semi-major axis of the Earth's orbit around the Sun (149,600,000 km).

Solar flare (eruption): A short and sudden brightening of the Sun's chromosphere and photosphere, usually around the sunspots. It usually lasts for 10-45 minutes, 9-10 times a day.

Solar prominence: An arc of heated gas that erupts from the surface of the Sun. A prominence can extend to nearly the diameter of the Sun itself. It consists of electrically charged particles that flow along the lines of magnetic force.

Solar wind: A stream of charged particles ejected from the Sun's corona, consisting almost exclusively of electrons and protons.

Sunspot: A patch of the surface of the Sun, where the magnetic field is much stronger than in the surrounding areas. Sunspots can be as large as 200,000 km in diameter, they can last for a few hours or several months.

Aurora: A temporary light phenomenon in the sky in the Arctic and Antarctic regions, caused by charged particles entering the atmosphere. It is generated by the excitation of oxygen and nitrogen atoms, its frequency is related to Solar surface activity (sunspot activity).

The Sun

  • chromosphere
  • granulation
  • sunspot
  • solar prominence
  • solar flare
  • corona


  • chromosphere
  • convective zone
  • 2 million K
  • radiative zone
  • core 14.5 million K
  • photosphere 6,000K
  • corona


– diameter: 1,392,000 km (109 Earths)​

– mass: 1.989 · 10³⁰ kg (333,000 Earths)

– average density: 1.4 g/cm³

– surface temerature: 5,780 K

– rotation period: 25.4 days

– luminosity: 3.85 · 10²⁶ W (6300 W/cm²)

Fusion process


  • chromosphere
  • convective zone
  • radiative zone
  • core 14.5 million K
  • photosphere 6,000K

Solar System


  • Sunspot
  • Sunspot
  • Solar flare
  • Solar flare
  • Solar prominence
  • Solar prominence
  • Corona
  • Corona


The Sun was considered a supernatural phenomenon and worshipped as a god in a number of ancient civilisations. In Egypt, it was worshipped as Amon, in Mesopotamia as Samas, and in Greece as Apollo. The Greek philosopher Anaxagoras offered the first scientific explanation in the 5th century BC; in his view the Sun was a glowing hot iron sphere. This unusual idea was considered blasphemous, and he was imprisoned for his views. After constructing his telescope, Galileo Galilei also studied the Sun and discovered sunspots. Later, Isaac Newton used a prism to split white light from the Sun into its components. Still later, William Herschel used this method when he discovered infrared radiation around 1800.

In his experiments in the 19th century, Joseph von Fraunhofer was the first to observe absorption lines in the solar spectrum, from which the chemical composition of the atmosphere could be determined. Hans Bethe developed the theory of nuclear fusion in 1939, which explains how energy is generated inside the Sun. The first space probes sent to observe the Sun were NASA’s Pioneer probes in 1959 and 1968. Orbiting the Sun at a distance equal to the Earth's, they thoroughly examined solar wind and explored the Sun’s magnetic field. The US-West German space probe Helios, launched in 1974, conducted research from within Mercury’s orbit. The Sun's X-ray radiation was examined by a space telescope from the Skylab space station.

After escaping the planets’ orbital plane, the Ulysses space probe studied the Sun, providing a great deal of new information about its polar regions. SOHO is one of the most important probes to research the Sun, always positioned between the Sun and the Earth. It has been taking pictures of the Sun since 1995, in both the visible and ultraviolet ranges. Recently, several new probes have been examining our star, which is very important, as solar activity has a profound influence on our weather. Utilising the energy of sunlight is gradually becoming more widespread: it is used for producing electricity by solar panels and solar power stations and for producing heat by solar collectors.

The Sun is an average star, a yellow dwarf. At 4.6 billion years old, it is now roughly halfway through its lifespan of about 12 billion years. It is composed of nearly three-quarters hydrogen, which is converted into helium by nuclear fusion in the Sun's core, thereby producing energy (high-energy photons). When its fuel supply is exhausted, it will shrink and its core will heat up sufficiently for the helium to convert into carbon. This process will result in even greater energy production; therefore, the star will expand to several hundred times its current size (so the Earth will probably be swallowed up). However, the Sun’s surface will be less hot, and it will become a red giant. This phase will not last long: when the fusion stops, the Sun’s internal pressure will decrease, and it will collapse due to its own gravity. It will then become an Earth-size, extremely dense white dwarf and will cool down after billions of years.

The Sun is not made up of solid material; it consists of plasma. This is why belts of different latitudes rotate at different rates. Its equatorial areas rotate every 25 days, while polar areas only rotate every 32 days. Its atmosphere is layered (made up of the photosphere, chromosphere and corona), and it gradually merges into the interplanetary medium. The corona becomes visible during solar eclipses.

99.87% of the mass of the Solar System is concentrated in its central star. The Sun has an enormous mass, so it has very strong gravity, which holds the solar system together and governs the movement of all the planets and smaller objects in it. The Sun emits a large amount of energy, mainly in the form of ultraviolet, visible and infrared radiation, but there is also a small amount of other types of radiation, like gamma rays, X-rays and radio waves.

Elementary particles (mainly protons and electrons) are also ejected from the Sun; these make up the solar wind. The core of the Sun has an estimated temperature of 14-15 million degrees K, a pressure of 3x10¹¹ atmospheres and a density of 155 g/cm³.

The core extends from the centre to about one-quarter of the solar radius and it works as a nuclear reactor, where energy is released in the form of high-energy photons, gamma rays and X-rays during the fusion of light elements into heavier ones. The fusion process involves the fusion of the nuclei of deuterium and tritium -both isotopes of hydrogen. Deuterium nuclei consist of one proton and one neutron, while tritium nuclei are made up of one proton and two neutrons. The reaction produces a helium nucleus, consisting of two protons and two neutrons.The reaction releases one neutron, as well as energy, in the form of free photons. During the collision, the forces of repulsion in the protons must be overcome. This is only possible if the hydrogen atoms move very fast, that is, if the temperature is extremely high.

The Sun can keep up the current level of radiation for another 6 or 7 billion years more. The core is surrounded by the radiative zone, which extends to about 70% of the Sun's radius. Photons often collide, become absorbed and are then emitted in this zone. It very often takes up to 10 thousand years for the photons to reach the surface. Large-scale convection takes place in the outer zone of the Sun, which takes up about 25-30% of the solar radius. This layer is therefore called the convective zone. Heat is transmitted to the photosphere by the flow of the zone's material. It is then emitted into outer space.

The Sun's atmosphere is mainly composed of lighter chemical elements: 71% hydrogen, 27% helium and 2% heavier elements. The nucleus contains 35% hydrogen.

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