About Sun

The Sun. A symbol of the life for somebody, a ball of glowing gas or a fascinating riddle for someone else. And for me? It’s a little bit all of that.

*vertically put equatorial radius of the Earth which is watched from the middle of the Sun in the medium distance Earth-Sun is shown from this viewpoint

The Sun is a star which is both situated in the middle of our system and is its biggest body (it concentrates 99,78% of the all substance in itself). Although it’s nearly the only leading source of the energy for us, it is a not too important star spectrum type G2V (yellow dwarf). It occurs at a distance of 10 kpc from the centre of the Galaxy in the space between two spiral branches with a low density of stars. One revolution around the centre of the Galaxy takes the Sun about 2 million years, moreover it make its own motion at a speed of 19,4 km.s^-1 in the direction of the constellation Hercules. The star itself rotates on its own axis and its period is 25,36 days. In view of the fact that it is a gaseous body, the speed of the rotation on the equator is higher than on the poles. In addition this speed depends on the in-depth position of an observed phenomenon (the deeper strata rotate quicker). The relative luminosity of the Sun on the terrestrial sky is -26,8 magnitude, the absolute luminosity is only 4,77 magnitude. This high apparent luminosity is caused by the small distance Earth-Sun called astronomical unit (1,496 . 10¹¹ m). The Sun is 328 900 times heavier than the Earth; its diameter is 109 times longer, so the density is 4 times smaller. It comes closer to the shape of a sphere lattened on the poles, which is caused by the steep temperature gradient in the polar area. The dimensions are constantly (in the dependence of the actions periodically repeated in the deep strata) changing. Chemically the Sun is consists of hydrogen which forms nearly 71,6% of the total material; helium (27%) and other elements (1,4%)- for the most part oxygen and carbon. There lacks only 5 earthly-known elements. They left no marks in the Sun spectrum because they probably occur in only very little number.

The solar atmosphere is stratificated, too. The photosphere is only 300 km wide stratum, it’s a visible surface of the star. The temperature is in the region of 6000 K and the density varies between 10^-4 and 10^-6 kg m-3. There are observable sunspots, faculae and granulation. Over the photosphere there is 14 000 km wide chromosphere. It is well observable in the H-alpha spectral line, it’s considerably reddish and has a typical filamenty structure called a chromosphere net. The temperature of the chromosphere grows with the height and it occasionally comes to 50 000K. The largest component of the sun atmosphere is called corona. It is a cloud of very rare gas and dust whose luminosity is about a million times poorer than the luminosity of the photosphere, so it’s observable no more than during a total solar eclipse. The corona gradually blends into an interplanetary gas and its density goes down. The temperature can come up to 7 million K, there is a temperature inversion, too. Electrically charged particles (electrons, protons and charged helium) escape from the corona into a interplanetary area, they’re known as a sun wind.

On the Sun there’s possible to observe lots of different formations. The sunspots are probably the most interesting of them all. They are dark places in the photosphere with a temperature lower than the ambient. Some of them are sizes 150 000km and it is possible to observe them with the naked eye. From the first stage- pore- is usually formed a spot with two parts: umbra (a dark core) and penumbra (a lighter edge). There is so-called Wilson’s phenomenon during a seeming movement of the sunspot (an influence of the sun rotation) in the direction of the rim of the disc. The sunspot seems to be a hollow where umbra forms a bottom and penumbra forms sloping sides. The sunspots move from 35°-30° to 15°-10° both north and south latitude during an eleven years cycle, apart from this belt there they occurs almost never. The other known objects in the solar atmosphere are faculae and floccules. They’re irregular formations around sunspots. Faculae are in photosphere, floccules are in chromosphere. In addition we can meet on the Sun with protuberance (a cloud of heavy and cold plasma in corona observable above the rim of the solar disc) and its projections on the disc- filamentary. A granulation is graining in photosphere and corona caused by a stream of hot gas risen to the surface. A sudden throwing of energy, an explosion in a space of strong magnetic field is called eruption. It causes a rise of a stream of gamma, roentgen, ultraviolet and visible radiation and an emission of a very big number of charged particles.

As far back as thousands years ago the people knew that we live just thanks to the Sun. The ancient Egyptians worshipped it as a god Re, Mesopotamians as Shamash, Greeks as Apollo, Indians as Surya. The Sun hasn’t lost its importance during the all human existence. It represents for us meal, heat, light,... It represents LIFE.

Used bibliography:
Odhalená tajemství Slunce (R.Kippenhahn)
Přehled astronomie (O.Hlad- J.Pavlousek)
vt-2004.fyzika.net
astro.troja.mff.cuni.cz/broz/slunce/

Author: Lenka Jakubčíková