During tens of thousands of years, man has observed the sun that heats and lights the earth: he looked at it and admired it; he loved it and worshiped it. He understood that every form of life as well as heat and energy depends on the sun and without it; there would be no humans, no plants, and no animals.

Then, pushed by curiosity and by his scientific mind, he began to wonder what exactly was this mysterious burning sphere, of what it was made, how it functioned.

It is like this, an object of worship; the sun became an object of research.

The sun in the universe:

    The sun was born 4 or 5 billion years ago. It is rather young compared to the universe whose age must be between 10 and 20 billions years.

The sun is a star, one of 100 billion populating our galaxy. At the universe scale, it is of absolutely no importance; astronomers classify it as a yellow dwarf star. At the scale of the solar system, on the other hand, it is a celestial body of paramount importance.

The majority of   stars belonging to our galaxy « The Milky Way » are concentrated in an area shaped like a disk, delimited by « arms » which are divided into spirals issued from a core. The sun occupies a peripheral position at approximately 28000 light years from the center of the galaxy, around   which it turns at a speed of 225km/s. It achieves a full revolution in about 300 millions years.

The current observations make it possible to suppose that the sun is alone and does not belong to a binary system. It is the central star of our planetary system, around which turns, to our current knowledge, 9 planets.


Life of the sun:

4 or 5 billion of years ago, a cloud of gas and dust condensed under the action of its own weight gave birth to the sun. Since then, the sun radiates. It is envisaged that it will live for approximately another 5 billion years. Then, it will end up undergoing an expansion phase and will become a red giant (it’s temperature on the surface will decrease by 6000K to 3000K), which, then, will condense to form a white dwarf (very hot).

Sun Characteristics

    Physical composition

    The diameter of the Sun is 1 392 400 km, it is approximately 110 times larger than that of the earth.     


  The mass of the sun is 333 000 times bigger than the earth, it is 2x10 30 kg. It contains more than 99.8% of the total mass of solar system ( Jupiter contains almost all of the remainder).

The density of the Sun is 1410 kg/m 3 , compared to the earth which that is 5520 kg/m 3.   The Sun is less dense than the earth, because the sun is made of gas, whereas the earth is made of solid matter. Surface temperature of the sun is about 6000K whereas its core temperature is around 15 000 000 K.

The Sun is a huge mass of burning gas. Gases that compose the sun are more concentrated in its center. The further from the center, the lesser the gas concentration is. These gases are in perpetual movement and form a bubbling and granularly surface

Beyond the core of the sun, which is the source of its energy, there is the photosphere which when we observe it, appears to be a perfect sphere. It is from this zone (5000km thick), that our solar energy arrives in the form of luminous and calorific rays and other invisible radiations (ultraviolet rays, infra-reds, X, gamma, Hertzian waves). Observations have shown that above the photosphere, there is another layer of 5000 km in width, the chromospheres.   It is kind of an ignited meadow where the blades of grass called specula, are incandescent jets of gas several thousand kilometers in height, in perpetual motion.

Still above, the corona extends on millions of kilometers. It is a brilliant aureole which surrounds our star and which can be observed only at the time of the total solar eclipses (photosphere prevents from seeing its glare in normal times).


Chemical composition

In the sun there is a lot of hydrogen (78,4 %???) and helium (19,8 %???), which constitutes 98.2 % of the total solar mass. There are also some traces of oxygen, carbon, nitrogen, neon, nickel, silica, sulfur and iron, which makes a total of 1.82%.   The solar chemical composition is estimated by observing solar spectrum absorption rays.

Sun mechanisms and motions:


            In the heart of the sun, the hydrogen nuclei are so compressed that they transform into helium, by thermonuclear fusion reaction, exactly like an H-bomb. Each minute, our star therefore converts into energy 240 million tons of its mass, according to the famous Einstein formulae: E = mc 2. This is how the formidable radiation of the sun is explained.


            The Sun achieves a full revolution around the galaxy in 300 millions years. It rotates on its axes with one period of 25 days for the equator and 30 days for the poles. It is by observing the displacement of the sunspots that the astronomers can determine these periods of rotation.

The photosphere is in perpetual agitation, from the extreme gas swirls that come and go unceasingly. One can see dark spots there corresponding to colder zones on the surface of the Sun and where the magnetic field extremely disturbs the passage of heat from the core towards the photosphere. When these sunspots grow bigger and reach their maximum intensity, one observes eruptions, protuberances and jets of charged particles.

Sun observatioNS:

  Although our star is approximately 150 millions kilometers away from the earth, its rays are still bright enough to damage our eyes. It is impossible to observe it directly through a telescope, thus the image is projected onto a white surface. This projection is studied to gather information about the photosphere (sunspot observations).




At the Kitt Peak observatory located in Arizona (USA) there is a heliostat. A mobile mirror that follows the sun and reflects the image of the sun through a tunnel down to a concave mirror located below. This concave mirror reflects on a pure plane mirror leading to a spectrograph. This spectrograph gives information on chemical composition and temperature of the sun.

    The corona can be observed, apart from total solar eclipses, with the help of an apparatus called: the chronograph. This apparatus invented in 1930 by the French astronomer Bern Lyot (1897 - 1952); artificially reproduced the conditions of an eclipse.

Corona pictures taken with a coronograph.


Since its launching in 1994, the satellite Soho (Solarand Heliospheric Observatory), resulting from a collaboration between ESA and NASA, has provided a lot of information concerning the sun.

Comparison with another star : Deneb


Deneb is a super-giant blue star that is around 1600 light years away from us. It is the most brilliant star in the Cygnus constellation and can always be observed in the summer sky.

Its mass is 25 times bigger than the sun. Its diameter is 60 times longer than the sun and its luminosity is equivalent to 6000 suns. The temperature on its surface is 10000K instead of 6000K for our sun. Fusion reactions occur very quickly, this explains why Deneb will explode in Supernovas by 200 000 years whereas the sun will turn into a red giant in 5 billion of years.


Exercise :

1)       What is the physical state of  the Sun ?

2)       What is called Solar corona?

3)       When and how can it be observed?

4)       Which part of Sun produces our light?



Le soleil Dossier hors série science et Vie Junior, oct 2002

Lippinott, L’astronomie , col : passion des sciences, Ed Gallimard

Tout L’Univers , tome 5,Ed Hachette

James Mitchell , L’Univers , Ed Larousse, 1976

Robin Kerrod, Etoiles et planètes , Ed Solar, 1979

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