Origin of the water molecule

The creation of the Molécule of Eau concerns a complex process, which proceeded as of the birth of the Univers and continues to proceed today. The plan of this article will be chronological; we will thus study initially the formation of the atoms of Hydrogène $\ mathrm {H}$ and of Oxygène $\ mathrm {O}$ , then the way in which these two element S could be assembled to form the water molecule $\ mathrm {H} _2 \ mathrm {O}$ .

The creation of the hydrogen atom

The Hydrogène is the simplest Atome which is: it is made up only of a Proton associated with a electron. It is also the atom most present in the universe, because it accounts for approximately 92% of its atoms and 70% of its mass.

The hydrogen atoms were created with the universe

According to the model of the Big Bang, the universe was created there is fifteen billion years; it does not cease since increasing and thus cooling.

At the first moments, the conditions reigning in the universe prevent the matter formation;
After $10^ {- 5}$ S, the temperature of the universe becomes sufficiently low (lower than $10^ {13}$ K) to allow the formation of protons and neutrons, electrons and photons. The protons, lighter, are much more numerous than the neutrons. These particles are attracted but remain remote because the temperature is still too strong.
After one minute: as the universe continues to dilate, its temperature drops. It reaches $10^ {10}$ K, so that protons and neutrons end up approaching to form cores. However, the protons being much more numerous, they remain in majority alone and form hydrogen cores ${} ^1_1 \ mathrm {H}$ . The others become cores of Deutérium ${} ^2_1 \ mathrm {H}$ (or deuteron), of helium 3 ${} ^3_2 \ mathrm {He}$ (two Isotopes of hydrogen and helium: they have the same number of protons but a number different of neutrons) and especially from Hélium ${} ^4_2 \ mathrm {He}$ , according to whether they are assembled with one or more neutrons. This reaction, called paramount Nucleosynthesis, stops after two or three minutes. Let us note that the formed cores are still not dependant with the electrons.
After 300.000 years: the temperature of the universe reached $3000$ K. It is sufficiently low to make it possible the cores to bind with the electron S. They then form Atome S, which have as many electrons as of Proton S. the Deutérium being unstable, it tends to be transformed into Tritium or Hélium 3 by gaining a Neutron or a proton. But the latter are too fragile and transform into helium 4.

Universe today

The universe kept about the same composition until today. It is made up to 99% of such elements, called light elements because of their low mass. One thus observes that it is composed to 92% of hydrogen and 7% of helium, the remaining share corresponding especially to the heavier elements which were created afterwards.

The creation of the oxygen atom

The Oxygène concerns a process much more complex: it is an atom made up of eight protons and of eight neutrons, it is sixteen times heavier than hydrogen. It belongs to the groups of the heavy elements (in opposition to the light elements ) and was not formed just after the birth of the universe, but after the birth of the first star S.

The creation of first stars

There exist several assumptions concerning the creation of stars. Will be presented here most commonly allowed.

The first stars of the universe were formed there is fourteen billion years. The universe is then bathed by a vast helium and hydrogen gas cloud, only made up. This cloud then split up in several masses which crumbled on themselves under the effect of the Gravitation. The Temperature and the Pression then increased out of arrow, until it occurs there the reactions of fusion which govern the birth of stars.

Characteristics of these stars

These first stars had the effect of being very massive: their Masse could reach several tens of solar masses.

Like all stars, they consumed their hydrogen by converting it into helium by reactions of fusion. Their size being very important, they consumed it relatively quickly (in about fifteen million years), just like the large current stars.

End-of-life of these stars

The end-of-life of these stars is comparable with that of average stars (like the Sun) or grosses of today.

Once consumed hydrogen, the sizes of these stars increases enormously. They become what one calls of the “giant reds”. at this stage of the life of a star, helium is consumed around the central part who contracted after the exhaustion of hydrogen, but its external envelope distends enormously: the star swells and can reach from ten to ten thousand times its original dimension.

Thus, the temperature and the Densité which reign in the center of star increase considerably.

The creation of the oxygen atom in the middle of stars

This increase allows that it occurs there reactions of fusion between the cores of helium present within the core, also called particles alpha ( $\ alpha$ ). These reactions require an higher temperature with $10^9$ K. the first element thus which can be produced in abundance is the Carbone 12 (six Proton S and six Neutron S). This carbon comes from the reaction of Fusion triples alpha which proceeds in two times:

an initial reaction of fusion: ${} ^4_2 \ mathrm {He} + {} ^4_2 \ mathrm {He} \ longrightarrow {} ^8_4 \ mathrm {Be}$ .
the Béryllium 8 product is very unstable (it disintegrates in $10^ {- 16}$ S); however, it happens that he enters in collision with another core of helium 4 before disintegrating; it is held then another reaction of fusion, modelled by the equation: ${} ^8_4 \ mathrm {Be} + {} ^4_2 \ mathrm {He} \ longrightarrow {} ^ {12} _ {\ 6} \ mathrm {C}$ .

By still adding a Particule alpha to the carbon core (what requires temperatures even stronger), one obtains the oxygen core (eight protons and eight neutrons). The reaction is written:

{} ^ {12} _ {\ 6} \ mathrm {C} + {} ^4_2 \ mathrm {He} \ longrightarrow {} ^ {16} _ {\ 8} \ mathrm {O}

The dispersion of oxygen within the universe

A star of this size finishes its life by an extremely violent explosion, the supernova .

It is during this explosion that are dispersed in space the elements created in the middle of star, like its residual hydrogen and its helium.

The oxygen O atom is thus created following reactions of fusion in the middle of the star S. Its formation started with first stars and continues nowadays.

The creation of the water molecule

Conditions of assembly of hydrogen and oxygen

Several very delicate conditions must be filled to allow the formation of the Molécule of Eau:

Of the conditions of temperature: the water molecule can be formed only in one variation in precise temperature. It cannot appear if the temperature is higher than a few thousands of degrees, because the extreme agitation of the atoms which reign then prevents the formation of molecules (bonds between the atoms do not resist). Thus, the water molecule cannot appear directly in the middle of the star S, which reaches several million degrees. In the same way, the medium should not be too cold because the scarcity of the atoms and their movements makes them difficult to associate. The temperature is thus a very important factor in the formation of water;
protection against the Radiation S , in particular the Ultraviolet radiation. Indeed, this very powerful radiation (produced by stars) bathes the universe and destroys the chemical bonds between the atoms: it thus dissociates the molecules. This radiation is very frequent in the universe. Water must thus appear in a particular environment, protected well from the ultraviolet radiation;
the presence of a chemical Environment precise containing hydrogen and especially of oxygen, rarer.

Cosmic stars, taps

The stars were formed all in the same way as first stars of the Univers, i.e. by collapse on itself, under the effect of the Gravitation, of a Nuage of gas primarily made up of Hydrogène and Hélium. Only difference, these clouds included/understood already heavy elements as the Oxygène (these elements heavy have a proportion of some thousandth; they will join young star or will gather by Accrétion in Planet S).

Let us recall that the stars of average mass to door finish their life in Géante red. Contrary to the core, the temperatures reigning on the surface of these red giants are relatively low (lower than 3000 K), which allows the formation of Molécule S, of which the water molecule $\ mathrm {H} _2 \ mathrm {O}$ , starting from the oxygen and hydrogen atoms contained in star.

The Molécule S created are ejected out of star by two processes:

the “ stellar Wind ”, a progressive ejection of mass towards the interstellar Environment. This wind is made possible by the weakness of the gravitational attraction and by the importance of the Flux of radiation coming from the remote center. The particle S ejected are called “dust of star”;
the Supernova E , already evoked.

Interstellar clouds

These supernovas, enormous explosions, give rise to interstellar clouds. They are cold and very dense condensations, they thus consist of solid particles, the interstellar “grains”. Their density can reach several thousands of particles by $\ mathrm {cm} ^3$ , and their temperature is worth approximately 20 K (- 253 °C).

These interstellar clouds of course comprise water molecules in the form of ice. One also detected there Silicate S, Graphite and perhaps of the Fullerène S.

Star dust present in the interstellar clouds acts like core of condensation, while being surrounded of coats of Glace.

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