Beauties of nature

tete-cheval-300x300It’s curious that modern man is probably less familiar with the night sky than were our ancestors 
100 000 years ago. Life in the cities tends to change our mode of existence which is now hardly away from our natural environment.

Yet everybody who looks up at the sky on a clear and dark night may wonder about the beauty of the nature far away from earth.

The reason being of stars is an unsolved problem but we can say something about where the stars come from and where they go.

Stellar genesis

Stars are forming within the clouds of gas and dust. These clouds are compounds of very tiny particles that absorb light like a fog.

A typical cloud is about 100 light years across, and the total mass of such a cloud can be as much as a million times the mass of the sun.

Sometimes, the obscuring particles of clouds are highly concentrated into what is called dark nebulae of which the horse-head nebulae is a striking example.

The gravitational attraction tends to collapse the nebular particles together. Thus with  time, the concentration of the particles in the nebulae rises. Whenever a gas becomes compressed and denser, its temperature tends to rise. The nebulae cut up himself in the region of high density and temperature. Gas near the center of the region of high density becomes hotter than gas near the surface. The difference of temperature causes a heat flow from the central region to the surface. The extra energy thus arriving at the surface is irradiated away into space: the proto-star has begun to shine with an orange-red color.

Convective-transportConvective transport of heat in a proto-star (red-coloration)

The process goes on until the central regions are hot enough so that nuclear processes which convert atoms of hydrogen into atoms of helium become important. Eventually the radiation process can be supplied by the nuclear process. At this stage, we can say that a star is born.

For example, the sun shines by converting hydrogen to helium. Deep in the interior of the sun, 600 million tons of hydrogen are converted to helium every second. The sun is so massive that it can survive about ten billion years before it exhausts its fuel supply.

Star formation is a continuous process. On average, one new bright star must be formed somewhere in our galaxy every 500 to 1000 years.

The stars form by accretion: clouds of gas and dust collapse under their own weight, causing an agglomeration of matter. But this process must stop when the star reaches eight solar masses according to the models. Beyond this limit, the star emits extremely intense ultraviolet radiation and prevents it from continuing to accumulate material.

However, massive stars do exist. Their mass can grow up to 120 times the Sun. This is still an enigma.

Different kinds of Stars

Within a region of high density of the nebulae, different quantities of gas conducts to stars with different masses. Large mass stars are brighter than those of small mass. At the time of their formation large mass stars are violet-blue and small mass stars are dark-red. Large mass stars die quickly because they quickly use up their supply of hydrogen atoms. It follows that large mass stars are only seen among groups that are formed recently. Old groups of stars have a red color since stars still shinning within them are stars of small mass with red color. On the other hand, young groups of stars are blue because they still contain stars of large mass that are blue.

The most massive stars have masses that are 50 to 100 times the mass of the sun, yet their luminosities are a million times greater than that of the sun.

Stars spectra

Stars are suns and like our Sun they are sending out energy all the time. Naturally their reserves are not inexhaustible, and so every star must have a definite life-cycle from birth to death. Like all hot object, stars emit a great number of electromagnetic waves. A spectroscope can separate and analyse those waves.

In the spectrum of stars, there are several black rays. Those rays correspond to a wave produced in the center region of the star and which has been absorbed by the gas in the outer part of the star.

black-rayBlack rays in the star’s spectrum

The kidnappers of the rays are hydrogen, helium, iron, oxygen, etc. From star spectrum it is possible to deduce the chemical composition of the gas. For example, the analysis of the spectrum of the Sun revels the presence of over 60 different chemical elements.

The star spectrum is an important source of information.

Evolution of stars

What happens when stars that use up all their supplies of hydrogen ? The hydrogen becomes exhausted first in the central regions of the stars and those regions begin to shrink. The temperature rises further in those regions. This brings other reactions into operation.

Temperature(°C) Elements
10 – 50 million He4
200 – 250 million C12, O16, N14, Ne20
1 billion Si28, Ca40
4 billion V51, Cr32, Fe56

Elements produced by nucleus reactions at different temperatures

The first reaction converts helium into carbon and oxygen and at still higher temperature other reactions start and generate other elements : sodium, magnesium, aluminium, sulfur, etc. The outer part of the star expands and the star becomes enormous. The star is at this stage a « Giant ». The color of the light radiated into space becomes red. At higher temperature irons, nickel, chromium and cobalt appear.

sun

H-R diagram

The properties of stars like his brightness, temperature, and weight are resumed in H-R diagram. At the beginning of their life and for most of their life, stars are members of the main sequence. Their further evolution depends on their mass.

Evolution of old stars

The processes that produce irons, nickel, chromium and cobalt give a limited quantity of energy to the star. The nucleus mechanism of production of different elements is not efficient when stars produce elements that have atomic weight of more than 60 because the cohesion force in the nucleus reaches a maximum for this atomic weight.

What the star can do then? Cool off and become a dead star? A star can « die » in this way if the mass of the star is about or less than the mass of the sun. A more massive star must lose his excess of material in order to be able to cool off.

White dwarf

Stars of moderate mass throw out their excess of material into space. In the center, the residual material becomes cool with a very high density. A cube with the size of a dice of this material is about a ton. This remarkable kind of star is known as white dwarf.

HR

Neutron star

For stars with a larger mass reaction is more violent. Those stars become unstable and explode like a nuclear bomb. This explosion is usually known as a supernova. The supernova is temporary as bright as the whole of the Milky Way (our galaxy that contains 100,000 million stars). This violent explosion also has a residue. A cube of the size of a dice is now about 100 million tons in this magnitude of density, electrons in the material collapse with the protons and produce neutrons. They become neutron stars.

Black hole

For the star over ten times as massive as the sun, there is a very rapid and dramatic collapse that conducts to what is called the black hole. In this case, the core of the star just fell down. The residue is so dense that even light can escape from it. Thus, it would be virtually undetectable. But in certain situations like after the explosion or when matter is polling into it, it is possible to detect its presence.

Two black holes can collapse if they are near. This event generates gravitational waves that have been recently detected by scientists.

Weigth residual Weigth for 1 cm3
about Mo white dwarf 1 ton
less than 10 Mo neutron star 100 million tons
more than 10 Mo black hole ?

The residue after « explosion » of the star ; Mo represents the sun’s weigth

All the material that we find on earth, in particular the carbon and oxygen (so important for the life on earth), is produced by stars. At the end of the life of starts, those materials are thrown into space.

In a way, we are dusts of stars.

condensation-1024x230

 

 

 

 

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