The Universe
Constitution of the Universe
Stars are not scattered randomly in the Universe; they are rather grouped into sets called galaxies. The Universe is essentially constituted of some billions of galaxies, which tend to be grouped in cluster of galaxies each formed sometimes of many billions of galaxies.
These clusters can themselves be organized in much greater clusters called super-clusters. In addition, the Universe contains also a certain number of strange stars called red giant, white dwarfs, neutron stars, black holes, quasars (galaxies very much brighter than ordinary galaxies) and exploding stars called novae and supernovae.
Dimensions of the Universe
The known or observable Universe is distributed over a space greater than 1023, km, i.e. about 10 billions of light-years (l.y.) (a light year is the distance covered, in vacuum, by light during one year: 1 l.y. =9.46 ´ 1012 km). The galaxies closest to us are at 2 ´ 106 l.y., while the farthest galaxies visible with optical telescope are at a distance of 3 ´ 106 light-years. Recent studies lead to the conclusion that clusters of galaxies are not distributed randomly, but over very wide regions of vacuum of a diameter of about 108 light-years.
If the expansion of the Universe started about 18 ´ 109 years, then the distance so covered by light is limited to 18 ´ 109 light-years.
The observable Universe is restricted around us as a halo which is a large sphere containing the oldest cold stars and has a radius of about 18 ´ 109 light-years. We cannot see stars beyond this radius, because their light has not had enough time to reach us.
The entire Universe is then infinite and unlimited because if it were finite or limited some questions are then raised:
-what would exist beyond these limits?
-why do not the regions beyond these limits belong to the observable Universe?
Our Galaxy the Milky Way
(click here to view a picture of milky way)
Observe the sky, during a clear night without moon. You see millions of stars of different degrees of brightness. We observe also a white cloud-like wisp, which constitute our Galaxy (this is a Greek word meaning Milky Way) formed of 100 billion stars distributed over a highly flattened disk of 105
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light years diameter. It has a form of spiral arms, which is the most frequent from of galaxy in the Universe.
Among the stars of the Milky Way, there is one star, dear to us, the Sun, which are 28000 light-years from the center of the Milky Way. The period of revolution of the Sun around this center is about 200 ´ 106 years, and its speed is of the order of 2200km/s.
At the center of the Milky Way, we find the central bulge, which is difficult to observe because of the clusters of stars, which are very dense and appear as clouds formed effectively of fluorescent gases called nebulae (which mean smog).
Finally there is the halo, which is a spherical region that surrounds all parts of the galaxy.
The mass of all the stars in the Milky Way is about is about 3 ´ 1041 kg.
Expansion of the Universe
By observing the sky, night after night, the stars appear motionless. On the scale of human time, the most stars undergo very small change. But on the scale of star time (millions of years), the stars are in motion with respect to each other, and their speeds are measured in km/s. due to their distance from the Earth, the motion of stars is not easily detected.
Two parameters, useful to determine the position of star or a galaxy, are its luminosity expressed in (W) and the variation of its brightness characterized by the spectral lines emitted.
Since 1914, Vesto Slipher has discovered that these lines undergo to Doppler Effect (change in the frequency of the wave, when emitted by a moving source), a red shift for fifteen galaxies. In 1925, he has obtained an experimental evidence of 45 galaxies.
These results have been interrupted by the American astronomer Edwin Hubble (1889-1953) and his student Humason as an Escape of the galaxy. The galaxies spread away like dots painted on a balloon which is being inflated. As the balloon which is inflated, an observer on each spot would see all the other spots moving away from him.
If the red shift is due to an escape of the galaxy, the latter possesses then an escape velocity.
By comparing the escape velocities of different galaxies, Hubble has obtained the most important scientific ideas of the twentieth century, which have changed the dominant hypothesis that restricts the Universe to our Galaxy, the Milky Way. According to this idea, the more remote the galaxy is the higher is its escape velocity. There is then a proportionality relation between the speed V of the galaxy and its distance to our Galaxy: V=H.d
This relation is known as Hubble’s law, and the constant H is called Hubble’s constant.
The value of H is not known precisely. Astronomers suggest two different values 50 and 80 km/s/Mpc, even though the first value (50 km/s/Mpc) is more accepted by most of them.
The term ‘Mpc’ means mega parsec; the parsec is the distance between the Earth and a star from which the radius of the orbit of Earth around the Sun is seen under an angle of one second. One parsec is equivalent to 3.26 light years or 3.084 ´ 1013 km.
1Mpc=3.26 ´ 106 light-years.
Hubble’s law is not valid for galaxies at small distances from our Galaxy, or those moving towards us (blue shift).
Birth, Life, and Death of Stars
On the scale of human time, most of stars undergo a very little change.
In the middle of the nineteenth century, a French astronomer wrote: “the stars are suns, which can differ from one another by their chemical constitution, but they all represent the same mode of existence. They are born, develop, and then die…”
The birth of stars
The stars are born from clouds of interstellar gasses, which contact under the effect of
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gravitational interaction. This contraction, called gravitational implosion, is accompanied by an elevation of temperature and liberation of energy over many years. The heated gas starts to radiate and a protostar is born.
The contraction goes on and the temperature continues to rise. When the latter reaches tens of millions of Kelvin, thermonuclear reactions can take place. Hydrogen, which represents the essential constitution of the heart of the star, starts to undergo nuclear fusion. Helium, carbon, nitrogen and oxygen are formed and if the mass of the star is sufficient, silicon and iron are also formed. The star is born.
Two essential types of stars are distinguished; the less massive stars with masses between 0.07 and two times the mass of the Sun, and the massive stars with masses greater than twice the mass of the Sun.
Life and death of stars
The life and the death of a star are due entirely to its mass. The larger the mass the more rapidly the star evolves. If the life of the Sun last 10 billion years, then that of a star 10 times greater in mass is 1000 times less.
A star is a natural thermonuclear power station. The fusion of hydrogen into helium, at its center, produces an energy that gives the star its brilliance.
The less massive stars burn their reserve of hydrogen slowly at their centers in a time between 1 and 20 billions of years. When all central the hydrogen has been converted into helium the star attains its greatest size, it implodes. This implosion provokes a rise in the temperature to 108 K sufficient to start the fusion of the helium nuclei.
Under the effect of radiation, the star blows up slowly to become a red giant (the sun will be a red giant in 5 billion years). After this stage, the star expulses in space a part of its envelope to be transformed into planetary nebulae around the inert heart called white dwarf. Its density is about 2700 kg.m-3.
Given that the heart of the white dwarf is very cold to maintain a thermonuclear reaction, the star does not stop its cooling and transforms into a black dwarf.
But if the white dwarf meets a second star, the latter is captured by the white dwarf. A thermonuclear reaction takes place transforming the white dwarf into a nova (new star).
The massive stars evolve differently. The heart of the star, crushed under the action of its weight, atoms attains a very high temperature, about 6 ´ 108 K. the reserve of hydrogen is rapidly spent, after about one million years. After the fusion of helium, the nuclear reactions continue and form many heavy nuclei: neon, magnesium, sulfur, silicon till iron, which is the stable element characterizing the ultimate stage of the heart of the star. Then the star is transformed into red supergiant.
After the total combustion, the star collapses. In a few seconds, its heart implodes to condense in a ball of nuclear matter. Reaching the heart of the star, the external layers expelled due the high pressing forces; the star becomes a supernova. The explosion of the supernova characterizes the end of the evolution of the massive stars. The atoms are destroyed in the heart of the star; the electrons and the protons are combined to form neutrons, and the star contracts rapidly to form a neutron star. Its radius is about 15 km and its density is enormous.
During the explosion of the supernova, the brilliance of the star increases suddenly many billions of times in few days, to decrease gradually during the coming months. Such supernova has been observed, in 1987, in the appendix of our galaxy called great cloud of Magellan at a distance of 170000 light-years. It was visible to the naked eye in the southern hemisphere. If the mass of a star is greater than 4 times that of the Sun, the gravity is so big that even the light emitted by the star cannot leave it. We are unable to see the star anymore; it is black. A body passing close to it deviates under the action of its very high gravitational field. But if the body comes very close to it, it will be attracted and swallowed by the star without ever having any possibility to leave it again. This star is called black hole.