Lecture 1.
Universe
The Universe is infinite. The material bodies that constitute the Universe are located in infinite space and the materiality of this space is not discussed. It is very important here to note that our imaginations about the Universe and its structure are based on the Metagalaxy, the part of Universe accessible for our investigation. The modern estimations of the age of Metagalaxy give about 15-18 milliard years.
To understand the structure of our Universe we need to consider some characteristics of the Universe, or more exactly only Metagalaxy, i.e. the part of Universe, available for surveying.
On the present stage of the evolution of Universe its substance is concentrated mainly in stars, however stars fill up only about 10-25 of all volume of Universe. The problem of forming the stars from the condensing interstar matter is not determined till now. The unceasing process of transformation of hydrogen to heavier elements, mainly helium, can indicate to the irreversible character of the evolution of Universe.
For astronomers of XVIII-XIX centuries the interstar medium was absolute vacuum. But at the beginning of XX century the German astronomer Hartmann proved that interstar medium is not vacuum, it is filled up with very rarefied gas. The distribution of interstar gas is very irregular. There are both extents with high density (ten times more than average density), named clouds, and extents with very rarefied gas. About the plane of our Galaxy average density is about (5-8)10-25 g/cm3 and it decreases immediately to the direction of the periphery of Galaxy. In spite of low density the interstar gas cannot be considered as vacuum. It is estimated as a compressing medium, continuum, to which the laws of gas dynamics are applied. This interstar gas can transfer the waves, is susceptible to complex turbulent movements and so on.
The hydrogen and helium are the leading chemical elements of the interstar gas. Besides gas the interstar medium contains so-called interstar dust. Its distribution in interstar space is extremely uneven. The dust consists of the microscopic particles less than 1 μm. These are represented with graphite, silicates, minute fragments of dirty ice and others, usually having the elongated form and approximately being orientated in one direction under the influence of the faint magnetic field. The temperature of the interstar medium is very low, about several degree of Kelvin.
In 1929, the American astronomer Edwin Hubble discovered that spectral lines nearly all galaxies, he observed, (except those situated most closely to our planet) are shifted to the red part of spectrum. He explained this situation with phenomenon of Doppler effect.
The gist of Doppler effect. If any object sends the light with the velocity C and wave length λ and object itself is immovable or moves with velocity V<<C, then observed wave length is equal to real wave length, i.e. λ. But if the object and observer move in opposite sides with relative velocity V, high enough, then observer measures the wave length with increase of Δλ, that shifts all light spectrum of an object to long wave, red section.
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On the base of calculations and considering Doppler effect Hubble received the result V = Hr staggering our imagination.
The velocity V of Galaxies moving away is proportional to their remoteness r or the further galaxy is situated the faster it withdraws.
Having accepted the Hubble law one can conclude that the reason for the scatter of the galaxies may be The Great Explosion known as The Big Bang. It gave the beginning to the modern Universe originated from initially small enough extent of substance. Hubble coefficient is a constant characterizing the velocity of the enlargement of the world space. This enlargement is isotropic, hence the Hubble law is correct for observations from any point of Universe.
On the base of Hubble constant it is possible to estimate the age of Universe.
t = 3.081019 /55 = 5.61017 sec = 17109 years (17 milliard years)
Later the additional acknowledgements of The Big Bang theory were received.
For example, You know that if something (like gas) increases, then obligatory its temperature decreases.
In accordance with this law the enlargement of Universe must be accompanied by the decline of the temperature. According to the estimations already in two hours after the explosion the temperature of Universe should be 108 K, in 100 years less than 106 K and 17 milliard years later, i.e. to-day, about 3K.
In conclusion one can repeat that all modern experiments and observations acknowledge that our Universe is not stationary system. It originated about 17-18109 years ago, supposedly after the great explosion. And up to date the process of its enlargement continues.
The stars and galaxies.
Stars. Contrary to the planets the stars are seen on the night sky as bright shining motionless points. But now due-to the modern achievements of science we know that it is only illusion. The stars as other sky bodies also move in the cosmic space, but owing to their colossal remoteness we perceive them motionless. The man looked at the sky with aided eye only in 1609 when Galileo Galilei created his first telescope. But much earlier looking at the sky the man could distinguish the stars from planets. He saw planets to be moveable among the fixed stars (The word planet from Greek is wanderer). Also the planets are more bright due-to their reflecting the sun's light.
The star appears as blob of the substance of Univerce shrinking under the action of the gravitational forces. Intensive compression causes the warming-up of the substance. At the result the density and temperature reach the values giving rise to the thermonuclear reaction, that raise the pressure and temperature more and more. The primary blob became the gaseous sphere (being in equilibrium state with such parameters as temperature and pressure) that is named as the star. The internal energy of the star originated mainly at the result of the reaction of the transformation of nuclei of hydrogen to nuclei of helium. As the quantity of hydrogen decreases the central zone of the star begin to shrink, that leads to the new increase of temperature. The internal structure of the star is essentially reconstructed. The star swells, however its nucleus continues to shrink. The external layers may come off forming the gaseous nebulae.
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But at the moment when the nuclear fuel comes to an end, the star begin to cool down. The internal pressure falls and its nucleus shrinks promptly enough.
Galaxies. These are the gigantic star-clusters. Their existence was known already in the second half of 18th century after the investigation of the English astronomer William Herschel, who first determined on the firmament the large circle with great number of the stars. It was our Galaxy. The assumption of William Herschel was acknowledged only after other cosmic objects, being outside our Galaxy, were found.
Our Galaxy is not a single star system in the Universe. There are many star diverse systems in the Universe that simultaneously have much resemblance.
The structure of our Galaxy.
Our own Galaxy the Milky Way is relatively well known. It is a typical star system with mass equal to 1012 of mass of the Sun. It is in the state of quasi-dynamic equilibrium. Its evolutionary stability is maintained by the intermingling of the stars during their movement in the general gravitational field of the system.
The Galaxy is like a strongly pressed disk with equatorial plane of symmetry and axis of symmetry normal to this plane and located in its centre. The Galaxy hasnt the clear-cut border that to a certain degree is conditional and its contour is accepted according to the gist and the task of our exploration.
The maximum star density (the number of stars per a cubic parsec) falls to the central parts of Galaxy, where star density is equal to several units per parsec. If we determine the border of Galaxy by the density 110-3 star per cubic parsec, then the diameter of Galaxy will be about 30 kiloparsec and thickness about 2.5 kiloparsec. The Sun is situated nearly exactly on the plane of symmetry and at the distance about 10 kiloparsec or 2/3 radius from the centre of Galaxy. The number of stars in our Galaxy is estimated as about 1011 (or 100 milliard different suns).
The Galaxy has an intricate interior structure. There are different collective objects there, such as double stars, star-clusters with ten to 2000 stars.
The Solar system
The solar system is a Sun and all objects that are in its gravitational field. These are 8 planets and their satellites, numerous comets, meteoroids and asteroids. All the planets rotate around the Sun in one direction with orbits close to be circular. The planes of these orbits practically coincide (to an accuracy of several degrees). The numerous accurate measurements permitted to determine that the orbits of planets are not circles but ellipses. The table displays the main information on Solar system.
