Knowledge of the world around is impossible without understanding of age of historical antiquities and how long there is a world – our Universe. Scientists have created a set of ways for determination of age of archeological finds and establishment of dates of historical events. Today on a chronological time scale both dates of eruption of ancient volcanoes, and time of the birth of stars whom we see in the night sky are celebrated. We will also tell today about the main methods of dating.
So far as concerns age of archeological finds, of course, all remember a radio-carbon method. It is, perhaps, the most known, though not only, a method of dating of antiquities. Known including thanks to constant criticism to which he is exposed. So it for a method for what and how it is used?
For a start it is necessary to tell that this method is applied, behind very rare exception, only for dating of objects and materials of a biological origin. That is age of everything that was once live. Moreover, it is about dating of the moment of death of a biological object. For example, the person found under blockages of the dwelling destroyed by an earthquake or the tree which is cut down to construct the ship. In the first case it allows to define approximate time of an earthquake (if it wasn’t known from other sources), in the second – approximate date of construction of the ship. So, for example, dated a volcanic eruption on the island Santorini, one of key events of ancient history, the possible reason of an apocalypse of a bronze age. For the analysis scientists have taken the branch of an olive tree found at excavation of volcanic soil.
Why the moment of death of an organism matters? Compounds of carbon, as we know, make a life basis on our planet. Live organisms receive it first of all from the atmosphere. With death carbon exchange with the atmosphere stops. But carbon on our planet, though occupies one cage of the table of Mendeleyev, however happens different. On Earth three isotopes of carbon, two stable – 12C and 13C and one radioactive, subject to disintegration, – 14C meet. So far the organism is live, a ratio of stable and radioactive isotopes in him the same, as in the atmosphere. As soon as carbon exchange stops, the amount of unstable isotope 14C (radio carbon) due to disintegration begins to decrease and the ratio changes. Approximately in 5700 years the amount of radio carbon decreases twice, this process is called a half-life period.
The method of radio-carbon dating has developed Uillard Libby. Originally he has assumed that the ratio of isotopes of carbon in the atmosphere in time and space doesn’t change, and the ratio of isotopes in live organisms corresponds to a ratio in the atmosphere. If so, then having measured this ratio in the available archaeological sample, we can define when it corresponded to atmospheric. Or to receive so-called “infinite age” if there is no radio carbon in a sample.
The method doesn’t allow to glance far in the past. Its theoretical depth – 70 000 years (13 half-life periods). Approximately during this time unstable carbon completely will break up. But a practical limit – 50 000–60 000 years. It is impossible more, equipment accuracy doesn’t allow. It is possible to measure age of “The ice person” by him, and to glance here in planet history before appearance of the person and it is impossible to define, for example, age of remains of dinosaurs already. Besides, a radio-carbon method – one of the most criticized. Disputes around the Turin shroud and analysis of a technique of establishment of age of a relic only one of illustrations of imperfection of this method. What is only costed by an argument about pollution of samples carbon isotope after the termination of carbon exchange with the atmosphere. Not always there is a confidence that the subject taken for the analysis is completely cleared of the carbon introduced later, for example by the bacteria and microorganisms which have lodged on a subject.
It is worth noticing that after the beginning of application of a method it has become clear that the ratio of isotopes in the atmosphere changed over time. Therefore scientists needed to create a so-called calibration scale on which change of content of radio carbon in the atmosphere is noted by years. Objects which dating is known have been for this purpose taken. Scientists were come to the rescue by a dendrochronology – the science based on a research of annual growth rings of wood.
In the beginning we have mentioned that there are exceptional cases when this method extends to objects of not biological origin. A characteristic example – ancient constructions in which construction solution not extinguished CaO lime was applied. At connection with water and the carbon dioxide which is contained in the atmosphere, lime turned into a CaCO3 calcium carbonate. Carbon exchange with the atmosphere in this case stopped from the moment of hardening of construction solution. In such a way it is possible to define age of many ancient constructions.
Remains of dinosaurs and ancient plants
Now we will talk about dinosaurs. It is known that an era of dinosaurs was rather small (of course, by the standards of geological history of Earth) an interval of time which has lasted 186 million years. The Mesozoic Era, so it is designated on a geochronological scale of our planet, has begun about 252 million years ago and has ended 66 million years ago. At the same time scientists have surely divided her into three periods: Triassic, Jurassic and cretaceous. And for everyone have defined the dinosaurs. But how? The radio-carbon method for such terms isn’t applicable. In most cases the age of remains of dinosaurs, other ancient beings, and also ancient plants determine by that in what breeds of the period they are found. If remains of a dinosaur have been found in breeds of the top Triassic, and it is 237-201 million years ago, so at this time the dinosaur also lived. Now there is a question how to define age of these breeds?
We already said that the radio-carbon method can be used not only for determination of age of the objects having a biological origin. But isotope of carbon has too small half-life period, and in case of determination of age of the same geological breeds it we won’t apply. This method, though is the most known, only one of methods of radio isotope dating. In the nature there are also other isotopes whose half-life periods are longer and known. And minerals which can be used for determination of age, for example zircon.
For determination of age by method uranium – lead dating is very convenient mineral. The zircon crystallization moment, similar to the moment of death of a biological object in case of a radio-carbon method will be a reference point for determination of age. Crystals of zircon are usually radioactive as they contain impurity of radioactive elements and first of all uranium isotopes. By the way, it would be possible to call a radio-carbon method and carbon – a nitrogen method as a product of disintegration of isotope of carbon is nitrogen. Here only what of the atoms of nitrogen which are in a sample were formed as a result of disintegration and what were initially there, scientists can’t determine. Therefore, unlike other radio isotope methods, it is so important to know change of concentration of radio carbon in the atmosphere of the planet here.
In a case about uranium – a lead method a product of disintegration is isotope which is interesting that it in a sample couldn’t be earlier or its initial concentration is initially known. Scientists estimate time of disintegration of two isotopes of uranium which disintegration comes to the end with formation of two various isotopes of lead. That is the ratio of concentration of initial isotopes and affiliated products is determined. Radio isotope methods are applied by scientists to izverzhenny breeds and show time which passed from the moment of hardening.
Earth and other celestial bodies
Apply to determination of age of geological breeds also other methods: potassium – argon, argon – argon, lead – lead. Thanks to the last it was succeeded to determine time of forming of planets of the Solar system and, respectively, age of our planet as it is considered that all planets in system were created practically at the same time. In 1953 the American geochemist Cler Patterson measured a ratio of isotopes of lead in samples of the meteorite which fell about 20-40 thousand years in the territory occupied now by the State of Arizona. Refining of assessment of age of Earth to 4,550 billion years was result. The analysis of terrestrial breeds gives figures of a similar order too. So, the stones found on coast of Hudson Bay in Canada have age of 4,28 billion years. And the gray gneisses located also in Canada (rocks, on the chemical composition close to granites and clay slates), long time holding leadership in age, had assessment from 3,92 to 4,03 billion years. This method is applicable to everything to what we can “will reach” in the Solar system. The analysis of samples of the moonstones brought to Earth showed that their age is equal to 4,47 billion years.
And here with stars all absolutely in a different way. They from us it is far. To get a star piece to measure its age, it is unreal. But, nevertheless, scientists know (or are sure) that, for example, the next to us a star the Proxima of the Centaur is only a little more senior than our Sun: to it 4,85 billion years, to the Sun – 4,57 billion years. And here diamond of the night sky Sirius absolutely teenager: to it about 230 million years. The pole star and that has less: 70–80 million years. Conditionally speaking, Sirius was lit in the sky at the beginning of an era of dinosaurs, and the Pole star already at the end. So from where to scientists the age of stars is known?
We can’t receive from far stars anything, except their light. But also it is already a lot of. Actually it is that piece of a star which allows to determine its chemical composition. Knowledge of what the star consists of, and is necessary for determination of its age. During the life stars evolve, passing all stages from protostars to white dwarfs. The thermonuclear reactions resulting in a star the structure of elements in it constantly changes.
Right after the birth the star gets on the so-called main sequence. Stars of the main sequence (our Sun also treats them) consist generally of hydrogen and helium. In the course of thermonuclear reactions of burning out of hydrogen helium content grows in a kernel of a star. A hydrogen burning stage – the most long period in star life. The star is in this stage about 90% of time allowed by it. Speed of passing of stages depends on the mass of a star: the it is more, the quicker the star contracts and quicker “burns down”. On the main sequence the star is until there is a burning out of hydrogen in its kernel. Duration of other stages at which burn out heavier elements, less than 10%. Thus, the star which is on the main sequence is more senior the it is more in it than helium less hydrogen.
Couple more it seemed hundreds years ago that we will never be able to learn structure of stars. But opening of the spectral analysis in the middle of the 19th century gave in hands to scientists the powerful tool of a research of far objects. Here only at first Isaac Newton at the beginning of the 18th century by means of a prism spread out this world to separate components of various chromaticity – a solar range. In 100 years, in 1802, the English scientist William Vollaston looked narrowly at a solar range and found in it narrow dark lines. He didn’t attach them great value. But soon already German physicist and the optician Josef Fraunhofer researches them and in detail describes. Besides, he explains them with absorption of beams gases of the atmosphere of the Sun. Except a solar range he studies a range of Venus and Sirius and finds similar lines there. They are found and in artificial light sources. And only in 1859 the German chemists Gustav Kirchhoff and Robert Bunsen made a series of experiments following the results of which they came to a conclusion that to each chemical element there corresponds the line in a range. And, therefore, on a range of heavenly bodies it is possible to draw conclusions on their structure.
The method has been adopted at once by scientists. And soon as a part of the Sun the unknown element which wasn’t found on Earth has been found. It was helium (from “гелиос” – the Sun). Only a few after him have found on Earth.
Our Sun for 73,46% consists of hydrogen and for 24,85% – of helium, the share of other elements is insignificant. By the way, among them there are also metals that speaks already not so much about age, and how many about “heredity” of our star. The sun – a young star of the third generation, and it means that it was formed of what remained from stars of the first and second generations. That is those stars in which kernels these metals have also been synthesized. In the Sun, for obvious reasons, it hasn’t occurred yet. The structure of the Sun also allows to tell that to him 4,57 billion years. To age of 12,2 billion years the Sun will leave the main sequence and will become the red giant, but already long before this moment life on Earth will be impossible.
The main population of our Galaxy are stars. The age of the Galaxy is determined by her oldest objects which managed to be found. For today the oldest stars in the Galaxy are the red giant HE 1523-0901 and “Mathusela’s Star”, or HD 140283. Both stars are in the direction of the Libra, and their age is estimated approximately in 13,2 billion years.
By the way, HE 1523-0901 and HD 140283 not just very old stars, are the stars of the second generation incorporating the insignificant content of metals. That is the stars relating to the generation preceding our Sun and its “peers”.
Other oldest object, by some estimates, is the spherical star cluster of NGC6397 which stars have age of 13,4 billion years. At the same time the interval between formation of the first generation of stars and the birth of the second is estimated by researchers at 200–300 million years. These researches allow scientists to claim that our Galaxy has age of 13,2-13,6 billion years.
As well as with the Galaxy, the age of the Universe can be assumed, having defined how many years to her oldest objects. Today the champion on age among objects known to us GN-z11 galaxy located in the direction of the constellation Big Dipper is considered. Light from a galaxy went 13,4 billion years, that is it has been let out later 400 million years after the Big Bang. And if light has done so long way, then the Universe can’t have smaller age. But how this term has been determined?
Number 11 in designation of a galaxy says that it has red offset of z = 11,1. The more this index, the further an object is from us, the there was longer light from it and that an object is higher. The previous champion on an age – Egsy8p7 galaxy – has red offset of z = 8,68 (it is removed from us 13,1 billion light years). The applicant for a precedence – UDFj-39546284 galaxy probably has z =11,9, but it so far is up to the end not confirmed. The Universe can’t have an age less these objects.
Slightly earlier we told about ranges of stars by which the composition of their chemical elements is determined. In a range of a star or a galaxy which moves away from us there is a shift of spectral lines of chemical elements in the red (long-wave) side. The an object is farther from us, the more its red offset. The offset of lines in the violet (short wave) side caused by approximation of an object is called blue or violet offset. One of explanations of this phenomenon is the ubiquitous doppler effect. For example, also reduction of tone of a siren of the machine passing by or a sound of the engine of the flying by airplane is explained by it. Operation and the majority of cameras of fixing of violations is based on the Doppler effect.
So, it is known that the Universe extends. And knowing the speed of her expansion, it is possible to define also age of the Universe. The constant showing with what speed of two galaxies, divided by distance in 1 Mpk (megaparsec), scatter in different directions, is called Hubble’s constant. But to define age of the Universe, scientists needed to learn its density and structure. For this purpose space observatories of WMAP (NASA) and Planck (European Space Agency) have been sent to space. Data of WMAP have allowed to define age of the Universe in 13,75 billion years. Data of the European satellite launched eight years later have allowed to specify necessary parameters, and the age of the Universe has been defined in 13,81 billion years.