Cosmic Abundance of Elements

Our solar system’s general chemical and isotopic makeup are the same as that of young stars and interstellar gas. Hence it is appropriate to call it a cosmic abundance of elements. Cosmic abundance of elements is defined as the approximate number of atoms of the different elements. The cosmic abundance of elements is determined by observing the chemical composition of the celestial bodies such as the stars in our galaxy, the gas clouds, the meteorites, the Moon, neighbouring galaxies and even the cosmic rays in the universe.

Stars and gas clouds

All atoms absorb and emit light. Each element does so at a particular wavelength. A spectroscope represents these characteristic wavelengths into an array of brightly coloured lines. These arrays from patterns unique to the elements they are emitted from. Thanks to this method, when scientists map the spectroscope of an unknown light source, they can analyse the patterns and determine which elements have emitted the light. Such a pattern emitted from a light source is called a bright line or an emission spectrum. If the light emitted passes through a gas cloud and the temperature of the gas cloud is lower than the source that is emitting the light, then the pattern is called a dark-line or absorption spectrum. This is because the gas will absorb according to its unique wavelength.

In this way, it is possible to determine the chemical composition of the stars emitting the light and the surrounding gas clouds. By simply observing the emitting and absorbing spectra, one can identify the elements in the stars and the proximate gas clouds. The dark line may result from the light travelling through surrounding gas clouds at a lower level than the star or the cooler outer layers of the star itself.

It is more complicated to determine the number of atoms present in the composition. For this, the whole physical structure of the star’s atmosphere needs to be determined. Studying and determining the composition of the sun is easier than other stars. But even with the sun, there are still quite many uncertainties regarding its chemical composition. The different spectroscopes of different stars are vastly varied. Earlier, this was thought to indicate a huge diversity in composition. But now, it has been found that the stars’ surface temperature determines the spectrum it emits.

Cosmic rays

Cosmic rays are high energy electrons and nuclei that reach the Earth from all directions in the galaxy. They yield only a minimal amount of information about their chemical composition. But one can study them to determine their source and thus the elements’ place of origin. 

It has been recorded that cosmic rays are richer both in the heavier elements and the lighter elements such as beryllium, lithium and boron than the stars. One very peculiar and interesting observation is that transuranium nuclei may be a part of cosmic rays. Uranium is element number 92 and is the largest naturally occurring element on Earth. The transuranium series is the collection of elements that must be artificially prepared on Earth. All the elements in this group of 20 are extremely unstable. This might indicate that these cosmic rays were not created too far back in time. 

Solar system

The Earth, the Moon, and meteorites can be directly analysed to determine their composition, although this forms a tiny part of the total bulk of the solar system’s chemical composition. One can study the Earth’s oceans and the surface too, but this forms a small fraction of the planet’s total landmass. But even in this small part, there are many variations. The composition of the innards of the Earth can be estimated by the earthquakes and the magnetic field generated by the planet’s interior.

The landing on the Moon’s surface brought back valuable samples that have been analysed to give an idea about the composition of the satellite. Comparing the findings with the composition of carbonaceous chondrites indicates that the Moon has undergone significant changes in its composition. A fractionation or segregation of elements has occurred in the lunar composition. 

Conclusion

The universe is not made up of a uniform distribution of elements. Not all cosmic bodies are made up of the same elements, and not all the constituent elements are observable. Even so, the chemical compositions of the various bodies resemble each other enough that it is useful to create a table that typifies these compositions. Such work was done by American physicists Hans Suess and Harold Urey in 1956. And despite it being nearly half a century old subsequent undertakings in the field have proved it to be true even today. The table shows that hydrogen and helium are the most common. The abundances decline as the atomic number increases. Light elements like lithium, boron and beryllium are rare. The table shows a peak in elements found near iron.