Spectroscopy

The visible light

The visible light from infrared to ultraviolet, is only a small range of the electromagnetic spectrum vibrations, however it has a special importance because it is the main information translated by our eyes on our environment.
Joseph von Fraunhofer is first noted in 1814, in the visible light of the solar spectrum, the so-called « de Fraunhofer lines ». This German optician and physicist was born March 6, 1787 in Straubing, Bavaria, and died in Munich, June 7, 1826. He invented the spectroscope in 1815 and was the first to study the diffraction of light by optical networks (Fraunhofer diffraction).
At this time we do not know the reason for the presence of the Fraunhofer lines in the visible spectrum of light. It is only much later in 1860, Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchhoff (1824-1887) discovered that the spectral lines of light emitted by an incandescent body, are a signature identifying this body. By observing the spectrum of sunlight, they recognize several chemical elements on Earth which, cesium and rubidium. The Sun therefore contains the same chemical elements as the Earth, it's a surprise. The history of the light passes through a series of milestones in physical, Christian Huygens and his wave theory in 1678, Young's experiment in 1801, the Fraunhofer lines in 1814, the Fresnel diffraction in 1815 that describes the wave nature of light, the solar spectrum Bunsen and Kirchhoff in 1850, Maxwell's electromagnetism in 1864, an unknown yellow line of helium in 1895, to Max Planck (1900) and Albert Einstein (1905) for corpuscular nature of light.

Image: Globular cluster Omega Centauri, image taken by the Wide Field Camera 3 (WFC3) Hubble in 2009. With a property of light, color, a few information is obtained. For example, the blue stars are young and hot, red stars are old and much less hot. The color also used to classify stars according to their spectral type with respect to temperature. The spectral types range from the violet to red more, that is to say, the hotter to the colder and are classified by the letters O B A F G K M. O and B stars are blue, A stars are white, F and G stars are yellow, the K stars are orange, M stars are red.
Credit image: NASA, ESA, and the Hubble SM4 ERO Team.

Image: German stamp commemorating the 200th anniversary of the birth of Fraunhofer (Germany, 1987). The spectrum emitted by an atom when it is heated, is discrete, it contains only a few rays. It appears then stripes of color (emission lines) on a black background. Conversely, the white sunlight partially absorbed (absorption lines) with heated atoms leads to a decrease in light intensity on the same wavelength as those issued lengths. After dispersion of the absorbed light, it appears dark or black stripes on an iridescent background (Fraunhofer spectrum, above). These properties of atoms and molecules are visible not only in the visible spectrum, but also in all the radiation, radio waves to gamma waves.

Spectroscopy to understand the universe

Spectroscopy provides information on the nature of our universe and brings us a lot of surprising information, beyond our expectations. Already with the color property of the light, a certain amount of information is obtained. For example, the blue stars are young and hot, red stars are older, less massive and much less hot, it is the same for galaxies, some are generally blue and so we can deduce their ages. Many other information are collected by the study of the electromagnetic spectrum. Spectroscopy is the analysis of the spectrum of visible light or not, i.e. all radiations, from radio waves to gamma waves.
By analyzing the light that reaches us of a celestial object, after diffraction, emission lines and absorption lines are observed. The emission lines and absorption lines represent the energy levels of atoms and therefore the atoms themselves, present in the layers crossed by the light. The emission lines are the lines of colors on black background, issued by the observed objet that the absorption lines are black stripes on iridescent background, so the colors absorbed by objects crossed by the light. And the emission lines are represented inversely with absorption rays.
Scientists can therefore reconstruct the chemical composition and also the quantity of elements of mater the analyzed object. Stars, galaxies, nebulae, quasars, gas clouds or interstellar dust are all objects analyzed by spectroscopy.
The thickness of the absorption or emission line provides information on the abundance of the element, "more the line is thick more elements there are."
Concerning distant objects we can deduce their movement through the Doppler effect Fizeau (1842). When the star is approaching the observer, the emission lines will be slightly shifted to the blue, when it moves away the lines will be red shifted. This is the same phenomenon is observed with the sound, when an ambulance passes, the sound of the siren is increasingly grave going away from us.
Thanks to this phenomenon can be observed a slight shift of star when exoplanet passes in front of it. This observation reveals the signature of the presence of an exoplanet orbiting a star. In addition to its presence we can deduce its mass, velocity, orbit,... ). By measuring the shift of the spectral lines of a star can also measure its gravitational field, to general relativity we know that there is a gravitational shift of the lines proportional to the mass of the body and its radius, it is used to analyze the white dwarf, more the body is massive, more the shift is important.

When a chemical is crossed by white light, the color spectrum that reaches us after diffraction, consists of black lines, which are characteristic absorption lines of chemical elements. These lines are the signature of the chemical elements traversed by the light. Thus, we can know the chemical composition of a star as the white light from the photosphere crosses ions in its atmosphere. For the same element, the absorption lines correspond to the emission lines (see the two spectra for lithium at the bottom of the right image). A chemical element absorbs radiation that is capable of emitting, and therefore the absorption and emission lines have the same wavelength. The black lines in the absorption spectrum of lithium correspond with the colored stripes of the emission spectrum.
Using spectroscopy we are able to reconstruct the rotation curve of a galaxy. In a galaxy, stars are concentrated toward the center and one might expect that the speed of the stars to the edge of the galaxy is smaller but it is not what is observed in spectroscopy, speed remains the same this confirms the presence of unknown matter (black). Spectroscopy is also used in the observation of gamma-ray bursts. Gamma ray bursts are very high-energy radiation, flashes that last a very short time, a few seconds, but enough to get a visible flash, very weak but the spectrum tells us about the signature elements of the surface of the star. This allows the use of these objects as probes to see the universe in his youth. With spectroscopy, we now know the chemical composition of the distant Universe. We identify the lines and we attribute them to certain atoms that allows us to say that the laws, constants and chemical elements are the same in all the observable Universe.
Many other reports were confirmed by spectroscopy as a measure of the temperature of the cosmic background radiation which is 2.7 ° K, and in the past the universe was hotter. Spectroscopy is not just the visible range of the electromagnetic spectrum, it is also applied in the field of low energy (radio waves) to high-energy waves (X, gamma).

NB: Diffraction is the behavior of waves when they encounter an obstacle that is not their completely transparent. The density of the wave is not preserved and the wave diffused by the object points. Diffraction is the result of the interference of waves scattered from each point.

Image: the spectrum of visible light ranging from infrared to ultraviolet, corresponds to wavelengths of 400 nm in the violet to 800 nanometers in red, that is to say of 4x10^-7 to 8x10^-7 meter. Between the wavelength (λ) and frequency (ν) exist the following relationship: ν = c / λ where c is the speed of light is about 300 000 m / s. In the electromagnetic spectrum of visible light from blue to magenta, each color is a visible light beam which is associated a value called wavelength. A set of lines separated according to their wavelength radiation, is called spectrum. White sunlight decomposed in a rainbow sky or dispersed by a prism forms a continuous spectrum because all colors are present. A lower frequency in the millimeter wave is "seen" molecules, a large number of lines tell us about the presence of molecules, however 40% of the lines have no equivalence with terrestrial molecules. These are the forbidden lines of certain atomic or molecular species. A forbidden line of transition is a spectral line emitted by atoms making energy transitions not normally allowed by the selection rules of quantum mechanics.

Modern spectroscopy

In 1814, Fraunhofer measured 570 black lines in the solar radiation composed of all the colors of the rainbow sky.
Modern spectroscopy can detect thousands of other lines, which are as much information about the source observed, due to the decomposition of the light. Much of our knowledge of the Sun comes from the spectral information. Emission lines and absorption lines represent the energy levels of the atoms present in the layers crossed by the light.
Why are there so many emission lines in the solar spectrum?
All chemical elements of the periodic table present on Earth, are also present at the surface of the Sun, in its atmosphere called photosphere. All elements but also all isotopes, stable or not, of each element will generate a line in the spectrum. Each chemical element has isotopes, these nuclear property is dependent on the number of protons. For example, the iron-56 (26 protons and 30 neutrons), which is the heaviest stable nuclide resulting from stellar nucleosynthesis has twenty isotopes (Fe 54, Fe 55, Fe 56, Fe 57, Fe 58, Fe 59, Fe 60,...).
In addition, in the solar furnace, all atoms are ionized, each ion iron (fe+, Fe2+, fe3+,...) represents a different chemical species, because its electric charge is different. All ions will generate different absorption spectrum on the solar spectrum when they will be traversed by the light coming from the center of the Sun.

In summary, in the solar spectrum can be seen, all the lines of all the energy levels of all ions of all the atoms and of all molecules that can be assembled in stellar nucleosynthesis. 40% of the lines have not been identified. These are the forbidden lines on certain atomic or molecular species. The information obtained in the analysis of the spectra is very rich and there is a definite link between spectroscopy and the atoms and molecules of the material universe. Thanks to the properties of light, the matter reveals us its signatures all over in the universe and they are identical (recognizable) even after having crossed time.


NB: Each atom is formed of protons and electrons. The energy of the electron in the repository may take several discrete values, called energy levels. When an electron moves from a high to a low level, it emits a photon whose energy is equal to the difference between the energies of the two levels. Thus, the light emitted takes a discrete value. This is what is called its spectrum. This allows to describe the atom as emitting or absorbing a certain amount of quantized energy (the photon). A ray of forbidden transition is a spectral line emitted by atoms making energy transitions not normally allowed by the selection rules of quantum mechanics.

Image: Each element has a set of lines in the electromagnetic spectrum. The spectrum of radiation emitted by the element is non-continuous and many rays represent the chemical element, is its signature. Here the spectrum of our Sun, on a scale of increasing wavelength from left to right and up and down along each strip. Each of 50 tranches cover 60 angstroms, for a complete visual spectrum across the range of 4,000 to 7,000 angstroms. Image created from a digital atlas (June 1984) with the Fourier transform spectrometer at the McMath-Pierce National Solar Observatory à Kitt Peak, Arizona.