Theory Electromagnetic Spectrum, Spectroscopy

Astronomy, from the beginning, has always been based on observation of visible light. It is only lately that it studied other radiations too. Range of these radiations is called the electromagnetic spectrum. The electromagnetic spectrum ranges from radio waves to gamma-rays. Visible light is just a part of it. Spectroscopy began in 1802 with English scientist William Hyde Wollaston who noticed that sunlight dispersed through a prism displayed mysterious dark lines. By 1817 Bavarian physicist Joseph von Fraunhofer independently rediscovered that and investigated. Eventually by 1859, German physicist Gustav Kirchhoff discovered that each atomic element leaves a unique set of spectral lines, initiating the use of spectrography like a tool to determine the composition of stellar objects

Theory

Technically, the idea of electromagnetic spectrum is linked to the concept of energy: energy is work performed by a system undergoing a change; energy may be transported from one point to another by, amongst others, mean of electromagnetic waves. Energy becomes a moving disturbance (a wave). This wave exists between both points. Then energy is transmitted to e.g. a body, or it is converted. Electromagnetic spectrum is the ensemble of these electromagnetic waves. Electromagnetic waves are an electric and a magnetic field continually interacting

Like waves in their common meaning, waves in physics have crests and troughs. Distance between two successive crests or two successive troughs is called the wavelength. The number of complete cycles of crests and troughs passing at one point in one second is called the frequency of the wave. Wave may superpose. Result may be either constructive (crests and troughs coincide, waves add and produce a higher wave) or destructive (crests and troughs are not in phase, waves annihilate each other). Different colors of the visible light as rainbow or a prism show them, are just the marks that different parts of white light carry different levels of energy. Red is the part of white light with least energy, violet with most). Colors in visible light point too to the fact that different parts of white light have different wavelengths. White light is just a composite wave

Still more technically, electromagnetic waves (of them light) are at the same time a wave and a particle. It is now well known that energy takes the particular form of photons which are particles created at atomic level by the originating body. This was studied by German physicist Max Planck (1858-1947). At the other end of the wave's travel, these same photons interact again at the atomic level, accomplishing their mission of transmitting the energy they carried. Photons are leaving originating body in a discontinuously fashion hence they are found at discrete points along their travel. Most striking consequence of this view is that energy as it travels from a body to another, or from a point of the Universe to another point, takes the form at the same time of a particle (the photons) and of a wave (the electromagnetic wave). The photon is discontinuous as the wave continuous. Hence electromagnetic spectrum may be seen too as different energies of photons transporting energy. Photons themselves are affected by such or such wavelength

Light, generally, can be described as a wave of electric and magnetic fields that vibrate in directions at right angles to each other and to their direction of travel. Usually, on a other hand, these fields can vibrate at all orientations. However, if they happen to vibrate preferentially in certain directions, the light is 'polarized.' Special filters can be used to absorb this polarized light, which is how polarized sunglasses eliminate glare. In space, the light emitted by stars, gas and dust can also be polarized in various ways that depend on magnetic fields

The Electromagnetic Spectrum Illustrated

When a material is heated to high temperatures, it releases energy in the form of light. The type, or wavelength, of that light is determined by what the material is, as well as its temperature. Practically, the electromagnetic spectrum is most usually described as a scale along which waves part according to their wavelength and their frequency. The scale ranges from, left, the radio waves, to, right, the high-energy rays (x-rays, gamma-rays). Shorter wavelengths of light possess higher energy, as longer ones lower

Each of the wavelengths allow a category of astronomy: radio waves are the domain of radio-astronomy, CMB -the remnant of Big Band is studied in the microwave range (which is used too by radars!). Infrared allows to sense heat-emitting objects, light of which is hidden in deep dust clouds. Infrared is bound too to be the next most used wavelength as it allows to plunge deeper into early Universe. Near infrared, the part of the spectrum closer to the visible light, allows to cooler red stars and to peer into dust as mid-infrared, a step further, is unveiling planets, comets and asteroids, dust warmed by starlight, or protoplanetary disks. Ultraviolet allows to study new born stars as they emit mostly in this wavelength. X-rays is the domain of energetic, interacting events like jets of particles, or stellar winds ramming into surrounding gas or material. Gamma-rays are due to the most energetic events in the Universe like supernovae, black holes, and gamma-ray bursts. X-rays and gamma rays point to some of the most extreme phenomena in the Universe, such as stellar explosions, powerful outbursts and black holes feasting on their surroundings. The high-energy sky is a dynamic light show, from flickering sources that change their brightness dramatically in a few minutes to others that vary on timescales spanning years or even decades. Not all these wavelengths reach Earth; only visible light and radio waves do as some ranges have so to be observed from other points of view -balloons, aircraft, sounding rockets, or satellites all surpassing the boundary at which these wavelengths are blocked

In terms of the color of any object, on Earth, it depends on how it interacts with light. White light is a series of colors, with red, orange, yellow, green, blue, indigo, and violet. Interaction of light with any object may be of the reflection type as light bounces back, scattering with light bouncing sideways, transmission, when light passes through, or, at last, absorption when light gives up its energy to the object. The color of a object on Earth depends of what color of light it reflects as all others are absorbed. A red apple, for example, is red because it reflects red and absorbs all other colors of visibile spectrum!

Spectroscopy

Spectrography has been born in the 19th century as the science to find into light emitted by an element distinctive features for this element. As white light is a composite wave, it may be dissociated into its different components; this is made with a prism which refracts different wavelengths at different angles. Either side of white light may be too dissociated into a spectrum trough more advanced techniques. Kirchhoff, one of the first scientists to thoroughly develop spectroscopy by second half of the 19th century formulated three laws about spectra:

• spectrum of a radiating body, liquid or highly pressurized gas is a continuous spectrum, an uninterrupted sequence of wavelenghts
• spectrum of a rarefied gas is an emission spectrum. The spectrum displays a set of isolated, discrete wavelenghts under the appearance of bright-colored lines. Such lines are termed "emission lines." Each set of lines is unique and specific to such or such element
• spectrum of a radiating source usually seen as continuous will, if passing through a cooler gas, have some of its wavelengths removed by the latter. These wavelengths correspond to characteristics of the cold gas. The spectrum displays a set of dark lines on a colored background. Such lines are termed "absorption lines."

Practically any celestial object emits ones of these spectra helping scientists to study it. Paradoxically Sun and stars emit absorption spectra only as energy coming from their cores are obliged to cross their surface layers before escaping and reaching us. Lines seen in their spectra are lines of elements present at their surfaces. Spectroscopy also allows, generally, not only to analyze the composition of celestial objects but to measure too the distance of them

Doppler Effect

Doppler effect is due to relative motion of the observer and of the radiation. Whether an observer is moving relatively to a wave coming to him, or a wave is moving relatively to an immobile observer, a shift occurs in the wavelength

• if the source and the observer are approaching each other, the wave will have a shorter wavelength. The wave will have a longer way to travel
• if the source and the observer are moving apart each other, the wave will have a longer wavelength. The wave has just to travel a shorter way

Amount of change in either direction is related to relative velocity between the source and the observer. The famous "redshift" is one of these shifts: when a celestial body -especially faraway galaxies- are receding from the observer, their wavelengths become longer. As electromagnetic spectrum ranges from large (red side) to short wavelengths (blue side), this means that the source wavelengths are shifted to the red part of the spectrum. Hence "redshift". Redshift of galaxies is mostly due to the expansion of the Universe. At the opposite, if the celestial body is coming closer to the observer, its wavelengths are shortened, i.e. they are shifted to the blue part of the spectrum (hence the term, less often used, "blueshift"). To calculate the redshift of one object allows to its velocity, thus its distance to a observer because the more a object if far away from us in the Universe, the swiftiest its speed

A good page about electromagnetic spectrum is found at http://imagers.gsfc.nasa.gov/ems/ems.html