Light, spectrum and luminosity
is the transmission of energy in the form of waves having both an electric and a magnetic component. The properties of the light (and all other electromagnetic radiation) are fully determined by its wavelength or frequency, which is the number of vibrations per second. The frequency is calculated by dividing the velocity of light, which is equal for all frequencies, by the wavelength, so the longer the wavelength, the higher the frequency.

The various colors of light, the colors of the rainbow, correspond with the various frequencies.

The human eye is only able to see a very small section of the total number wavelengths of electromagnetic radiation. The largest part of the so called electromagnetic spectrum is invisible to the naked human eye.

The longest waves are radio waves, whcih are used to transmit radio and TV signals. The wavelength of radio waves varies from kilometers to decimeters.

After that microwaves are the next longest. They are used for radar and microwave ovens. The infrared part of the spectrum provides for night vision and health lamps. Then there is the little part of visible light, from red, yellow and green via blue to purple.

In the shorter wavelengths we go from ultraviolet light, that tans our skin, to röntgen radiation (or X-rays) and finally the dangerous gamma-radiation that is released with nuclear reactions.

Spectrum.
When electromagnetic waves are ordered in accordance with their frequency or wavelength, this ordered array is called the electromagnetic spectrum. A source of radiation such as the stars never produces just one frequency of electromagnetic wave, but rather emits a mixture of waves of many different frequencies. These spectra may be resolved, or separated, by instruments such as prisms or grating spectrometers.

In principle the electromagnetic spectrum extends from zero, the short wavelength limit of the gamma-ray end of the spectrum, to infinity, the long wavelength limit of the radio end of the spectrum. Visible light, the portion of the spectrum to which the eye is sensitive, occupies a narrow band. The radiation dispersed by a prism actually extends farther in both directions but light outside of this range cannot be detected by the human eye.

The spectrum of stars.
When light shines on matter, the radiation energy is partly absorbed by the molecules of that matter. This is caused by the transition of electrons from a lower energy level to a higher energy level, when a photon is absorbed by the atom.

From experiments we know that every type of molecule or chemical compound absorbs only the radiation of a specific frequency, that is characteristic for that type of molecule. For example red colored matter absorbs mainly blue light from incoming sunlight, the red color frequencies are reflected back. So by measuring the absorbed and reflected frequencies of a mixture of all frequencies that shines on an object we can determine exactly which elements the object is made of.

Stars, like our sun, emit a full range of  frequencies, but the atoms in the cooler outer layers of the atmosphere of the star absorb certain frequencies. When the spectrum of starlight is measured, it shows various dark lines, the absorption lines. These lines reveal the presence of particular elements in the atmosphere of the star.

Luminosity.
The observed brightness of a star not only depends on its emission intensity, but also on its distance, far off stars seem less bright than nearer. For observational purposes a brightness indication (or magnitude) is used, which is the relative brightness as seen from Earth.
The luminosity, or intrinsic intensity, is an indication of the energy radiated by a star. It varies with the temperature of the surface of the star as well as with its radius. The light from stars with a higher surface temperature (like blue giants) is of a higher luminosity than that of colder ones (like red giants). Also the luminosity of  larger stars is higher than that of smaller ones (like blue giants compared to white dwarfs).

Hertzsprung-Russell diagram.
When the temperature of stars is plotted on a graph against their luminosity, you will find that the majority of the stars lie on a narrow diagonal band, which is called the main sequence.
In such a Hertzsprung-Russell diagram, the main sequence band extends from hot stars of high luminosity in the upper left corner to cooler stars of low luminosity in the lower right corner. All stars in the main sequence are in their steady state, they shine very stable. At the end of its lifetime a star leaves the main sequence.
When the stars in a cluster are plotted in such a H-R diagram, the concentration of stars in a certain region of the diagram gives an indication of the stellar evolution in the cluster.

Spectral classes of stars.
 The astronomers have made a classification of the stars, based upon their spectrum and luminosity. The seven main classes are designated by the letters O, B, A, F, G, K and M.The color of a star gives an impression of its surface temperature, red colored stars are much cooler than our sun, the blue ones much hotter. The temperature depends on energy source of the star (the main element present in the star) which is also found in the star spectrum. Class Color Temperature Energy source O blue-white 35 000 Kelvin ionized helium B blue-white 21 000 Kelvin helium A white 10 000 Kelvin hydrogen F creamy 7 000 Kelvin ionized calcium G yellow 6 000 Kelvin calcium K orange 4 500 Kelvin titanium oxide M red 3 000 Kelvin titanium oxide

The history of the light theory.
One of first scientific theories of light, that of Christiaan Huygens in the 17th century, indicated that light is a wave-like phenomenon, comparable with the waves of an ocean.

Shortly after, it was Isaac Newton who rejected this theory, he stated that light was a flow of particles in a linear motion.

Two centuries later the theory of James Clarck Maxwell, showed that light is an electromagnetic wave of mutual perpendicular oscillating electric and magnetic fields, both of the same amplitude.

Albert Einstein amongst others, re-introduced the idea that light is a flow of particles, now called photons.

And only since around 1950 the generally accepted, but very complex, theory of quantumelectricdynamics explains that light sometimes behaves like waves and sometimes like a flow of particles. The electrical fields are equal to the force exercised on the electric charge and the magnetic fields are equal to the force exercised on an elementary magnetic particle.

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