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The spectral class of a star is a short code primarily summarizing the ionization state, giving an objective measure of the photosphere's temperature. Most stars are currently classified under the Morgan–Keenan (MK) system using the letters O , B , A , F , G , K , and M , a sequence from the hottest ( O type) to the coolest ( M type).
A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On a Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence. [a]
A-type star In the Harvard spectral classification system, a class of main-sequence star having spectra dominated by Balmer absorption lines of hydrogen. Stars of spectral class A are typically blue-white or white in color, measure between 1.4 and 2.1 times the mass of the Sun, and have surface temperatures of 7,600–10,000 kelvin.
The spectral signature of an object is a function of the incidental EM wavelength and material interaction with that section of the electromagnetic spectrum. The measurements can be made with various instruments, including a task specific spectrometer , although the most common method is separation of the red, green, blue and near infrared ...
[2] [3] Soon the term referred to a plot of light intensity or power as a function of frequency or wavelength, also known as a spectral density plot. Later it expanded to apply to other waves, such as sound waves and sea waves that could also be measured as a function of frequency (e.g., noise spectrum, sea wave spectrum).
Atmospheric windows are useful for astronomy, remote sensing, telecommunications and other science and technology applications. In the study of the greenhouse effect, the term atmospheric window may be limited to mean the infrared window, which is the primary escape route for a fraction of the thermal radiation emitted near the surface.
Multispectral imaging measures light in a small number (typically 3 to 15) of spectral bands. Hyperspectral imaging is a special case of spectral imaging where often hundreds of contiguous spectral bands are available. [5]
The Star-Spectroscope of the Lick Observatory in 1898. Designed by James Keeler and constructed by John Brashear.. Astronomical spectroscopy is the study of astronomy using the techniques of spectroscopy to measure the spectrum of electromagnetic radiation, including visible light, ultraviolet, X-ray, infrared and radio waves that radiate from stars and other celestial objects.