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Stellar spectra - star spectrum

Hydrogen atoms in the Sunís atmosphere absorb bands from the background continuous emission spectrum generated by the Sun itself: "Astronomy! A Brief Edition," J. B. Kaler, Addison-Wesley, 1997.



Almost 40 spectral absorption lines appear above in this 20-angstrom-wide section of the of the Sunís spectrum. ďIĒ stands for a non-ionized, or neutral, ion of an element; ďIIĒ represents a singly ionized instance of an element (i.e., it's atoms have lost only one electron due to thermal excitation). The blacker the line, the more a particular element in the background continuous emission spectrum has been absorbed, indicating that that element is relatively more present in the Sunís atmosphere (through which all the radiation from the Sun itself must pass in order to leave the Sun). Thus iron (Fe) is much more present in the Sunís atmosphere than are the more rare elements yttrium (Y), neodymium (Nd), and lanthanum (La), whose absorption lines are very weaker. E. C. Olson, Mt. Wilson Observatory.



The Spectral types of stars



All objects in the universe radiate energy in the form of fluctuating electric and magnetic fields. One characteristic of this electromagnetic (EM) radiation is its wavelength. A bodyís total radiation can be separated into its component wavelengths by passing it through a prism or a set of microscopic, parallel lines ruled very closely together on transparent material (a "diffraction grating.") The entire collection of wavelengths that a body radiates -- as well as their dispersion into component wavelengths by a prism or a diffraction grating -- is called that bodyís "spectrum" (pl., spectra).

From longest to shortest wavelengths, the entire electromagnetic spectrum consists of radio waves, microwaves, infrared (heat), visible light, ultraviolet light (the shortest visible light), x-rays, gamma rays, and finally cosmic rays. The latter three pass through solids, their penetration increasing as their wavelength decreases. Stars radiate all wavelengths, visible light being just a tiny fraction of their entire EM spectra.

If an object is warm enough, it radiates in the part of the spectrum humans can see known as visible light. It turns out that incandescent solids and gases under high pressure radiate all visible wavelengths, known as a "continuous spectrum." The cores of stars are gases under immense pressure that emit just such continuous spectra of light, the white light from our Sun being an example.

The continuous emission spectrum from a starís core is selectively absorbed on its journey to us by elements in both that starís and our own atmosphere. The particular wavelengths absorbed are characteristic of the absorbing element and of its temperature. The resultant "absorption spectrum" is a continuous rainbow with dark lines or gaps in it where those particular wavelengths were absorbed. These absorption lines describe a starís temperature, the makeup of its atmosphere, and, by any displacement of a line from the wavelength exactly characteristic of its absorbing element, the starís velocity relative to us (its Doppler shift). Over 25,000 lines are known that correspond to elements in stars.

Stars can be categorized by these spectral signatures characteristic of their surface temperature and directly related to their color. From hottest to coolest, starsí spectral types and associated colors are O and B (blue), type A (white), type F (yellow-white), type G like the Sun (yellow), K (orange), and M (red).

This Stellar spectra and the Spectral Types of Stars page and much of this 600-page website are excerpted from the personalized Fine Art Book You and the Universe.

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© Carl Woebcke: Stellar spectra and the spectral types of stars., 1991-2017. All rights reserved.