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Astronomical Spectra Astronomical Spectra We've talked about collecting and focusing light using mirrors and lenses. You can learn much more about the structure of light-emitting objects by breaking up their light into a spectrum. We will discuss: Refraction and Diffraction Types of Spectra Features of Spectra Spectral Classification Uses for Spectra Refraction Remember chromatic aberration? A prism breaks up light into its spectrum for the same reasons that light focused by a lens has chromatic aberration. Refraction works on the principle that different energies of light have different “bending” properties in glass, water, etc. The greater the energy, the more bending that takes place. In these days, astronomical spectra are not generally created by means of prisms. Diffraction Light can also be broken up into a spectrum by lines that are ruled on a surface very close together (e.g. 10,000 parallel lines ruled in each centimeter). These ruled surfaces, better known as diffraction gratings, are more commonly used than prisms to create astronomical spectra. Diffraction works on the physical principle of interfering light waves. The shorter the wavelength (greater energy) the less the light is diffracted. Instruments A spectroscope is a spectrum-making device that usually includes everything between a slit at one end and the viewing eyepiece at the other. When the resultant spectrum is recorded either with a photographic plate or electronically (as opposed to viewed with the eye), the device is called a spectrograph. http://www.exploratorium.edu/spectroscope/ Different Types of Spectra I A continuous spectrum with a number of fine dark lines (i.e. missing colors) is called an absorption spectrum. A spectrum with more radiation at some points than at neighboring points is an emission spectrum. Different Types of Spectra II A hot, sufficiently dense gas or a hot solid object produces a continuous spectrum with no dark spectral lines. A hot, diffuse gas produces bright spectral emission lines. A cooler, diffuse gas in front of a hotter source of a continuous spectrum produces dark spectral absorption lines in the continuous spectrum. Spectra of Astronomical Objects Stars will usually have absorption spectra - the interior core is sufficiently dense to create a continuous spectrum and the stellar atmosphere “steals” light from this spectrum giving rise to absorption. Planets shine by reflected sunlight so their spectra will be absorption in nature. Galaxies and nebulae usually have emission spectra because of the geometry of the stars related to the gas. Most astronomical objects do not have continuous spectra because there are so many patches of dust to get in the way. Features of Astronomical Spectra The absorption or emission lines in astronomical spectra correspond to certain elements. Which element is excited is dependant on the average energy of the light hitting the atoms in the star. The average energy of the star’s light is dependant on temperature. The strength of the emission does not reflect the abundance of that element in the star, but merely which elements get excited by the light. Most stars have similar compositions. Absorption and Emission Explained... Quantum mechanics tells us that only certain size electron orbits are allowed around the dense nucleus of an atom. An electron can drop from a high energy orbit to a lower energy orbit, emitting a single photon of light. An electron can absorb a photon and go into a higher energy orbit. Oh Be A Fine Girl Kiss Me (Spectral Classification) Through a very complicated relationship the lines you see in the spectrum are related to the temperature of the object. 3/ 2 2πm kT i +1 = i +1 N 2Z − χ / kT e e i N n Z h2 i e i O Strong He Thus, based on the presence B Strong H and strength of various A Strong H elemental lines astronomers F Moderate H can lump stars into classes Temp G Strong Ca based on their temperature. K Strong Metals M Strong Molecules Other Uses of Spectra Spectroscopy is possibly one of the most useful tools in astronomy! Spectral lines shift as objects move. Spectra can be used to determine how fast something is moving. For most distant galaxies this can also be used to determine how far away they are. Detection of Extrasolar planets. Detection of relics from the big bang.
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