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Stellar Spectra Caroline Fletcher

Stellar Spectra

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Stellar Spectra. Caroline Fletcher. Spectroscopy. The process of dispersion of an object's light into its component colors. Dispersion is the spatial separation of a white light into components of different wavelengths (different colors) - PowerPoint PPT Presentation

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Page 1: Stellar Spectra

Stellar SpectraCaroline Fletcher

Page 2: Stellar Spectra

SpectroscopyThe process of dispersion of an object's light into its

component colors.Dispersion is the spatial separation of a white light into

components of different wavelengths (different colors)Originally the study of the interaction between

radiation and matter as a function of wavelength (λ).Physicists classify light waves by their energies

(wavelengths). Labeled in increasing energy, we might draw the entire electromagnetic spectrum as shown in the figure below:http://lectureonline.cl.msu.edu/~mmp/applist/Spectrum/s.htm

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History1814 Joseph Frauenhofer, an optician in Munich, discovered

many dark lines crossing the spectrum of the Sun.In 1814, Fraunhofer invented the spectroscope.

This instrument is used to measure the properties of light. These are usually used to conduct a spectral analysis to identify

materials. A Spectroscope works by breaking light into the different wavelengths. Spectroscopes are made of prisms, as light passes through the glass, the different wavelengths slow down by different amounts and are bent into their colors.

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1910 Annie Jump Cannon and others working at Harvard developed an empirical scheme for classifying the spectra.

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1930s Cecilia Payne-Gaposchkin applied quantum mechanical calculations to stellar spectrum and demonstrated the importance of temperature to the appearance of a spectrum and showed that stars are almost entirely H.She became the first woman to become a full

professor at Harvard.

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Three types of Spectra

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Continuous SpectrumA spectrum in which all wavelengths are present

between certain limits. It is produced by electrons undergoing free bound-

transitions in a hot gas. Free bound- transition is the emission of radiation when a

free electron (not attached to an atom) is captured by an ion. The recombination may be to an excited energy level, with

the emission of a photon, after which the electron cascades down through the excited states to the ground state, producing emission lines characteristic of that ion or atom.

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Rainbows How do they form?

Caused by drops of water falling through the air. This creates a natural continuous spectrum.

Why are they bent, and always in the same order?Roy G. Biv (Red, Orange, Yellow, Green, Blue,

Indigo, and Violet.The light exits the raindrop at a 40-42 degree angle

away from the angle it entered the raindrop. The violets and blues bend at a 40 degree angle, and

the oranges and reds bend at a 42 degree angle. This is due to the light being bent as it enters and then

again when it exits the water drop.

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Emission SpectraEach element's emission spectrum is unique.

Spectroscopy can be used to identify the elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.

The emission spectrum characteristics of some elements are plainly visible to the naked eye when these elements are heated. For example, when platinum wire is dipped into a strontium nitrate solution and then inserted into a flame, the strontium atoms emit a red color. Similarly, when copper is inserted into a flame, the flame becomes green.

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Absorption SpectrumOver 100,000 absorption lines are visible in

the Sun's spectrum.Stars have absorption line spectra.

We can think of stars as a hot continuum source with a "cool" atmosphere of absorbing gas.

The chemical make-up of the stellar atmosphere determines the wavelengths that get absorbed.

Dark hydrogen absorption lines appear against a continuous visual spectrum, the light in the spectrum absorbed by intervening hydrogen atoms. From "Astronomy! A Brief Edition," J. B. Kaler, Addison-Wesley, 1997.

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Stellar spectral sequenceThe appearances of the spectra of stars is

dependent on the temperature of the star.

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