3COLOR and SPECTRAL CLASS The light emitted by stars consists of a mixture of all colors, but our eyes (and brain) perceive such light as being white or tinged with pastel color.In fact, different stars have varying amounts of each color in their light; this causes stars to have different colors.Most people, however, have never noticed that stars come in a variety of colors.When light from the Sun (or any other star) is passed through a prism, it is separated into its component colors -- a continuous spectrum.
4When a beam of white light is passed through a prism, it is broken up into a rainbow-like spectrum.
5COLOR and SPECTRAL CLASS If the spectra of different stars are analyzed, it is found that the intensity of the various colors differs from star to star.Relatively cool stars have their peak intensity in the red or orange part of the spectrum.The hottest stars emit blue light most strongly.In other words, the color (or wavelength, ) of the maximum intensity depends upon the temperature of the star.The star is not necessarily the color of the max-imum intensity; in fact, there are no green stars.
7In the late 1890’s, Wien and Rayleigh had unsuccessfully attempted to formulate an equation expressing the intensity of electromagnetic radiation as a function of wavelength and the temperature of the source.In 1900, Planck derived the equation empirically.By December of 1900, Planck had derived the equation from fundamental principles.Max Planck
8Planck’s Law Intensity of Radiation vs. Wavelength The intensity (I) of electromagnetic radiation at a given wavelength () is a complicated function of the wavelength and the temperature (T).
9Planck’s Law Intensity of Radiation vs. Wavelength
10Planck’s Law Radiation Intensity vs Planck’s Law Radiation Intensity vs. Wavelength at 3000oK (Note Peak in Infrared)
11Planck’s Law Radiation Intensity vs Planck’s Law Radiation Intensity vs. Wavelength at 6000oK (Note Peak in Visible)
12Planck’s Law Radiation Intensity vs Planck’s Law Radiation Intensity vs. Wavelength at 10000oK (Note Peak in Ultraviolet)
13Planck’s Law Actual Radiation Intensity vs Planck’s Law Actual Radiation Intensity vs. Wavelength at 3000, 6000, and 10000oK
14Planck’s Law Intensity of Radiation vs Planck’s Law Intensity of Radiation vs. Wavelength; Normalized Intensity vs. Wavelength
15Planck’s Law Normalized Intensity vs Planck’s Law Normalized Intensity vs. Wavelength at 3000, 6000, and oK
16Planck’s Law Normalized Radiation Intensity vs Planck’s Law Normalized Radiation Intensity vs. Wavelength at Various Temperatures
17Planck’s Law Normalized Radiation Intensity vs Planck’s Law Normalized Radiation Intensity vs. Wavelength at Various Temperatures
18where ET = total energy radiated per unit area over all wavelengths, Stefan-Boltzmann LawET = T4where ET = total energy radiated per unit area over all wavelengths,and = J / cm2 s K4ET
19Wilhelm Carl Werner Otto Fritz Franz Wien 1864 - 1928
20In 1896, Wilhelm Wien unsuccessfully attempted to derive what is now known as Planck’s Law. However, he did notice a relationship between the temperature of a glowing object and the wavelength of its maximum intensity of emission.The result of his investigation is now known as Wien’s Displacement Law.Wilhelm Wien
21Wien’s Displacement Law: The peak of the emission spectrum of a glowing object is a function of its temperature. The hotter the object, the shorter the peak wavelength.
22Wien’s Displacement Law Gives lmax as f(T), which allows us to calculate the temperature of a star if we know the wavelength of its maximum emission, which is easy to measure from its spectrum.From Planck’s Law, take dI/dl, set = 0.Then, lmaxT = 106 nmK.Example: lmax for the Sun = 502 nm.Therefore, T = 5770K = 5500C.
23The three types of Spectra: Continuous, Emission Line, and Absorption Line
24Sodium Absorption Lines: The sodium vapor “subtracts out” the yellow lines from the continuous spectrum emitted by the source.
25As an excited hydrogen atom returns to its ground state, it emits the extra energy in the form of a photon with a certain wavelength.
26Each energy transition within an atom gives rise to a photon of a particular wavelength.
27Solar Spectrum (Original Drawings by Fraunhofer)
28Absorption lines in a star’s spectrum reveal the presence of elements and compounds.
29Absorption Spectrum of the Sun Continuous SpectrumAbsorption Spectrum of the SunBright-line Spectrum of SodiumBright-line Spectrum of HydrogenBright-line Spectrum of CalciumBright-line Spectrum of MercuryBright-line Spectrum of Neon
30The “Inverse Square” Law: When light from a point source travels twice as far, it covers four times the area, and is therefore only one fourth as bright.