Atoms & Light (Spectroscopy)

Blackbody Radiation A. Blackbody = a hot solid, hot liquid, or hot high density gas that emits light over a range of frequencies - stars are almost blackbodies B. Radiation emitted by a blackbody 1. graph of intensity emitted vs. wavelength

I. Blackbody Radiation A. Blackbody = a hot solid, hot liquid, or hot high density gas that emits light over a range of frequencies - stars are almost blackbodies B. Radiation emitted by a blackbody 1. graph of intensity emitted vs. wavelength 2. max = wavelength of maximum intensity emitted by a blackbody

II. A Brief History of Spectroscopy A. Issac Newton (1666) - passes sunlight through a slit and a prism => full rainbow of colors (continuous spectrum) B. Joseph Fraunhofer (1814) - passes sunlight through slit & a diffraction grating - finds 100s of dark lines in sun’s spectrum - labels darkest lines A, B, C, D, E, F, G, H, K C. Robert Bunsen & Gustav Kirchhoff (1859) - Vaporize chemical elements & take the spectrum of the light that is emitted

C.Robert Bunsen & Gustav Kirchhoff (1859) 1. Vaporize chemical elements & take the spectrum of the light that is emitted => spectrum is a series of bright lines - unique set of lines for each chemical element 2. Identify unknown samples by bright line patterns 3. Recognize that sodium's two bright lines have the same wavelength as Fraunhofer dark D lines

4. Kirchhoff’s 3 Laws of Spectral Analysis a. Hot solids, hot liquids, and hot high density gases => Continuous Spectrum b. Hot low density gases => Bright (Emission) Line Spectrum c. Light from a continuous spectrum source passing through a cooler low density gas => Dark (Absorption) Line Spectrum

Three types of Spectra Continuous: from glowing solids or very compressed gases, such as the photosphere of the Sun Emission: from hot, glowing gases that are rarefied (not very compressed, such as an emission nebula or features in the solar atmosphere Absorption: a combination spectrum produced by a continuous light source passing through cool gases. The gases “take what they want” from the spectrum. Examples: planetary atmospheres, stellar spectra

Continuous Spectrum

Emission Spectrum

Absorption Spectrum

D. Niels Bohr (1913) 1. Spectral lines (both bright & dark) are due to electrons in atoms changing energy => electron allowed only certain energies 2. Structure of the hydrogen atom - proton (+) at nucleus & electron (-) outside - atom diameter = m - proton diameter = m 3. Energy level diagram for the electron of a hydrogen atom a. Electron absorbs a photon - goes to higher energy level - photon must have correct energy => dark (absorption) line spectrum b. Electron emits a photon - goes to lower energy level => bright (emission) line spectrum

D. Niels Bohr (1913) 1. Spectral lines (both bright & dark) are due to electrons in atoms changing energy => electrons allowed only certain energies

Spectral lines can be used as a sensitive star thermometer. Star Temperatures

From the study of blackbody radiation, you know that temperatures of stars can be estimated from their color—red stars are cool, and blue stars are hot. However, the relative strengths of various spectral lines give much greater accuracy in measuring star temperatures. Spectral Lines and Temperature

The strength of the hydrogen Balmer lines depends on the temperature of the star’s surface layers. –Both hot and cool stars have weak Balmer lines. –Medium-temperature stars have strong Balmer lines. Spectral Lines and Temperature

Each type of atom or molecule produces spectral lines that are weak at high and low temperatures and strong at some intermediate temperature. The temperature at which the lines reach maximum strength is different for each type of atom or molecule. Spectral Lines and Temperature

Astronomers classify stars by the lines and bands in their spectra. –For example, if it has weak Balmer lines and lines of ionized helium, it must be an O star. Temperature Spectral Classification

The star classification system now used by astronomers was devised at Harvard during the 1890s and 1900s. One of the astronomers there, Annie J. Cannon, personally inspected and classified the spectra of over 250,000 stars. Temperature Spectral Classification

The final classification includes seven main spectral classes or types that are still used today: –O, B, A, F, G, K, and M “Oh, Be A Fine Guy/Girl, Kiss Me!” Temperature Spectral Classification

This set of star types—called the spectral sequence—is important because it is a temperature sequence. –The O stars are the hottest. –The temperature continues to decrease down to the M stars, the coolest. For further precision, astronomers divide each spectral class into 10 subclasses. –For example, spectral class A consists of the subclasses A0, A1, A2,... A8, and A9. –Next come F0, F1, F2, and so on. Temperature Spectral Classification

These finer divisions define a star’s temperature to a precision of about 5 percent. –Thus, the sun is not just a G star. –It is a G2 star, with a temperature of 5,800 K. Temperature Spectral Classification

The figure shows color images of 13 stellar spectra—ranging from the hottest at the top to the coolest at the bottom. Temperature Spectral Classification

Color spectra as converted to graphs of intensity versus wavelength with dark absorption lines as dips in the graph. –Such graphs show more detail than photos and allow astronomers to quantitate data.. Temperature Spectral Classification

Notice also that the overall curves are similar to blackbody curves. The wavelength of maximum is in the infrared for the coolest stars and in the ultraviolet for the hottest stars. Temperature Spectral Classification

Compare the figures and notice how the strength of spectral lines depends on temperature. Temperature Spectral Classification

III.The Doppler Effect A. Doppler Effect for sound - source of sound moving away => hear longer - source of sound moving toward => hear shorter - amount of shift in wavelength => speed toward or away B. Doppler Effect for light - star's spectral lines shifted - shift to longer (Red Shift) => star moving away - shift to shorter (Blue Shift) => star moving toward - amount of shift => star’s speed toward or away