What can we learn from light? Temperature Energy Chemical Composition Speed towards or away from us All from the spectrum! Temperature and Radiation Why do different objects give off different forms of light? They have different temperatures! Temperature depends on the motion of atoms & molecules Fast motion -> High temp. Slow motion -> Low temp. Temperature, Heat, or Thermal Energy? Temperature: intensity of thermal energy Heat: amount of thermal energy Two objects can be at the same temperature, but have different amounts of heat or thermal energy Kelvin Temperature Scale Zero Kelvin (written 0 K) is absolute zero (-459.7ºF) No heat energy Water freezes at 273 K, boils at 373 K. K = ºC + 273.2 Temperature and Color Higher temp = higher E= higher f = shorter λ What color has shorter wavelength? Blue/Violet! Opposite of faucet handles Spectrum: Wavelength vs. Intensity Visible: Intensity vs. wavelength plot: 400 nm 700 nm 1
They show the same information Dips in color = black lines Temp. and Color: Blackbodies A blackbody absorbs and re-emits all light that falls on it Idealized objects But still a good model for stars They don t actually look black (misnomer) Color depends on their temperature Dips in intensity Intensity vs. wavelength for three objects of different temperatures. The curves are high in the middle and low at either end. These objects emit most intensely at middle wavelengths. The total area under each curve is proportional to the total energy emitted. Hotter object emits more total energy than cooler objects. Wavelength of maximum intensity depends on temperature. The hotter the object, the shorter the wavelength of max. intensity 2
Temperature determines the color of a glowing black body. The hotter object emits more blue light than red, and thus looks blue. The cooler object emits more red than blue, and consequently looks red. Temp. and Color: Blackbodies Hottest Object Coolest Object Blackbody Radiation Lecture Tutorial: page 57-60 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. Temperature, Heat, or Thermal Energy? Temperature: intensity of thermal energy Heat: amount of thermal energy Two objects can be at the same temperature, but have different amounts of heat or thermal energy Comparing Spectra Peak at shorter wavelength = higher temperature Higher temperature = bluer in color Larger total area under curve = higher total energy output 3
Example: Solar Spectrum Hydrogen Lines Lower E, Lower f, Longer λ Visible! Higher E, Higher f, Shorter λ Visible Hydrogen Spectrum Lines: Balmer Series Spectral Classification Get spectral type from line features, predict temperature Subdivisions within each letter: 0-9 0 is hottest, 9 is coolest Sun is a G2 star (hotter than a G8 star) Long λ = Low E Short λ = High E 4
Spectral Classification O, B, A, F, G, K, M Oh Be A Fine Girl/Guy Kiss Me Only Boring Astronomers Feel Good Knowing Mnemonics Subdivisions 0-9 Sun is a G2 star Predict temperature to 5% Intensity Actual Spectrum from SDSS Balmer Lines Wavelength The The How does light tell us the speed of a distant object? Definition: The change in wavelength of radiation due to relative radial motion between the source and the observer. Real Life Example of Doppler Effect The change in the pitch of a siren on a police car, fire truck, or ambulance as it zooms past (sound waves) Astronomers deal with the Doppler Effect of light waves 5
When something which is giving off light moves towards or away from you, the wavelength of the emitted light is changed or shifted Star V=0 Light Wave When the source of light is moving away from the observer the wavelength of the emitted light will increase. We call this a redshift. When the source of light is moving towards the observer the wavelength of the emitted light will appear to decrease. We call this a blueshift. Radial means along line of sight happens only if the light source is moving towards you or away from you. Direction of shift tells us direction of light source s motion V=0 6
Amount of shift tells us speed of source s motion Stationary Doppler Shifts Redshift (to longer wavelengths): The source is moving away from the observer Blueshift (to shorter wavelengths): The source is moving towards the observer Moving Away Away Faster Moving Toward Toward Faster Δλ= shift in wavelength λ 0 = wavelength if source is not moving V = velocity of source c = speed of light Lecture Tutorial: Pg. 73-77 Work with a partner or two Read directions and answer all questions carefully. Take time to understand it now! Come to a consensus answer you all agree on before moving on to the next question. If you get stuck, ask another group for help. If you get really stuck, raise your hand and I will come around. Chapter 7 Recap Atoms, electron energy levels, absorbing & emitting light Temperature & color Types of spectra: absorption, emission, continuous Spectral Classes: OBAFGKM Doppler shift & speed 7