Astro 201: Sept. 14, 2010 Read: Hester, Chapter 4 Chaos and Fractal information on class web page On-Line quiz #3: available after class, due next Tuesday.
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Astro 201: Sept. 14, 2010 Read: Hester, Chapter 4 Chaos and Fractal information on class web page On-Line quiz #3: available after class, due next Tuesday before class HW #3 on line Today: – Light
LIGHT Astronomers study the Universe by analyzing the light from cosmic objects LIGHT: Maxwell’s equations predict that moving charged particles will produce periodic waves of electric and magnetic field
Bright at all wavelengths Bright at specific wavelengths Bright at all except some specific wavelengths
An example of a Continuum Spectrum: Blackbody A “blackbody” is an object that absorbs all light that falls on it It radiates light which has a distinctive spectrum, specified by its temperature The peak of the spectrum for blackbodies with T=98.6 F is in the infrared part of the spectrum T=6000 degree black bodies have peaks in the visible part of the spectrum (e.g. the SUN) Incandescent light bulb filament: 2000-3000 deg.
Stars have spectra that are more or less like those of blackbodies
Produced when light interacts with atoms A nucleus, consisting of protons and (usually) neutrons, is surrounded by a cloud of electrons. Absorption and Emission Lines in Spectra:
electron proton Behavior on subatomic scales is governed by QUANTUM MECHANICS. Rule: electrons can only exist in orbits of particular energy. (Small orbit = low energy, big orbit = high energy). HYDROGEN: One Proton, one electron.
Electron falls from high energy to low energy orbit: energy is carried away by a photon. λ=656.3 nm 121.6 nm 102.6 nm Photon has a fixed ENERGY, corresponding to a specific WAVELENGTH.
Consider a hot, low density glob of hydrogen gas. λ = 656.3 nm (3→2) λ = 486.1 nm (4→2) λ = 434.0 nm (5→2) Light is emitted with wavelengths or frequencies corresponding to energy jumps between electron orbits.
1) Hot, low density gas produces an emission line spectrum. Spectrum of hydrogen at visible wavelengths. 3→25→24→2
Carina Nebula: a cloud of hot, low density gas about 7000 light-years away. Its reddish color comes from the 656.3 nm emission line of hydrogen.
A cool, low density glob of hydrogen gas in front of a light source. Light is absorbed at specific wavelengths corresponding to energy jumps between electron orbits.
2) Cool, low density gas produces an absorption line spectrum. Spectrum of hydrogen at visible wavelengths. 3←25←24←2
If a light source is moving TOWARDS you, the wavelength appears shorter (called “blueshift”). If the light source is moving AWAY from you, wavelength is longer (called “redshift”).
Doppler shifts are easily detected in emission or absorption line spectra.
Figure 4.17: Doppler shifts of astronomical objects
Size of Doppler shift is proportional to radial velocity: Δλ = observed wavelength shift = λ-λ 0 λ 0 = wavelength if source isn’t moving V = radial velocity of moving source c = speed of light = 300,000 km/sec
One of the nifty applications of the Doppler effect has been the detection of Planets orbiting other stars: “Extrasolar Planets” The star and planet orbit around their “center of mass” periodic red and blue shifts of the lines in the star’s spectrum To date: about 358 planets have been discovered, mostly larger than Jupiter
Another nifty application of the Doppler effect : Doppler radar to predict weather Not so nifty: Police radar guns to catch you speeding