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Light and how we observe the Universe How do atoms emit and absorb light? Each atom has a specific pattern of allowed orbits. Electrons jump between allowed orbits http://spaceplace.nasa.gov/ir-photoalbum/en/ Why is the sky blue at mid-day and red at sunset? The sky on Earth appears blue because blue (and violet) photons are scattered as they collide with air particles. The sky on Earth appears red at sunset because the light must pass through a lot of atmosphere. Earth’s Atmospher e Brainstorm! 1) Why does our sky appear to be mostly blue, and not violet, at mid-day? 2) What color would our sky be if atmospheric particles were slightly larger? 3) Why is the sky black on the moon? Why do stars have a particular color? All objects emit electromagnetic radiation. Color depends on temperature. Energy Output Surface Temperature "Color" In science, the standard measure of temperature is the Kelvin. 0 Kelvin = ─273o C = ─460o F Object Human Body Sun Blue Star Kelvin 310.15 K 5800 K 35000 K Fahrenheit 98.6 ºF 9,980 ºF 62,540 ºF Energy, Temperature, & Wavelength Stefan-Boltzman Law Wien’s Law E = T4 T = 3,000,000 max 2T → 16E 3T → 81 E 4T → 256 E Every star emits photons in all colors. The color emitted most is related to the surface temperature. Brainstorm! 1) What do you think the surface temperature of a red star would be? 2) A blue star has a wavelength of maximum emission at 434 nm. What is the surface temperature of this star? Brainstorm! 1. If the Doppler shift is used to measure the radial velocity, how do we measure the tangential velocity? 2. What is the effect of the Doppler shift for light coming from the sun? The Magnitude Scale First introduced by Hipparchus (160–127 B.C.): • Brightest stars: ~1st magnitude • Faintest stars (unaided eye): 6th magnitude More quantitative: • 1st magnitude stars appear 100 times brighter than 6th magnitude stars • 1 magnitude difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!) Example: Magnitude Difference Flux Ratio 1 2.512 2 2.512 × 2.512 = (2.512)2 = 6.31 … … 5 (2.512)5 = 100 For a magnitude difference of 0.41 – 0.14 = 0.27, we find an flux ratio of (2.512)0.27 = 1.28 Betelgeuse Magnitude = 0.41 mag Rigel Magnitude = 0.14 mag The Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): mv = –1.42 Full Moon: mv = –12.5 Sun: mv = –26.5 The Absolute Magnitude A star’s absolute magnitude Mv is the apparent magnitude it would have if it were at a distance of 10 parsecs (32.6 lightyears) from Earth. – The Sun’s absolute magnitude is Mv = 4.8 – Sirius: Mv = +1.4 – Betelgeuse: Mv = -5.1 • • Apparent magnitude tells us nothing about the luminosity of the objects, but it tell us how difficult it is to see the objects in the sky. Absolute magnitude, on the other hand, is directly related to the luminosity of the object. But it does not tell us how bright they appear in the sky. Determination of Distance Stellar Parallax Knowledge of the distance to the stars is crucial for our determination of the luminosity of stars… • Current technology allows us to determine the distance accurately to within a few hundred light-years. • Hipparcos mission (European Space Agency) measured the stellar parallax of roughly 100,000 stars with precision of a few milli-arcseconds. So, it can measure distance of star up to 1,000 light-years away… Many methods are used to determine distance Spectroscopy Kirchhoff’s Laws of Radiation 1. A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum. Kirchhoff’s Laws of Radiation 2. If light comprising a continuous spectrum passes through a cool, low-density gas, the result will be an absorption spectrum. Light excites electrons in atoms to higher energy states, The frequencies of light correspond to the transition energies absorbed from the continuous spectrum. Kirchhoff’s Laws of Radiation 3. A low-density gas excited to emit light will do so at specific wavelengths and thus produce an emission spectrum. Light excites electrons in atoms to higher energy states, Which transition back to lower states, emitting light at specific frequencies. The Spectra of Stars Inner, dense layers of a star produce a continuous (blackbody) spectrum. Cooler surface layers absorb light at specific frequencies. => Spectra of stars are absorption spectra. Thermal source plus cool gas Lines of Hydrogen Most prominent lines in many astronomical objects: Balmer lines of hydrogen The Balmer Lines Transitions from 2nd to higher levels of hydrogen n=1 H H H The only hydrogen lines in the visible wavelength range 2nd to 3rd level = H (Balmer alpha line) 2nd to 4th level = H (Balmer beta line) … Absorption Spectrum Dominated by Balmer Lines Modern spectra are usually recorded digitally and represented as plots of intensity vs. wavelength. Emission nebula, dominated by the red H line Emission Spectra (“nonthermal”) from hot, transparent gases Spectral Lines from Sunlight Wollaston (1802) – sunlight through slit to prism – spectrum = rainbow with holes Fraunhofer (~1812) – cataloged over 600 dark lines Doppler-shifted absorption spectra % shift in wavelength = speed of source, as % of Doppler-shifted absorption spectra % shift in wavelength = speed of source, as % of Doppler-shifted absorption spectra Shorter wavelengths implies it’s moving toward us are shifted by about 10 units Wavelengths (Angstroms) out of 4000, or 1 part in 400. Therefore this object is moving toward us at Dispersion Splitting up light in its spectral components achieved by one of two ways: • differential refraction – prism • interference – reflection/transmission grating – fourier transform – (Fabry-Perot) 71 Diffraction grating