Week 2 of notes!
Week 2 of notes! astronomy 113
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This 5 page Class Notes was uploaded by Morgan Owens on Wednesday February 3, 2016. The Class Notes belongs to astronomy 113 at George Mason University taught by Pesce in Winter 2016. Since its upload, it has received 144 views.
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Date Created: 02/03/16
Week two notes: The spectrum: Shorter waves = higher the energy Visible light is only a small component of EM energy Electromagnetic Radiation and Spectrum Blackbody an object which absorbs all EM radiation which strikes it and is heated. Energy is reremitted Amount at each wavelength depends on temperature There is linkage between the temperature and energy Blackbody Curves: temperature profiles of intensity of black body at different wavelengths An object emit energy at a rate proportional to the 4th power of its temperature (in Kelvin, absolute scale) Wein’s Law: relationship between color peak and temperature found by Wien in 1893 As temp increases, the peak wave length being emitted becomes shorter. Very useful for determining temperature of surface of stars since size and brightness doesn’t need to be known Peak wave length = 0.29(cm)/T(K) Spectra: Fraunhofer: solar spectrum has dark lines (spectral lines) Kirchoff Bunsen: spectra of each element has characteristics pattern of spectral lines Element: a fundamental substance which can’t be broken into more basic chemicals Spectral analysis led to discovery of new elements (cesium and rubidium) In 1968 there was a solar eclipse, scientist saw helium on sun 27 years before detected on Earth Each element has a characteristic spectrum, so by observing a spectrum of an astronomical object, we can determine types of elements. Kirchoffs Laws: 1. A hot object, or hot dense gas produces a continuous spectrum, (no lines, a black body spectrum) 2. A hot rarified (low density) gas produces emissions lines (bright features) 3. A cool gas in front of a continuous source of light produces absorption (dark) lines. [Absorption happens if background is hotter than foreground gas, emission happens if background is cooler] Why do spectra occur? Rutherford: atoms consist of positively charges, massive nucleus, orbited by tiny negatively charged electrons Nucleus: protons(+) and neutrons (x) attract electrons () The Bohr model: He understood mathematically and physically that e can have specific orbits (n=1, 2, 3, 4), to move from one level to another the e must lose or gain a specific amount of energy. In order to go from a low orbit to a higher orbit e must gain energy To go from a high orbit to a low orbit, e must lose energy When e moves from one level to another and it releases energy and gives off a color. Orbit 3 to orbit 2 = red orbit 4 to orbit 2= blue Doppler Shift Doppler Shift: spectral lines shifted due to motion Doppler shift is applied to sounds and light because light is a wave Motion towards source (or source toward you) compresses wavelength= shorter wavelength= bluer light Motion away from source (or source away from you) stretches wavelength = longer wavelength= redder light What this tells us is that if we are moving towards or away from the object, or the object is moving away or towards us which will then tell us how fast the object is moving. The Nature of Stars Parallax: for “nearby” stars – measures distance with parallax 1 AU, you measure the star at one time and then 6 months later (the earth is on the other side of the sun) you measure the star again, and then plug numbers into the formula D=1/p (arcsec)[pc] 1 pc when p=1 arcsec, 1 pc= 206265 AU = 3 x 10^13 P = very small!! ^ Won’t be on test but important to understand concept of parallax Brightness: most fundamental measure is the apparent magnitude (m), based on the response of the human eye We categorize brightness of starts with numbers, as number becomes bigger the object becomes fainter. +1 bright+6(naked eye limit) 1 magnitude = 2.5 times real brightness, (so 1 to 6 is 100 times difference, 2.5^5(number of distance) = 100) The problem is that apparent magnitude is not the “real” brightness of the object, the brightness decreases with the distance squared (^2), if you triple the distance the brightness decreases by 1/9 Absolute Magnitude (M): the apparent brightness an object would have if it was places at 10pc Luminosity: the Absolute Magnitude is related to Luminosity, the physical brightness of an object Steller Temperatures: Measure spectrum of the star with a photometer, which measures light intensity and filters to measure intensity at different “bands”; At different temperatures, different elements produce different emission line, you can measure temperature this way too. Spectral types: a stars surface temp. Determined from the color index or spectral line strengths In 1920’s Cecilia Payne classified stars based on spectral features visible (and ordered them by surface temperature) O B A F G K M ^Hottest^Coolest Sun= G2 35,000 degrees K 3000 degrees K Blue Red The colors are Just like blackbodies color scale Types of Stars The Hertzspring Russell Diagram: * on first exam, know this well find diagram online or in the book and know it really well, will be on the first exam 1. Really important when it comes to understanding stars, in the 2. y axes = Absolute Magnitude bottom= faint, top = bright 3. X axes= surface temperature, hot on left, cool on right There is a pattern when color index is plotted against absolute magnitude We notice that surface temp and magnitude are related, and 90% are on main sequence, we call them dwarf stars, M type most} on main sequence O type – rare} on main sequence Giants: Low surface temp far from sun, Cool objects radiate less energy than hot objects per surface area, so for giants to be so bright they must be huge! 3,000 6000 degrees, red color cause of low surface temp and huge radius Super Giants: even bigger and brighter, 1% of stars, if put into solar system it would reach to mars from center. 9 % of stars are white dwarfs, high surface temp but faint. Binary Stars: two stars are gravitationally bound, orbital motion of binaries shifts spectral lines (Doppler shift) Eclipsing Binaries: we see stars along their orbital plane, causes effects in light curve, total eclipses allow us to measure radii of stars and orbital speed Stellar Masses: How do you measure mass? Newton’s adaption of Kepler’s Law, mass luminosity relationship: on the main sequence low mass stars are faint (m type) and high mass have high luminosity on main sequence. Contact Binaries: Roche Lobe sphere of gravitational influence, In binary stars their Roche lobes are touching (contact binaries) transfer mass to one another Stellar Motion: Proper motion: true motion on the plane of the sky If were looking at star in the sky, we might see one star moving in relationship to another star that is more stationary. Flat and two dimensional, star is moving left, right, up, down. On the plane of the sky. Nearby stars Radial Motion: “3D” motion along the lineofsight Can be seen with farther stars Motion in and out, coming towards or away from you Can combine two motions, and can be only done for nearby objects
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