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Week 2 of notes!

by: Morgan Owens

Week 2 of notes! astronomy 113

Morgan Owens
GPA 3.5

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These notes cover what was discussed in the second class of Astronomy. There are also important things that will be on the first exam that the professor said we should start studying now!
Intro to Astronomy
Class Notes
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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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