1Discovery of White Dwarfs—1 Oct• Adams’ discovery• Magnitude, apparent & absolute• Why are white dwarfs much smaller than stars like the sun?• Normal gas & degenerate gas Sirius A & Bhttp://chandra.harvard.edu/photo/2000/0065/0065_optical.jpg• Homework 4 and 5 are due on Wed, Oct 8. No late papers.• Test 2 is on Fri, Oct 10.– Covers material through class on Fri, Oct 3.– It will be mostly on material not covered on Test 1.– Practice test (with answers) is on the web.– Last 15min of class of Wed, Oct 8 will be Missouri club.• Observing session next week is cancelled. go to public observing on 10-11 (link on syllabus) if you are interested.• I will be in Chile to set up the Spartan Infrared Camera during the week of 13-17.– Jack Baldwin will teach for me.Luminosity and flux• Luminosity = amount of energy per second (Watt) produced by the star–L=R2T4• Flux = energy per second received by a detector on earth (Watt/m2) –F=L/D21. As viewed from Earth, which is the faintest star?A. SunB. VegaC. Sirius2. As viewed from a distance of 10 pc from each star, which is the faintest star?8542.1×10280.512.7×10-80.0Vega2.75×10-6[pc]Distance[Lsun][W][fVega][W/m2]2313.95.2×10109.0×10273.9×1026Luminosity1.1×10-71400Flux1.4-1.45Sirius4.8-26.7SunAbsolute magApparent magStarUse fluxUse luminosityApparent & Absolute Magnitude• Apparent mag is a logarithmetic expression of flux• If the apparent mag changesby -2.5, the flux is brighter by a factor of 10.– If the apparent mag changes by +2.5, the flux is fainter by a factor of 10.1. The apparent magnitude of a star is +2.5. Its flux isA. 2.7×10-6W/m2.B. 2.7×10-7W/m2.C. 2.7×10-8W/m2.D. 2.7×10-9W/m2.E. 2.7×10-10W/m2.2. The apparent magnitude of a star is +5. Its flux is8542.1×10280.512.7×10-80.0Vega2.75×10-6[pc]Distance[Lsun][W][fVega][W/m2]2313.95.2×10109.0×10273.9×1026Luminosity1.1×10-71400Flux1.4-1.45Sirius4.8-26.7SunAbsolute magApparent magStar2Apparent & Absolute Magnitude• Apparent mag is a logarithmetic expression of flux• If the apparent mag changesby −2.5, the flux is brighter by a factorof 10.• Fluxes and magnitudes of two stars A and B–fB/fA= 10−(mB−mA)/2.5–mB−mA= −2.5 log(fB/fA)8542.1×10280.512.7×10-80.0Vega2.75×10-6[pc]Distance[Lsun][W][fVega][W/m2]2313.95.2×10109.0×10273.9×1026Luminosity1.1×10-71400Flux1.4-1.45Sirius4.8-26.7SunAbsolute magApparent magStarApparent & Absolute Magnitude• Apparent mag is a logarithmetic expression of flux– If the apparent mag changes by -2.5, the flux is brighter by a factor of 10.• Absolute mag is a logarithmetic expression of luminosity– Abs mag of a star is its apparent mag if the star is moved to a distance of 10 pc.– If the abs mag changesby -2.5 mag, the luminosity is brighter by a factor of 10.1. The absolute magnitude of a star is -2. Its luminosity isA. 2.1×1026 W.B. 2.1×1027 W.C. 2.1×1028W.D. 2.1×1029 W.E. 2.1×1030 W.8542.1×10280.512.7×10-80.0Vega2.75×10-6[pc]Distance[Lsun][W][fVega][W/m2]2313.95.2×10109.0×10273.9×1026Luminosity1.1×10-71400Flux1.4-1.45Sirius4.8-26.7SunAbsolute magApparent magStarSirius A and Sirius B• We are Walter Adams of the Mt. Wilson Observatory in 1914. We are studying the double star Sirius A and B. (Sirius A & B orbit each other.)• Sirius B is much fainter than Sirius A.http://chandra.harvard.edu/photo/2000/0065/0065_optical.jpgWalter Adams 1876-19561. Sirius B may be faint for two reasons. It may be small or it may be A. farther awayB. closerC. coolerD. hotter31. Sirius B may be faint for two reasons. It may be small or it may be A. farther awayB. closerC. coolerD. hotter• Adams found that Sirius A and B have about the same color. Therefore Sirius B is smaller.Sirius A and Sirius B• Adams found that Sirius A and B have about the same color. Therefore Sirius B is smaller.–L=R2T4• How much smaller is Sirius B?• Apparent mag of Sirius A is −1.5• Apparent mag of Sirius B is 8.71. The mag of Sirius B is approximately steps of 2.5 fainter than that of Sirius A.A. 4B. 5C. 6D. 102. The flux of Sirius B is approximately fainter.A. a factor 10B. a factor of 100C. a factor of 1000D. a factor of 10,000.http://chandra.harvard.edu/photo/2000/0065/0065_optical.jpgSirius A and Sirius B• Adams found that Sirius A and B have about the same color. Therefore Sirius B is smaller.–L=R2T4• How much smaller is Sirius B?• Apparent mag of Sirius A is −1.5• Apparent mag of Sirius B is 8.71. The mag of Sirius B is 4steps of 2.5 fainter than that of Sirius B.2. The flux of Sirius B is approximately fainter.A. a factor 10B. a factor of 100C. a factor of 1000D. a factor of 10,000.• The radius of Sirius B is 1/100 that of Sirius A.– Sirius B is about the size of the Earth.http://chandra.harvard.edu/photo/2000/0065/0065_optical.jpgSummarizing question• Why was finding of Sirius B’s spectral class crucial to discovery of white
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