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MSU AST 207 - 10-07

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1Discovery of White Dwarfs—7 Oct• Hertzsprung-Russell diagrams• Magnitude, apparent & absolute• Adams’ discoverySirius A & Bhttp://chandra.harvard.edu/photo/2000/0065/0065_optical.jpgTest 1• Statistics– Top quartile: 24, 80%– Median: 20, 67%– Bottom quartile: 16, 53%Test 10510152025300265278104RankGrade2Luminosity 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?Sun8542.1×10280.512.7×10-80.0Vega2.75×10-6[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 magStar3Apparent & 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)•Try it–If mBis −2.5 more than mA, mB−mA= −2.5, andfB/fA= 10−(−2.5)/2.5 = 101 = 10.– If B is brighter by a factor of 10, fB/fA=10, andmB−mA= −2.5 log(10)= −2.5.Apparent & 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 magStar4Hertzsprung-Russell Diagram of a star cluster M15• Observations of M15 show– Main-sequence or dwarf stars– Giants– Horizontal-branch stars– White dwarfs are too faint for these observations.1. Stars in region I and II areA. MS & MSB. MS & GC. G & MSD. G & G2. The luminosity of a star at I is __ times the luminosity of a star at II.A. 1000B. 100C. 10D. 1/10E. 1/1000IIIHBHertzsprung-Russell Diagram of nearby stars1. Stars in region X areA. DwarfsB. GiantsC. White


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MSU AST 207 - 10-07

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