# lec17 - Lecture 17 Properties of Stars Purdue University,...

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Unformatted text preview: Lecture 17 Properties of Stars Purdue University, Astronomy 364 1 Lecture 17 Absolute Magnitude Lecture 16 Purdue University, Astronomy 364 2 A measure of the luminosity of a star The absolute magnitude of a star is defined as the apparent magnitude of the star if it were at a distance of d = 10 pc: M ≡ − 2.5log 10 F 10 pc F Vega ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − 2.5log 10 F F Vega ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ − 2.5log 10 F 10 pc F ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = m − 2.5log 10 d 10 pc ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 2 ∴ M = m − 5log 10 d 10 pc ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ Example: The Sun Lecture 16 Purdue University, Astronomy 364 3 The visual luminosity is 3.86x10 26 W. At a distance of 1 AU, the solar flux in the visible band is F = L 4 π d 2 = 3.86 × 10 26 4 π (1.496 × 10 11 ) 2 ≈ 1370 W m − 2 So, the apparent visual magnitude of the Sun is m = − 2.5log 10 F F Vega ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − 2.5log 10 1370 2.76 × 10 − 8 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − 26.75 The absolute visual magnitude of the Sun is M = m − 5log 10 d 10 pc ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = − 26.75 − 5log 10 1 2062650 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ = 4.83 Distance Modulus Lecture 16 Purdue University, Astronomy 364 4 m − M = 5log...
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## This note was uploaded on 04/23/2011 for the course ASTR 364 taught by Professor Cui,weik. during the Spring '11 term at Purdue.

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lec17 - Lecture 17 Properties of Stars Purdue University,...

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