# Apparent brightness refers to the amount of a stars

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Chapter 15 / Exercise 09
An Introduction to Physical Science
Shipman/Wilson
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Apparent brightness – refers to the amount of a star’s light which reaches per unit areac.ii.1.The farther away the star is, the fainter it appears to usc.ii.2.How much fainter it gets obeys an inverse square lawc.ii.3.It apparent brightness decrease as the (distance)2c.iii.The apparent brightness of a star depends onc.iii.1.How much light it is emitting: luminosity (L) wattsc.iii.2.How far away it is: distance (d) metersc.iv.Apparent Brightness = L/4(pi)d2d.Measuring Distance to Starsd.i.Parallax – apparent wobble of a star due to the Earth’s orbiting of the Sund.i.1.Right triangle d.i.1.a.D = 1 AUd.i.1.b.P = parallax angle d.i.1.c.Measure in radsd.i.1.d.Convert p into arcsecd.i.1.d.i.D = 206,265 AU/p (now measured in arcsec)e.Measuring Distance to Starse.i.1 parsec = 206,265 AU – 3.26 light yeare.ii.d = 1/pe.iii.is p is in arcsec and d is in parsecs, a star with a parallax of 1 has a 1 parsec distance 2/13/131.Review HW & Online Quizzes a.Multiple choice with a handful of FRQ – NO MATH2.Colors of Starsa.Stars come in many different colors
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Chapter 15 / Exercise 09
An Introduction to Physical Science
Shipman/Wilson
Expert Verified
b.The color tells you the star’s temperature according to Wien’s Lawb.i.Bluer means hotter 3.Spectral Type Classification Systema.OBAFGKMa.i.Oh Be A Fine Girl/Guy, Kiss Me!a.i.1.G type – 5000-6000 K (Our Sun)a.i.2.O is the hottest (bluest) M is the coolest (reddest)a.i.3.Email a pneumonic of my own creation ***a.i.4.50,000 K -------------- 3,000K4.Spectral Type of Starsa.Defined by: absorption lines in a star’s spectrum from different elementsb.Not determined by a star’s compositionb.i.All stars are primarily H & Hec.Determined by Surface temperaturec.i.Dictates the energy states of electrons in atomsc.ii.The types of ions or molecules which existc.iii.Determines the number and relative strengths of absorption lines in the star’s spectrumc.iv.Discovered by Cecilia Payne in 19355.Stellar Massesa.Mass is the single most important property of any stara.i.At each stage of a star’s life, mass determinesa.i.1.What its luminosity will bea.i.2.What its color/spectral type will beb.The mass of a star can only be measured directly byb.i.Observing the effect gravity from another objectc.This is easily done for two stars which orbit each other – binary star6.Binary Starsa.Optical Doublesa.i.Two unrelated stars which are in the same area of the skyb.Visual Binariesb.i.A binary which is spatially resolved ie two stars are seen (Sirius)c.Spectroscopic Binariesc.i.A binary which is spatially unresolved, ie only one star is seen; the existence of the second star is inferred from the Doppler shift in spectral linesd.Eclipsing Binariesd.i.A binary whose orbital plane lies along our line of sight, thus causing “dips” in the light we seee.The stars orbit each other via gravityf.Newton’s version of Kepler’s Third Law: P2= 4(pi)2a3/ G (m1+m2)g.Measure orbital period of binary (P) and distances between the stars (a), then calculate the masses of the stars (m1+m2)7.The Hertzsprung-Russell Diagrama.A very useful diagram for understanding starsb.We plot two major properties of stars
b.i.Surface Temp (x) vs Luminosity (y)b.ii.