Imagine three identical stars (meaning that they have the same intrinsic luminosity). The apparent magnitudes of the stars are given below; which star is the most distant?A) m=8B) m=10C)m=12Quiz Question: MagnitudesImagine three identical stars (meaning that they have the same intrinsic luminosity). The apparent magnitudes of the stars are given below; which star is the most distant?A) m=8B) m=10C)m=12Star C has the largest apparent magnitude, hence it is the most distant.Quiz Question: MagnitudesThe absolute magnitude of a star is its luminosity. The apparent magnitude of a star is its perceivedbrightness.Brightnesses, Luminosities, & Magnitudes•How do astronomers measure the distances to nearby stars?•How do we get stellar masses from binary stars?•How do we classify stars?QuestionsFig. 16.3Using the motion of the Earth to measure distances to starsRecall: 1" = (1/60) of 1'1' = (1/60) of 1°Distance (parallax seconds) = 1/parallax(")1 parallax second = 1 parsec = 3.26 light years = 3.1x1016md(pc) = 1/p(")Stellar Parallax: Measuring the Distances to Nearby StarsThe closest star to the SunSunProxima CentauriEarthGolfBallGrain of SandGolfBall1m 270kmProxima Centauri is a little closer to the Sun than the average separation between stars in the Milky Way.Case Study: Proxima Centaurid(pc) = 1/p(“)What is the distance, in parsecs, to Proxima Centauri, given that it’s parallax angle is 0.76“?A) 0.76 pcB) 1.3 pcConcept Question: ProximaCentauri Parallaxd(pc) = 1/p(“)What is the distance, in parsecs, to Proxima Centauri, given that it’s parallax angle is 0.76“?A) 0.76 pcB)1.3 pcd(pc) = 1/0.76“ = 1.3 pcConcept Question: ProximaCentauri ParallaxAlthough you cannot see it, stars are moving in all 3 dimensions.α Cen22 km/s-20 km/s(blueshift)30 km/s net motionSunStellar Motions: α CentauriAstronomers use the relative motion of stars in binary systems, along with Kepler’s laws, to measure the masses of stars. (Approximately half of all stars are in binary systems.)Fig. 16.09 & Math. Insight 16.4RestpositionsDetermining Stellar Masses: Binary StarsRestpositionsWhich star is more massive?A)AB)BConcept Question: Binary Stars MassesRestpositionsWhich star is more massive?A)A—Star A has a slower orbital velocityB)BConcept Question: Binary Stars MassesStars are classified based on their spectra, which are determined by their temperatures.Fig. 16.041. Stars are like blackbodies: color ⇒temperature2. Spectra absorption features allow a finer classificationStellar ClassificationStudy this tableStellar Classification: Spectral SequenceStudy this tableOhBeAFineGirlKissMeStellar Classification: Spectral SequenceFig. 16.10The Hertzprung-Russell
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