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Download Measuring the Distances to the Stars: Parallax What sets the parallax limit?
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Measuring the Distances to the Stars: Parallax Parsec (pc) = distance to star for which angle p = 1 arcsec = 3.26 LY Hwk 2 due Sept 23 Hwk 3 not yet assigned, but due Sept 30 Midterm 1 Wed, Oct 2 http://sci2.esa.int/interactive/media/html/sec23p1.htm What sets the parallax limit? Fuzzy images A field of stars …blurred by Earth’s atmosphere. Old limit for parallax distances: 20-50 parsecs 1 What sets the parallax limit? Fuzzy images Our Galaxy A field of stars 50 pc around Sun …blurred by Earth’s atmosphere. Old limit for parallax distances: 20-50 parsecs 30,000 pc Sharp Images from Space Our Galaxy A field of stars 50 pc around Sun Hipparcos Old limit for parallax distances: 20-50 parsecs 30,000 pc Hipparcos (1989-1993): 100-200 parsecs (1σ =1 milliarcsec = 1kpc) 2 Coming Attraction Our Galaxy A field of stars 50 pc around Sun GAIA spacecraft: Dec 2011 launch Old limit for parallax distances: 20-50 parsecs Hipparcos (1989-1993): 100-200 parsecs (1σ =1 milliarcsec = 1kpc) 30,000 pc GAIA: 8.5 kpc 8.5 kpc around Sun Pulsating Variable Stars period Time (days) Î P-L relation PeriodLuminosity Relation Luminosity Î Apparent brightness Î • These stars regularly expand & contract. • Like a big spring. • Change in size Î • change in temperature • change in luminosity [Fig. 14.5] • discovered in Magellanic Clouds • calibrated locally, using (statistical) parallaxes 3 Luminosity Î Pulsating Variable Stars M3 10 6 4 3 T (thousands degs.) The Instability Strip • In populations of old stars. • For example, Globular Clusters • Occurs during He burning Measuring Distances inside the MW • Parallax • Pulsating variables • Main sequence fitting for clusters • Calibrate with Hyades (moving cluster method = pp. 919-922) vt = vr tan φ µ= d= vt d vr tan φ µ 4 Measuring the MW Rotation Curve • In principle, for stars, clusters, etc: + • measure distance d and vr • assume circular orbit d d • For H I 21cm, CO, etc. radio emission: Sun • Only can measure vr • Use tangent point method • Only works inside R0 [CO Fig. 24.25] R0 • Outside Ro • Must use other methods. • Must know actual distance + velocity. Mostly radio data with tangent point method Galactic Rotation Curves Young clusters (CO emission) Merrifield 1992 R0 (take your pick) • Most data points interior to R0 are from tangent point method. • Beyond R0 most data points are young objects with known distances & motions. • But Merrifield used variation of apparent thickness of H I disk with longitude (complicated). (old data) Amaral et al. 1996 [CO 24.25] (from old1985 paper, illegible symbols ) R0 (used 7.9) R0 Mostly tangent point CO emission from star-forming regions × = tangent point o = planetary nebulae + other young objects 5