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Astronomy 101 Lecture # 7 Today: Magnitude Terminology Photometry Applications Reading: 10 Astronomy 101 Lecture # 7 Magnitude Terminology Instrumental Magnitude: measured with a particular telescope+detector combination with respect to arbitrary zero point and uncorrected for the atmospheric conditions; physically meaningless. Differential Magnitude: difference between instrumental magnitudes of two stars in the same image; corrects for instrumental response and atmospheric conditions; gives a true measure of relative brightness. Calibrated Magnitude: physically meaningful brightness of a star calibrated relative to the known flux standard (e.g. Vega); typically obtained by calculating differential magnitude w.r.t. a known standard star. Astronomy 101 Lecture # 7 Magnitude Terminology Apparent Magnitude: the same as calibrated magnitude; measures the observed brightness of a star Absolute Magnitude: calibrated magnitude of star as it would be observed from a distance of 10pc. Bolometric Magnitude: measure of the total flux from a star, integrated over the entire spectrum; could refer to apparent of bolometric magnitude Color Magnitude: instrumental, calibrated (apparent) or absolute magnitude measured with color filters Color or Color Index: difference between color magnitudes of the same object obtained in different filters, usually defined to that larger positive color index indicates a redder object Apparent Magnitude to Flux Conversion Specific flux in the center of band X: F X = f , X 10 −m X / 2.5 Total flux in band X: F X =F X W X Astronomy 101 Lecture # 7 Stellar spectra are roughly blackbodies. ● ● ● ● Color index, ●e.g. BV, is a good measure of blackbody temperature. ● Astronomy 101 Lecture # 7 Color –Color Diagram for Normal Stars From C&O (Fig. 3.10) T Astronomy 101 Lecture # 7 ColorMagnitude diagram combining the stars closer than 10pc and the stars with apparent magnitudes less than 3. Data is from Hipparcos catalog. Image from: www.jb.man.ac.uk/.../sample/stars/hrdiag.gif Astronomy 101 Lecture # 7 Effects of Reddening Astronomy 101 Lecture # 7 Effects of Reddening mV − M V =5 log 10 d AV 10pc AV = extinction coefficient Image Credit: Mathis, Rumpl, and Nordsieck (1977) Astronomy 101 Lecture # 7 Effects of Reddening mV −M V =5 log10 d 10pc AV AV = extinction coefficient To determine AV we need to observe extinction at different wavelengths to find the color excess, for example E B−V = A B− AV since the ratio AB/AV is more or less constant Image Credit: www.iras.ucalgary.ca/~volk Astronomy 101 Lecture # 7 Effects of reddening Astronomy 101 Lecture # 7 Effects of reddening Astronomy 101 Lecture # 7 Omega Centauri (globular) Pleiades (open cluster) Credit:W. Keel (U of Alabama) Credit: Harvard Smithsonian Center for Astrophysics S n ai M n ue eq ce Main Sequence al t n o z i r Ho Branch Blue Strugglers Red G iant Bran ch B G A equ in S Ma e enc Astronomy 101 Lecture # 7 The distance to Pleiades is 135pc. Estimate the distance to M3. Astronomy 101 Lecture # 7 Metallicity definition: [ Fe/ H ]=log 10 N Fe NH −log 10 N Fe NH solar [Fe/H] = 0 means solar metallicity (Z = 0.019) [Fe/H] > 0 means metalrich stars (Z > 0.019) [Fe/H] < 0 means metalpoor stars (Z < 0.019) Effects of Metallicity HR Diagram of Globuar Cluster M3 Theoretical Cluster Isochrones Image Credit: Demarque et al. 2004 Image Credit: Sandage 1970 Astronomy 101 Lecture # 7 HR diagram of M92 [Fe/H] = 2.2 HR diagram of NGC 6535 [Fe/H] = 1.7 Image Credit: Rosenberg et al. 2000 How would taking metallicity into consideration affect your estimate for the distance to M3? Photometric Redshifts obs=0 1 z How would taking metallicity Astronomy 101 Lecture # 7