Evolution of Imaging in Astronomy: From Photographic Plates to CCDs

1. Review: how we record images? • Photographic plates were the long-time standard (until c. 1982). Harvard has world’s largest collection (500,000 plates giving 100y of coverage full sky; our DASCH project will digitize them! • See DASCH video website (http://hea-www.harvard.edu/DASCH/) • Now we use Charge Coupled Devices (Tf6-19), like our CCD on the Clay Telescope (EL2): 1K x 1K pixels, 11microns each! (see blackboard explanation for how they work…) • We also put spectrographs at focus of telescope (Tf6-20) to diffract the light and disperse into a spectrum to reveal both the continuum shape (and thus BB temperature) and also spectral lines Oct. 23, 2007

2. Using filters (B,V,R) for stellar temp. measure • Demonstrate (in class) a continuum (roughly BB source) light source shining thru prism (disperse light to spectrum) and effect of interposing blue (B), green (V) and red (R) filters • EL2 measure of flux of Albireo thru B and V filters allows you to measure the (approx.) temp. of both stars from ratio of fluxes: • Temp. ~ (Flux thru B filter)/(Flux thru V filter); T-f17-9: • See also T-f17-7 & T-f17-8 for illustrations of temp. vs. filters… Oct. 23, 2007

3. Magnitude scale for flux and luminosity • Since stars visible to naked eye range over factor of ~100 in apparent brightness, and with telescopes we can extend this to stars a factor of ~108 fainter still, we use logarithmic scales (or power of 10) since it greatly compresses the large range • Apparent magnitude defined in given band (e.g. B, V) as log of flux in that band, with scale such that 5 mags = 100X flux: mB = B = -2.5log(FB) + constB, where constB = value which gives B = 0 for star Vega (calibration standard), and also mV = V = -2.5log(FV) + constV, where constV gives V=0 for Vega Etc. for U (ultraviolet), R (red), I(near InfraRed) bands • Absolute magnitude defined as app. mag. in given band for star observed at fixed distance of D = 10pc, so MV = mV - 5 log Dpc + 5, where Dpc is dist. in pc of star with app. mag. mV Oct. 23, 2007

4. Distance modulus and magnitudes • We can “invert” previous equation to give the distance modulus, or difference between apparent and absolute magnitudes, which is of course a direct measure of distance from the inverse square law: m – M = 5log Dpc -5, where m & M are in given band (e.g. V) & D in pc Why the “-5”? Because definition of M was value m obs. at D = 10pc! Why the “5log D” ? Because Flux = Lum/4πD2(inverse square law) and because m = -2.5 log (Flux)= -2.5log(L) -2.5log(1/4πD2) = M + 5 log Dpc • Get practice in manipulating m and M values! This is what you need to get M (and thus L) for Vega in EL2… Oct. 23, 2007

5. What powers the Sun (and stars)? • Now finally ready to dig into the stars and ask what is their source of luminosity? • Can’t be gravitational “settling” (e.g. Sun shrinking slowly, as Helmholtz thought), and tapping into potential energy of gravity, E = GM/R, since Sun is not shrinking and if it were, would not shine long enough • Can’t be “chemical burning” since energy per atom available from “chemistry” is only ~1 eV ~10-19Joules & Sun Luminosity of L ~ 4 x 1026 Joules/sec would then “burn” 4 x 1045 atoms/sec but Sun has only ~1057 atoms to burn, so lasts only for lifetime ~1057/4 x 1045 seconds or ~3 x 1011 = 104 years! • So it’s nuclear fusion (continuous H bombs in central Sun )…. Oct. 23, 2007