Oct. 23, 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 ( 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, 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, 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 ~10 8 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: m B = B = -2.5log(F B ) + const B, where const B = value which gives B = 0 for star Vega (calibration standard), and also m V = V = -2.5log(F V ) + const V, where const V 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 M V = m V - 5 log D pc + 5, where D pc is dist. in pc of star with app. mag. m V

Oct. 23, 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 D pc -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/4πD 2 (inverse square law) and because m = -2.5 log (Flux) = -2.5log(L) -2.5log(1/4πD 2 ) = M + 5 log D pc 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, 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 ~ Joules & Sun Luminosity of L ~ 4 x Joules/sec would then “burn” 4 x atoms/sec but Sun has only ~10 57 atoms to burn, so lasts only for lifetime ~10 57 /4 x seconds or ~3 x = 10 4 years! So it’s nuclear fusion (continuous H bombs in central Sun )….