2 Apparent Magnitude System of Hipparchus Group of brightest stars 1m Stars about ½ as bright as 1m 2mStars about ½ as bright as 2m 3m•Naked Eye Limit 6m
3 Apparent Magnitude19th century photographers learn how eye responds to light (Pogson)Doubling the brightness is not perceived as a doubling by the eyeEye response is logarithmicRatio of 100 in brightness corresponds to a Difference of five magnitudesDm of 5 100X in lightDm of 1 2.512X in light
4 Some Apparent Magnitudes SunFull MoonVenus at brightestSiriusNaked Eye LimitFaintest Objects +30.0Hubble
5 Learning the Brightness Is a star bright...Because it really is a bright star?Because it is close to the Earth?Stellar brightness depends onLuminosityDistance
9 Stellar Parallax When p is measured in arcsec and d is measured in parsecsOne parsec:206,265 AU3.26 light years
10 Stellar Parallax Nearest star to Sun (largest parallax) a Cen p = 0.7 arcsecLimit of accurate parallax 200 pcs (angles of arcsec)Hipparcos satellite (120,000 stars measured to arcsec)
11 Absolute MagnitudeThe magnitude a star would have at 10 parsecs from the Sun.The apparent (m) and absolute (M) magnitudes of a star at 10 pcs are the same.M, m, and d are related. Knowing two allows you to compute the third.
12 Putting the Pieces into Place Ejnar Hertsprung1911Henry Norris Russell1913
16 Luminosity Class implies Size Consider the Sun and CapellaThe SunG2V M=5CapellaG2III M=0
17 Luminosity Class implies Size Equal sized pieces of each star are equally brightCapella is 100X brighter (5 magnitudes)Capella must have 100X as much areaSurface area radius2Capella must be 10X larger than Sun.
18 Luminosity Class in the Spectrum SupergiantA3GiantA3Dwarf