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parallax:difference in apparent position in an object viewed along 2 different lines of sight; measures stellar distance; need position of star at 2 points of year (Jan, July) farther star (large d) = smaller parallax angle (p). d (in parsec) =1/p (in arcsec). larger p = closer star. nearest star is proxima centauri. ground based telescopes measure p>.02., Hipparcos measure distance of stars accurately within 500pc,stars farther away can’t be measured with parallax, other less accurate measurements can be used apparent brightness:brightness of a star as it appears to our eyes. depends on luminosity & distance to the star.luminosity:measures total light energy output per second. the farther a star is, the more its light is spread out → less apparent brightness. brightness &distance follow the inverse square law. 3 times farther away → ⅓^2 brightness → 1/9 as bright.apparent magnitude:relate the magnitude to the apparent brightness b in logarithmic scale; measureof apparent brightness of the star. backwards scale → small number is brighter. 6 magnitudes, 1 is strongest. 1st magnitude are 100x brighter than 6th. very bright objects have negative magnitudes. dim objects have large magnitudes. absolute magnitude:apparent magnitude of a star places at a distance of -10pc, compares stars luminosities. stars like sun & low luminosity stars a common. most stars in universe are less luminous than the sun. very luminous stars are rare review:hot objects radiate at all wavelengths, hotter objects = shorter wavelengths, emit more energy than cooler objects of same size. hottest to coolest: white/blue, yellow, redabsorption lines:colors missing from the sunlight earth receives spectrums are unique least to most spectral lines: hydrogen, helium, oxygen, iron continuous:complete rainbow of colors emission lines:bright lines against dark background absorption lines: dark lines among the colors of the continuous spectrum. laws of radiation. blackbody emits light at all wavelengths. atoms in gas cloud absorb light of certain, specific wavelengths, producing dark lines in spectrum. atoms in gas cloud re emit the absorbed light at the same wavelengths it was absorbed. cooler surface layers absorb light at different frequency.stellar spectra: based on strength of hydrogen lines. hydrogen lines: lower temperatures→ weak lines, high temperature: more electrons are ionized, weak lines. line strengths can measure a star’s surface temp. OBAFGKM= hot → cold., subtypes g0 hottest, g9 coolest). measuring stars diameters: size can be determined by luminosity & temp. hot objects emit more power. stars of the same size, hotter one emits most/bluest light. bigger objects emit more power. for stars of the same temp, larger star = great luminosity. Luminosity = (power/area) x stars surface area. HR diagram.plots luminosities v surface temp. study correlations between luminosity & temp. temperature decreases to the right. 90% of stars are in the main sequence, 70% if nearby stars are red dwarfs. mass determined main sequence stars. normals stars that fuse H to HE in the cores. different chemical composition & ages cause scatter of main sequence. high mass= high luminosity **for main sequence stars only. upper right: red giant -- cool & luminous, high mass. <1% are giants, <.01%