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Chapter 15

Chapter 15

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PHYS 183
Tracy Webb

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15: Surveying the Stars 15.1 Properties of Stars  all stars have a lot in common with the sun  form in clouds of gas & dust with roughly same chemical composition of the sun  stars differ in size, age, brightness & temperature  luminosity o brightness of a star depends on distance & how much light it emits o apparent brightness: the brightness of a star as it appears to our eyes  amount of power reaching us per unit area o luminosity: how bright stars are in an absolute sense regardless of distance  amount of power that is being emitted into space o inverse square law for light: apparent brightness = luminosity / 4π x 2 distance  only works if the starlight follows an uninterrupted path to earth  passes through stellar dust, etc. but we can measure the effect of it o can measure the luminosity of a star through luminosity = 4π (distance) x brightness o when we perceive a star’s brightness, our eyes only measure in the visible region of the spectrum, so end up calculating the visible light luminosity only o most direct way to measure star’s distance is with stellar parallax o stellar parallax: the small annual shifts in a star’s apparent position caused by earth’s motion around the sun o 1 parsec (pc): distance to an object with a parallax angle of 1 arcsecond o d (in parsecs) = 1 / p (in arcsecs) o parallax = first reliable technique astronomers developed for measuring distances to stars  only technique telling us stellar distance without assumptions about nature of stars o state the luminosity of stars relative to the luminosity of the sun  most 6 -4 luminous stars = 10 Lsunleast luminous stars = 10 L sun o dim stars are more common than bright stars  sun is in the middle of the range of luminosity o magnitude system: originally classified stars according to how bright they look to human eyes (brightest stars got 1 magnitude); o modern magnitude system: defines absolute magnitudes as a way of describing stellar luminosities & also uses apparent magnitudes; each difference of 5 apparent magnitudes represents a factor of exactly 100 in brightness (star with magnitude of 1 is 100 times brighter than magnitude 6 star)  brightest star, Sirius, has apparent magnitude of -1.46  surface temperature o surface temperature different than temperature of a star’s core (surface directly measureable, interior inferred) o easier than measuring luminosity since distance doesn’t affect measurement  determined through star’s colour or spectrum o stars come in different colour because emit thermal radiation  dependent upon temperature o measure surface temperature by comparing star’s apparent brightness in 2 different colours of light o interstellar dust affects apparent colours of stars, so use spectral lines  highly ionized spectral lines must be fairly hot  spectral lines of molecules relatively cool o spectral type: determined from spectral lines present in star’s spectrum  hottest stars are blue, called spectral type O followed by B, A, F, G, K, M in order of declining surface temperatures  each one is subdivided into subcategories (B0, B1,… B9)  larger the number, cooler the star  sun is a G2 star, hotter than G3 but cooler than G1 o coolest stars are 3000 K, hottest stars are 40 000 K, the sun is 6000 K  stellar masses o more difficult to measure than surface temperature or luminosity rd o most dependable way relies on Newton’s version of Kepler’s 3 law  can only be applied when we can observe one object orbiting another & requires we measure orbital period & average orbital distance of orbiting object o generally can only apply this to measure masses only in binary star systems o binary star system: systems in which two stars continually orbit one another rd  to measure masses apply Kepler’s 3 law if measure orbital period & separation o visual binary: pair of stars that we can see distinctly as the stars orbit each other  to measure orbital period, observe how long each orbit takes o eclipsing binary: pair of stars that orbit in the plane of our line of sight  when neither star eclipsed, see the combined light of both stars  when one eclipses the other, apparent brightness drops since some light blocked  to measure orbital period, measure time between eclipses  able to measure separation easily based on direct measurements of velociti
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