lecture13

# lecture13 - Astronomy 3: The Nature of the Universe...

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Astronomy 3: The Nature of the Universe Professor Alice Shapley Lecture 13: Measurements of Stars Stellar Life Cycles (NGC 1499 Image credit: Markus Noller, Deep Sky Images)

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Logistics Quiz #7: due Monday, May 23rd, 10 pm. Available at CCLE website https://ccle.ucla.edu/course/view/11S-ASTR3-2 Structure and Motion of Spiral Galaxies.”
Review from Last Time The Sun: Energy source (nuclear fusion, proton-proton chain); solar activity and magnetic fields. Measurements of stars: Luminosity vs. apparent brightness (flux); Distance; Parallax as a distance indicator.

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Our Goals for Learning • How luminous are stars? • How hot are stars? • How massive are stars? Not all stars are exactly like the Sun !
Luminosity: Amount of power a star radiates (energy per second) The unit of power is: 1 Joule per second = 1 Watt Apparent brightness (flux): Amount of starlight that reaches Earth - the energy per second per square meter. The unit of apparent brightness is: 1 Joule per second per square meter = 1 Watt / meter 2 So, apparent brightness (flux) of a star depends on LUMINOSITY & DISTANCE . Everything we know about stars deduced from light we receive

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Luminosity/App. Brightness of Stars Apparent brightness or Flux refers to the amount of a star’s light which reaches us per unit area . At distance, d , same luminosity is spread out over an invisible sphere of increasing area with distance from source, A=4 π d 2 . The amount of luminosity per unit area decreases, the farther the star is, so the fainter it appears, according to an inverse square law – i.e., its apparent brightness decreases as the (distance) 2 . Apparent Brightness = Flux = L / (4 d 2 ) Units: Watts/m 2 Luminosity – the total amount of power radiated by a star into space. The apparent brightness (flux) of a star depends on two things: How much light it is emitting: luminosity ( L ) [watts] How far away it is: distance ( d ) [meters]
d (parsecs) =1/p (arcsec) p can be as small as 0.01 arcsec. Nearest star has p =0.7 arcsec. How far away? 1 parsec is the distance that gives a parallax angle of 1 second of arc = 3.26 light years = 206265 AU

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Every object emits thermal radiation with a spectrum that depends on its temperature How hot are stars?
1) Hotter objects emit more light at all wavelengths Stefan-Boltzmann Law: Luminosity per square meter = constant x T 4 2) Hotter objects emit light at shorter wavelengths (higher frequencies) Wien’s Law: T (K) = 2,900,000/ wavelength (nm) Laws of Thermal Radiation

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Lines in a star’s spectrum correspond to a spectral type that reveals its temperature. Spectral type (letter/number) is shorthand for temperature. (Hottest) O B A F G K M (Coolest) STELLAR SPECTRA Hottest stars Coolest stars Letters (A-O) assigned over 100 years ago before temperatures were known. Had to be re-ordered to make sense. Still a useful aid to memory.
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## lecture13 - Astronomy 3: The Nature of the Universe...

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