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Putting it all together: L vs. M relation for main sequence stars

Putting it all together: L vs. M relation for main sequence stars.

ayanna-mcmahon
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Putting it all together: L vs. M relation for main sequence stars

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  1. Putting it all together: L vs. M relation for main sequence stars • With now masses M derived (from binaries), as well as stellar radii (which check validity of BB spectral determination), we can finally see how stellar luminosity varies with stellar mass for main sequence stars: L ~ M3.5 (see Tf17-21): • This in turn will allow us to measure stellar lifetime on the main sequence, since lifetime (as for Sun) must depend on both L and M since lifetime ~(amount available to burn)/(rate of burning): Lifetime ~ M/L ~ M/ M3.5 ~ 1/ M2.5 So lower mass main sequence stars live longer! (See BOX 19-2 in your text! We’ll re-visit this) Review Mass vs. Lum. along main seq.: Fig. on p. 459 of text Nov. 8, 2007

  2. How would we verify stellar age vs. Mass? • LOOK at an H-R diagram (naturally; they are “good” for almost everything!) • If low mass stars really DO live longer, and if stars in a cluster are ALL born at same time, what do we expect to see for distribution of star numbers on main sequence – from upper left of H-R diag. (most massive) to lower right (least massive)? • We expect a cutoff: no stars left if their lifetime is shorter than age of cluster. • This will be main topic for Nov. 20 class: stellar evolution! • So if now “deduce” that stars have finite lifetimes, they must not live forever and so must be BORN. • HOW to find where stars are born? Look for YOUNGEST stars, which are the most massive ones… Nov. 8, 2007

  3. Finding a stellar nursery… • The most massive stars (shortest lived, we just deduced) are found in “messy” environments O and B stars (upper m-s stars) are embedded in giant clouds of gas and dust, which are “lit up” as emission nebulae by the excitation and ionization of the powerful radiation (star light) from the nearby O and B stars, as in Orion A image (Tf18-2) Nov. 8, 2007

  4. How to excite a Nebula… • Hit gas with energetic (e.g. UV) photons, and it becomes excited, or even ionized: • Result? Emission lines from the excited gas. This is itself a signature of star formation region: the nebula results from the very stars being born… • DL3 will explore emission vs. abs. lines with Kirchoff’s Laws Nov. 8, 2007

  5. What is effect of the gas and dust on color? Reddening • Just as for Sunsets/Sunrise and red sky from preferential scattering of blue light, the dusty clouds of gas around young stars redden star light • Color index B-V is displaced to red (larger value) proportional to how much dust along line of sight • Result? A B star, for example, no longer looks blue; its normal main sequence B-V = -0.2 becomes instead B-V = 0 so it would be mistaken for an A star like Vega! But…it spectrum reveals its true Spectral Type, and so instead the “extinction” E(B-V) = (B-V)observed – (B-V)true = 0 – (-0.2) = +0.2 Tells us how much dust is in the way. We therefore learn stars are born in dusty, dense gas clouds… Nov. 8, 2007

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