1. Nov. 8, 2007 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 ~ M 3.5 (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/ M 3.5 ~ 1/ M 2.5 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

2. 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…

3. 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)

4. 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

5. 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…