Measuring the Stars (Part II) A Quantitative Way to Characterize Stars: Color (Temperature) vs. Magnitude (Luminosity) Brighter Hotter A “Hertzsprung-Russell” (“H-R”) Diagram (Stellar Luminosity vs. Temperature) (Done here for a “random” selection of stars… …No apparent pattern…) H-R Diagram for stars lying within ~5 parsecs of our Sun (Our Sun is actually hotter & brighter than most stars in a typical galactic neighborhood…) H-R Diagram for the 100 brightest(-looking) stars in our Night Sky (…but our Sun is not very bright compared to the typical stars one sees in the sky!) (…in other words, compared to the “celebrities” of the star kingdom…) The Main Regions of the H-R Diagram (“Cartoon View”) Red Dwarfs A condensing Protostar “hops” onto the Main Sequence… …becoming an official Star as its core Nuclear Fusion initiates! With its permanent Main Sequence “status” (position) depending upon its Mass! (Generally) Life-Long Main Sequence Position, as a function of Stellar Mass: (vs. Mass of our Sun) Typical Stellar Abundances – by Mass of the Star (…but why are Low-Mass stars so much more common…?) Of “All” Milky Way Stars (roughly): ~9% White Dwarfs (“dead”) ~1% Red (Super-)Giants (“dying”) ~90% Main Sequence (“living”) Some Reasons for the great abundance of Low-Mass stars… 1) Obviously requires less material to make (many!) Low-Mass Stars, than High-Mass ones; 2) Even though High-Mass stars have more “fuel” (Hydrogen)… They “burn through it” (fusion) much sooner! Stellar Luminosity increases dramatically as Star’s Mass grows Larger…! “Main Sequence Turn-Off” as Stars Die… o “Sun-like” (G-Type) Stars, ~ × Sun : Live ~10 Billion Years ; o “Blue Giant” (O-Type) Stars, ~ × Sun : Live (“only”) ~25 Million Years ; ( Not even as long ago as the Dinosaurs!) o “Red Dwarf” (M-Type) Stars, ~ . × Sun : Live ~1 *Trillion* Years! (…older than the Universe , so far…!) Spectacularly Bright, Massive Stars Die “Young”!