here she comes again when she's dancing 'neath the starry sky she'll make you flip here she comes again when she's dancing 'neath the starry sky I kinda like the way she dips The Cars, My Best Friend's Girl

Assignments: Read Chapter 31, Sections 3 and 4 (pp. 505-513) Do your Observing Lab (note new due date)

In Class: -------------- review: Now we started this whole mess by looking at diagrams of clusters and seeing that they all looked different - can we explain them now? -- Of course - the differences are all due to one parameter --- AGE - stars of different mass evolve differently - in particular, stars of higher mass spend less time on the MS - if all stars are born in a cluster at the same time (big if, by the way, but let's just go with it) - most massive stars will move off MS first - they use up their supplies fastest - start their RG stages earlier - result: MS tips more upright - there is a "turn-off" in the MS - where is the turnoff for a given cluster? - at the position of the stars that have just run out of H in their cores. So how old is the cluster? - however long it takes for that star to run out of H - O stars: 10 MYR or less - B stars: 100 MYR - A stars: 1 BYR - F stars: 5 BYR - G stars: 10 BYR - K stars: 30 BYR (longer than lifetime of the universe) - M stars: 200 BYR So you can date a cluster by its turnoff - also by presence of RGs - and WD's if the cluster is that old - note that it takes a LONG TIME to make a WD - WD is the end product of a LOW MASS (i.e., long-lived) star - only the oldest clusters will have them - Globular Clusters have them - they must be ancient (they are) -------------------------------------------- Milky Way - known by anyone looking up that the distribution of stars in the sky is _not_ uniform - areas where there are few stars -- Cancer, Cetus - where there are lots more stars - Perseus, Orion, Canis Major - in a really dark site, (or on a moonless, really clear night @Obs) - see a band of faint light across the sky - not hard to imagine it's the combined light of many faint stars - easy to see even with binoculars - obvious in a telescope Galileo saw in in 1610 - !Kung tribesman call it the "backbone of the sky" - Romans term "Via Lachea" comes from Greek mythology - milk from the breast of Hera: goddess of the skies - progenitor of the present-day term "Milky Way" Johann Lambert (1749) and Thomas Wright (1750) recognized that the universe had structure - non-isotropic distribution of stars in the night sky means - non-isotropic distribution of stars in space - "an ecliptic of the fixed stars"