Tiger! Tiger! burning bright In the forests of the night, What immortal hand or eye Could frame thy fearful symmetry? In what distantdeeps or skies Burnt the fire of thine eyes? William Blake, Songs of Experience, The Tiger

Assignments: Read Chapter 25, Sections 4 and 5 Don't forget about the Observing Lab!

In Class: ----------------- star clusters: we've done individual stars to death, so let's start looking at clusters of stars. - talked about stars liking to be members of binary systems - also true for larger aggregations, too - stars like partners - star clusters are found all over the sky - not all stars are members of clusters, but many are - probably an accident of birth - stars like to form in groups, it seems whatever the cause, we have a bunch of stars all together in space - i.e., all at the same distance from us. - Hertzsprung made use of these beasts to create H-R diagrams for clusters without knowing the distance - y-axis is just flux. - directly related to luminosity, since f = L/4piR2 - but not luminosity - still works just fine, since all of the distances are the same But what if we knew the luminosity of a G star? - how? - maybe from measuring the flux from G stars whose distance we know - then we could figureo ut the distance to a Pleiades G star and get the distance t all of hte stars in the cluster - then we could relabel the axis with luminosity - that is, we can assign luminosities to stars in the cluster with measured fluxes. - aha! then now we can calculate the distance to the cluster. - how? f = L/4piR^2 Can do this with any star: - measure spectrum --> get spectral type - look up luminosity for MS stars of that spectral type - measure flux - used f = L/4piR^2 to get the distance >>>> only dangerous part is assuming MS star - most are, but some are off the MS - giants or white dwarfs - if you mistakenly assumed that these were MS stars - you would calculate the wrong distance With a cluster it's easier - lots of stars define the MS - pretty unambiguous - lots of chances to check it, too - compare F star flux with F star luminosity - compare K star flux with K star luminosity, etc. unfortunate name: spectroscopic parallax - spectroscopic because you need spectral type - but there's no parallax measurement about it - DISTANCE is determined by the distance dimming, or inverse square law ONLY Using this method, you can construct proper H-R diagrams for lots of clusters of stars - using luminosity as the y-axis - by definition, the MS will be in the same place for each cluster. - so the H-R diagrams of all clusters should look the same They don't Pleadies star cluster - well defined Main Sequence - nearly all stars on it Praesepe - still a pretty good Main Sequence - most stars on it - shorter, though -- really hot stars missing - and look at those giants Messier 67 - very short Main Sequence - still lots of stars on it - extends only up to spectral type F or so - lots of red giants and stuff with large radii What's going on here? remember how we once thought all stars were different because of their myriad of different spectral lines? - we're in the same boat again with clusters - they should all look the same, but they don't just as we searched for a single parameter or variable to explaiin all of the differences in all of the spectra (and found it --> temperature) we would like to find a single factor responsible for all of the variation we see in the structure of the HR diags for all of these clusters Need to explain: - presence/absence of hottest stars - length of the MS - absence presence of giant stars What is it about a cluster of stars that could differ from cluster to cluster - I can think of three possibilities 1) composition - long thought to be the culprit (as it was with star spectra) - maybe you need something special to make hot stars - zinc or something that allows hot stars to stay stable - maybe contaminants in the coolest stars force them to "puff up" and become giants 2) formation mechanism - how stars form has been (and still is) poorly understood - maybe the conditions under whch the stars formed are only occasionally conducive to the formation of the hottest stars maybe failed hot stars are the giants 3) age - stars don't live forever - clusters are groups of stars that all formed together - maybe we're just looking at the same cluster in different times of life. - consider nursery school/college/retirement home for people - all would look very different - yet all are snapshots of common-aged people at different times in their lives - if the nursery schoolers stuck together in a few years, they'd look like college students.