When Two Worlds collide The anger and the pain Of all those who remain Two worlds collide Who will be left alive No place to hide Iron Maiden, When Two Worlds Collide

Assignments: Study for Wednesday's Exam

In Class: last time, Hubble had shown that the Andromeda Nebula is REALLY far away and therefore must be a HUGE galaxy - equal in size to the MW Hubble does this with observations of Cepheid variables from the 100-in Hooker Telescope at Mt. Wilson - measurements show that notonly is the Andromeda galaxy not part of our galaxy, but it's way far away from our galaxy - i.e., not just next door - the space between our galaxy and the Andromeda galaxy is HUGE even when compared to these huge galaxies --> The universe is really big, AND really empty - we thought that galaxy was empty (e.g., big spaces between stars; what junk there is between the stars is diffuse -- nearly a vacuum) - AND THAT'S THE POPULATED PART - immense voids between the galaxies where there's nothing (I mean really nothing) Measurements of Cepheids in other nearby galaxies soon followed and it because clear within a short time that the MW was simply one of a huge number of galaxies separated by vast distances in a universe that is MUCH bigger than previously thought. ------------------------- With Hubble's measurements, extragalactic astronomy was born Let's now look at the extragalactic universe from a modern perspective - types of galaxies - spirals - numerous; probably 75% of recognized galaxies (N.B.:see discussion after dwarfs) - MW is one -- a big one - classification based on morphology - tightly wrapped spiral arms (Sa) - more loosely wrapped (Sb, or Sc) - presence of a bar in the middle - SBa, SBb, and SBc - ellipticals - fewer, but some are _really_ big - classified by flatness (E0 is round, E7 is a football) - no spiral arms - rotation is not as well-defined as in spirals - stars tend to orbit in different directions - irregulars - don't fit well into classification sequence - kind of jumbled morphology - generally pretty small - it may be that these are the most numerous in the universe, but they're faint so we can't find many of them --------ASIDE on Malmquist bias------------ luminous objects are easier to see if you're counting objects - say, --to find out the percentage of all stars that are O stars --to find out the percentage of all gaxs that are elliptical you can see luminous objects that are more distant - you end up counting the luminous objects over a larger volume - end up with a larger number of luminous objects you can't see the less luminous objects that far away - they're too faint - only count nearby objects of low luminosity So your ratio of the number of objects of each type is not an accurate measure of the real ratio out in space Need to calculate the ratio from a distance-limited sample choose a distance at which you can see all of the objects you're interested in. - often, this is very hard to do ------END ASIDE----------------------------- Despite the issue of the irregulars, most study is confined to spirals and ellipticals - probably because they have well-defined structure whereas irregulars are, well, irregular - also because spirals and ellipticals are typically bigger Main physical differences between spirals and ellipticals - spirals have plenty of dust; ellipticals have very little dust - E's are mainly stars, with not much in between - S's have lots of gas and dust in their disks - spirals have more younger stars - sort of follows from the preceeding statement - more dust and gas around, more opportunity to form stars - star formation in the recent past means younger stars - younger stars also mean bluer stars - can form stars of any type - but only where star formation is current will you find O and B stars - they just don't live that long - in E's no star formation has taken place recently - therefore E's have no O and B stars - stars in E's are low mass ones - the others, if they were ever there, are long gone - more like globular clusters in this way - with only low mass stars, light from E's is redder - does this mean spirals are younger than ellipticals - NO NO NO NO NO ! - at least not necessarily - it might just mean that spirals are always making stars - whereas ellipticals just aren't doing it anymore - mabye spirals have been doing this for a long time - it's just that the current crop of stars is younger --> does not mean that the galaxy is young - in fact, current thinking suggests that ellipticals may form from collisions between spirals, thereby making the spirals the old ones, and the E's the youngsters Interactions and mergers though the space between gax's is huge compared to their size, they've been around a long time, and odds are prety good that every gax in the universe has had an interaction interactions are strange beasts - virtually no "contact" between the galaxies - galaxies are mostly empty space - stars don't collide - big deal is the change in gravity - changes orbits of stars - sweeps out gas and dust - possible outcomes - ships that pass in the night - might stretch out galaxy - strong interactions "ANTENNAE" galaxies - merging remnants - large tails of ejected material - often an "explosion" of star formation - near disruption "CARTWHEEL" galaxy - large amount of ejected gas in a ring -> called "ring" galaxies - cannabalism - small galaxy digested by larger one - stars asimilated into larger galaxy's motion - maybe how some of the really big ellipticals were built clusters of galaxies - though gax are separated from each other by vast distances compared to their own sizes - they're not completely randomly distributed in space - i.e., the number of gax/cubic meter of space is not constant - on even larger scales, galaxies appear to cluster - groups of thousands of gax in some rich clusters - separation between galaxies, even in clusters, is still huge - BUT the empty space between clusters is even bigger - structure on all scales in our universe -- stars like to be in clusters -- stars and clusters like to be in galaxies -- galaxies like to be in clusters of galaxies - basically, it's gravity at work -- gravity sucks -- hates smooth distributions of material in space -- pulls stuff together whever it can