It was a one-eyed, one-horned, flyin' purple people eater, one-eyed, one-horned, flyin' purple people eater A one-eyed one-horned, flyin' purple people eater Sure looks stange to me. Sheb Wooley, Purple People Eater

Assignments: Problem Set #5 Solutions are now available Problem Set #6 due Thursday, 1 April, 5:00pm Check out the HST Image of Disks Around Young Stars

In Class: OK, so the galaxy is rotating and it's big and massive and there's a lot of it we can't see, especially on the outside but let's look at the parts we can see: the galaxy is not rotating like a solid body outer parts take longer to go around than the inner parts -->"DIFFERENTIAL ROTATION" what are the implications of "differential rotation" for an extended object like the galaxy? - radial structure is smeared out into a spiral pattern - so you might expect to see spiral structure - another way to get spiral structure is with a "spiral density wave" - divide the gax into a series of annular zones - each zone represented by an ellipse - inner zones move faster than outer zones, so the ellipses get out of phase with each other --> creates places in the disk where theere's more material -- stuff from inner zones piles up with stuff from outer zones. - enhancement in density in a spiral pattern (hence the name). spiral structure, density waves density of stars and space debris increases in the arms site of molecular cloud formation - gather together a lot of gas and dust are higher density - at higher densities atoms can form into molecules - molecules would normally be blasted apart by photons - but inside a dusty cloud, it's dark - so molecules can form, and don't get destroyed - over time most of the cloud becomes molecular --> hence the term "molecular cloud (mc)" molecular clouds - you've seen them in absorption - places where there appear to be no stars - they also emit light - but they're cold, so the energy of the photons they emit is low --> long wavelength ---> radio waves - and their density is low - spectral line emission - contain lots of molecular hydrogen, H_2 - more than 90% of mass - molecular hydrogen doesn't emit well at all - traces of over 100 different molecules - common ones: CO, NH3, H2CO - exotic ones: long carbon chains -- HC_11N ethyl alcohol glycine (amino acid) - the chemistry of the ISM is really rich and remarkable given that even the dense parts are almost empty, and it's so cold - dust - essentially really big molecules - may have formed from the aftermath of previous generation of stars, e.g., planetary nebulae, ejecta from SN MC's are important because they are where the next generation of stars is born - well-shielded nurseries, when gravity can act - creates dense knots called "cloud cores" - as the density increases, gravity just gets stronger - runaway accretion or gravitational collapse - mass falls to the center - builds up there and forms a star - timescale 100000 yr from onset of collapse - simple model: mass accretes until interior pressures are large enough and T is high enough to start fusion --> a star is born Problems with the simple model rotation - if the cloud core is spinning - some of the material will not fall to the center but instead onto a disk - the centrifugal force of the rotation will balnce the pull of gravity for some material - find disks in young stars - can be seen in the infrared cuz they're cooler than the star's surface - young stars are said to have "infrared excesses" ie more infrared light than should be coming from a stellar blackbody - indicates presence of a disk - all forming low-mass stars have it - dunno about high mass stars - progenitors of our SS? - HST has imaged them outflows - jets and outflows seen toward nearly all young stars - not understood theoretically - jets emanate perpendicularly from the disks