Where timorous knowledge stands considering, Audacious ignorance hath done the deed. Samuel Daniel, c. 1600

Assignments: Problem Set #6 due Thursday, 1 April, 5:00pm

In Class: ---------------- review: action in the spiral arms star formation because of compression of material molecular cloud formation gravitational collapse -> stars get made tricky parts: rotation -- makes disks outflows -- some of the gas falling in gets spat back out - not well-understood theoretically - jets emanate perpendicular to the disk - crash into other (probably unrelated) stuff lying around - light up "Herbig-Haro objects" another complication: multiplicity --> stars mostly form in clusters - interactions between stars play a role as well - gravitational competition for material - jets and outflows stir up the medium formation of big stars mucks things up - lots of luminosity - lots of high energy photons - dissociated molecules - even ionizes H atoms --> makes HII regions - makes cloud turbulent - lots of motion --> hard for gravity to work - However, shock waves might actually compress material that isn't yet vaporized - might help the star formation process by increasing the density - net effect unclear Eventually SN blow things up -- everything nearby is blasted to bits, -- end of star formation (at least locally) -- means that a typical massive star forming region last only about 10 million years after it forms its first O star - destroyed by its own success -- on a larger scale, SN might actually foster star formation - by scooping up material and compressing it. - shells of compressed material might be the next site for star formation ------------------------ while the spiral arms are more active than the spots between the sprial arms - the really active part of our galaxy is toward the center - densities are a lot higher near the galactic center - stars are actually closer together there - there's more dust and gas between the stars - and it's warmer there Consequently the galactic nucleus is a pretty active place Unfortunately, it's also invisible optically - need to look with longer wavelengths, infrared, radio - what we know if from these long wavelength investigations - Infrared studies - lots of individual stars packed really close together - lots of warm gas between the stars - xray and gamma rays indicate huge stars so closely packed together that they're feeding each other material - radio studies - immense amounts of gas over larger scales - lots of fuel for star formation - ring surrounding the center - 6pc inner edge; 25pc outer edge - some is spiraling into the very center of the gax - net picture - something is eating the galaxy! - stuff is spiraling down a hole - can there be a black hole at the center of the galaxy? - find black holes by measuring their mass and size - small massive objects are black holes -measuring mass - best way -- find something orbiting whatever you want the mass of - acceleration due to gravity = GM/R^2 - centrifugal acceleration due to circular orbit = v^2/R - these are equal is the orbit is stable - M = v^2R/G - we've done this for the Earth around the Sun and the Sun around the galaxy We can apply this to anything orbiting some mass, so let's try at the galactic center - measure speed of gas - one side approaching - other side receding gas moves with speed 700 km/s within 20000 A.U. of the center M = v^2 R/G v = 7 x 10^5 m/s R = 20000 x 1.5 x 10^11 = 3 x 10^15 m M = (7x10^5)^2 3 x 10^15/6.67 x 10^-11 = 2.2 x 10^37, or 10^7 M_o 10 million solar masses of material within 20000 AU that's a high density there are no stars within 20000 AU of the Sun we do not see that mass of stars in the galactic center - not even close only other real possibility is a massive black hole Note: stellar black holes are 3-10 solar masses this one's 10 million solar masses - same kind of beast, but MUCH bigger - it's probably the remains of many earlier epochs of massive star formation - massive stars don't live that long - then blow up and die, leaving NS or stellar BH - after a few generations of star formation, there are a lot of these -- they collide and coalesce -- make one big BH - still being fed - gas directly - gas converted to stars and then eaten Will this monster eat our whole galaxy? - probably not - it would take a long time for all of the mass in our gax to fall down that hole - e.g., we're pretty far away