The center of our galaxy is not near at hand among the bright stars that define the southern constellations for they are but a few hundred light years away. The center of our galaxy, we have found, is more than 25,000 light years distant. The billions of stars in that nucleus together make the extended white glow in the southern Milky Way which we call the Sagittarius star cloud. Harlow Shapley, Of Stars and Men

Assignments: Read Chapter 32, Section 4 (pp. 525-530) Check out this cool fish-eye view of the southern Milky Way Problem Set #5 due 25 March, 5:00 pm

In Class: Lambert and Wright were able to see the Universe as a flattenned structure but were unable to quantify this - how big? - what aspect ratio? ------------ ASIDE on the term UNIVERSE Universe means everything there is - to the Greeks, it was the planetary system - stars were just dots on the outermost sphere circling the Earth they were not distant Suns, so the universe didn't have to be big - structure of the universe meant positions and motions of the planets - a small-scale universe (and therefore Man had a large presence) - once it was realized that stars are probably distant Suns (late 1600's) the universe got much bigger SS was a small piece structure of the universe meant 3-D distribution of stars - a large-scale universe - the Sun is just one of many stars - therefore how important are we? - we know know that the distribution of stars around us is just our Galaxy, the Milky Way, which is only one of a huge number of galaxies out in space - therefore, we make a distinction between our Galaxy -- the system of stars around us and the Universe -- the system which includes not only our galaxy, but all others as well - the universe in this sense is truly huge But we're getting ahead of ourselves - in the 19th century, the galaxy and the universe were the same thing - there was nothing beyond the confines of our galaxy - hence when people back then said that the universe had a flattenned shape, they meant that the galaxy was flattenned - only in our modern conception is there a distinction between galaxy and universe END ASIDE -------------- William Herschel (1738-1822) made the first quantitative measurements of the shape of the universe - Herschel's first attempt to determine the distribution of stars in space was simple: 1) assume all stars have the same luminosity - he didn't know about different kinds of stars 2) assume that different fluxes came from distance effects only. - he measured fluxes and positions of stars - from this calculated the 3-D picture of the universe - a few simple results: 1) from the fact that more stars are found in a band around the sky, Herschel concluded we live within a flattenned structure, like a disk. - looking through the disk, see lots of stars - looking out of the disk, see fewer stars - especially true for faint (aka distant) stars - actually why the Milky Way looks the way it does 2) looking in the disk plane, the number of stars is roughly constant in every direction. - we must be at the center of the disk - if not, we would see more stars in one direction (toward the center) 1and fewer in the opposite direction (away from center) Immanuel Kant (late 1700's; yes the philosopher) realized that if the universe is a disk, it must be rotating Why? - without rotation, stars would all collapse together under their mutual gravity just as the planets in the Solar System must revolve around the Sun so too, the stars of the Milky Way must revolve around their common center In fact, Kant realized that if you took even a spherical distribution of matter and set it rotating - it would flatten to a disk In fact, the idea that the universe was a magnified version of our Solar System was philosophically inviting - Moon revolves around Earth - Earth revolves around Sun - Sun revolves around the center of the universe --> attractive symmetry in that By the end of the eighteenth century, accepted theory suggested 1) universe was disk-shaped 2) only a few hundred ly thick 3) extended for a few thousand ly 4) Sun was more-or-less at the center 5) rotated to keep from collapsing Note that it was only a theory - parallaxes to even the nearest stars had not yet been measured - nothing was known about the way stars generate luminosity - or that stars might have luminosities different from the Sun Still, the view held into the early twentieth century Kapetyn universe Shapley and the spatial distribution of globular clusters (GC's) - globular clusters are easy to recognize - for some reason, they seem to be found more often above and below the disk plane - most stars are in the disk plane (that more of less defines the disk) - GC's are a bit odd Harlow Shapley notices that there are more GCs in one direction of the sky than in other directions -- says, "this is odd" - if we're at the center of the universe - shouldn't there be the same number of GCs in any direction - what's so special about this one particular direction where there are lots of GCs? - spatial distribution of GC's - not centered on us - 30,000 ly away toward Sag. - we'll do a lab on this, so we won't talk more about it now If the center of the GC distribution isn't here, then maybe the center of the universe isn't here, either --> finding the true extent and new center of the Gax Reconciling the old and new views of the Gax the role of dust in our Galaxy - extinction - dust dims light - like a fog - technically it scatters light out of our path - often measured in magnitudes - 1 magnitude of dust will make a star shining through it one magnitude fainter than if there were no dust scientific implications: more than half of the gax is hidden by dust - optical wavelength studies are limited --> must be a lot of dust in the gax How to "get around" the dust - don't use optical light - long wavelength light isn't as affected by dust - consider this: - airplane radar (radio ranging) works on rainy days when clouds block optical light - why? cuz radio waves travel straight through clouds Optical and infrared - optical light shows the Milky Way band - doesn't really define a direction for the center - you're not looking at the whole GAX - can't see all of it - like living in LA - can't figure out where downtown is because you can't see very far in any direction - seeing the bulge in the infrared - infrared "cuts through" dust - see the whole galaxy - easy to figure out where the center is - not here In fact, the optical is among the worst windows to use to look at the MW