Images of broken light, which dance before me like a million eyes They call me on across the universe The Beatles, Across the Universe

Assignments: Read Chapter 34, Sections 1-4 (pp. 566-574) Check out Vesto Sliper's 24-inch Alvin Clark refractor at the Lowell Observatory

In Class: -------- we're now talking about the structure of the universe on the really largest scales - distances so vast that light takes millions and often billions of years to go from point to another - e.g, even the nearest big galaxy, the Great Gax of Andromeda, is 2 million light years away. - the nearest rich cluster of galaxies, the Virgo cluster, is 50 million ly away - we'll even start using a new term more appropriate to these sizes -- "megaparsec," or million parsecs - 20 or so times the size of our galaxy On these truly grand scales, we'll stop talking about the things in the universe and instead start talking about the structure of the universe itself - shift in perception - think of the universe as one big entity, within which there is structure - rather than that the universe is just the agglomeration of the things in it - sometimes referred to as "top-down" thinking, rather than "bottom-up" - so now instead of viewing galaxies as individual systems, we wee them as "test particles" which can tell us about the structure of the universe - we look at where they're located and how they move This is just what Edwin Hubble started to do once he showed that the spiral nebulae were indeed galaxies in a giant universe. The process of measuring the properties of the spiral nebulae was begun before the debate over their nature had been settled One of the leaders in the technical process of obtaining information on these objects was Vesto Slipher - really an engineer more than a scientist - built a better mousetrap - in this case, the mousetrap was a spectrograph - problem: spiral nebulae are really faint - and to make matters worse, to get a spectrum of these objects you take this really faint light and spread it out into a spectrum - now you're taking the few photons you get and sorting them by wavelength - get very faint spectrum - solution: get more photons solution #1: more glass; make a bigger telescope - expensive, but this is what Mt. Wilson Obs did -- 100-in on line in 1917 solution #2: waste less; build a better spectrograph - use fewer mirrors; shinier optics - less expensive, but trickier - what Lowell Obs in Flagstaff, AZ did - Slipher was a master spectrograph builder - for awhile, he could do better with his 24-in telescope than the Mt Wilson crowd could do with a telescope with 16 times more collecting area (they ultimately caught up). - measured spectra of spiral nebulae - found them to be star-like spectra (evidence that they're star systems, or gax) - found the spectral lines shifted - doppler shift - gax are moving - some very fast - faster than any other celestial object -------------------------- New York Times article 19 January 1921 - not previously attempted or thought possible <-- too faint - requires long exposure "with the most powerful instrumental equipment" - exposed for two weeks - open shutter at night, close it at dawn, repeat - two weeks is the limit of "dark time" (i.e., no Moon) - necessary to disperse the light <-- make a spectrum to see spectral lines - measure the amount they are shifted <-- doppler effect - 1,100 mile/s (almost 1% of the speed of light) - most distant and enormously large <--- Slipher's bias; Hubble hasn't yet done the Cepheid work - estimate of distance - based on velocity and age of Earth (age of universe?) - assumption is that if it isn't that far away, then sometime after the Earth was formed, this thing came crashing through our neighborhood - Earth (then) was a billion years old (give or take) - 1 x 10^9 yr x (3.16 x 10^7 sec/yr) = 3.16 x 10^16 sec - distance = speed x time = 1,100 miles/sec x 3.16 x 10^16 sec = 3.48 x10^19 miles or 5.6 x 10^22 m or 1.8 million pc == 1.8 Mpc - wild ass guess, and though there's no way he could've known it at the time, more-or-less right There it is in the NYT; "All the News that's fit to Print" - next time you complain that you're learning nothing of use, remember that you've just learning how to read articles in the NYT ------------------------- Not only are these objects really moving fast, but - almost all are moving away from us - very curious - odd enough that they are moving so fast - now we find that the vast majority are moving away from us - WILL TURN OUT TO BE REALLY IMPORTANT IN A BIT Hubble's contribution to this mess was in determining the distances to as many of these galaxies as possible - that's why he was measuring Cepheids in the Andromdea galaxy in the first place - while measuring velocities is technically difficult - ie., you need great equipment - interpretation isn't all that difficult - doppler shift --> recesson velocity distance measurement, on the other hand, is much more interpretive - Cepheids are pretty straightforward, but you can't measure Cepheids in more than a few of the most nearby galaxies - either too faint, or too crowded a field to pick them out (most often, both) - need to resort to "secondary" distance indicators (a lot of these are WAGs, too) Examples of secondary distance indicators - Hubble pulls a page from history and employs Herschel's method - estimates the "average" luminosity of a gax - he can measure total flux from a bunch of galaxies - for a few, he can also get distance (say from Cepheid measurements) -- then he can calculate the luminosity for at least a few galaxies - based on this, he assumes all gax have the same luminosity - compares flux with assumed luminosity to get distance - this is very dangerous for the same reason it was dangerous for Herschel - gax aren't all the same luminosity Hubble knew (or at least suspected) this and tried to get around it - calculated the average brightness of all of the gax in a cluster (maybe average gax have the same L) on balance, it works ok, but not great