Greetings from Ithaca New York. My work takes me places and has now landed me in the " Land of Sagan" which is a good thing. Now that got me thinking about the size of our universe or more accurately the size and shape of our galaxy. I was reading my Astronomy magazine about the REAL structure of our galaxy. I had to chuckle about all the revisions Man has made to our little slice of the Cosmos. Hey so lets take a look at this thing called the Milky Way Galaxy or more simply the Galaxy( depends on what camp you hang out in). For me I can think of better things to argue about than the correct name of our encampment of stars. The way on back machine is set for ancient Greece right around 500-370 BC. There were two Philosophers named Anaxagoras and Democritus Now these two folks proposed that the Milky Way was made up of stars. That was a far better than the Mythological belief that some how some baby unknown to Hera (It was Zeus's illegitimate child) latched on to her breast and when she happened to notice Hey wait a minute that is not my Kid! She jerks the baby off her breast and milk shot out across the sky. While this might be plausible (snicker) I am going to go with the two Greek guys for now. Lets go forward a few years to 384 -322 BC to Aristotle's day Now where he got this idea I have no clue, but here is what he thought. Aristotle thought the Milky Way were a fiery exhalation from nearby stars were large numerous and tightly packed (that has a bit of a ring of truth to it). It seemed to always be a scale problem with the ancients. It was just very hard to imagine the universe as a unbelievably huge place. The ignition happened in the atmosphere. Now that might needs some work, at least that's what the Arab astronomer, Alhazen thought some 1300 years later he measured the apparent movement of more distant stars and deduced that the Milky Way was really far out because it had no parallax about this same time there were several theories floated about what that thing was. we now know they were all hitting around the bulls-eye they were everything from it is a sphere of stars to a large quantity of nebulous stars whet ever those are but the point is here these early Astronomers were using the information at hand and science to explain what they saw. Now keep in mind here the telescope had not been invented yet so these were all from visual observations.
1608 brought about the invention of the telescope and here is where this Milky Way thing really started to take off. Galileo observed countless stars within the Milky Way. Thomas Wright and later Immanuel Kant (1750s)pictured here.In a treatise, Kant deduced the Milky Way was a rotating disk of stars. It had the same kind of orbiting mechanics as the solar system. We are starting to get the picture now. Kant also went on to say he believed these Spiral nebulas being discovered were island universes of themselves. Galaxies/nebula located outside our Galaxy.
But hey! What does that picture look like? William Herschel took a stab at it in1785 He did it the old fashioned way; he started counting! Herschel and his sister Caroline divided the task into different areas. What a chore that was. At the end of the count they came up with some interesting conclusions. One was the counts were consistent where ever you looked so the solar system must be in the center of the Galaxy. Lord Rosse had just built a huge scope for the day and was taking a fancy to these spiral nebula until he notice individual points of light within the "nebula" That dang Kant might be on to something! Well it took almost72 more years before the island universe theory got some real traction. Now we put those two versions together and we come to Edwin Hubble. He discovered these were indeed galaxies and in the shape of a spirals barred spirals or ellipticals etc.using the famed Hooker Telescope on Mount Wilson California. He also believed that our galaxy was a spiral galaxy.This occured between the nineteen twenties and thirties.That is what I was taught growing up. Along with all of this the diameter has increased from a few miles (Aristotle) to 120,000 light years! From millions of stars to nearly 400 billion.
So Now Ladies and Gentlemen We have now got a difinative view of what our galaxy really looks like (until they change it ). We hang our hats in a barred spiral. This spiral has 4 arms two more prominent than the other two. For years we thought it had 4 major arms.We call the Orion spur of the Sagittarius arm home. Sorry Herschel old buddy. Nice try though to be sure! Now how in the world do we know this is the real deal?
New equipment Got to love new toys! We started looking at the Milky Way in the infrared and other wave lengths that allows us to see through all the dust and mess to the core of the Milky Way . How cool is that? We have a pretty good and accurate map of the home Galaxy right now but scientists are still wondering about definitive arm structure etc... and Now the mystery is solved or has it just opened up a whole new box of questions needing to be answered.It is a wild and exciting world we live in. What a cool time to be an Astronomer! So I guess there is no crying after spilled milk after all. Sorry Hera!
Clear skies and great seeing too!