How do we know that we are one of millions of other stars in a spiral arm of the Milky Way Galaxy?  While we may have taken this on faith by reading articles or textbooks, we can actually “see it for ourselves” by observing the sky in August.  All we need is a star chart, a pair of binoculars, a small telescope, and a clear, dark sky.

First, find the Milky Way.  It is a cloudy haze of light that begins on the southern horizon near the tail of Scorpius, rises north past the “tea pot” of Sagittarius, passes through Scutum, Aquila, and Cygnus, then dives through Cassiopeia and Perseus into the northwestern horizon.  It continues to flow beneath our feet (visible during wintertime), encircling us.  We can conclude that we are within this band of light called “The Milky Way.”   

Next, with binoculars, scan the Milky Way.  You will see that it actually consists of hundreds of stars (millions actually).  You will also see many tight groups of stars and hazy patches of light.  These are the open star clusters (such as M7, M11 and NGC6823) and the star-forming nebulae (such as M6 and M8).  There are also planetary nebulae and other collections of dust and debris (visible with a telescope or photographs).  These objects are only present in the band of the milky way due to the effects of gravity and “angular momentum” as they orbit around a common center.  We therefore can conclude that the milky way is not just a “band;” it’s a disc, viewed by us from the side!

Finally, with a telescope, look at the numerous globular clusters in the August sky. These include the bright M4 in Scorpius, M13 in Hercules, and M15 in Pegasus; but don’t miss the small by striking NCG6934 and NGC7006 in Delphinus. These ancient “globes,” each containing hundreds of thousands of stars, appear above and below the disc of the Milky Way.   They also seem to cluster around a point near the southern horizon, in Sagittarius.  They do this because they are orbiting a common center of gravity: the galactic center of the Milky Way!  Likewise, the entire disc of the milky way, including us and all the stars, star clusters, and nebulae that we can see are also orbiting around this same galactic center.

So, our own observations tell us that we are located within a disc of stars (the “Milky Way”) orbiting at some distance away from the galactic center (in Sagittarius).  Compare the diagram below to the map on the back of the newsletter to see how our observations can infer the structure of our galaxy.  

And, in fact, careful observations and detailed measurements by professional astronomers have  confirmed that we are located in the Milky Way’s “Orion band,” some 30000 lightyears from the galactic center (see the figure below). 

Jim