Our Sun and the Local System of stars are part of a much larger disc-shaped collection of many billions of stars, gas, and dust that are bound together by mutual gravitational attraction to a vast flattened system that turns like a great pinwheel in space. A simplified sketch of our galaxy is presented above. There is a dense bright central region or nucleus and spiral arms extend from the nucleus outward into space to form a flat disk. These arms become increasingly thin until they are imperceptible. Our Earth and the solar system is embedded within the great disk that is our galaxy, and from our vantage point within this disk (and toward the edge of the great wheel), the plane of the galaxy appears as a great glowing arch in the night sky -- the Milky Way. All stars that we can see with the naked eye and almost all stellar material that is visible to our telescopes is concentrated within this galactic disk or plane, as it is called. The immense mass of stars and light has been known to man since time began through many names: River of Heaven, River of Light, Silver Street, Winter Street, Shining Wheel and The Ashen Path.
Figures A, B, and C below illustrate the general features of the Galaxy. Our Sun is located very much toward the edge or rim of the galactic disk, rather than toward the center. Keep in mind that we are embedded deep within the plane of the galaxy and that there are countless stars above and below us as well as toward the center and rim of the galactic disk. However, by far the greatest concentration of light and stellar matter appears to us in the direction of the Galactic Center and Anti-Center, as we look through or along the actual plane of the galaxy. If we look (from the Earth) in the direction of the North or South Pole of the Galaxy, we are not peering through the countless stars concentrated in the disk, but rather through a relatively thin sheet of stars between us and the intergalactic void beyond.
Because we are situated so far out on the galactic disc, there is also a great difference in what we see when we look toward the Center as opposed to the Anti-Center of our galaxy. There is much less material between us and the rim or edge of the galactic plane than there is toward the galactic center. When we gaze toward the galactic center (GC), we receive the combined light from all the stars between us and that center as well as the light stemming from the stars in the galactic disc beyond the center. In fact, as we look into the GC, we receive light (at once) that has been travelling to reach us for very different periods of time.
Keep in mind that although it takes some 9 minutes for light to reach us from the Sun, it takes a period of around four years for us to receive light from even the nearest of stars. When we consider what we see as we gaze toward the GC, it becomes difficult to comprehend. We are looking at starlight that may have been travelling to reach us for 50-70,000 years! In other words we are looking at stars as they were a long, long time ago. We are looking into the past at the universe then. Who knows if these stars even exist now and, if so, what kind of light they give off today. We will not know for another 50,000 years of so. The stars in the anti-center direction are not so distant from us and we have a more up-to-date idea of how the galaxy is when we look in this direction.
Copyright (c) 1997-99 Michael Erlewine