From dark rural sites, one of the most eye catching features of the summer nighttime sky is the hazy band of light that we normally call the Milky Way. On these late July nights the Milky Way is best seen during the hours around midnight, when it stretches from the northeastern horizon, through the prominent constellation of Cygnus, the swan, that flies high overhead, on down through the teapot-shaped constellation of Sagittarius that rides above the southern horizon, and then on southward past that to the southwestern horizon.
Even the most casual of inspections will show that the Milky Way is not a uniform band of light. There are some areas that are distinctly brighter than others, and indeed the Milky Way looks brightest (and thickest) around Sagittarius. Meanwhile, around Cygnus the Milky Way appears to split into two branches, with each of these extending on southward to Sagittarius before the western branch ends while the eastern branch extends into the bright and thick section near that constellation. The dark area between these two branches has often been called the "great rift."
While the Milky Way (under that name or others) has, certainly, been known since antiquity, its actual physical nature did not start to become apparent until the Italian astronomer Galileo Galilei began examining it with his primitive telescope during the early 17th Century and found that it comprises multitudes of dim stars. A pair of binoculars will reveal the same thing today, showing that while the individual stars are too dim to be viewed with the unaided eye, collectively they produce the hazy band of light.
We now know that this band of light is actually the combined light of uncountable multitudes of distant stars that constitute the plane of our galaxy. We are located within that galactic plane, and the individual stars we see scattered across the nighttime sky are, in comparison to the stars making up the band of light, relatively nearby to us. Were we to travel to a distant location in our galaxy, our sun and these nearby stars would merge into that plane.
Once this basic nature of the Milky Way was realized, the general assumption among astronomers was that our sun was located somewhat near the center of the galaxy, and various star counts along the Milky Way (including those conducted by the well-known British astronomer William Herschel in the late 18th Century) appeared to reinforce this. However, the early 20th Century American astronomer Harlow Shapley discovered that, in addition to all the stars, there is a significant amount of dust in the Milky Way, and once this is taken into account we find that we are nowhere near the center of the galaxy. We are in fact located some 28,000 light-years away from the center – in other words, over half-way from the center to the edge – and the center itself lies in Sagittarius.
Just like dust clouds during a dust storm can obscure objects behind them, so, too, do the dust clouds in the Milky Way hide stars that are located beyond them. The great rift is in fact such a dust cloud, located at a relatively nearby 300 light-years away from us, and it effectively obscures the light of the multitudes of Milky Stars beyond it. As we look along the Milky Way we see other such clouds, both large and small, scattered along it; one such dust cloud located just northwest of the galactic center in Sagittarius has the general shape of an upside-down horse.
We see similar appearances when we examine other galaxies that are presented to us edge-on, and from our vantage point within the galactic plane we in fact see our galaxy edge-on as well. From here in the northern hemisphere where Sagittarius rides fairly low above the southern horizon, the view can be a bit misleading, but from the southern hemisphere, where Sagittarius rides high during the nighttime hours this time of year, the view of the Milky Way as an edge-on galaxy is dramatic and unmistakable.
The presence of all this dust means that study of more distant regions of the galaxy becomes quite difficult, and indeed the galactic center is completely obscured by massive amounts of dust that are in the way. There are occasional clear areas that do permit us to see distant sections of the galaxy here and there; one of these, located just to the northwest of the spout of the Sagittarius teapot, was identified and studied quite extensively by the German-American astronomer Walter Baade back in the 1940s and is usually referred to as "Baade's Window."
While this dust is generally opaque to what we call visible light, it is more transparent to light in other parts of the spectrum, for example, infrared and radio waves, as well as x-rays. The opening up of these various regions of the spectrum for study of the universe during the past few decades, often involving spacecraft since our atmosphere blocks much of this light from reaching Earth's surface, has among many other things made direct study of the galactic center possible.
We have found that the galaxy's center is an active and violent place. There appears to be a very large, super-massive black hole in the center, which is constantly swallowing up passing stars and clouds of ionized gas. The density of stars is much higher there than it is in our part of the galaxy, and due to all the active and violent goings-on the radiation environment throughout that entire region would very likely be lethal to any kind of life as we know it.
Out here in the suburbs, as it were, things are much quieter, and life on at least one world thrives abundantly. At least some of that life can view the goings-on elsewhere, and be grateful for the opportunities we do have to live in such a quiet place.