During this time of the year, the hazy band of light we call the Milky Way is well seen from dark, rural sites during the nighttime hours, being high in the east during the evening.
It passes almost directly overhead, stretching from north to south during the hours around and after midnight. Even a pair of binoculars will reveal that the light of the Milky Way is made up of stars. We now know that it is the combined light of multitudes of uncountable stars that constitute the "disk" of our galaxy.
The Milky Way is brightest, and widest, around the constellation of Sagittarius, which rides at its highest above the southern horizon around midnight during early August. When we look in this direction, we are looking toward the center of our galaxy, which we now know to be located some 26,000 light-years away from us.
We can't actually "see" the center of our galaxy, since there is too much dust in the way blocking the view. With the unaided eye we can see some of this dust, which looks like clear areas in and around the Milky Way. In actuality, these are large dust clouds that block out the light from the stars behind them.
This dust is relatively transparent in the infrared, as well as to radio waves and x-rays. If we examine the center of the galaxy with instruments sensitive to these parts of the spectrum -- including with telescopes in Earth orbit above the atmosphere--- we can detect the goings-on there.
During the mid-1970s, a very strong source of radio waves was discovered at the galactic center, which has since been dubbed "Sagittarius A*" (pronounced "Sagittarius A-star"). From studies of the motions of various stars and other objects near the center, astronomers have determined that Sagittarius A* has from 2 million to 4 million times the mass of our sun. But all of this is contained within a region that's about the size of Mercury's orbit around the sun. Something that massive, but that small, can only be one thing -- a black hole, an object so dense that the escape velocity is faster than the speed of light.
Normally when the term "black hole" is utilized, we are referring to a very massive star that has exploded as a supernova. The black hole is what's left when the remnant of the supernova collapses. Sagittarius A*, however, is a much bigger beast -- and the term "super massive black hole" has been coined to describe something of this nature. We have seen evidence for such objects in the centers of other galaxies, and astronomers now feel that super massive black holes reside in the centers of most, if not almost all, large galaxies.
Like any black hole, Sagittarius A* grows by swallowing other objects, including stars that come too close, and over time, other black holes. When it does swallow an object, it gives off radiation (primarily x-rays, since the material of the object being swallowed heats up enormously due to friction). Although we've occasionally detected small bursts of x-rays from Sagittarius A*, for the most part it has been relatively quiet ever since it was first discovered.
However, a snack appears to be headed its way. In late 2011, a team of European astronomers led by Stefan Gillessen (at the Max Planck Institute for Extraterrestrial Physics in Germany) announced the discovery of a dusty gas cloud -- since dubbed "G2" -- that appeared to be headed almost directly toward the galactic center. At that time, Gillessen's team determined that G2 -- which seems to be about three times as massive as Earth, although stretched out over a length about 100 times the Earth-sun distance -- would be making its closest approach to Sagittarius A* around September. More recent observations suggest this will likely be pushed back until about March.
It appears, though, that the passage of G2 around Sagittarius A* has already started. From our perspective on Earth, G2 will pass behind the black hole. As it approaches, it will appear to be moving away from us. However, a small portion of the cloud -- about 7 percent -- now seems to be approaching us, which suggests that a small portion has completed the "turnaround" of its close approach.
The bulk of G2's passage around Sagittarius A* is still to come, of course. So, what will happen and what will we see? There is some possibility that a small star may lie enshrouded within the gas cloud. If so, its gravity will keep most of the cloud from being swallowed by the black hole.
But if there is no star, then a significant amount of material will be swallowed -- which in turn could produce large numbers of strong x-ray flares as this material heats up before being swallowed. This period of enhanced flare activity from Sagittarius A* could potentially last months, perhaps even a few years.
We don't have long to wait.
Alan Hale is a professional astronomer who resides in Cloudcroft. He is involved in various space-related research and educational activities throughout New Mexico and elsewhere. His website is earthriseinstitute.org.