The hazy band of light that we call the Milky Way is one of the most prominent sky features that graces the evening sky during the summer months. Back in the early 17th Century the Italian astronomer Galileo Galilei examined the Milky Way with his primitive telescope and saw that it is composed of numerous stars, too dim to see individually with the unaided eye but collectively visible as that hazy band of light.
Today we know that the Milky Way is the combined light of almost uncountable multitudes of stars that constitute the plane of our galaxy — which also goes by the name of the Milky Way.
Our galaxy is made up of more than just stars, of course. Even a casual examination of the summertime Milky Way will show dark regions, which we now know to be large clouds of dust that block off the light of the stars behind them. With telescopes we can see small dust clouds that are even darker and denser, and we also see clouds of glowing gas as well.
The visible light that we detect with our eyes — either unaided, or with telescopes — is only a small fraction of what we call the electromagnetic spectrum. There are many other forms of light that our eyes cannot detect, both higher frequency forms like ultraviolet, x-rays, and gamma-rays as well as lower frequency forms like infrared and radio waves. Earth's atmosphere blocks many of these forms of light from reaching the surface, so our ability to examine the surrounding universe in these other forms of light often had to wait until we were able to launch satellites that could carry the appropriate detectors. With the opening up of these various other regions of the electromagnetic spectrum to our study over the past few decades, our view of the surrounding universe is far more comprehensive and complete than it ever could have been earlier.
We thus now know that our galaxy contains far more material than just what we can detect with visible light. In addition to the glowing gas that we can detect with normal telescopes — the glowing of which is caused by radiation from nearby stars that ionizes, or strips electrons from, the atoms within that gas— there is a lot of neutral gas, i.e., gas that has not been ionized, between the stars.
The distribution of all this gas and dust throughout the galaxy is certainly not uniform. There are regions where one or more of these various elements are thick, and other regions where they are thin. Our solar system appears to be located in one of the regions where much of this material is indeed thin, in fact, we are located within what could be called a bubble within the interstellar gas. This region that we're in has even been called the local bubble and it is about 300 light-years across.
This local bubble is far from empty, of course, as it includes the sun and all the other various other nearby stars. But although the density of the interstellar neutral gas is quite low compared to other regions of the galaxy, there is other material present as well, including relatively high amounts of thin but very hot gas that gives off large amounts of x-rays. It was the presence of these x-rays, in fact, that gave away the existence of this local bubble; the neutral gas in our galaxy absorbs these, and the fact that we are nevertheless able to detect them means that the amount of neutral gas in our region of the galaxy is quite low.
What could have caused this local bubble? The most obvious explanation is that a supernova explosion, sometime between several hundred thousand and a few million years ago, swept out a lot of the neutral gas and carved out the bubble; such an event could also have produced the hot x-ray glowing gas that we detect. It is possible that an object in the constellation Gemini that is known as Geminga, which gives off both gamma-rays and x-rays and which is now suspected to be a neutron star, i.e., the small core of a star that has exploded as a supernova, contributed at least in part to the formation of the local bubble. Geminga is located a few hundred light-years away from us and appears to have exploded approximately 300,000 years ago.
The current consensus among astronomers is that, in addition to the Geminga supernova, other supernovae have also contributed to forming the local bubble. Meanwhile, there are other such bubbles that are relatively nearby to us; one of these is located in the general direction of the bright star Antares in the constellation Scorpius, which is now prominently placed in our southern sky during the evening hours. Antares is apparently located within this bubble, and so are a lot of the very hot and massive stars that Antares is traveling together with through space. It is quite likely that one or more supernova explosions from this traveling group of stars has created this bubble, and in fact Antares itself will likely explode as a supernova sometime in the not-too-very-distant future.
In addition to the local bubble and the various other bubbles around us, there also appear to be various tunnels through the surrounding neutral gas that connect the bubbles, as well as chimneys that extend from the bubbles out of the plane of the galaxy. These features may also be due to supernova explosions, although there are obviously things we have yet to learn about the exact processes involved. In any event, though,from this life we live within our own little bubble, it is quite apparent that there is far more to our galaxy than what initially meets the eye.
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 web site is http://earthriseinstitute.org.