'Super bubble' that crowns the Galaxy
Interstellar matter is violent and frothy -- quite different from the textbook description of cold dust clouds floating serenely among warm tenuous gases.
This has been dramatically illustrated by the recent, and totally unexpected, discovery of the "super bubble." It is the largest object yet found in the Galaxy.
This mass of superheated gases is surrounded by a cooler and denser gaseous shell over 1,600 light years in diameter. At a temperature of 3.5 million degrees F., its gases hold a staggering amount of energy -- 10 times the total energy generated by the Sun since its formation 4.6 billion years ago. Extrapolating backward from the bubble's current rate of expansion, the discoverers -- Webster Cash of the University of Colorado and Philip Charles of the University of California at Berkeley -- figure it must be at least 3 million years old.
This particular super bubble is located near Cygnus, the Swan. If it were visible, which it is not, the bubble would fit above the constellation somewhat like a crown. This new feature is about 6,000 light years from our solar system and on the opposite side of a large dust cloud called the Rift of Cygnus.
The energy source underlying the bubbles is thought to be supernovae exploding stars. These are massive, very bright stars which evolve rapidly to a point where they throw off much of their matter in a tremendous explosion. According to one estimate, a supernova emits as much energy in one second as the Sun produces in 10 years. Drs. Cash and Charles believe such explosions are frequent enough to keep the interstellar medium violently mixed. Shock waves from these cosmic detonations create regions of superheated gases in the space between the stars. These regions have been detected before, but on a much smaller scale then that of the super bubble.
"Before, it was assumed that the supernovae went off randomly and the shock-wave pattern would interact to form a series of 'tunnels' of superheated gas," Dr. Cash explains, adding "now we know that this assumption was wrong." Instead, the very existence of super bubble suggests that there is a complex interaction between these shock fronts and the process of star formation.
The astronomers explored the possibility that the super bubble may have been caused by a single extraordinarily large stellar explosion. But the details of the bubble structure are not what would expected if that had been the case. Instead, the scientists think that the giant bubble has been blown up gradually by as many as 100 individual supernovae.
Given the average rate of supernovae explosions for galaxies similar to ours, the probability that such a large number of supernovae should occur ramdomly in this particular region of space is quite small. However, the Rift is a dense cloud of dust and gas, 600 x 1,200 light years across, which contains enough material to makes several million stars. Dr. Cash believes that the edge of the bubble, passing through the Rift, induced formation of a large number of new stars. Of these, he thinks a certain percentage ultimately became supernovae, providing the energy to blow the bubble even larger.
Besides the super bubble itself, there is other evidence to support this contention. Shining through the obscuring clouds of the Rift is a group of unusual stars -- the "Cygnus OB2" association. These are extremely bright young stars. They are so bright, in fact, that were the Rift not obscuring them, they would be as bright as the Pleiades even though 15 times farther away. Because they are a million years younger than the super bubble itself, Dr. Cash believes their information was probably stimulated by its impact.
Also, one of the newest stars known is in the vicinity of the super bubble's edge. The two astronomers have subsequently found a similar star in a smaller super bubble located in the constellation orion.
"This is a very efficient process for the forming of new stars. Odds are that the sun was formed at the edge of such a super bubble 4.5 billion years ago ," Dr. Cash says.
Super bubbles were not discovered sooner because they are so hot they emit x-rays rather than visible light. Only in recent years, with the orbiting of x-ray observing satellites, have astronomers been able to study the sky at all thorougly, using such short wave lengths.
However, astronomers have long known that something unusual was going on in these regions. They are characterized by twisting filaments of glowing hydrogen gas. Usually such filaments are energized by extremely bright stars and astronomers had long been puzzled by the lack of such an association in Cygnus and Orion. Now it looks as though a massive, but previously unknown galactic feature is involved. In fact, Drs. Cash and Charles believe that as much as 10 percent of the interstellar medium in our Milky Way Galaxy may consist of super bubbles.