SP-466 The Star Splitters
 
 

11

SUPERBUBBLES

 

[99] It has been known for some time that supernovas profoundly affect the interstellar medium over distances of several light years, sweeping interstellar gas up into shells that glow in radio, optical, and X-radiation for tens of thousands of light years. In the past few years it has become apparent that the effects of a supernova are much more dramatic. They evidently play a major role in the formation of solar systems such as our own, as well as the structure and evolution of the galaxy as a whole.

Some of the latest evidence comes from data collected by the A-2 X-ray experiment aboard the first NASA HEAO. In the course of a search of the HEAO I data for X-ray emission from normal stars, a group of scientists came across a previously uncatalogued source. The X-ray map showed that the X-rays did not come from a point-like source, as might be expected from a star. Rather, they were spread out over a large region. The scientists thought they had discovered a new supernova remnant, that is, a cloud of hot gas generated by a supernova explosion.

However, when they looked at X-ray data from adjacent parts of the sky, they found to their surprise that the new source was much larger than a normal supernova remnant. The cloud of gas, which has a temperature of several million degrees, stretches across more than a thousand light years; it contains a gaseous mass equal to that of several hundred thousand suns. Supernovas can produce bubbles of hot gas. A dozen or more of these have been observed. But the bubbles from supernovas have diameters ranging from several light years to slightly more than a hundred light years. The Cygnus superbubble is 10 times larger than the large supernova remnants, occupies 1000 times more volume, and contains 20 times more energy.

The X-rays do not tell the whole story. Radio and optical observations show that the superbubble is surrounded by a shell of cooler hydrogen gas and a network of glowing gaseous filaments. The shell is expanding at a rate of about 10 km per second, or about 20000 miles per hour as it is pushed outward by the pressure of the hot gas in the superbubble. From the dynamics of the expansion and the observed size of the superbubble, its age can be estimated at about three million years. Near the center of the bubble is a closely bunched group of bright young stars. These stars are surrounded by a smaller bubble of gas that, because of its relatively low temperature (1OOOO or 20000 degrees), shows up best in radio band, where it is one of the strongest sources in the sky. The Cygnus superbubble is not an isolated example. Radio, optical, and X-ray astronomers have discovered a number of [100] objects, called supershells, rings, spurs, etc., that are most probably superbubbles in various stages of evolution.

What are superbubbles? How are they produced? Where do they get their energy? The Cygnus superbubble has an unusually large number of massive stars inside it, in what is called the Cygnus OB2 association. Are they somehow responsible for the superbubble? It seems very unlikely. Observations of the energy produced in the form of radiation and "winds" of stellar gas by these and similar stars show that there are not enough stars, massive or otherwise, inside the superbubble to explain its origin. Furthermore, the stars in the Cygnus OB2 association are only about I million years old, whereas the superbubble must have been expanding for about 3 million years. Clearly, the massive stars did not produce the superbubble. On the contrary, we shall see later that they may have been produced by the superbubble!

What about a single giant explosion, a super-supernova? After all, we now know of stars that are over a hundred times as massive as the Sun, such as the monstrous star in the southern constellation of Carina. Could they explode and release the energy of 20 supernovas? This idea, intriguing though it might be, does not square with the facts. A super-supernova energetic enough to carve out the superbubble would produce far more X-radiation than is observed.

A more efficient method for making a superbubble is to do it a little bit at a time. As Webster Cash and Philip Charles have put it, "A dynamite blast will create a hole very quickly, but a man with a shovel can, over a longer period of time, accomplish the same goal expending far less energy." What is needed then is something intermediate between stellar winds (which is like digging a hole with a spoon-it would take too long) and a supersupernova. The obvious solution is a series of supernova explosions. But how can you get a chain reaction of supernovas? And how would they create a superbubble? A combination of careful observation and ingenious theory has led to the following rough scenario.

A giant cloud of gas, containing enough matter to make millions of stars, forms in interstellar space. Perhaps by chance, perhaps as a result of the processes that caused the formation of the cloud in the first place, one or more massive stars form in the vicinity of the cloud. As this first generation of stars ages and become giant stars, their outer layers evaporate at high speeds. This wind of stellar material pushes away the matter in the surrounding space, creating a hot bubble.

Less than a million years after entering the giant phase, the massive stars begin to explode. The blast waves from these supernova explosions rush through the bubble, inflating it and driving it at high speeds into the cloud from which the stars formed. The compression of matter along the periphery of the cloud triggers the formation of a new generation of stars. The more massive stars in this generation evolve rapidly and explode, further....

 


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HEAO 1 X-ray image of the Cygnus superbubble.

HEAO 1 X-ray image of the Cygnus superbubble. The bright yellow and orange colors outline a giant glowing ring of hot gas that is over 1000 light years in diameter. The dark lane down the middle of the superbubble is caused by absorption from a dust lane known as the Great Rift of Cygnus. The dark spots at the lower middle and lower right are areas where strong X-ray stars appear by coincidence in the same field of view. (Courtesy Stuart Bowyer)

 

...inflating the bubble, until it reaches superbubble size. The superbubble would continue to grow until it literally ran out of gas. This would happen when it reached the outer edges of the galaxy or until it collided and coalesced with another expanding superbubble. Calculations indicate that the galaxy is laced with tunnels of hot rarified gas produced by the coalescence of superbubbles. Our own solar system is apparently inside such a tunnel.

In this view, superbubbles are produced by one of the most awesome chain reactions imaginable: a chain reaction of supernova explosions. What controls it? Could it sweep through a galaxy? Given the right conditions, that is, enough gas clouds to sustain the chain, it could. Some astronomers believe that the spiral arms of galaxies are just that, a trail of stars formed in the wake of a wave of supernovas that rushed through the galaxy.

Although not everyone agrees with this method of making galactic spiral arms, there is independent evidence that the blast of supernovas can trigger star formation. Several cases have been found in which new stars are [102] distributed in an arc behind an expanding cloud of matter that has evidently been shocked by a powerful blast, probably from a supernova that occurred a million or more years ago. An odd mixture of radioactive isotopes found in meteorites indicates that a blast wave from a supernova triggered the collapse some 4.5 billion years ago of the cloud that was to become our solar system.

Supernovas, then, are the explosions that drive the galactic engine forward in its evolution. Without them, the formation of stars might occur much less frequently, and the recycling of the heavy elements necessary for life might occur much more slowly.

 

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