M82, then, is the case of an exploding galaxy. The energy expended is equivalent to that of five million super novas formed in rapid succession, like uranium atoms undergoing fission in an atomic bomb-though on a vastly greater scale, to be sure. I feel quite certain that if there had been any life anywhere in that galactic nucleus, there isn't any now.
In fact, I suspect that even the outskirts of the galaxy may no longer be examples of prime real estate.
Which brings up a horrible thought… Yes, you guessed it!
What if the nucleus of our own dear Galaxy explodes?
It very likely won't, of course (I don't want to cause fear and despondency among the Gentle Readers), for explod ing galaxies are probably as uncommon among galaxies as exploding stars are among stars. Still, if it's not going to happen, it is all the more comfortable then, as an intellec tual exercise, to wonder about the consequences of such an explosion.
To begin with, we are not in the nucleus of our Galaxy but far in the outskirts and in distance there is a modicum of safety. This is especially so since between ourselves and the nucleus are vast clouds of dust that will effectively screen off any visible fireworks.
Of course, the radio waves would come spewing out through dust and all, and this would probably ruin radio astronomy for millions of years by blanking out everything else. Worse still would be the cosmic radiation that might rise high enough to become fatal to life. In other words, we might be caught in the fallout of that galactic explo sion.
Suppose, though, we put cosmic radiation to one side, since the extent of its formation is uncertain and since consideration of its presence would be depressing to the spirits. Let's also abolish the dust clouds with a wave of the speculative hand.
Now we can see the nucleus. What does it look like without an explosion?
Considering the nucleus to be 10,000 light-years in diameter and 30,000 light-years away from us, it would be visible as a roughly spherical area about 20' in dia meter. When entirely above the horizon it would make up a patch about %5 of the visible sky.
Its total light would be about 30 times that given off by Venus at its brightest, but spread out over so large an area it would look comparatively dim. An area of the nucleus equal in size to the full Moon would have an average brightness only 1/200,000 of the full Moon.
It would be visible then as a patch of luminosity broad ening out of the Milky Way in the constellation of Sagit tarius, distinctly brighter than the Milky Way itself; bright est at the center, in fact, and fading off with distance from the center.
But what if the nucleus of our Galaxy exploded? The explosion would take place, I feel certain, in the center of the nucleus, where the stars were thickest and the effect of one pre-supemova on its neighbors would be most marked. Let us suppose that 5,000,000 supernovas are formed, as in M82.
If the nucleus has pre-supemovas separated by 5 light years in its central regions (as estimated earlier in the chapter, for galaxies capable of explosion), then 5,000,000 pre-supernovas would fit into a sphere about 850 light years in diameter. At a distance of 30,000 light-years, such a sphere would appear to have a diameter of 1.6', which is a little more than three times the apparent di ameter of the full Moon. We would therefore have an ex cellent view.
Once the explosion started, supernova ought to follow supemova at an accelerating rate. It would be a chain reaction.
If we were to look back on that vast explosion millions of years later, we could say (and be roughly correct) that the center of the nucleus had all exploded at once. But this is only roughly correct. If we actually watch the ex plosion in process, we will find it will take considerable time, thanks entirely to the fact that light takes considerable time to travel from one star to another.
When a supernova explodes, it can't affect a neighbor ing presupemova (5 light-years away, remember) until the radiation of the first star reaches the second-and that would take 5 years. If the second star was on the far side of the first (with respect to ourselves), an additional 5 years would be lost while the light traveled back to the vicinity of the first. We would therefore see the second supernova 10 years later than the first.
Since a supemova will not remain visible to the naked eye for more than a year or so even under the best condi tions (at the distance of the Galactic nucleus), the second supemova would not be visible until long after the first had faded off to invisibility.
In short, the 5,000,000 supemovas, forming in a sphere 850 light-years in diameter, would be seen by us to appear over a stretch of time equal to roughly a thousand years.
If the explosions started at the near edge of that sphere so that radiation had to travel away from us and return to set off other supernovas, the spread might easily be 1500 years.
If it started at the far end and additional explosions took place as the light of the original explosion passed the pre supernovas en route to ourselves, the time-spread might be considerably less.
On the whole, the chances are that the Galactic nucleus would begin to show individual twinkles. At first there might be only three or four twinkles a decade, but then, as the decades and centuries passed, there would be more and more until finally there.might be several hundred visible at one time. And finally, they would all go out and leave behind dimly glowing gaseous turbulence.
How bright will the individual twinkles be? A single supemova can reach a maximum absolute magnitude of - 17. That means if it were at a distance of 10 parsecs (32.5 light-years) from ourselves, it would have an appar ent magnitude of -17, which is 1/10,000 the brightness of the Sun.
At a distance of 30,000 light-years, the apparent magni tude of such a supemova would decline by l0 magnitudes.
The apparent magnitude would now be -2, which is about the brightness of Jupiter at its brightest.
This is quite a static statistic. At the distance of the nucleus, no ordinary star can be individually seen with the naked eye. The hundred billion stars of the nucleus just make up a luminous but featureless haze under ordinary conditions. For a single star, at that distance, to fire up to the apparent brightness of Jupiter is simply colossal. Such a supemova, in fact, burns with a tenth the light intensity of an entire non-exploding galaxy such as ours.
Yet it is unlikely that every supemova forming will be a supemova of maximum brilliance. Let's be conservative and suppose that the supemovas will be, on the average, two magnitudes below the maximum. Each will then have a magnitude of 0, about that of the star Arcturus.
Even so, the "twinkles" would be prominent indeed. If humanity were exposed to such a sight in the early stages of civilization, they would never make the mistake of think ing that the heavens were eternally fixed and unchangeable.
Perhaps the absence of that particular misconception (which, in actual fact, mankind labored under until early modern times) might have accelerated the development of astronomy.
However, we can't see the Galactic nucleus and that's that. Is there anything even faintly approaching such a multi-explosion that we can see?
There's one conceivable possibility. Here and there, in our Galaxy, are to be found globular clusters. It is estimated there are about 200 of these per galaxy. (About a hundred of our own clusters have been observed, and the other hundred are probably obscured by the dust clouds.)
These globular clusters are like detached bits of galactic nuclei, 100 light-years or so in diameter and containing from 100,000 to 10,000,000 stars-symmctricary scat tered about the galactic center.
The largest known globular cluster is the Great Hercules Cluster, M13, but it is not the closest. The nearest globular cluster is Omega Centauri, which is 22,000 light-years from us and is clearly visible to the naked eye as an object of the fifth magnitude. It is only a point of light to the naked eye, however, for at that distance even a diameter of 100 light years covers an area of only about 1.5 minutes of arc in diameter.