Part I Of Time And Space 10. A Galaxy At A Time
Four or five'vears ago there was a small fire at a school two blocks from my house. It wasn't much of a fire, really, producing smoke and damaging some rooms in the base ment, but nothing more. What's more, it was outside school hours so that no lives were in danger.
Nevertheless, as soon as the first piece of fire apparatus was on the scene the audience had begun to gather. Every idiot in town and half the idiots from the various con tiguous towns came racing down to see the fire. They came by auto and by oxcart, on bicycle and on foot. They came with girl friends on their arms, with aged parents on their shoulders, and with infants at the breast.
They parked all the streets solid for miles around and after the first fire engine had come on the scene nothing more could have been added to it except by helicopter.
Apparently this happens every time. At every disaster, big or small, the two-legged ghouls gather and line up shoulder to shoulder and chest to back. They do this, it seems, for two purposes: a) to stare goggle-eyed and slack-jawed at destruction and misery, and b) to prevent the approach of the proper authorities who are attempting to safeguard life and property.
Naturally, I wasn't one of those who rushed to see the fire and I felt very self-righteously noble about it. How ever (since we are all friends), I will confess that this is not necessarily because I am free of the destructive in stinct. It's just that a messy little fire in a basement isn't my idea of destruction; or a good, roaring blaze at the munitions dump, either.
If a star were to blow up, then we might have some thing.
Come to think of it, my instinct for destruction must be well developed after all, or I wouldn't find myself so fascinated by the subject of supernovas, those colossal stellar explosions.
Yet in thinking of them, I have, it turns out, been a piker. Here I've been assuming for years that a supernova was the grandest spectacle the universe had to offer (pro vided you were standing several dozen light-years away) but, thanks to certain 1963 findings, it turns out that a supemova taken by itself is not much more than a two inch firecracker.
This realization arose out of radio astronomy. Since World War 11, astronomers have been picking up micro wave (very short radio-wave) radiation from various parts of the sky, and have found that some of it comes from our own neighborhood. The Sun itself is a radio source and so are Jupiter and Venus.
The radio sources of the Solar System, however, are virtually insignificant. We would never spot them if we weren't right here with them. To pick up radio waves across the vastness of stellar distances we need something better. For instance, one radio source from beyond the Solar System is the Crab Nebula. Even after its radio waves have been diluted by spreading out for five thousand light-years before reaching us, we can still pick up what remams and impinges upon our instruments. But then the Crab Nebula represents the remains of a supernova that blew itself to kingdom come-the first light of the explo sion reaching the Earth about 900 years ago.
But a great number of radio sources lie outside our Galaxy altogether and are millions and even billions of light-years distant. Still their radio-wave emanations can be detected and so they must represent energy sources that shrink mere supemovas to virtually nothing.
For instance, one particularly strong source turned out, on investigation, to arise from a galaxy 200,000,000 light years away. Once the large telescopes zeroed in on that galaxy it turned out to be distorted in shape. After closer study it became quite clear that it was not a galaxy at all, but two galaxies in the process of collision.
When two galaxies collide like that, there is little likeli hood of actual collisions between stars (which are too small and too widely spaced). However, if the galaxies possess clouds of dust (and many galaxies, including our own, do), these clouds will collide and the turbulence of the collision will set up radio-wave emission, as does the turbulence (in order of decreasing intensity) of the gases of the Crab Nebula, of our Sun, of the atmosphere of Jupiter, and of the atmosphere of Venus.
But as more and more radio sources were detected and pinpointed, the number found among the far-distant',ga laxies seemed impossibly high. There might be occasional collisions among galaxies but it seemed most unlikely that there could be enough collisions to account for all those radio sources.
Was there any other possible explanation? What was needed was some cataclysm just as vast and intense as that represented by a pair of colliding galaxies, but one that involved a single gallaxy. Once freed from the neces sity of supposing collisions we can explain any number of radio sources.
But what can a single galaxy do alone, without the help of a sister galaxy?
Well, it can explode.
But how? A galaxy isn't really a single object. It is simply a loose aggregate of up to a couple of hundred billion stars. These stars can explode individually, but how can we have an explosion of a whole galaxy at a time?
To answer that, let's begin by understanding that a galaxy isn't really as loose an aggregation as we might tend to think. A galaxy like our own may stretch out 100,000 light-years in its longest diameter, but most of that consists of nothing more than a thin powdering of stars-thin enough to be ignored. We happen to live in this thinly starred outskirt of our own Galaxy so we accept that as the norm, but it isn't.
The nub of a galaxy is its nucleus, a dense packet of stars roughly spherical in shape and with a diameter of, say, 10,000 light-years. Its volume is then 525,000,000,000 cubic light-years, and if it contains 100,000,000,000 stars, that means there is I star per 5.25 cubic light-years.
With stars massed together like that, the average dis tance between stars in the galactic nucleus is 1.7 light-years - but that's the average over the entire volume. The den sity of star numbers in such a nucleus increases as one moves toward the center, and I think it is entirely fair to expect that toward the center of the nucleus, stars are not separated by more than half a light-year.
Even half a light-year is something like 3,000,000,000, 000 miles or 400 times the extreme width of Pluto's orbit, so that the stars aren't actually crowded, they're not likely to be colliding with each other, and yet...
Now suppose that, somewhere in a galaxy, a supernova lets go.
What happens?
In most cases, nothing (except that one star is smashed to flinders). If the supernova were in a galactic suburb in our own neighborhood, for instance-the stars would be so thinly spread out that none of them would be near enough to pick up much in the way of radiation. The in credible quantities of energy poured out into space by such a supemova would simply spread and thin out and come to nothing.
In the center of a galactic nucleus, the supernova is not quite as easy to dismiss. A good supernova at its height is releasing energy at nearly 10,000,000,000 times the rate of our Sun. An object five light-years away would pick up a tenth as much energy per second as the Earth picks up from the Sun. At half a light-year from the supernova it would pick up ten times as much energy per second as Earth picks up from the Sun.
This isn't good. If a supernova let go five light-years from us we would have a year of bad heat problems. If it were half a light-year away I suspect there would be little left of earthly life. However, don't worry. There is only one star-system within five light-years of us and it is not the kind that can go supemova.
But what about the effects on the stars themselves? If our Sun were in the neighborhood of a supernova it would be subjected to a batb of energy and its own temperature would have to go up. After the supernova is done, the Sun would seek its own equilibrium again and be as good as before (though life on its planets may not be). However, in the process, it would have increased its fuel consump tion in proportion to the fourth power of its absolute tem perature. Even a small rise in temperature might lead to a surprisingly large consumption of fuel.
It is by fuel consumption that one measures a star's age.
When the fuel supply shrinks low enough, the star expands into a red giant or explodes into a supernova. A distant supenova by [email protected] the Sun slightly for a year might cause it to move a century, or ten centuries closer to such a crisis. Fortunately, our Sun has a long lifetime ahead of it (several billion years), and a few centuries or even a million years would mean little.
Some stars, however, cannot afford to age even slightly.
They are already close to that state of fuel consumption which will lead to drastic changes, perhaps even supernova -hood. Let's call such stars, which are on the brink, pre supernovas. How many of them would there be per galaxy?
It has been estimated that there are an average of 3 supemovas per century in the average galaxy. That means that in 33,000,000 years there are about a million super novas in the average galaxy. Considering that a galactic life span may easily be a hundred billion years, any star that's only a few million years removed from supemova hood may reasonably well be said to be on the brink. if, out of the hundred billion stars in an average galac tic nucleus, a million stars are on the brink, then 1 star out of 100,000 is a pre-supern6va. This means that pre supemovas within galactic nuclei are separated by average distances of 80 light-years. Toward the center of the nu cleus, the average distance of separation might be as low as 25 light-years.
But iven-at 25 light-years, the light from a supemova would be only 1/2:-,o that which the Earth receives from the
Sun, and its effect would be trifling. And, as a matter of fact, we frequently see supemovas light up one galaxy or another and nothing happens. At least, the supemova slowly dies out and the galaxy is then as it was before.
However, if the average galaxy has I pre-supemova in every 100,000 stars, particular galaxies may be poorer than that in supernovas richer. An occasional galaxy may be particularly rich and I star out of every 1000 may be a pre-supernova.
In such a galaxy, the nucleus would contain 100,000, 000 pre-supemovas, separated by an average distance of 17 light-years. Toward the center, the average separation might be no more than 5 light-years. If a supemova lights up a pre-supernova only 5 light-years away it will shorten its life significantly, and if that supernova had been a thousand years from explosion before, it might be only two months from explosion afterward.
Then, when it lets go, a more distant pre-supemova which has had its lifetime shortened, but not so drastically, by the first, may have its lifetime shortened again by the second and closer supernova, and after a few months it blasts.
On and on like a bunch of tumbling dominoes this would go, until we end up with a galaty in which not a single supernova lets bang, but several million,perhaps, one after the other.
There is the galactic explosion. Surely such a tumbling of dominoes would be sufficient to give birth to a corusca tion of radio waves that would still be easily detectable even after it had spread out for a billion light-years.
Is this just speculation? To begin with, it was, but in late 1963 some observational data made it appear to be more than that.
It involves a galaxy in Ursa Major which is called M82 because it is number 82 on a list of objects in the heavens prepared by the French astronomer Charles Messier about two hundred years ago.
Messier was a comet-hunter and was always looking through his telescope and thinking he had found a comet and turning handsprings and then finding out that he had been fooled by some foggy object which was always there and was not a comet.
Finally, he decided to map each of 101 annoying ob jects that were foggy but were not comets so that others would not be fooled as he was. It was that list of annoy ances that made his name immortal.
The first on his list, Ml, is the Crab Nebula.. Over two dozen are globular clusters (spherical conglomerations of densely strewn stars), Ml 3 being the Great Hercules Clus ter, which is the largest known. Over thirty members of his list are galaxies, including the Andromeda Galaxy (M31) and the Whirlpool Galaxy (M51). Other famous objects on the list are the Orion Nebula (M42), the Ring Nebula (M57), and the Owl Nebula (M97).
Anyway, M82 is a galaxy about 10,000,000 light-years from Earth which aroused interest when it proved to be a strong radio source. Astronomerp. turned the 200-inch telescope upon it and took pictures, through filters that blocked all light except that coming from hydrogen ions.
There was reason to suppose that any disturbances that might exist would show up most clearly among the hydro gen ions.
They did! A three-hour exposure revealed jets of bydro gen up to a thousand light-years long, bursting out of the galactic nucleus. The total mass of hydrogen being shot out was the equivalent of at least 5,000,000 average stars.
From the rate at which the jets were traveling and the distance they had covered, the explosion must have taken place about 1,500,000 years before. (Of course, it takes light ten million years to reach us from M82, so that the explosion took place 11,500,000 years ago, Earth-time just at the beginning of the Pleistocene Epoch.)
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.
Now let us say that Omega Centauri contained 10,000 pre-supemovas and that every one of these exploded at their earliest opportunity. There would be fewer twinkles altogether, but they would appear over a shorter time in terval and would be, individually, twice as bright.
It would be a perfectly ideal explosion, for it would be unobscured by dust clouds; it would be small enough to be quite safe; and large enough to be sufficiently spectacular for anyone.
And yet, now that I've worked up my sense of excite ment over the spectacle, I must admit that the chances of viewing an explosion in Omega Centauri are just about nil.
And even if it happened, Omega Centauri is not visible in New England and I would have to travel quite a bit south ward if I expected to see it high in the sky in full glory and I don't like to travel.
Hmm... Oh well, anyone for a neighborhood fire?
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