The Black Cloud

by Fred Hoyle

  “Is there anything wrong, do you think?”

  “Nothing so far as I can see.” Marlowe took a magnifying glass out of his pocket and scanned carefully over the plate.

  “Looks perfectly all right. No plate defects.”

  “Tell me why you’re so surprised, Dr Marlowe.”

  “Well, isn’t this what you wanted me to look at?’

  “Not by itself. It’s the comparison with a second plate that I took a month later that looks so odd.”

  “But this first one is singular enough,” said Marlowe. “You’ve had it lying in your drawer for a month! Pity you didn’t show it to me right away. But of course, you weren’t to know.”

  “I don’t see why you’re so surprised by this one plate, though.”

  “Well, look at this dark circular patch. It’s obviously a dark cloud obscuring the light from the stars that lie beyond it. Such globules are not uncommon in the Milky Way, but usually they’re tiny things. My God, look at this! It’s huge, it must be the best part of two and a half degrees across!”

  “But, Dr Marlowe, there are lots of clouds bigger than this, especially in the region of Sagittarius.”

  “If you look carefully at what seem like very big clouds, you’ll find them to be built up of lots of much smaller clouds. This thing you’ve got here seems, on the other hand, to be just one single spherical cloud. What really surprises me is how I could have missed anything as big as this.”

  Marlowe looked again at the markings on the plate.

  “It is true that it’s in the south, and we’re not so concerned with the winter sky. Even so, I don’t see how I could have missed it when I was working on the Trapezium in Orion. That was only three or four years ago and I wouldn’t have forgotten anything like this.”

  Marlowe’s failure to identify the cloud — for this is undoubtedly what it was — came as a surprise to Jensen. Marlowe knew the sky and all the strange objects to be found in it as well as he knew the streets and avenues of Pasadena.

  Marlowe went over to the sideboard to renew the drinks. When he came back, Jensen said:

  “It was this second plate that puzzled me.”

  Marlowe had not looked at it for ten seconds before he was back to the first plate. His experienced eye needed no ‘blinker’ to see that in the first plate the cloud was surrounded by a ring of stars that were either absent or nearly absent in the second plate. He continued to gaze thoughtfully at the two plates.

  “There was nothing unusual about the way you took these pictures?”

  “Not so far as I know.”

  “They certainly look all right, but you can never be quite sure.”

  Marlowe broke off abruptly and stood up. Now, as always when he was excited or agitated, he blew out enormous clouds of aniseed-scented tobacco smoke, a South African variety. Jensen marvelled that the bowl of his pipe did not burst into flames.

  “Something crazy may have happened. The best thing we can do is to get another plate shot straight away. I wonder who is on the mountain tonight.”

  “You mean Mount Wilson or Palomar?”

  “Mount Wilson. Palomar’s too far.”

  “Well, as far as I remember, one of the visiting astronomers is using the 100-inch. I think Harvey Smith is on the 60-inch.”

  “Look, it would probably be best if I went up myself. Harvey won’t mind letting me have a few moments. I won’t be able to get the whole nebulosity of course, but I can get some of the star fields at the edge. Do you know the exact co-ordinates?”

  “No. I phoned as soon as I’d tried the plates in the “blink”. I didn’t stop to measure them.”

  “Well, never mind, we can do that on the way. But there’s no real need to keep you out of bed, Knut. Why don’t I drop you at your apartment? I’ll leave a note for Mary saying I won’t be back until sometime tomorrow.”

  Jensen was excited when Marlowe dropped him at his lodging. Before he turned in that night he wrote letters home, one to his parents telling them very briefly of the unusual discovery, and another to Greta saying that he believed he’d stumbled on something important.

  Marlowe drove to the Observatory offices. His first step was to get Mount Wilson on the phone and to talk to Harvey Smith. When he heard Smith’s soft southern accent, he said:

  “This is Geoff Marlowe. Look, Harvey, something pretty queer has turned up, so queer that I’m wondering if you’d let me have the 60-inch for tonight. What is it? I don’t know what it is. That’s just what I want to find out. It’s to do with young Jensen’s work. Come down here at ten o’clock tomorrow and I’ll be able to tell you more about it. If you’re bored I’ll stand you a bottle of Scotch. That’s good enough for you? Fine! Tell the night assistant that I’ll be up at about one o’clock, will you?”

  Marlowe next put through a call to Bill Barnett of Caltech.

  “Bill, this is Geoff Marlowe ringing from the offices. I wanted to tell you that there’ll be a pretty important meeting here tomorrow morning at ten o’clock. I’d like you to come along and bring a few theoreticians along. They don’t need to be astronomers. Bring several bright boys … No, I can’t explain now. I’ll know more tomorrow. I’m going on the 60-inch tonight. But I’ll tell you what, if you think by lunch-time tomorrow that I’ve got you out on a wild-goose chase, I’ll stand you a crate of Scotch … Fine!”

  He hummed with excitement as he hurried down to the basement where Jensen had been working earlier in the evening. He spent some three-quarters of an hour measuring Jensen’s plates. When at last he was satisfied that he would know exactly where to point the telescope, he went out, climbed into his car, and drove off towards Mount Wilson.

  Dr Herrick, the Director of the Observatory, was astonished to find Marlowe waiting for him when he reached his office at seven-thirty the following morning. It was the Director’s habit to start his day some two hours before the main body of his staff, “in order to get some work done’, as he used to say. At the other extreme, Marlowe usually did not put in an appearance until ten-thirty, and sometimes later still. This day, however, Marlowe was sitting at his desk, carefully examining a pile of about a dozen positive prints. Herrick’s surprise was not lessened when he heard what Marlowe had to say. The two men spent the next hour and a half in earnest conversation. At about nine o’clock they slipped out for a quick breakfast, and returned in time to make preparations for a meeting to be held in the library at ten o’clock.

  When Bill Barnett’s party of five arrived they found some dozen members of the Observatory already assembled, including Jensen, Rogers, Emerson, and Harvey Smith. A blackboard had been fitted up and a screen and lantern for showing slides. The only member of Barnett’s party who had to be introduced round was Dave Weichart. Marlowe, who had heard a number of reports of the abilities of this brilliant twenty-seven-year-old physicist, noted that Barnett had evidently done his best to bring a bright boy along.

  “The best thing I can do,” began Marlowe, “is to explain things in a chronological way, starting with the plates that Knut Jensen brought to my house last night. When I’ve shown them you’ll see why this emergency meeting was called.”

  Emerson, who was working the lantern, put in a slide that Marlowe had made up from Jensen’s first plate, the one taken on the night of 9 December 1963.

  “The centre of the dark blob,” went on Marlowe, “is in Right Ascension 5 hours 49 minutes, Declination minus 30 degrees 16 minutes, as near as I can judge.”

  “A fine example of a Bok globule,” said Barnett.

  “How big is it?”

  “About two and a half degrees across.”

  There were gasps from several of the astronomers.

  “Geoff, you can keep your bottle of whisky,” said Harvey Smith.

  “And my crate, too,” added Bill Barnett amidst the general laughter.

  “I reckon you’ll be needing the whisky when you see the next plate. Bert, keep rocking the two backwards and forwards, so that we can get some idea o
f a comparison,” went on Marlowe.

  “It’s fantastic,” burst out Rogers, “it looks as if there’s a whole ring of oscillating stars surrounding the cloud. But how could that be?”

  “It can’t,” answered Marlowe. “That’s what I saw straight away. Even if we admit the unlikely hypothesis that this cloud is surrounded by a halo of variable stars, it is surely quite inconceivable that they’d all oscillate in phase with each other, all up together as in the first slide, and all down together as in the second.”

  “No, that’s preposterous,” broke in Barnett. “If we’re to take it that there’s been no slip-up in the photography, then surely there’s only one possible explanation. The cloud is moving towards us. In the second slide it’s nearer to us, and therefore it’s obscuring more of the distant stars. At what interval apart were the two plates taken?”

  “Rather less than a month.”

  “Then there must be something wrong with the photography.”

  “That’s exactly the way I reasoned last night. But as I couldn’t see anything wrong with the plates, the obvious thing was to take some new pictures. If a month made all that difference between Jensen’s first plate and his second, then the effect should have been easily detectable in a week — Jensen’s last plate was taken on 7 January. Yesterday was 14 January. So I rushed up to Mount Wilson, bullied Harvey off the 60-inch, and spent the night photographing the edges of the cloud. I’ve got a whole collection of new slides here. They’re not of course on the same scale as Jensen’s plates, but you’Il be able to see pretty well what’s happening. Put them through one by one, Bert, and keep referring back to Jensen’s plate of 7 January.”

  There was almost dead silence for the next quarter of an hour, as the star fields on the edge of the cloud were carefully compared by the assembled astronomers. At the end Barnett said:

  “I give up. As far as I’m concerned there isn’t a shadow of a doubt but that this cloud is travelling towards us.”

  And it was clear that he had expressed the conviction of the meeting. The stars at the edge of the cloud were being steadily blacked out as it advanced towards the solar system.

  “Actually there’s no doubt at all about it,” went on Marlowe. “When I discussed things with Dr Herrick earlier this morning he pointed out that we have a photograph taken twenty years ago of this part of the sky.”

  Herrick produced the photograph.

  “We haven’t had time to make up a slide,” said he, “so you will have to hand it round. You can see the black cloud, but it’s small on this picture, no more than a tiny globule. I’ve marked it with an arrow.”

  He handed the picture to Emerson who, after passing it to Harvey Smith, said:

  “It’s certainly grown enormously over the twenty years. I’m a bit apprehensive about what’s going to happen in the next twenty. It seems as if it might cover the whole constellation of Orion. Pretty soon astronomers will be out of business.”

  It was then that Dave Weichart spoke up for the first time.

  “I’ve two questions that I’d like to ask. The first is about the position of the cloud. As I understand what you’ve said, the cloud is growing in its apparent size because it’s getting nearer to us. That’s clear enough. But what I’d like to know is whether the centre of the cloud is staying in the same position, or does it seem to be moving against the background of the stars?”

  “A very good question. The centre seems, over the last twenty years, to have moved very little relative to the star field,” answered Herrick.

  “Then that means the cloud is coming dead at the solar system.”

  Weichart was used to thinking more quickly than other people, so when he saw hesitation to accept his conclusion, he went to the blackboard.

  “I can make it clear with a picture. Here’s the Earth. Let’s suppose first that the cloud is moving dead towards us, like this, from A to B.

  Then at B the cloud will look bigger but its centre will be in the same direction. This is the case that apparently corresponds pretty well to the observed situation.”

  There was a general murmur of assent, so Weichart went on:

  “Now let’s suppose that the cloud is moving sideways, as well as towards us and let’s suppose that the motion sideways is about as fast as the motion towards us. Then the cloud will move about like this. Now if you consider the motion from A to B you’ll see that there are two effects — the cloud will seem bigger at B than it was at A, exactly as in the previous case, but now the centre will have moved. And it will move through the angle AEB which must be something of the order of thirty degrees.”

  “I don’t think the centre has moved through an angle of more than a quarter of a degree,” remarked Marlowe.

  “Then the sideways motion can’t be more than about one per cent of the motion towards us. It looks as though the cloud is heading towards the solar system like a bullet at a target.”

  “You mean, Dave, that there’s no chance of the cloud missing the solar system, of it being a near-miss, let us say?”

  “On the facts as they’ve been given to us that cloud is going to score a bull’s eye, plumb in the middle of the target. Remember that it’s already two and a half degrees in diameter. The transverse velocity would have to be as much as ten per cent or so of the radial velocity if it were to miss us. And that would imply a far greater angular motion of the centre than Dr Marlowe says has taken place. The other question I’d like to ask is, why wasn’t the cloud detected sooner? I don’t want to be rude about it, but it seems very surprising that it wasn’t picked up quite a while ago, say ten years ago.”

  “That of course was the first thing that sprang to my mind,” answered Marlowe. “It seemed so astonishing that I could scarcely credit the validity of Jensen’s work. But then I saw a number of reasons. If a bright nova or a supernova were to flash out in the sky it would immediately be detected by thousands of ordinary people, let alone by astronomers. But this is not something bright, it’s something dark, and that’s not so easy to pick up — a dark patch is pretty well camouflaged against the sky. Of course if one of the stars that has been hidden by the cloud had happened to be a bright fellow it would have been spotted. The disappearance of a bright star is not so easy to detect as the appearance of a new bright star, but it would nevertheless have been noticed by thousands of professional and amateur astronomers. It happened, however, that all the stars near the cloud are telescopic, none brighter than eighth magnitude. That’s the first mischance. Then you must know that in order to get good seeing conditions we prefer to work on objects near the zenith, whereas this cloud lies rather low in our sky. So we would naturally tend to avoid that part of the sky unless it happened to contain some particularly interesting material, which by a second mischance (if we exclude the case of the cloud) it does not. It is true that to observatories in the southern hemisphere the cloud would be high in the sky, but observatories in the southern hemisphere are hard put to it with their small staffs to get through a host of important problems connected with the Magellanic Clouds and the nucleus of the Galaxy. The cloud had to be detected sooner or later. It turned out to be later, but it might have been sooner. That’s all I can say.”

  “It’s too late to worry about that now,” said the Director. “Our next step must be to measure the speed with which the cloud is moving towards us. Marlowe and I have had a long talk about it, and we think it should be possible. Stars on the fringe of the cloud are partially obscured, as the plates taken by Marlowe last night show. Their spectrum should show absorption lines due to the cloud, and the Doppler shift will give us the speed.”

  “Then it should be possible to calculate how long the cloud will be before it reaches us,” joined in Barnett. “I must say I don’t like the look of things. The way the cloud has increased its angular diameter during the last twenty years makes it look as if it’ll be on top of us within fifty or sixty years. How long do you think it’ll take to get a Doppler shift?”

  “
Perhaps about a week. It shouldn’t be a difficult job.”

  “Sorry I don’t understand all this,” broke in Weichart. “I don’t see why you need the speed of the cloud. You can calculate straight away how long the cloud is going to take to reach us. Here, let me do it. My guess is that the answer will turn out at much less than fifty years.”

  For the second time Weichart left his seat, went to the blackboard, and cleaned off his previous drawings.

  “Could we have Jensen’s two slides again please?”

  When Emerson had flashed them up, first one and then the other, Weichart asked: ‘Could you estimate how much larger the cloud is in the second slide?”

  “I would say about five per cent larger. It may be a little more or a little less, but certainly not very far away from that,” answered Marlowe.

  “Right,” Weichart continued, “let’s begin by defining a few symbols.”

  Then followed a somewhat lengthy calculation at the end of which Weichart announced:

  “And so you see that the black cloud will be here by August 1965, or possibly sooner if some of the present estimates have to be corrected.”

  Then he stood back from the blackboard, checking through his mathematical argument.

  “It certainly looks all right — very straightforward in fact,” said Marlowe, putting out great volumes of smoke.[1]

  “Yes, it seems unimpeachably correct,” answered Weichart.

  At the end of Weichart’s astonishing calculation, the Director had thought it wise to caution the whole meeting to secrecy. Whether they were right or wrong, no good could come of talking outside the Observatory, not even at home. Once the spark was struck the story would spread like wildfire, and would be in the papers in next to no time. The Director had never had any cause to think highly of newspaper reporters, particularly of their scientific accuracy.

  From midday to two o’clock he sat alone in his office, wrestling with the most difficult situation he had ever experienced. It was utterly antipathetic to his nature to announce any result or to take steps on the basis of a result until it had been repeatedly checked and cross-checked. Yet would it be right for him to maintain silence for a fortnight or more? It would be two or three weeks at least before every facet of the matter were fully investigated. Could he afford the time? For perhaps the tenth time he worked through Weichart’s argument. He could see no flaw in it.