The Day We Found the Universe (12 page)

With the Crossley checked out, Curtis at last turned his attention to the mysterious nebulae. Keeler and others at Lick had previously amassed a photographic library of around one hundred nebulae and clusters using the Crossley. By the summer of 1913, Curtis boosted that number to more than two hundred. “Many of these nebulae show forms of unusual interest,” he jotted down in his observatory report. “The great preponderance of the spiral form becomes more and more striking with the progress of the survey.” He was beginning the process of identifying and cataloging the nebulae, particularly the spirals, in hope of detecting patterns that would lead to revealing what they were. His descriptions conveyed the rich diversity in their appearance: A spiral could be either “patchy,” “branched,” “irregular,” “elongated oval,” or “symmetrical.” For the moment, he was merely recording what he saw, not venturing to discuss what they might be.

It was tiring work. “Crossley still has its old reputation of using up more energy than any other instrument on the hill,” Curtis told a colleague. Despite the improvements he had made on the telescope, it was still difficult to reach the eyepiece at certain positions. “If you got a little bit sleepy at night, it was dangerous, because it went down a great many feet [from the observing platform] to a floor in the basement,” said one of the telescope's later users. One wisecracker suggested the only way to observe with the Crossley in comfort was to fill the dome with water and observe from a boat.

When he first started his study, Curtis assumed that the spirals were comparable to the size of a modest cluster of stars, spanning no more than several hundred lightyears in width. It was a reasonable assumption. Over at the Mount Wilson Observatory, with its new 60-inch reflector, George Ritchey had begun to photograph the spiral nebulae and was concluding they were a mix “of smooth nebulous material and also of soft star-like condensations or nebulous stars.” He surmised he was seeing a collection of developing stars—a good-sized cluster but certainly not an entire “island universe.”

But Curtis began to doubt this viewpoint as he gathered more evidence with the Crossley. Some of the first hints surfaced when he rephotographed a number of nebulae that Keeler had previously imaged. By comparing his most recent spiral pictures with those gathered years earlier, he hoped to see how the swirling clouds had rotated. The amount of motion measured was going to help him judge their distance. But Curtis didn't detect any sign of movement, not a smidgeon “rotatory or otherwise,” he reported. “As the spirals are undoubtedly in revolution—any other explanation of the spiral form seems impossible—the failure to find any evidence of rotation would indicate that they must be of enormous actual size, and at enormous distances from us.” It would simply be impossible to measure a shift by sight alone if the spiral were considerably larger and at the same time pushed far off into space.

An edge-on galaxy photographed by Heber Curtis in 1914,
showing the dark lanes of dust and gas within the disk
(Copyright UC Regents/Lick Observatory)

Even earlier Curtis started reporting that some of the spirals he photographed—the ones so tilted they were seen edge-on—resembled “the Greek letter Ф… for lack of a better term”: an oval ring crossed by a straight dark line. He expressly mentioned them in his research notes: NGC 891 “shows dark lane down center,” he jotted down. And NGC 7814 was described as small but with a dark lane “beautifully clear.”

This was at a time when Yerkes astronomer E. E. Barnard was also acquainting astronomers with myriad “dark nebulae” within the Milky Way. Barnard was gathering exquisite photographic evidence that the coal-black regions within the Milky Way that appeared to be devoid of stars (“holes in the heavens,” Herschel called them) were actually clouds of cosmic gas and dust—colossal streams of inky darkness without the hint of a glow. Curtis immediately connected this finding to his work: The dark lanes he was sighting in the spirals had to be “due to the same general cause that produces certain occulting effects in our own galaxy….” The dark bands were almost certainly matter—but matter that wasn't glowing.

This also explained why no spiral was ever seen in certain areas of the celestial sky, aptly named the “zone of avoidance.” Spiral nebulae were very exclusive objects; they tended to huddle around the north and south galactic poles, as if shunning the long white swath of the Milky Way. Astronomers had long scratched their heads over this peculiar distribution. If spirals were truly the birthplaces of new stars, why weren't they found in the richest star fields? Why were the spirals found in only those sectors of the sky where stars were scarce? Not one spiral had ever been spotted in the thick of the Milky Way. Curtis cleverly deduced that this cosmic quarantine was only an illusion: If his dark-banded spirals were truly distant galaxies, then the Milky Way, too, must have its own dark band. All the dark gaseous clouds within the Milky Way were collectively acting like an opaque wall, making it impossible to see the spirals that resided beyond this obstruction, keeping the spirals hidden. “[The] great band of occulting matter in the plane of our galaxy … serves to cut off from our view the distant spirals lying near the projection of our galactic plane in space,” explained Curtis. And that couldn't happen unless the spirals were very far off.

To Curtis this argument made perfect sense, but he was presenting the idea at a time when most astronomers still thought of the vast expanses between the stars as a pristine emptiness and the Milky Way as transparent as a glass window. His reasoning wasn't as readily accepted as he had hoped.

Curtis spent much of the 1910s on this fight—gathering data, giving lectures, coming up with fresh new arguments. He gathered clues as if he were a cosmic sleuth. “Were the Great Nebula in
Andromeda
situated five hundred times as far away as at present,” reasoned Curtis, “it would appear as a structureless oval…with [a] very bright center, and not to be distinguished from the thousands of very small, round or oval nebulae found wherever the spirals are found. There is an unbroken progression from such minute objects up to the Great Nebula in
Andromeda
itself; I see no reason to believe that these very small nebulae are of a different type from their larger neighbors.” But his mounting certainty that the spirals he photographed, both large and small, were all distant galaxies strewn through space was based solely on circumstantial evidence. He had convinced his colleagues at Lick, which came to be identified as a stronghold of island-universe supporters, but the majority of astronomers still preferred to think of all the stars and nebulae as inhabiting one great system, the Milky Way. Curtis was absolutely right, but convincing the wider community of astronomers was an entirely different matter.

And then something interesting…and very unusual…happened.

On July 19, 1917, some three hundred miles southeast of Mount Hamilton, George Ritchey was taking a routine photograph of a spiral nebula with the 60-inch reflector at the Mount Wilson Observatory. It was the fourth in a series of long-exposure photos he had been taking of NGC 6946 over the previous seven years. This time, though, he noticed a new pinpoint of light in the spiral's outer region. It had to be a nova, for this “new star” wasn't in any of his previous pictures. More important, this nova was distinctly different from the dazzling one that had flared up in Andromeda thirty-two years earlier. This one was very, very
faint
.

The unforgettable nova that briefly blazed within Andromeda in 1885 had reached a brightness that could be discerned by the naked eye (just barely); the nova in NGC 6946, on the other hand, was about sixteen hundred times dimmer. Ritchey knew he had caught the nova fairly early in its burst; a plate taken just a month earlier with another telescope showed no extra speck of light whatsoever. Telegrams announcing the new find were quickly sent out to other observatories.

Curtis likely received the report with a sinking heart, for he had sighted similar novae months earlier. On the very day that Ritchey's telegram reached Lick, Curtis was actually at his desk, drafting a paper on three faint novae he had discovered in other spiral nebulae. He had been sitting on the news since March, when he first observed the flare-ups. He was being very careful, holding off any announcement until he was sure that the outbursts were not simply variable stars reaching their maximum brightness. His caution kept him from the prize of first announcement.

The first nova that Curtis spotted was in NGC 4527, an elongated spiral located in Virgo. By checking plates of this region made earlier at the Harvard, Yerkes, and Lick observatories, Curtis confirmed that no star had been visible in the spiral over the previous seventeen years. The tiny dot on his photo reached around fourteenth magnitude (some sixty thousand times dimmer than the stars in the Big Dipper). And in the course of his plate search, he came upon two additional faint novae: this time in M100 (also known as NGC 4321), a spectacular spiral in Coma Berenices viewed face-on. One of these novae had flared in 1901, the other in 1914. “That both these novae should have appeared in the
same
spiral is especially worthy of note,” reported Curtis. By the time Curtis announced his finds in July 1917, though, all three of these novae had completely disappeared. But he made sure to point out in his bulletin that the new stars “must be regarded as having a very definite bearing on the ‘island universe’ theory.”

With such startling news from both Mount Wilson and Lick, nova hunting spread like wildfire among the top U.S. observatories. Going to old astronomical plates and searching for novae became the craze, and new candidates were found right away. The list was getting longer week by week. “Such is the progress of Astronomy in the wild and wooly West,” joked one Mount Wilson astronomer. Curtis was tremendously excited by all the discoveries. Every time he found a new nova in a spiral, he'd go through the observatory and show off the plate, like some proud papa in a hospital maternity ward.

Curtis soon had a big enough sample of novae to make a judgment call: He suspected that the 1885 outburst in Andromeda, as well as the 1895 one in Centaurus, were rare and exceptional celestial events. Curtis guessed that their spectacular radiance had misled astronomers into thinking the novae's host nebulae had to be close by. He suggested that nova bursts actually came in two varieties: The rarer ones were big and spectacular (now known to be stars blowing apart), while the ones seen more often were less energetic (determined later to be a flaring off the surface of a white dwarf star). And since the majority of the novae being sighted in the spirals more resembled the ordinary novae seen periodically within the Milky Way, he concluded that the spiral nebulae had to be
millions
of lightyears distant, in order for those novae to appear so dim. He said as much to the Associated Press. He boldly told its reporter that the nova bursts he had discovered occurred some 20 million years in the past, meaning the nebulae had to be 20 million lightyears distant for the light to be reaching us now. (With 1 lightyear equaling about six trillion miles, that's more than a hundred million trillion miles.) For Curtis the faint novae were bona fide proof that the nebulae resided far beyond the borders of the Milky Way. But Curtis was championing this idea too early, before the physics could explain it. Many of his fellow astronomers were still fairly skeptical, unwilling to conjure up new celestial creatures willy-nilly. For them “Occam's Razor” prevailed, the long-standing rule of thumb established by the English philosopher William of Occam in the fourteenth century.
“Pluralitas non est ponenda sine necessitate
,” declared Occam, which can be translated as “plurality must not be posited without necessity.” Best to choose the simplest interpretation over an unnecessarily complex one—unless forced to do otherwise. One type of nova was far more preferable than two.

Arrows point to the novae discovered by Heber Curtis
in photos of NGC-4321 taken in 1901 and 1914.
(Copyright UC Regents/Lick Observatory)