• • • • v. • ■-■.'. ■ .•• •• ' • • . , . . •• • 'IK H

I felt her presence, by its spell of might, Stoop o'er me from above; The calm, majestic presence of the Night, As of the one I love.

I heard the sounds of sorrow and delight, The manifold, soft chimes, That fill the haunted chambers of the Night Like some old poet's rhymes.

Barnard's career and struggles prove a very important point: it is never a large telescope alone, which is the seed for great discoveries: it is the eye behind the telescope - the mind of the astronomer. In an obituary published by the Royal Astronomical Society, we read: "Barnard is to be reckoned as one of the greatest observers of all time, and his work may be compared with that of Tycho Brahe, J.D. Cassini, and the Herschels."

Photographic surveys of the night sky had begun in earnest. The photographic surveys of Keeler (1898-1900), Perrine (1901-1903), Curtis (1909-1918) with the Crossley reflector at the Lick Observatory in California hugely increased the number of known nebulae. Added to this were the Mount Wilson surveys by Ritchey and by Pease (1917, 1920). It was a monumental leap forward, since Henry Draper had secured the first successful photograph of a nebula - the Great Nebula in Orion, in 1880. Keeler conservatively estimated that at least 120 000 nebulae were within reach of the Crossley 36-inch reflector. Many of the exquisite photographic plates secured by Keeler are reproduced in this chapter (Figures 97-110).

By the end of the nineteenth century - when galaxies were routinely photographed in long exposure ("deep") photographic plates, there was a growing conviction among a few astronomers that many of the nebulae, the spirals in particular, were extragalactic: outside of our Shrouds of the Night Milky Way Galaxy. For these astronomers, such objects were not nearby gaseous clouds in

166 our Galaxy, but instead were believed to be "island Universes:" galaxies in their own right.

Shrouds of the Night

176 Figure 106 [419]

The great controversy about the nature of the spiral nebulae was not really resolved until Hubble had made his observations in the mid-1920s of Cepheid variables in some nearby galaxies, including the Andromeda Spiral. (Cepheid stars vary regularly in brightness over a period of time. The astronomer Henrietta Leavitt discovered that the brightest such variable stars have the longest periods. It is possible to determine the distance of a Cepheid variable star from its observed period.)

Meanwhile, astronomers had begun to use spectrometers to measure the velocities of the nebulae from their Doppler shifts. (For a galaxy that is moving away from us, the features in its spectrum are Doppler shifted towards the red end of the spectrum by an amount proportional to the velocity of recession.) By 1917, Vesto Slipher at Lowell Observatory had measured velocities for twenty-five nearby nebulae and it was already clear that some of these velocities (up to 1100 km/s) were much larger than the typical velocities of the stars near the Sun. The average velocity of his twenty-five nebulae was 570 km/s which, as Slipher wrote, "is about thirty times the average velocity of the stars. And it is so much greater than that known of any other class of celestial bodies as to set the spiral nebulae aside in a class to themselves."

In 1917, Slipher correctly interpreted this large average motion as indicating that we [i.e. the solar system] have some drift through space. For us to have such motion and the stars not show it means that our whole stellar system moves and carries us with it. It has for a long time been suggested that the spiral nebulae are stellar systems seen at great distances. This is the so-called "island Universe" theory, which regards our stellar system and the Milky Way as a great spiral nebula which we see from within. This theory, it seems to me, gains favor in the present observations.

We would have expected Slipher's velocities to have had an enormous impact on the controversy, but for some reason they did not. Was the community simply suspicious of the quality of the data? This would be surprising, because several other observers had also made measurements which agreed quite well with Slipher's. Or were these very large velocities just too The Dawning much for the community to absorb at the time? 181

As Allan Sandage wrote in the Hubble Atlas, "Everyone, once his belief is set, will rationalise the facts to suit himself."

By the 1930s, the large velocities of the nebulae had clearly been accepted: Hubble had made his momentous discovery of the expanding Universe (which included some of Slipher's measurements).

There is an interesting parallel with the saga about dark matter in the Universe. In the mid-1930s, Fritz Zwicky measured the motions of galaxies within a cluster of galaxies known as the Coma cluster; he showed very convincingly that there had to be far more mass in the Coma cluster than could be accounted for by the individual galaxies. Herein lay the fingerprints for dark (nonluminous) matter; one of the most exciting arenas of astronomical research today. However, this astonishing discovery by Zwicky seemed to have made almost no impact on astronomers at the time. It took another thirty-five years before the subject of dark matter in the Universe came to life. Maybe again Zwicky's discovery was just too much for the community to absorb. Discoveries must be made at the right time if they are to be embraced by the community. There are no prizes for making discoveries too early.

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