BIRR CASTLE AND THE NGC
This is the final talk I gave in the series covering the cataloguing of deep sky objects used by astronomers such as ourselves and the subsequent understanding of those objects. By deep sky objects we mean things which are outside the solar system and not stars as such. They are stars, though. They were originally known as nebulae, meaning clouds, because our story started long before anyone knew what these things were. Now of course we know that deep sky objects are reflection or emission nebulae (the stellar birthplaces), galaxies (huge concentrations of stars), star clusters (smaller concentrations of stars all truly associated with each other, almost certainly born in the same nebulae), and planetary nebulae and supernova remnants (dying stars). It's obvious from that that nebulae tell the whole story of stellar evolution, and that's why it's so important to understand what they are, and to understand what they are astronomers had to start by looking at them.
For the final link in the chain enter now a group of people, the first being William Parsons. Born in 1800, he was the son of the 2nd Earl of Rosse. The family seat was Birr Castle at Parsonstown in Ireland. William was educated at home in science and engineering, but graduated from Oxford in mathematics. At the age of 23 he became MP for King's County, in Ireland, and shorty afterwards joined the Royal Astronomical Society.
Like Herschel he built his own telescopes and cast his own mirrors, which wasn't an easy process. Mirrors of the time were not glass but a mix of copper and speculum, a type of tin, and the material was very brittle and hard to work. Parson completed a 36 inch telescope in 1830, to the same design as those used by the Herschels, but it was another nine years before he had a mirror for it. In the meantime he married Mary Field in 1836. She was a Yorkshire heiress, and with her money and the privileges he already had, Parsons' financial future was secure when he inherited the title of 3rd Earl of Rosse and the castle.
The 36 inch telescope was used to study 'nebulae', but it just wasn't big enough. Messier had used mainly refracting telescopes, but even today it's much harder to make a large lens than a large mirror. Every time you do something with light - reflect it, bend it, pass it through something - you lose some of it. A good quality lens needs multiple elements to correct abberations, with subsequent light loss. As Herschel had already realised, for deep sky observation a simple reflector is best, and the bigger the 'light bucket' the better, but the speculum mirrors were so inferior to today's glass ones that telescopes of not only very large diameter but also enormous focal length were required to try to improve the image quality. Not for our forebears the compact, short focal length, easy to manage telescopes we take for granted today.
So Parsons set about building a bigger telescope. The mirror was 72 inches - six feet - in diameter, and it took six attempts to produce two of them. He wanted two mirrors so the telescope wouldn't be out of action when the mirror needed cleaning. The tube was fifty eight feet long. The telescope, instead of being on a fully rotating frame, to Herschel's fairly standard design, was mounted between two walls aligned north-south. It could not be turned more than about fifteen degrees, but some of Parsons' equipment had, not suprisingly, suffered at the hands of the Irish weather and this was his way of affording some protection. The enormous telescope became known as the 'Leviathon' of Parsonstown.
The Leviathon's first light, the term used for a telescope's first outing, saw it turned on Castor, the double star in Gemini, in February 1845. Two months later Parsons observed M51, the Whirlpool Galaxy, and for the first time its spiral arms were recorded in intricate detail. By 1850 fourteen 'nebulae' were observed to be spiral like the Whirlpool, and there was a strong suspicion that these were galaxies outside our own.
The
idea wasn't new. In 1750 the Englishman Thomas Wright had published a book proposing
that the Milky Way was a huge slab of stars, and if that was so then perhaps
the 'nebulae' were, too. This concept was given further credance by Immanuel
Kant, The East Prussian philosopher and mathematician. Herschel had certainly
read Wright's book, but was unable to prove anything. In fact proof was to come
from something other than pure observation.
At about the same time as Parsons and the other astronomers who worked at Birr Castle were making their observations, other astronomers were discovering that the spectrum of light could be analysed to determine chemical elements present in those stars, and that all stars were similar. We'll see the significance of this later.
Willian Parsons died in 1867 but hos observatory did not die with him. His work was continued by his son Lawrence, the 4th Earl, and the Leviathon held the title of World's Largest Telescope for 75 years. Lawrence actually prefered the 36 inch, which he rebuilt as an equatorial, but his own personal field of interest was the Moon, and for that he didn't need the unwieldy Leviathon. The enormous telescope was, however, still doing much useful work.
Several other astronomers worked at Birr Castle from time to time and one of these was John Louis Emil Dreyer. Like William Herschel, Dreyer had anglisised his name. He was born Johannes, in Copenhagen, in 1852. He graduated from the university there, went to Birr Castle in 1874 and stayed for four years, working with the Leviathon and observing nebulae.
After Dreyer moved on he published a supplement of 1000 objects to John Herschel's General Catalogue. He was intending another supplement, but in 1882 he became director of Armagh Observatory. It was an observatory close to bankruptcy. Dreyer had no useful telescope and no assistants. Instead of publishing his own supplement, the Royal Astronomical Society asked him the collect all the work there was on nebulae. Until this time there had been a multitude of lists. There were Messier's and Herschel's, Dreyer's own and countless others from astronomers both professional, amateur and anywhere in between. Some of the data was accurate and some was not. Dreyer's amalgamation was published in 1888, as the 'New General Catalogue of Nebulae' or NGC. It contained 7840 objects. There were also two supplements, the 'Index Catalogues'. The first, of 1529 objects, was published in 1895, the second, of 5386 objects, in 1908. Between these catalogues there are 14755 deep sky objects with either NGC or IC numbers, and these are the catalogues we use extensively today, a legacy going back two hundred and fifty years to Messier, whose 109 objects are contained in the NGC but always refered to as M...
However
this gets us no nearer to finding out what Rosse's spiral structures were. For
that we must turn to William Huggins, another astronomer of independant means
who had built his own observatory. It was Huggins who studied the spectral lines
of stars, and when he did the same for nebulae, he confirmed that while some,
like the Orion and Crab Nebulae, had the spectra of hot gasses, Rosse's spirals
had stellar spectra. They were definitely made up of stars, but outside the
Milky Way....?
Two things happened to provide that final proof, both in America. The first, at the Harvard College Observatory, was Henrietta Leavitt's discovery of a method of measuring the distance to a type of variable star known as a Cepheid Variable, published in 1908. She found a number of these stars in the Small Magellanic Cloud, but at the time no-one seemed to appreciate the significance. Instead they concentrated on globular clusters, objects within our own galaxy. It wasn't till 1923 that Edwin Hubble, using the 100 inch Mount Wilson reflector to photograph the Andromeda Galaxy, or Nebula as it was still called then, accidentally found a Cepheid variable in it, and the distance to it, although at the time an underestimate, placed it far beyond the most distant star or globular cluster in the Milky Way. Paradoxically, it was the Mount Wilson telescope that took the World's Biggest title from the Leviathon.
So what started as a list of fuzzy things likely to be mistaken for comets ended as the key to our understanding of not only the structure of the Universe but also its size and ultimately its age.
As a postscript, the Leviathon fell into disuse when Dreyer left Parsonstown and was dismantled in 1908 on the death of Lawrence, but in the 1980s the 7th Earl set about its restoration, working in part from photographs taken by William's wife Mary. Work was completed in 1999 when a new mirror made of nickel coated aluminium alloy was put in place, and now visitors can see the telescope which first saw deep sky objects as something more than uniform, uninteresting 'nebulae'.