Andromeda – From nebula to galaxy

The word galaxy derives from the Greek word galaxias meaning milky one, which was the ancient Greek term for the Milky Way that indistinct band of stars visible across the night sky in areas that don’t suffer from too much light pollution. Today galaxy is used as the general term for the very large groups of stars scattered around the universe. Current estimates of the total number of galaxies range from 2×1011 to 2×1012 or even more. Confronted by these vast numbers it is oft easy to forget that less than one hundred years ago we still thought that our galaxy, the Milky Way Galaxy, was the entire universe. This changed on 1 January 1925 when H.N. Russell read a paper by Edwin Hubble to the American Association for the Advancement of Science, which established that spiral nebulae were in fact separate galaxies. The path through the history of astronomy leading up to that epoch defining paper in 1925 goes back almost one thousand years and in what follows I shall briefly outline some of the important stations, nearly all of which concern our nearest galactic neighbour Andromeda, along that path.

The word nebula comes from the Latin and means a cloud, mist, fog, smoke, vapour, exhalation, as you can see the definition is fairly nebulous. In astronomy it can be traced back to Ptolemaeus’ Mathēmatikē Syntaxis or as it is more commonly known The Almagest. In this founding work of Western astronomy Ptolemaeus lists a total of six astronomical nebulae without giving them any great attention. All of Ptolemaeus’ nebulae were in fact indistinct star clusters too far away to be resolved with the naked eye. The first so-to-speak true nebula, the Andromeda nebula, was recorded by the Persian astronomer Abd al-Rahman al-Sufi, usually just referred to as Al Sufi, in his Book of Fixed Stars (Arabic: ‫كتاب صور الكواكب‎ kitab suwar al-kawakib) around 964 CE. He describes and illustrated the Andromeda nebula as a little cloud before the mouth of the Arabic constellation Fish.


Al Sufi’s drawing of the constellation Fish with the Andromeda nebula in fount of it mouth

Amongst his other early telescopic observations Galileo showed that the Ptolemaic nebulae resolved into many unseen stars when viewed through the telescope. In 1612, it was, however, Galileo’s telescopic rival, Simon Marius who first turned his telescope on the Andromeda nebula and saw that it didn’t resolve into stars when viewed through his telescopic lenses. In his Mundus Iovialis (1614) Marius described what he saw as follows:

Among them the first is that with the spy-glass, from 15 December 1612 I discovered and observed a fixed star with a certain wonderful shape that I cannot find in the entire heavens. It is near the third and northernmost [star] in the belt of Andromeda. Without the instrument the same is seen as some sort of little cloud; and with the instrument no distinct stars are seen as in the nebular star in Cancer and other nebular stars, but rather only white rays, which the closer to the centre the brighter they come out; in the centre there is a dull and pale light; and its diameter is about a quarter of a degree. About the same brilliance appears when a bright candle is observed through a clear lantern from a long distance.


Simon Marius from the frontispiece of the Mundus Iovialis Source: Wikimedia Commons

The research into nebulae came of age first in the eighteenth century with the work of the French comet hunter Charles Messier (1730–1817). In order to make it easier for comet hunters to distinguish potential comet sightings from other indistinct and nebulous object in the night sky, Messier began to compile a catalogue of the positions and appearance of all such objects that he detected during his nightly vigils. His work, the final version of which was published in 1781 and is now known as the Messier Catalogue, contains a list of 110 Messier objects, in his time nebulae and star clusters. The Messier objects are now known to be 39 galaxies, 5 planetary nebulae, 7 other types of nebulae and 55 star clusters. The Andromeda nebula, the discovery of which Messier, ignorant of Al Sufi’s book, falsely attributes to Marius, is Messier object M31.


Charles Messier, French astronomer, at the age of 40 Source: Wikimedia Commons

Although Messier’s catalogue was compiled to assist comet hunters in differentiating potential comets from other faint celestial objects it is usually regarded as an early example of so-called deep sky astronomy; that is the study of objects well outside the solar system. The man who first practiced deep sky astronomy systematically was William Herschel, who together with his sister Caroline, methodically map the heavens quadrant for quadrant recording with his 20 foot reflecting telescope all of the non-stellar objects he could find. Caroline and he recorded 2400 nebulae in three catalogues.


William and Caroline Herschel polishing a telescope lens, 1896 Lithograph. Source: Wellcome Collection via Wikimedia Commons

They categorised the objects that they recorded into eight classes: (I) bright nebulae, (II) faint nebulae, (III) very faint nebulae, (IV) planetary nebulae, (V) very large nebulae, (VI) very compressed and rich clusters of stars, (VII) compressed clusters of small and large [faint and bright] stars and (VIII) coarsely scattered clusters of stars. Extended by his son and later John Dreyer, Herschel’s catalogue became the New General Catalogue (NGC) of 7840 deep sky objects in 1888. The NGC numbering is still used for most of the objects recorded therein. In 1785 Herschel observed a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the great nebula.

In 1750 the English astronomer Thomas Wright (1711–1786) published his An Original Theory on New Hypothesis of the Universe in which he was the first to correctly describe the shape of the Milky Way Galaxy. He also speculated that the faint nebulae where in fact distant galaxies. However, his very perceptive thoughts remained speculations that he was unable to verify.


Thomas Wright in 1737 Source: Wikimedia Commons


Illustration of groups of stars, from An original theory or new hypothesis of the Universe, plate XVII Source: Wikimedia Commons

Interestingly his speculations were taken up by the German philosopher Immanuel Kant (1724–1804) and further developed in his anonymously published Allgemeine Naturgeschichte und Theorie des Himmels (Universal Natural History and Theory of Heaven) (1755). At the time neither Wright’s nor Kant’s theories received much credence but with hindsight both have been praised for their perceptiveness.


Title page of Kant’s Allgemeine Naturgeschichte und Theorie des Himmels Source: Wikimedia Commons

In 1850, William Parsons, using the largest reflecting telescope constructed in the nineteenth century the Leviathan of Parsonstown, was able to identify the spiral structure of the Andromeda nebula for the first time. This was just one of a series of spiral nebula, in reality galaxies, that he was able to identify.


The largest telescope of the 19th century, the Leviathan of Parsonstown. Source: Wikimedia Commons

In 1864 William Huggins, a pioneer in stellar spectroscopy, noted that the spectrum of Andromeda differs from that of a gaseous nebula. The spectrum, as observed by Huggins, had the same characteristics as the spectrum of individual stars leading he to conclude that Andromeda was in fact stellar in nature.


Sir William Huggins, by John Collier Source: Wikimedia Commons

We have already come a long way from Al Sufi’s first record of a small cloud. In 1887, Isaac Roberts, who thought that spiral nebula were solar systems in the process of forming, took the first-ever photograph of Andromeda.


Isaac Roberts’ picture of the Great Nebula in Andromeda Source: Wikimedia Commons

In 1912 the American astronomer, Vesto Slipher, measured the rotational velocity of Andromeda using spectroscopy at 300kilometres per second the highest yet measured velocity.


V.M. Slipher, astronomer at Lowell Observatory from 1901 to 1954. Source: Wikimedia Commons

In 1917 Heber Curtis observed a nova in Andromeda and discovered eleven more in the photographic record. These were on average ten magnitudes weaker that others observed in the heavens. Based in this data he estimated that Andromeda was 500,000 light-years distant. Curtis now proposed the island universes hypothesis i.e. spiral nebulae are actually independent galaxies.


Heber Doust Curtis poses before the Crossley telescope. Source: Wikimedia Commons

On 26 April 1920 Heber Curtis and Harlow Shapley held the so-called great debate at the Smithsonian Museum of Natural History on the nature of spiral nebulae. Curtis argued that they were distant independent galaxies, Shapley that they were much smaller and much nearly and thus within the Milky Way galaxy, which was the entire universe. This debate raised the question to the priority question in astronomy.


Portrait of Harlow Shapely Source: Wikimedia Commons

In 1922 Ernst Öpik measured the distance of Andromeda using the velocity of stars. His estimate was 1,500,00 light-years.


Ernst Julius Öpik Source: Wikimedia Commons

As I said in the opening paragraph Edwin Hubble finally settled the mater when he measured the distance of Andromeda using Cepheid variable stars and proved conclusively that Andromeda was not a nebula inside the Milky Way but a separate galaxy. With this result the age of galactic astronomy was born.


Studio Portrait of Edwin Powell Hubble. Photographer: Johan Hagemeyer Source: Wikimedia Commons

Of interest the method of determining distances using Cepheids was developed by Henrietta Swan Leavitt, one of the Harvard computers, investigating thousands of variable stars in the Magellanic Clouds in 1908; she published her results in 1912.


Henrietta Swan Leavitt working at her desk in the Harvard College Observatory Source: Wikimedia Commons


Early photograph of ‘Pickering’s Harem’, as the group of women assembled by Harvard astronomer Edward Charles Pickering, who were dubbed as his “computers”. The group included Leavitt, Annie Jump Cannon, Williamina Fleming, and Antonia Maury. Source: Wikimedia Commons

The story of Andromeda’s historical journey from Al Sufi’s nebula to Curtis’ galaxy illustrates very nicely how scientific knowledge grows over time with generations of researchers with differing interests and motivations contributing directly and indirectly to that growth.

Post amended 11 January 2018




Filed under astrophysics, History of Astronomy, History of science, Uncategorized

16 responses to “Andromeda – From nebula to galaxy

  1. Interesting that Messier is famous for his non-cometary objects, when he was a comet-hunter, while many famous discoverers of comets, such as Kohoutek, discovered them accidentally and actually work mainly on other things.

  2. The Wikipedia caption ate Ernst’s umlaut!

  3. As a person unfamiliar with the study of astronomy, i found your article very interesting and thought-provoking. It really illustrates how knowledge builds over the centuries in increments until a decisive answer is proven (at least for the time being!).

    I was amazed 1) that so much could be discovered with (relatively) primitive instruments,2) that mathematical calculations played such a key role in each step, and 3) that we did not know (definitively) that we lived in a universe of multiple galaxies till 1925!!!

    So much of what we take for granted today is of relatively recent origin.

    Thank you for your insight and very readable writing style!

    I also could not help but wonder what the neighbors said, when the astronomer first put up the Great Leviathan!

  4. The term “galaxy” was coined by the English astronomer, Thomas Wright (22 September 1711 – 25 February 1786), who in 1750, published,  “An Original Theory on New Hypothesis of the Universe,” and in which he proposed disc shaped galaxy.

    • I doubt that Thomas Wright coined the term galaxy but if you know better than please tell more. My etymological dictionary says that it first came into English, meaning the Milky Way, in the 14th century from Latin via French. It also say the first use in the modern sense of galaxies plural was in 1848 but doesn’t give the source for this date.

  5. If I remember correctly, Thomas Wright, Immanuel Kant and several others suggested early on that the faint nebulae might be distinct collections of stars,rather like our own galaxy is that right?

    • Yes, you and Asis Chaudhuri are both correct and I should have included Wright’s and Kant’s cosmological speculations in my post. I shall now amend it to correct this lapsus.

  6. This post underlines what a service you provide. Great stuff.

  7. “If I remember correctly, Thomas Wright, Immanuel Kant and several others suggested early on that the faint nebulae might be distinct collections of stars,rather like our own galaxy is that right”

    Yes. However, they had no real scientific support for their ideas. Conclusive proof was Hubble’s discovery of Cepheid variables in the Andromeda Galaxy.

  8. Laurence Cox

    I was a bit surprised that Andromeda wasn’t noted by the Greeks as it is a naked-eye object. I can remember seeing it some 45 years ago in a location with particularly dark skies just to the north of London.

  9. David L. DiLaura

    I think the early image of William and Caroline Herschel polishing a telescope lens from the Wellcome Collection is rather fanciful, no? Though he did make some small lenses, he never worked on so gigantic a piece of glass. He did produced soome large mirrors, however.

    • Yes, the picture is totally fanciful.

      • It look s like a mirror made of speculum metal see: for an example, in which case they could be repolishing it to take the tarnish off.

      • You are probably right about it being a speculum metal mirror but they could equally well be grinding and polishing a freshly cast mirror, something that Caroline describes in her autobiography; She used to sieve to horse dung in which they bedded to mirrors for grinding and polishing.
        However the picture is totally fanciful for several different reasons. Firstly they did this work in a workshop installed in the kitchen, in the cellar of the house in Bath and not in the living room on the first floor. Secondly they would wear work clothes and not the fine garments depicted to carry out this work.
        I chose this picture because it displays William and Caroline as equals whereas the real existing portraits of William show him as a fine Georgian gentleman and those of Caroline show her as a wizened old maid making him look very superior.
        In real life Caroline was, due to childhood illness, stunted in growth (4ft 3 ins) and facially disfigured.

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