We Had a Name for ‘Galaxies’ before We Knew They Existed

Astronomy can be a difficult subject for newcomers. Like any field of science, it has its own jargon and buzzwords, as well as terms whose meanings can be not only strange, but downright counterintuitive.

The most obvious is the use of the word by astronomers metal to designate any element heavier than helium. Lithium? Metal. Oxygen? Metal. Carbon? It is also a metal, as far as astronomy is concerned.

Using a single term to cover these elements heavier than helium makes sense, because the universe is mostly made up of lighter elements, so lumping everything else into one group makes calculations easier. I wish our astronomical ancestors would have chosen a better term, but here we are.

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There are many other examples. The words that really irritate me are not only confusing but also outdated. These are terms we should abandon in favor of others that better reflect our more modern understanding. Population I, II and III stars are a good example. They actually refer, in order, to stars with more metals, stars with fewer metals, and stars that were the very first in the universe, which is ridiculously confusing.

But it is to our credit that astronomers sometimes abandon terms when they become obsolete. It may just take a long time.

For example, I remember reading about “spiral nebulae” in older books when I was much younger. Nebula is a fairly generic term; it’s Latin for “fog” and refers to any sort of diffuse deep space object we see in the sky. Some, like the Great Orion Nebula, have little overall shape, while others, like the Ring Nebula, are very structured. We now know that they are all huge clouds of gas and dust, and although they have a wide range of very different structures and origins, grouping them together as cosmic blurs is useful for understanding them because they have many characteristics in common.

But what about spiral nebulae?

“Spiral nebula” is a centuries-old term with a rich scientific history. While stars (usually) appear as sharp points through a telescope, nebulae are extensive and sometimes indistinct. A few centuries ago, when telescopes became large enough to resolve more structural details in nebulae, some were pinwheel-shaped, with one or more spiral arms wrapping around a somewhat brighter core.

One of the best examples of such a strange body was M51, the 51st object in the catalog of the great French comet hunter Charles Messier, who compiled a list of fluffy, boring objects he found that he thought could be confused with the comets he held dear and scoured the sky for. (The irony: Today, this list includes many of the brightest and most beautiful deep space objects in the sky.) When English astronomer William Parsons, third Earl of Rosse, observed M51 in 1845 using his 6-foot reflecting telescope, an instrument so large at the time that it was called the “Leviathan of Parsonstown,” he saw that it had spiral arms, and other observations confirmed it. It became known as the Whirl Nebula, the first of many spiral nebulae to be characterized as such.

Astronomers postulated that nebulae were nascent planetary systems (which we now know feature spirals of a different type) or stars that collided and spewed matter, among other things. Hypotheses abounded. Most were fanciful; none fully explained what was seen. But this began to change at the beginning of the 20th century.

At that time, the commonly accepted idea was that the Milky Way represented the entire observable universe. Everything we saw in the sky was inside the Milky Way, which obviously included the spiral nebulae. Many, perhaps most, astronomers thought that these were actually nebulae in the classical sense, clouds of gas inside our Milky Way.

But in the 1920s, doubts about this interpretation multiplied. Astronomer Heber Curtis noted that the Andromeda Nebula, one of the most famous of these spiral nebulae, easily visible to the naked eye in dark skies, was home to several novae. These were stars that suddenly became much brighter than usual, then disappeared over the weeks. Novae were not well understood at the time but had often been observed in the Milky Way. Andromeda’s were much weaker than normal, however, implying they were a great distance away. Andromeda also had dark pathways similar to those of the Milky Way throughout its spiral structure. And observations indicated that Andromeda had a significant Doppler shift, which meant it was moving very quickly relative to the Milky Way, a special property for such an object.

This led to the Great Debate, a formal formal debate over whether these nebulae were inside the Milky Way (an idea championed by astronomer Harlow Shapley) or, as Curtis argued, “galaxies” in their own right. The term galaxy itself dates back at least 600 years, long before anyone had a clue, and is derived from the Greek galaxiesmeaning “milky”, which of course refers to our own Milky Way.

It was only a few years later that a team led by Edwin Hubble demonstrated that the spiral nebulae were in fact at great distances, millions of light years from us, and that they were in fact galaxies. Soon, most astronomers had changed their minds about the true scale of the universe, which was far bigger than they had thought. Eventually, the galaxy became the generic handle for all these objects, no longer reserved for just the Milky Way.

This brings us back to the term nebula. I remember very well that, when I was a child, I still heard astronomers talking about the “Andromeda Nebula”, which I now find rather funny. This term was obsolete half a century ago, so any astronomer who used it at the time was probably doing so out of habit.

This led me to a wonderful discovery while researching this column. Looking at old issues of Scientific American, I came across an article titled “The Dynamics of the Andromeda Nebula,” written by famed astronomer Vera Rubin (the namesake of the recently activated Vera C. Rubin Observatory). Note its terminology; she wrote the article in 1973! This confirms my memory of seeing the term used when I was younger. I will add that Rubin was born in 1928, years after the Great Debate. Astronomers of the time still called them nebulae – astronomer Henry Norris Russell called the Milky Way a spiral nebula in empty space. SciAm article in 1929 – so just like me, Rubin probably grew up hearing the term used, and it became a habit.

I’m not sure when the old term finally fell into disuse, but it couldn’t have been much later. Although I saw it here and there when I started reading about astronomy as a child, the galaxy was much more common.

Words matter. What we call something shapes our understanding, how we think about it, how we see it. A nebula now means something quite different to a modern astronomer than a galaxy, and rightly so!

It’s worth thinking about how our terminology guides our thinking, and it’s definitely worth questioning certain terms when they cause more confusion than clarity or keep us stuck in outdated notions best left in the history books.