Amongst everything else that happened in 2023, a key anniversary of a huge leap in our understanding of the Universe passed largely unnoticed – the centenary of the realisation that not only was our Sun one of many stars in the Milky Way galaxy but that our galaxy was one of many galaxies in the Universe.
I had been watching the approaching anniversary for over a decade, thanks to teaching the cosmology section of the introductory astronomy course at the University of Auckland. My lectures come at the end of the semester and each October finds me showing this image – with its “October 1923” inscription – to a roomful of students.
The image was captured by the astronomer Edwin Hubble, using the world’s then-largest telescope, on top of Mt Wilson, outside Los Angeles. At first glance, it may not even look like a picture of the night sky: raw photographic images are reversed, so stars show up as dark spots against a light backgrounds. However, this odd-looking picture changed our sense of where we live in the Universe.
My usual approach when I share this image with my students is to ask for a show of hands by people with a living relative born before 1923. It’s a decent-sized class and this year a few of them had a centenarian in the family. However, I would get far more hands a decade ago when I asked about mere 90 year olds. And sometime soon no hands will rise at this prompt and I will have to come up with a new shtick. But it is remarkable to me that there are people alive today who were born before we understood of the overall arrangement of the Universe.
For tens of thousands of years, the Milky Way – the band of light that stretches across the dark night sky – would have been one of the most striking sights in the sky on a dark night once you stepped away from the fire.
Milky Way — via Unsplash
Ironically, the same technological prowess that has allowed us to explore the farthest reaches of the Universe also gives us cities and electric lights. I always ask whether my students have seen the Milky Way for themselves with another show of hands and each year quite a few of them disclose that they have not. I encourage them (and everyone) to find chances to sit out under a cloudless, moonless sky and take in the full majesty of the heavens as it slowly reveals itself to you as your eyes adapt to the dark.
In the meantime, though, we make do with a projector and a darkened lecture theatre.
It was over 400 years ago that Galileo pointed the first, small telescope at the sky. In that moment the apparent clouds of the Milky Way revealed themselves to be composed of many individual stars. By the 1920s, we understood that our Sun is a star and that the Milky Way is a collection of billions of stars, with our Sun inside it. But the single biggest question in astronomy in 1923 — which, with hindsight, became known “Great Debate” — was whether the Milky Way was an isolated island of stars in an infinite and otherwise empty ocean of space, or if it was one of many such islands, sprinkled across the sky.
In other words, for Hubble and his contemporaries the question was whether our galaxy was the galaxy, or one of many?
More specifically, the argument was whether nebulae, which are visible as extended patches of light in the night sky, were themselves galaxies or contained within the Milky Way. These objects, almost all of which are only detectable in telescopes, had been catalogued by astronomers as they mapped the sky with increasingly capable instruments. There are many kinds of nebulae, but the white nebulae had the colour of starlight and looked like little clouds through the eyepiece. Since the 1750s these had been proposed as possible galaxies. But until 1923 nobody knew with certainty whether they were small objects on the outskirts of our galaxy – or much larger, far more distant objects on the same scale as the Milky Way itself.
To human observers, the largest and most impressive of the nebulae is Andromeda. This was this object at which Hubble had pointed his telescope in October 1923. Hubble was renowned for his ability to spot interesting details in complex images [1] and after the photographic plate was developed his eye alighted on a little spot that had not been present in an earlier observation [2].
Hubble’s original guess was that this was a nova, a kind of star that sporadically flares in brightness by a factor of 1,000 or more, so he marked it and a couple of other candidates with an “N”. However, after looking back at images that he had already taken and monitoring the star through the following months Hubble came to realise that he had found a Cepheid variable – a star whose brightness changes rhythmically over weeks or months.
Stars come in a huge range of sizes and big stars are millions of times brighter than little ones, so simply looking a star in the sky tells us little about its distance from us. But Cepheids have a useful property [3]: brighter Cepheids takes longer to pass through a single cycle than their smaller siblings.
Imagine a group of people holding torches (flashlights if you are North Americans) each of which has a bulb with its own distinctive brightness. If this group fans out across a field at night and turns on their torches, we cannot tell how far away each person simply by looking at the resulting pattern of lights. Is that torch faint because it is further from us than most, or because its bulb is dimmer than most? But if each person were to flash the wattage of their bulbs in Morse Code we could estimate distances by comparing their apparent brightness (since distant objects appear fainter) to their actual intensity (which is encoded in the flashing light).
In the case of Cepheids they are not flashing in Morse code; instead, nature provides us with the requisite information via the time it takes for their brightness to vary from maximum to minimum and back to a maximum again.
Hubble used this knowledge to estimate the distance to Andromeda. While the number he found was lower than the best present-day estimates it was still large enough to show that it was far from the Milky Way and this roughly the same size as our galaxy.
The immediate implication, given that Andromeda is the brightest of the many nebulae we see in big telescopes, was that our Milky Way was neither alone nor unique in the Universe. Thus we confirmed that our galaxy was just one of an almost uncountable number of islands in the ocean of space – and the full scale of the cosmos yielded to human measurement for the first time, through Hubble’s careful lens on a curious star.
A modern image (made by Richard Gentler) of the Andromeda galaxy with a closeup on what is now called “Hubble’s star” taken using the (appropriately enough) Hubble Space Telescope, in the white circle. A “positive” image from Hubble’s original plate is shown at the bottom right.
Illustration Credit: NASA, ESA and Z. Levay (STScI). Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
[1] Astronomers in Hubble’s day used a gizmo called a “Blink Comparator” that chops quickly between two images viewed through an eyepiece, so objects changing in brightness draw attention to themselves by flickering.
[2] In most reproductions of the original plate I am hard put to spot it all, even more so when it is projected on a screen in a lecture theatre. A bit of mild image processing makes it a little clearer, but it hardly calls attention to itself.
[3] This “period-luminosity law” had been described just 15 years earlier by Henrietta Swan Leavitt and it is still key to setting the overall scale of the Universe.