The Latin word nebula means ‘mist’, and originally a nebula was any sort of misty patch in the sky. Today it is more precise, referring to an interstellar cloud of dust and gas; and we shall see in a moment why the name evolved.
An issue that came to the fore around 1920 was the question of the nature of a particular group of nebulae – spiral nebulae.
It was know that a number of these spiral nebulae were moving away from us, at a significant speed. It was possible to find this out by using a property of waves called the Doppler effect.
This effect is familiar in sound waves, when we hear a rise in the pitch of the siren of a car heading toward us, and then the pitch starts to fall as the car overtakes us and fades away into the distance ahead.
The same effect happens with light, where the equivalent of a sound’s pitch is the colour of light – its position in the spectrum that runs between blue at one end and red at the other. If a light-source is moving towards us, there is a shift towards the red end of the spectrum; and if it is moving away, then the shift is toward the blue.
What we actually see with distant stars are absorption lines, where some of the light pouring out from the star’s core has been absorbed in its outer gaseous layers during the first part of its journey towards us. A particular absorption line tells us that a particular colour in the light has been absorbed by a corresponding chemical element. This is how we can tell what stars are made of, by looking for which colour lines are missing from the light they send to us. And similarly, if there was a planet orbiting the star, we could learn about its atmosphere from the absorption lines of the starlight passing through it.
The clue to the motion of the star comes from the position of these absorption lines in the spectrum. If they are shifted towards the red, as compared with a star of similar composition, then we deduce that the star moving away from us.
The explanation of the effect came in 1842 from the Austrian Christian Doppler. Six years later Hippolyte Fizeau in France found the first spectral lines to show a redshift, and in 1864 the English astronomer Sir William Huggins made the first calculation of a speed.
Huggins worked at home in South London, in his own private observatory, assisted by his wife Margaret, an able researcher in her own right. Huggins was a skilled photographer, the first to use the new dry plate process, and he was fortunate to have a professor of chemistry – William A. Miller – as a neighbour, to help identify the elements in the spectra. They found two main types of spectra. Some of the misty patches in the sky (like the Orion Nebula) had pure emission spectra, the hallmark of a gas; while others (such as the spiral nebula M31 in Andromeda) had the type of spectra that is characteristic of stars.
In 1912 Vesto Slipher at the Lowell Observatory at Flagstaff, Arizona, measured the redshifts of the spectrum of 15 spiral nebulae and found that all but three are receding from us.
Slipher’s finding opened up the debate about the nature of the spiral nebulae. Huggins had shown that they had the spectra of stars, and now it was clear that some of them were receding from us at immense speeds. The explanation which we have today is that these nebulae are complete galaxies, and the mistiness is due to the myriad of stars of which they are composed; and today we speak of spiral galaxies and reserve the word ‘nebula’ for the real mist-like objects, the clouds of dust and gas that drift in space.
But at the time there was an argument, with some astronomers maintaining that everything was contained within our own galaxy, the Milky Way.
One of the leading proponents of this latter view was Harlow Shapley, working at Mount Wilson Observatory in California. Shapley’s track record on galaxies was a good one, as he was the first man to show that our galaxy, the Milky Way, was much larger than had been previously believed – and that the sun was far from the centre of the Milky Way. (We will find out how he demonstrated this in the next part of this series.)
Shapley had found his way into astronomy by a rather indirect route. Born on a farm in Missouri, he dropped out of school and studied at home, and then got a job as a newspaper reporter covering crime stories. He decided to catch up with his school education, and did so rapidly. He planned to go to the University of Missouri to study jourmalism, but found that the opening of the university’s School of Journalism had been postponed. So he looked for another subject. The first one in the course directory was Archaeology, which he later said that he found difficult to pronounce – so he opted instead for the subject after that; which was Astronomy. A brilliant career followed, taking him to Princeton and then Mount Wilson.
In 1920 Shapley debated the nature of spiral nebulae with Heber D. Curtis, who had been studying nebulae at the Lick Observatory for eighteen years. The Great Debate was held at the Smithsonian Museum of Natural History in Washington, DC. Curtis argued that objects like the nebula in Andromeda were ‘island universes’ (a term originally coined by the philosopher Immanuel Kant, who had himself believed that the nebula lay beyond the Milky Way). Curtis won, and today we speak of the Andromeda Galaxy, one amongst a vast number of such island universes.
But Shapley had already a success that nothing could tarnish. He had discovered the nature of our galaxy, and our place within it. And a further success that would highlight his name in the history of astronomy was that another aspect of his work was – as we shall see – of key importance for Edwin Hubble.