Section 3.1.1: How did astronomers figure out what galaxies were?
Scientists and philosophers had looked up at the Milky Way for thousands of years and theorised that it was made up of countless unresolved stars. However, it wasn’t till the 17th century when astronomers first used telescopes to observe the night sky that this theory was proved right. One of the earliest attempts to map out the structure of the Milky Way galaxy was published in 1785 by William Herschel (most famous for discovering the planet Uranus) who, with his sister Caroline, counted the number of stars seen in different directions.
The structure of the Milky Way as published by William Herschel in "On the construction of the heavens" (credit: Philosophical Transactions of the Royal Society, vol.75 (1785) )
Around the same time that William Herschel was trying to figure out the shape of the Milky Way, astronomers were also systematically observing and cataloguing the night sky, discovering objects that did not fit into their categories of star, planet or comet in the process. In 1774 Charles Messier published his first catalogue of fuzzy (or diffuse) celestial objects that he had discovered while searching for comets. These objects were often referred to as ‘nebulae’ and the final Messier catalogue contained 103 such nebulae, named M1 to M103 (these names may be familiar to you as popular targets for amateur astronomy observations).
One of the first known sketches of a spiral galaxy (M51) drawn by Lord Roose in around 1845
By the start of the 20th century, better telescopes had been used to observe the night sky, resulting in first sketches and then photographs of some nebulae in much more detail. However, it was still unknown whether these nebulae were part of our own galaxy, the Milky Way, or whether they were separate entities. In 1920 a “Great Debate” about the scale of the Universe took place between two astronomers, Harlow Shapley and Heber Curtis, who argued about the size of the Milky Way and the nature of these nebulae. This argument was resolved in 1924 when Edwin Hubble worked out the distances to two nebulae using a technique pioneered by Henrietta Leavitt, proving that they were at vast distances outside of the Milky Way.
The first known photograph taken of another galaxy (M31) captured by Isaac Roberts in the late 19th Century (credit: A Selection of Photographs of Stars, Star-clusters and Nebulae, Volume II, The Universal Press, London, 1899)
Today we have catalogued millions of galaxies thanks to projects such as the Sloan Digital Sky Survey, which has observed a third of the sky from New Mexico. We see galaxies all over the sky, even in what seem at first to be empty patches of sky. All we have to do is keep our telescope pointing there long enough to capture the light from these faint galaxies; essentially taking a long-exposure photograph of the sky. This is what astronomers did to obtain the famous Hubble Ultra Deep Field image, as illustrated in the video below:
Zooming in to the Hubble Ultra Deep Field (credit: STScI)
The picture below shows the Hubble Ultra Deep Field itself. This image contains around 10,000 galaxies in a patch of sky the equivalent size of a two pence piece at the far end of a basketball court! Using images like this one, we can work out that there must be over a hundred billion galaxies in the Universe.
The Hubble Ultra Deep Field (credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI) )