Four days ago on the 26th of April, it was the 100th anniversary of the Great Debate, which is also known as the Shapley-Curtis Debate. The debate was held at the Smithsonian Museum of Natural History and it concerned the nature of the so-called nebulae (i.e. a giant cloud made out of gas and dust in space) and the size of the Universe. Shapley had the belief that the distant nebulae are relatively small and lay within our Galaxy, while Curtis that they are independent galaxies, which implies that they are large and distant. Before checking what happened in the Great Debate, it is worth having a look at the evolution of Astronomy as a science which led to the Great Debate, through the timeline of the Universe.
History of the Milky Way Galaxy
The establishment of the heliocentric model (i.e. the Sun is the center of the world) for what it was believed back in the 16th century to be the Universe, was the beginning of the evolution of Astronomy. Already in the early 17th century the famous Italian astronomer Galileo Galilei used a telescope to observe the night sky and discovered that it is composed of a huge number of faint stars. Additionally, during this period, many astronomers reported they were also able to observe faint nebulae in the sky, but they could not interpret both their nature and if they are part of our Galaxy, or not.
In 1750 the English astronomer Thomas Wright in his book An Original Theory, or New Hypothesis of the Universe, by using existing observations suggested that our Galaxy consists of many stars bound by gravity, and that the whole galaxy most likely rotates in a similar way as the Solar System, but on a much larger scale. Based on Wright’s model, the Milky Way Galaxy is a spherical shell of stars, with the Sun on its center. Additionally, according to Wright the faint nebulae are galaxies, thus they were located outside our own. The ideas of Thomas Wright influenced Immanuel Kant and in 1755 he presented his view on the nature of the Universe. According to Kant the stars are a system in order, in the form of a lens-shaped disk, which rotates about its center, following Newton’s laws of motion. The observed nebulae according to Kant are distant galaxies, with the same structure as our own.
The first attempt to map the Milky Way, was undertaken by William Herschel and his sister Caroline. In 1785, they constructed what was back then the world’s largest telescope (diameter of 1.2 m) and used it for a period of twenty years to observe and count stars along 683 lines of sight. Their observations were made by using two assumptions.
The first was that stars are distributed uniformly through space and they do not extend beyond our Galaxy, while the second was that their telescope could resolve all stars within the boundaries of the Milky Way. According to their findings the Galaxy has a flattened shape and the Sun is near the center.
Towards the end of the 18th century French astronomer Charles Messier, created a catalog with the 109 brightest objects that have nebular structure, while William Herschel published a catalog with 5,000 objects. In 1845 William Parsons, the 3rd Earl of Rosse constructed the largest telescope of its era (diameter 1.83 m). Lord Rosse, was able to observe the known nebulous structures and he was able to distinguish between elliptical and spiral structures. Additionally, he was able to distinguish individual point sources in these structures (i.e. stars in other galaxies).
Half a century later, the Dutch Astronomer Jacobus Kapteyn between 1901 and 1920s, used a similar but refined search method as Herschel and concluded that our Galaxy has an ellipsoid shape with the Sun close to the center. Harlow Shapley based on cataloging star clusters came up with a totally different result, where the Galaxy is a flat disk, with the Sun being far from the center.
The progress of technology led to the construction of spectrographs and in 1912 the American astronomer Vesto Slipher made spectrographic studies of the brightest spiral nebulae to determine their composition. Slipher found that the spiral nebulae move away from us with velocities that were not consistent with star velocities he had measured. In 1917 Heber Curtis observed nova S (i.e. a star that suddenly increases its luminosity tremendously and then fades away to its former obscurity within months, or years.) in the Andromeda galaxy (back then known as M31 or “Great Andromeda” nebula). Curtis found 11 more novae in Andromeda and found that they were much fainter than those observed in our Galaxy. Thus, he became a supporter of the model that suggested that the mysterious observed nebulae are independent galaxies.
The Great Debate in 1920
On April 26, 1920 a debate was held at the Smithsonian Museum of Natural History, with the topics being, how big is the Milky Way and how distant are spiral nebulae. Shapley supported that our Galaxy is 100 Kpc (parsec is a unit to measure distance in Astronomy and it’s equal 3.0857×1016 m) across (three times larger than the actual size), and that the distance to the Andromeda Nebula is only 10 kpc, hence it is part of our Galaxy. Curtis supported that our Galaxy has a size of 10 kpc (three times smaller), while he estimated that the Andromeda nebula is at a distance of 150 kpc (five times smaller), thus it is external.
The Great Debate was actually inconclusive. Even though Shapley had better arguments, he was actually on the wrong side. Curtis on the other hand was on the right side, but his arguments were weak. Also, both got the distance to the Andromeda Nebula wrong (778 kpc) and also they were not able to explain the motion of the nebulae. What actually created confusion on both sides was a report from Adriaan van Maanen, where he claimed that if M101 was outside of our Galaxy then it is moving at speed greater than the speed of light. Actually, no one else was able to reproduce van Maanen’s findings, thus proving that he made something incorrect in his data analysis.
From Hubble to Hubble
The solution to the problem was given three years later, when Edwin Hubble using the new 100-inch Mount Wilson telescope, was able to resolve the outer parts of some spiral nebulae and identified variable stars there. By using these stars he was able to measure the distances to these nebulae, thus finding that they are too far to be part of our Galaxy. Furthermore, in 1936 Hubble produced a classification system for the morphology of the galaxies, a system that is used to this day. In the upcoming decades, with the explosive evolution of technology, our knowledge in Astronomy was at the same pace. Thus, 70 years after the Great Debate, NASA launched into orbit the Hubble Space Telescope (HST) on the 24th of April 1990. Just like Edwin Hubble, HST has broadened our view of the Universe, since we were able to get data from galaxies that are too far away to be observed with ground-based telescopes.
Famous women astronomers and their role in its evolution
So far the only woman that was mentioned, was Caroline Herschel. But the question of, what was the role of women in the development of Astronomy is something that plenty of people ask. In Harvard Observatory Edward Charles Pickering, hired around 80 women, for processing astronomical data and they were known as Harvard Computers. One of the most famous women that was in Harvard during that period was Annie Jump Cannon. During her career in Harvard she classified around 350,000 stars (more than anybody else), she discovered around 300 variable stars and novas and she was also one of the creators of the Harvard classification star system. Henrietta Swan Leavitt was initially hired for cataloging and measuring the brightness of stars. Through her work though she was able to discover a relation between the luminosity and period of a special class of variable stars known as Cepheid variables. This period-luminosity relationship is known as Leavitt’s Law and provided a tool for measuring distances accurately (e.g. Edwin Hubble proved that the Andromeda galaxy is not part of our Galaxy by using Leavitt’s law). It should be mentioned here that all women that were working at Harvard Observatory were getting paid between 0.25-0.50$ per hour (much less than men) and they were criticized for being “out of their place” (i.e. home). Additionally, most of them were for their whole careers assistants, since they were women.
Astronomy fun facts
William Herschel was born in Hannover Germany and he moved to England not as an astronomer, but as a music teacher. Apart from creating the first map of the Universe, he discovered planet Uranus on 13th of March 1781, and on the 11th of January 1787 Uranus’ satellites Oberon and Titania. Furthermore, on the 28th of August and 17th September he discovered two satellites in Saturn, Enceladus and Mimas.
Caroline Herschel discovered many comets and she was the first woman who received the golden medal from the Royal Astronomical Society.
The famous French mathematician, physicist, astronomer and philosopher, Pierre-Simon Laplace, in 1796 suggested that the observed “nebulae”, might be forming planetary systems.