Last time I talked about the Great Debate of 1920, and about Edwin Hubble’s discovery that Galaxies lie beyond the Milky Way. The 1920s changed over view of the Universe – they made it much larger! This time I’m going to quickly outline the basic types of galaxies and the kind of sizes and distances we are dealing with.
Galaxies are usually grouped by their appearance. You may be familiar with spiral galaxies, for example. In fact there are two types of spiral galaxy: those with bars through their middle, and those without. You also have elliptical galaxies, which are basically big blobs of stars. Finally there are irregular galaxies, i.e. galaxies that don’t seem to be one shape or another really. There are examples of each of these types shown below – taken from the Galaxy Zoo data, of course!
The different shapes of galaxies tell us something about their properties, and we’ll deal with each type of galaxy in the next few blog posts. For now I thought I’d end with another of Hubble’s ideas. When he saw these different types of galaxies he tried to understand the different shapes as an overall evolution. He thought that elliptical galaxies might evolve into spirals as time went by. The Hubble ‘tuning fork’ diagram is shown below.
Hubble called the elliptical galaxies ‘early’ galaxies and the spirals ‘late’ galaxies. Galaxies do not move left across the diagram as they evolve, but still the diagram is a nice way to visualise the varying shapes of galaxies relative to one another. Understanding the shapes – or morphologies – of galaxies are a huge part of the motivation behind the Galaxy Zoo project. you can learn more about it on our science pages.
[UPDATE: This post has been modified from its original form to correct some errors on my part.]
This is the first in a new series of blog posts under the title of ‘Galaxies 101’. These posts aim to explore the history and basics of the science of galaxies. I’ll be covering some of people who helped us understand these ‘Island Universes’ as well as some of the basics that would be taught during a first year undergraduate galaxies course at university.
It is fortunate that these posts are beginning in the week of the 90th anniversary of The Great Debate which occurred on April 26th, 1920. The Great Debate – or the Shapely-Curtis Debate – took place at the Smithsonian Museum of Natural History between two eminent astronomers, Harlow Shapley and Heber Curtis. Shapely was arguing that the ‘spiral nebulae’, that were observed at the time, were within our own Galaxy – and that our Galaxy was the Universe. He also argued that the Sun was not at its centre. Conversely, Curtis argued that the Sun was at the centre of our Galaxy but that the ‘spiral nebulae’ were not inside our Galaxy at all. He suggested instead that the Universe was much larger than our Galaxy and that these nebulae were in fact other, ‘island’ universes.
Below is a drawing of the ‘spiral nebula’ M51. This is an observation by Lord Rosse, drawn in 1845 using the 72-inch Birr Telescope at Armagh Observatory in the UK.
With 90 years of hindsight we can now say that Shapely and Curtis were both right and wrong. The Sun is not at the centre of the Galaxy and the Galaxy is only one of hundreds of billions of galaxies in the Universe. But how was the argument resolved? The answer, in part, comes from a very famous name in astronomy: Hubble.
Less a decade after the Great Debate took place, Edwin Hubble used the largest telescope in the world – the 100-inch Hooker Telescope on Mount Wilson – to observe Cepheid variable stars in the Andromeda Nebula/Galaxy. Cepheid variables are a type of pulsating stars whose pulsation periods are precisely proportional to their luminosities. This makes Cepheid variable stars a ‘standard candle’ – an object where the brightness is a known quantity. If you can observe the apparent brightness of a standard candle, then you can determine its distance by a simple inverse square law. Since Cepheid variable stars have pulse rates proportional to their luminosity, if you can measure the pulse rate of a Cepheid variable anywhere in the Universe, then you can determine how far away it is. This is what Edwin Hubble did in 1925 and he calculated the distance to Andromeda as 1.5 million light years.
At the time, Shapely thought that our Galaxy was around 300,000 light years across and Curtis believed it was around 30,000 light years. Hubble’s measurement placed Andromeda well outside our galaxy and showed that Curtis was correct in thinking that the ‘spiral nebulae’ could indeed be other galaxies. Today we think the Milky Way is about 100,000 light years across and that Andromeda is 2.5 million light years away.
The discoveries of the 1920s started a whole new adventure for astronomy. The Universe had gotten a lot bigger and was about to expand much, much more. It is important to remember that Shapely, although wrong about the nature of the nebulae, did correctly assert that the Sun was not at the centre of the Galaxy. This is the kind of Copernican shift that makes people think about things differently and it is important to realise that the issues discussed during the Great Debate were complex. For our benefit though, the Great Debate is a starting point for exploring the relatively new study of galaxies. Humanity’s view of the Universe, and our place within it, has changed an awful lot since 1920. The study of galaxies has had a lot to do with that.
[Andromeda image credit: Robert Gendler]