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The Distances of the Spiral Nebulae: A Huge Challenge! The ‘Great Debate’ Curtis vs Shapley (1920): are the nebulae big, even comparable to our own Milky Way (as Curtis argued)? or are they small satellites of the Milky Way (as Shapley maintained)? Shapley’s arguments, on varied lines of evidence, carried the day (but he was wrong). How Can We Determine Their Distances? A Naïve Approach Why not just take some photographs that demonstrate that the spiral nebulae are full of stars? The extreme faintness of those stars would tell us that even the largest, closest nebula (like the Andromeda nebula, M31) is very far away. Surely that would resolve the question! Unfortunately it was not that simple. Meet M31, Our Nearest Big Neighbour Just visible (as a fuzzy patch) to the unaided eye – the most remote object we can see directly. The Problem Andromeda (a.k.a. M31), the closest big galaxy, is 2 million light years away. This makes the constituent stars very faint At the very start of the 20th century, telescopes and photographic techniques were simply not able to resolve even the brightest giant stars in M31 The dots of light you see sprinkled around the image are foreground stars, in our own Milky Way and quite close to us. Close examination of the M31 nebula itself does not show myriads of member stars, just a general ‘blur’ of light. How About Now? Even now, the Hubble Space Telescope would just barely be able to detect a sunlike star in M31, our very nearest galaxy neighbour. Studying the stars in more remote galaxies is of course even more challenging. It’s Not Quite as Bad as it Sounds! There are, of course, lots of stars that are much brighter than the sun! With big enough telescopes, these intrinsically brighter stars (giants and supergiants) can be seen and studied. By the 1910’s, the 100-inch Hooker telescope – the world’s largest – at Mt Wilson (Los Angeles) was able to just detect the very brightest stars in M31. The Hooker Telescope A Second Problem, However How do we convince ourselves that the dots of light that we see in a nebula are stars, not just (say) ‘Jupiters in formation’? Here’s an Insufficient Answer Suppose we study the spectrum of one of the dots of light, and discover (say) that it is comparable to the spectrum of the Sun. This will not prove that it is a star! Why not? It might simply be reflecting the light from the hypothetical ‘protostar’ that is forming at the middle of the nebula. (Analogy: here in the Solar System, the light we get from Jupiter has a spectrum like the Sun: it’s reflected sunlight!) Anyway, they are so faint that astronomers couldn’t get a spectrum, which requires spreading out the light. So this approach was not possible. A Much Better Idea Let’s monitor the nebula to see if one of the dots of light behaves in a way that only a star does! Exactly what do we mean by that? Should we watch to see if the ‘star’ evolves? Of course not! – that takes millions of years! We need a more practical short-term approach. The Eventual Solution: Identify Variable Stars It was discovered by Hubble that some of the dots of light in M31 vary in brightness in a way that only certain types of very bright stars do. So they are clearly not ‘Jupiters in formation,’and the nebulae are not solar systems at birth. As we will see, that settled the question: the nebulae are galaxies like the Milky Way!