The Great Debate



A nebula is a giant cloud of gas and dust in outer space. And when I say giant, I really mean giant. The so-called “pillars of creation” nebula is several light years long. Given that light travels over six trillion miles in a year, that’s pretty huge. The Earth suspended in most nebulae would be invisible, not even a speck of dust in the big picture. There’s a reason why astronomers get that reverent far-away look in their eyes and wax poetic about the vastness of the universe. It’s huge out there, man. And nebulae are some of the most attractive deep-space objects you can look at, so let’s learn about them. It’s definitely fun to wear Star Wars t shirts, but why not learn a little about outer space while we’re at it?


“Nebula” comes from the Latin word for cloud, which also means fog. It’s the root of the English word “nebulous,” meaning something mysterious or difficult to discern. These objects are enormous clouds with vague edges, hence “nebula” for something cloudy or foggy. Nebulae often have uncertain edges, and you’d never be able to define exactly where they stop. Other interstellar objects have this problem as well: VY Canis Majoris, one of the largest known stars, has historically had its “size” adjusted several times as scientists have puzzled out how to define its boundaries. In fact, all large interstellar objects were once referred to as nebulae, until the early 20th century when we began to understand more about outer space.


That understanding came from something called the “Great Debate” between two astronomers named Harlow Shapley and Heber Curtis. You have to understand the context here to really understand what the debate was about. At that point in time, it was thought that the Milky Way galaxy was the whole of the universe. We didn’t even know about other galaxies back then. So the idea of galaxies outside of our own was totally new. Other galaxies had been observed, but they just looked like big gas clouds. So we thought, for example, that the Andromeda galaxy was a giant nebula and not a galaxy of its own.


Shapley was a more old-timey astronomer who thought that the “nebulae” in outer space were just big gas clouds inside of the Milky Way. Curtis, however, thought that the nebulae were their own self-contained galaxies, which would mean that they were stupendously huge and very far away. Shapley was arguing for the old thesis that the Milky Way was the entire universe. It was not until after the Great Debate that astronomers slowly began to realize what was going on. There were nebulae, yes, but also distant galaxies containing billions of stars and, indeed, nebulae of their own.


One more fun fact: on a clear night sky, in a region with low light pollution, the Orion Nebula can be seen with the naked eye. If you live in the United States, you can go to the extreme northeast of Maine to a dark sky preserve to see what the night sky looks like without any light pollution. The night sky is also much more clear in deserts. If you’re willing to travel to a region with low light pollution, you can see the nebulae and the stars in the same glory that people did thousands of years ago, before electric lights and pollution made it so hard to see the stars.

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