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and quickly crashing the new website. For the next six hours the Zookeepers running the site worked frantically to get the site back up and running. When the site finally reappeared, volunteers rapidly began signing up, and by the end of the first day more than 70,000 galaxy classifications were being done every hour—more than Schawinski had managed in his heroic week. Each galaxy was examined independently by many volunteers, enabling the Zookeepers to automatically identify and discard incorrect classifications. This made the results comparable to careful classification by professional astronomers. Although the rate of galaxy classification gradually slowed from its peak of 70,000 per hour, Galaxy Zoo’s first classification of galaxies was complete after just a few months. That gave Schawinski the data he needed to finish his project. Verdict: yes, the conventional wisdom about spirals versus ellipticals was wrong, and some ellipticals really do contain a lot of newly formed stars.

Galaxy Zoo began with Schawinksi’s questions, but over time the site has expanded to address a much broader range of questions. Many discoveries have been made serendipitously, when some Zooite (as the participants call themselves) has noticed something unusual in a photo, as in Hanny van Arkel’s discovery of the voorwerp. A second, more complex example of serendipitous discovery is the story of the “green pea” galaxies. This story illustrates the potential of citizen science even better than the voorwerp, and so I’ll recount it here. Incidentally, my account draws on a marvelous article written by one of the Zooites, Alice Sheppard, which you can find referenced in the “Notes” at the end of the book.

On July 28, 2007, two weeks after the Zoo first opened, a poster to the Galaxy Zoo forum named Nightblizzard posted a picture of a fuzzy green galaxy, noting that it was unusual for galaxies to be green. A couple of weeks later, on August 11, 2007, someone else posted a picture of a strange green galaxy. It was unusually bright, and the poster, named Pat, asked if the galaxy might be a quasar. No one was quite sure.

The next day, on August 12, a third poster, the ubiquitous Hanny van Arkel, found another of the strange green galaxies. Van Arkel dubbed the galaxy a “green pea,” and posted it to the forum with a message titled “Give peas a chance!” Other Zooites thought this was hilarious, and started to dig up peas of their own, adding them to the “pea soup” taking shape on the forum. For several months the discussion thread grew. At first it was mostly people adding objects, or making pea jokes (“peas stop”). But people also asked thoughtful questions. What exactly were the peas? Why hadn’t anyone heard of them before? One poster commented: “They talk about stars, galaxies, nebulae, planets, etc. in astronomy courses, but they never mention the peas. It must be a big secret among professional astronomers. They probably want all the peas for themselves to eat.”

At first, pea collection was just a fun hobby for the Zooites. But as the collection of peas grew, so too did the mystery surrounding them. Some turned out to be ordinary stars or nebulae. But a few of the green galaxies still stood out as unusual. The Zooites figured out—I’ll describe how shortly—that some of the pea galaxies were surrounded by incredibly hot, ionized oxygen gas. That was unusual for a galaxy. What were these small, green, highly luminous galaxies, surrounded by hot, ionized oxygen? And why had nobody ever heard of them before?

Let me pause here to explain how the Zooites figured out that the peas were surrounded by hot, ionized oxygen. It’s an interesting piece of science, and illustrates just how serious some of the Zooites were becoming. Obviously, they couldn’t determine that oxygen was present by going and visiting one of the galaxies. Instead, they figured it out by teaching themselves a technique called spectral analysis. We don’t need to go into the details of how spectral analysis works, but the basic idea is quite simple. It’s based on what’s called the spectrum