Reinventing Discovery - Michael Nielsen [71]
Figure 7.2. The first of the green pea galaxies, found by Galaxy Zoo forum member Nightblizzard in July, 2007. The green pea is in the center. Like all of the peas, it looks quite nondescript, and if you’re not familiar with galaxies, it’s tempting to think it’s just another elliptical galaxy, or maybe a star. But many of the Zooites became quite expert at analyzing galaxy images, and it wasn’t long before they realized the peas were unusual. Credit: Sloan Digital Sky Survey.
The reason the spectrum of a galaxy is important is because it allows astronomers to figure out what the galaxy is made of. This may sound surprising, but again the idea is quite simple: when you heat up a material, say, sodium, it tends to glow with a particular mixture of colors. That’s why sodium streetlamps glow with a very particular yellow-orange color. It turns out that every material—not just sodium, but oxygen, hydrogen, carbon, and any other—has its own unique spectrum, that is, glows with a characteristic mixture of colors. The spectrum of a material is thus a bit like a signature, and by looking closely for such signatures in a galaxy’s spectrum it’s possible to figure out what the galaxy is made of. It’s one of the more remarkable discoveries of science: by looking carefully at the color of distant objects we can figure out what they’re made of, and even how hot they are, since heating a material up changes its characteristic spectrum slightly. The SDSS made high-quality spectra available for all Galaxy Zoo galaxies, and it was by looking closely at the spectrum of the green peas that the Zooites figured out that some of the peas were surrounded by hot, ionized oxygen gas.
(I can’t resist digressing to mention the marvelous fact that the substance helium was actually discovered using spectral analysis! In 1868, the astronomers Pierre Jules César Janssen and Joseph Norman Lockyer independently observed that the spectrum of the sun had features unlike any substance ever seen on Earth. They deduced, correctly, that they were seeing the first sign of a new chemical substance. But it wasn’t until almost 30 years later that a chemist named William Ramsay discovered helium on Earth.)
Enough about spectral analysis; back to Galaxy Zoo and the mystery othat theeas. By this point—December 12, 2007—Zookeeper Kevin Schawinski had become intrigued by these strange galaxies. He decided to take a closer look at the peas. He ran some tests and quickly confirmed that they were indeed a new type of galaxy.
You might think the professional astronomers would now move in and take over the project. After all, the amateurs at Galaxy Zoo had just discovered an entirely new class of galaxy! But the pros, including Schawinski, were busy with other things, including Hanny’s Voorwerp, and they didn’t take over straight away. Instead, what happened next was a remarkable piece of science driven by the amateurs. The tone was set by a Zooite named Rick Nowell. Nowell went back through all the pea images that had been posted to the Galaxy Zoo forum, and systematically identified 39 objects that looked like they might be the new type of galaxy. Inspired by Nowell’s list, other people started to make their own lists, and began debating what criteria should be used to distinguish this new type of galaxy from similar-looking objects, such as green stars. The tone of the project began to change, becoming focused on getting to the bottom of the pea mystery. People found red galaxies with characteristics similar to the green peas, but further away. More and more, the discussion focused on detailed properties of