Reinventing Discovery - Michael Nielsen [67]
Puzzled, on August 13 she posted a note to Galaxy Zoo’s online forum, asking if anyone knew what the blue blob might be.
No one knew.
Tests were done. The blob wasn’t some kind of blemish in the photograph, it was real. Observations were made at other telescopes to get more detailed information, including observations with the powerful William Herschel telescope in the Canary Islands. Those observations showed that the blue blob was at about the same distance from the Earth as the galaxy hovering above it, which meant the blob was enormous, tens of thousands of light-years in diameter. More experts were called in, none of whom had ever seen anything like it.
Figure 7.1. A black and white reproduction showing the strange blob first noticed by Hanny van Arkel. In the original color image the blob was a striking blue, and contrasted vividly with the galaxy above. Credit: Sloan Digital Sky Survey.
The mystery mounted. More and more people began speculating about what the blue blob could be. The object was dubbed Hanny’s Voorwerp, after the discoverer and the Dutch word for object.
Slowly, an explanation of the voorwerp took shape, an explanation connecting the voorwerp to the staggeringly bright objects known as quasars. To understand that explanation, we first need to back up a bit and talk about quasars. As you may know, quasars are among the strangest and most mysterious objects in the universe. They are incredibly bright: a quasar the size of our solar system can shine as brightly as a trillion suns, outshining a giant galaxy like our Milky Way many times over. Fortunately for us, the nearest quasars are hundreds of millions of light-years away—if a quasar turned on a few light-years away, it would fry the Earth.
When quasars were first discovered, in 1963, it was a mystery how such comparatively small objects could shine so brightly. It took astronomers and astrophysicists many years to understand and agree on what is going on, but by the 1980s it was widely accepted that quasars are powered by solar system–sized black holes at the center of galaxies. Those black holes devour surrounding matter—stars, dust, you name it—while other matter swirls around the black hole, not quite falling in, but accelerated to near the speed of light. That enormous acceleration produces vast quantities of energy, some of which is emitted as light. It’s that light that we see on Earth as the quasar. But while this rudimentary picture of quasars is now widely accepted, many fundamental questions remain unanswered.
With that understanding of quasars in mind, let’s come back to the voorwerp. As the people at Galaxy Zoo puzzled over what the voorwerp might be, they considered many possible explanations, and gradually closed in on a simple explanation that seemed to fit all the facts: the voorwerp is a quasar mirror. The idea is that about 100,000 years ago, the galaxy near the voorwerp contained a quasar. That quasar has since switched off, for reasons unknown, and we no longer see it. But while the quasar was still shining, the light from the quasar was heating up gas inside a nearby dwarf galaxy, and causing it to glow. It’s that glowing gas that we now see as a blue blob, and that’s why we can think of the voorwerp as a quasar mirror. In fact, it’s really a huge collection of mirrors, distributed over a vast