X-Ray Specs

Columbia astrophysicist Chuck Hailey is building a telescope to view stars so hot they’ve never been seen.

by David J. Craig Published Spring 2010
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Black-hole hunters

Illustration: Geoff McCormack

Hailey’s invention capitalizes on a curious characteristic of hard X-rays: They lose their penetrative power when they hit a flat surface at a very shallow angle, in which case they’ll reflect off the surface, like a rock skipped off of water. NuSTAR’s glass cylinders will be positioned nearly parallel to incoming X-rays. The cylinders are skinnier at their tail ends, forcing X-rays to glance off their inner surfaces, not once but twice, reflecting the X-rays inward about a quarter degree each time.

“X-rays have very limited reflective properties,” says Hailey, who is Columbia’s Pupin Professor of Physics, “but they’re made of photons, and just like any photons, they’ll bounce off a mirror. You just have to catch them on their sweet spot.”

Astronomers have been using cylindrical mirrors to focus high-energy wavelengths of light since the late 1960s. But no telescope has ever achieved the tiny reflection angles necessary to focus hard X-rays. NASA’s Chandra X-ray Observatory, for example, employs four cylinders nested together less tightly than are NuSTAR’s, enabling Chandra to focus light only in the soft X-ray spectrum. NuSTAR is unique also for having more cylindrical shells than any existing telescope. Its 133 layers of glass would cover a Little League baseball diamond if spread out on the ground. That represents an exposure area three times as large as Chandra’s and it will enable NuSTAR to produce images of stars that emit even the faintest trace of hard X-rays.

Chuck Hailey at Columbia’s Nevis LabsThe drawback of using a giant aperture is that resultant images won’t have exquisite resolution. Every photographer understands this trade-off: if you’re shooting in low light, you must sacrifice some detail to get any picture at all. But this fits the exploratory nature of NuSTAR’s mission. Whereas NASA launched its Chandra and Hubble observatories in the 1990s in part to make highly detailed pictures of phenomena already observed — which requires moderating the amount of light entering a telescope at any moment — NuSTAR’s goal is to make useful, but not necessarily beautiful, images of celestial objects never seen before.

Hailey and his NuSTAR colleagues, who include physicists from the California Institute of Technology and Stanford University, view themselves as pioneers exploring a new band of the light spectrum, identifying phenomena that future X-ray missions will observe more carefully. (Their project, as part of NASA’s Small Explorer class of missions, has a budget cap of about $110 million; large NASA satellites like Chandra and Hubble cost 10 to 20 times as much.) “We won’t be making sharply focused pictures like the big dogs,” says Hailey. “For us, the name of the game is to be the first into the hard X-ray band, to pluck the low-hanging fruit in terms of discoveries, and to demonstrate that our method for focusing hard X-rays works.”

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Keep up the great work, Chuck!


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