A new space telescope is set for launch Wednesday on a quest to explore the inner workings of black holes, the mechanisms driving some of the most mighty explosions in the cosmos, and the poorly understood processes heating the sun's corona to more than 1 million degrees.
These phenomena reveal their secrets via X-rays – a form a light substantially more energetic than visible light and capable of piercing enshrouding clouds of cosmic dust. Known as the Nuclear Spectroscopic Telescope Array (NuSTAR), the $170 million X-ray observatory will allow scientists to observe activity around black holes and other sources of X-rays with greater sharpness and clarity than any telescopes currently available.
Its trick is a 33-foot-long mast that extends out from the spacecraft in space. Two sets of optics on the end of the mast will deliver the X-rays they pick up to detectors on the spacecraft in a tightly focused group. As a result, NuSTAR not only will be able to create images 10 times sharper than those from other orbiting X-ray observatories, but it will also be able to see X-ray sources 100 times fainter.
This means that its targets will be "some of the hottest, densest, most energetic phenomenon in the universe," says Fiona Harrison, an astrophysicist at the California Institute of Technology in Pasadena and the project's lead scientist.
This includes regions near the point of no return for matter falling into supermassive black holes at the centers of large galaxies. As dust and gas get closer to this so-called event horizon, the matter is compressed and heated, eventually reaching the point where it emits radiation as X-rays. Existing X-ray telescopes can detect emissions from fairly close to the event horizon. But NuSTAR is designed to detect more-energetic X-rays, in effect giving it a window on conditions even closer to the event horizon.
NuSTAR will also open new vistas on supernovae – some of the most powerful explosions known and events that are thought to produce all the chemical elements in the universe heavier than hydrogen and helium. NuSTAR scientists will in effect act as a cosmic bomb squad, teasing from the expanding cloud of element-rich matter ejected new details about how the explosions happen, says Daniel Stern, project scientist for NuSTAR at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
To do this, the team plans to train the telescope on two supernova remnants: Cassiopeia A and SN 1987A.
In Cass A's case, the type of star that exploded is unknown. But supernova 1987A involved a star some 20 times the sun's mass. When a star at least 10 times the sun's mass explodes, the object that remains after a supernova is a neutron star – essentially a solid core of neutrons with as much mass as the sun but packed into an object the size of Manhattan. But if a far more massive star goes supernova, the result is a black hole – an object so dense and with gravity so intense that not even light travels fast enough to escape it.
The Chandra X-ray Observatory has detected a neutron star at the heart of Cass A, but it can't detect key features of the expanding remnants of the progenitor star that can help answer key questions scientists have regarding supernovae explosions. Scientists have not yet located anything at the core of SN 1987A, but one hypothesis is that a neutron star could be shrouded in dust, making it impossible to see at visible wavelengths.
The broad outlines of supernovae explosions for massive stars are fairly well established, notes Robert Kirshner, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
For stars like the one involved in Supernova 1987A, in essence, the star runs out of fuel. When the star burns its initial stock of hydrogen, it begins fusing helium. That process continues until the star progressively burns its way through increasingly heavy elements up to iron. By then, however, the star lacks the energy needed to fuse iron. This slams the brakes on fusion reactions, and the forces vanish that allow the star to remain a large gas ball.
What remains of the star begins a runaway collapse that continues until the core is nothing but compressed neutrons, which can collapse no farther. Temperatures and pressures are so high at this point that fusion briefly resumes, forming even heavier elements, including silver, gold, and titanium. The collapse comes to a sudden halt; all the gravitational energy gathered in the collapse is released, hurling the layers into interstellar space.
A second broad category of supernovae results from overfeeding. A white dwarf in a binary system can attract material from its companion if the companion is close enough. Once the white dwarf's mass grows beyond about 1.38 times the sun's mass, pressures and temperatures grow so high at the core that they trigger a runaway nuclear blast.
But the details of these explosive processes are poorly understood, says Steven Boggs, an astrophysicist from the University of California at Berkeley on the NuSTAR science team. For instance: Where does the process begin? How do the runaway reactions proceed through the core of some supernovae? How does the explosion disrupt the star?
Researchers will tease out these details by looking at a product of that final fit of fusion as the star gets set to shed its shells – titanium-44.
By mapping the distribution of titanium-44 across the expanding debris cloud – and then comparing that map with different models of supernovae explosions – researchers can in effect reverse the process and see how the titanium would have been distributed in the progenitor stars during the blasts. And by measuring the speed at which individual clumps of titanium are traveling, researchers can glean information about other details of the explosion.
"Between those two, we have a lot of information about the nuclear physics" involved in the initial blasts, he says.
NuSTAR also will be a powerful tool for studying black holes of all sizes – from the black holes that supernovae produce to the supermassive black holes that lurk in the cores of massive galaxies like the Milky Way and can be billions of times the mass of the sun.
The craft will hunt both for the X-rays emitted as black holes compress and heat matter, as well as high-energy X-rays emanating from black holes' "poles" in beams of charged particles accelerated nearly to the speed of light by intense magnetic fields.
In one experiment, NuSTAR will team up with NASA's orbiting Fermi Gamma-ray telescope to measure the rotation speed of supermassive black holes in galactic cores.
Launch is scheduled for 11:30 a.m. Eastern Daylight Time from the US Army's facilities at Kwajalein Atoll in the Pacific. NuSTAR is tucked into a package about 4 feet wide and 7 feet long, allowing it to be lofted using a rocket from Orbital Sciences Corp. that is launched from underneath a Lockheed L-1011 jumbo jet.