Weaver is sensitive to concerns about using a nuclear device. He proposes that it would be detonated in deep space, where, accord- ing to his simulation, neither the explosion nor the radioactive fallout would pose any threat to Earth. The real issue, according to Weaver, is how much advance notice of a nearby asteroid is obtained and how long it would take to execute a mission; deflecting an asteroid could require a two-year warning. Yet even with an appeal to the nuclear op- tion, there is currently no feasible defense against asteroids more than a mile wide. Fortunately, they rarely impact Earth. The last occurrence was the celestial celebrity that brought about the end of the dinosaur age 65 million years ago. ­ —Kirsten Fox Ghost of Christmas Past On Christmas day, 2010, the Burst Alert Telescope onboard NASA’s Swift satellite, running software developed at Los Alamos, detected a new type of gamma-ray burst (GRB). GRBs are exactly what they sound like—quick bursts of gamma rays—often followed by less energetic radiation. Al- though the gamma-ray component typically lasts less than a minute, this particular burst lasted a half hour before being followed by an x-ray afterglow. Its emission spec- trum, too, contained a blend of familiar and unfamiliar features: One part resembled an energetic jet of matter and another part resembled a supernova; both of these are frequently associated with stellar explo- sions and the expanding shock waves that accompany them. But this particular GRB had one feature that could not be so neatly explained as originating with an exploding star: shortly after the shock wave broke out of the star, it appeared to run into an unexpected outer shell of material, emitting a burst of ultraviolet, visible, and infrared light. Evidently the GRB, now known as the Christmas Burst, represents a rare astro- nomical event. The cause of the Christmas Burst is the subject of lively debate in astronomy circles. One leading hypothesis is that the GRB originated in a binary star system in which a red giant star with a core made of helium closely orbited a neutron star. Eventually the stars spiraled into each other, producing the outburst. This scenario was originally pro- posed in 1997 by Los Alamos computational physicist Chris Fryer, who has continued to pioneer theoretical efforts to better study its characteristics. If Fryer’s scenario is indeed the cause of Christmas Burst, then based on the burst’s brightness, the collision must have taken place in a distant galaxy. An alternate proposal involves a small, comet-like object falling onto a neutron star and producing a much dimmer burst, in which case it must have occurred closer, within our Galaxy. Normally this could be resolved by simply looking for a galaxy in the location where the GRB was observed, but only an inconclusive hint of a glow was found. Indeed, Fryer and his team submit- ted a proposal to take a longer-exposure image of that region using the Hubble Space Telescope, while still pursuing other ways to identify the source of the unusual GRB. During the year following the burst, Fryer and Los Alamos colleague Wesley Even put the distant stellar collision hypothesis to the test, using a sophisticated computer simula- tion to calculate the emission spectrum pro- duced by such a collision. Working with an international team of researchers running a variety of simulations, they were able to confirm that the observed Christmas Burst matched the neutron star-red giant collision model studied by Fryer and his team. For most of its “life,” a star produces energy by nuclear fusion, with hydrogen nuclei in its core fusing together. But even- tually, the hydrogen in the core fuses into helium, which is unable to fuse with itself to generate additional heat and pressure. Without that source of pressure, the helium core begins to collapse under its own weight while the outer layers of the star, spurred by nuclear reactions outside the core, expand outward. The result is known as a red giant. For massive stars, the helium core eventually gets hot enough to ignite further nuclear fusion and thereby produce This artist’s conception of the Christmas Burst astronomical event shows the merger of a neutron star with a red giant star, with the resulting body collapsing to a black hole (central dot). The collapse produces an extremely energetic jet of matter that interacts first with the collapsing stellar core (central sphere) and then with the outer layers of the red giant star (surrounding red swirl) that were previously cast outward by the approach of the neutron star. Los Alamos scientists recently performed a computer simulation to demonstrate how this event produced the complex pattern of radiation observed. CREDIT: Aurore Simonnet, Sonoma State University, and NASA 1663 los alamos science and technology magazine june 2012 23