Kaboom!

Chaco Culture National Historical Park is located well off the beaten path:

Chaco Canyon is located in northwestern New Mexico. The preferred and recommended access route to the park is from the north, via US 550 (formerly NM 44) and County Road (CR) 7900, and CR 7950.

From the north, turn off US 550 at CR 7900--3 miles southeast of Nageezi and approximately 50 miles west of Cuba (at mile 112.5). This route is clearly signed from US 550 to the park boundary (21 miles). The route includes 5 miles of paved road (CR 7900) and 16 miles of rough dirt road(CR 7950).

From the south, two routes access Chaco from Highway 9, which runs between Crownpoint, Pueblo Pintado, and Cuba. Both routes can vary from very rough to impassable. Not recommended for RVs. If you are traveling from the south, please call ahead for the latest conditions.

In fact, Chaco Canyon is about as isolated a location as can still be driven to in the lower 48 states. I love visiting there. One of my fondest high school memories is of our Astronomy Club making the long trip to camp overnight at the park. The night was moonless, perfectly clear, and utterly devoid of any skyglow from nearby cities. (There are no nearby cities. Even the nearest gas station is 21 miles away.) We spent a wonderful evening looking at a sky full of amazingly bright stars while perched on the rim of the main kiva of Pueblo Bonita. It is the only time in my life when I can recall being able to find my way around easily by starlight.

Even with its dark skies, Chaco Canyon might seem like an odd place for an astronomy club excursion. But Chaco Canyon is a favorite visit for pagans and archaeoastronomers, both of whom look for astronomically significant pictograms and building orientations in ancient ruins. We chose the vernal equinox for our visit, and verified that, yes, the pueblo appears to be aligned precisely with sunrise on the vernal equinox. We also hiked several miles down the canyon to see the famous Crab Supernova pictogram:

Source: National Park Service

This photograph is taken looking almost straight up at the underside of an overhanging cliff. (The objects that resemble broken pots are actually bird's nests.) It took some doing for the artist in question to climb up to the overhang, produce his work of art, and sign it (so to speak.) I don't know how one proves such things, but some archaeoastronomers believe this is a rendition of the Crab Supernova of 1054. The age of the pictogram is about right.

The Crab Supernova of 1054 was one of the most spectacular explosions ever witnessed with the naked eye. According to Chinese accounts, the supernova appeared on July 4th of that year and was visible in daylight for 23 days. It remained visible to the naked eye for almost two years. Oddly enough, neither the Arabs nor the Europeans left any unambiguous record of this display.

In 1731, the British astronomer John Bevis discovered a small nebulosity in the constellation Taurus. It was rediscovered by Charles Messier in 1758 and briefly mistaken for the return of Halley's Comet. Messier was apparently prompted by this experience to begin his famous catalog of celestial objects, with the Crab Nebula becoming Messier-1. There the matter remained until 1921, when Knut Lundmark made the connection to the supernova seen by the Chinese in 1054. By 1930, it was clear that the Crab Nebula was a most peculiar object, and it was eventually found to be emitting radiation in almost every part of the electromagnetic spectrum, from radio waves to X-rays.

In 1968, a pulsar with the remarkably short period of 33 milliseconds was detected in the Crab Nebula. This pulsar was quickly identified with an optical pulsar, CM Tauri, the only optical pulsar so far discovered. This pulsar is a rotating neutron star left over from the supernova of 1054.

Supernovae are not the only kind of stellar outburst. Stellar eruptions cover a broad range of energies, and (not surprisingly) the smaller outbursts are much more common. At the low end of the scale, UV Ceti stars experience periodic flares similar in energy to those of the Sun. However, a UV Ceti star is a red dwarf, which is a very small, very faint star that emits most of its energy in the infrared. Its visible luminosity is typically 1/100,000 that of the Sun. As a result, a solar-sized flare can increase the brightness of the star by six magnitudes, or a factor of more than 200. Like a solar flare, the flare of a UV Ceti star is probably caused by reconnection of a twisted magnetic field with the release of large amounts of magnetic energy. The magnetic field itself originates in the very deep convection present in these stars. UV Ceti stars are extremely common; there are several within thirty light-years of the Sun.

Further up the scale, one encounters dwarf novae, which exhibit sudden increases of brightness by a factor of 5 to 250 every few days or weeks. All known dwarf novae are binary systems in which matter is being torn from a red giant by its white dwarf companion. The hydrogen-rich matter that accumulates on the white dwarf is slowly compressed and heated to the ignition point for hydrogen fusion. But because of the high surface gravity of the white dwarf, the hydrogen is very dense when it ignites, and it does not burn smoothly as in the core of a normal star. Instead, it burns explosively, which accounts for the dwarf nova outbursts.

There is some evidence that there is a continuum of behavior between dwarf novae, with frequent, modest outbursts, and ordinary novae, with rare, powerful outbursts. The underlying mechanism appears to be similar. Novae increase in brightness by up to a factor of a million and remain bright for days to months. There are typically a few novae visible to the naked eye each decade, and several telescopic novae are observed in the Andromeda galaxy each year.

Supernovae are not just very bright novae. The mechanisms are different. Supernovae are divided into several types based on their observational characteristics. Type Ia supernovae are similar to ordinary novae in that the seat of the explosion is a white dwarf that has accreted matter from a companion star. In fact, some astronomers believe that ordinary novae can eventually become supernovae. However, the thermonuclear runaway in a Type Ia supernova occurs when the white dwarf reaches the Chandrasekhar limit of 1.4 solar masses, and it involves the entire body of the star, not just a thin skin of hydrogen on its surface. The entire mass of the white dwarf is converted to iron-peak elements, which releases all the nuclear energy available in the star at once. This is enough to completely disrupt the star, producing an expanding cloud of hot iron and nickel and no remnant.

Because the Type Ia supernovae all occur in stars with similar mass and composition (carbon-oxygen white dwarfs of 1.4 solar masses) the explosions are remarkably uniform in their characteristics. Type Ia supernovae have become important standard candles for plumbing the depths of the cosmos. Until a few decades ago, they were also believed to be the brightest explosions in the Universe.

All other supernovae are believed to result from the collapse of the silicon core of a very massive evolved star. In such a stellar core, temperatures are so high that copious quantities of neutrinos are produced and escape from the star. This energy loss causes the core to contract and heat even further. When the electron density becomes high enough, electrons begin to combine with protons to produce neutrons -- a highly endothermic reaction that destabilizes the core, triggering the collapse. The collapse is halted when the core reaches nuclear density, about 10¹⁴ grams per cubic centimeter. The gravitational energy of this collapse is converted into a brief, very intense pulse of neutrinos that heats the mantle surrounding the core, reversing its collapse and sending the outer layers of the star into space in a spectacular explosion. A tiny but massive neutron star is left behind, which typically has the high rotation rate and powerful magnetic field required to become a pulsar.

Because core collapse can occur in stars with a variety of masses, core-collapse supernovae are not uniform in characteristics and do not make good standard candles. They are also somewhat fainter than Type Ia supernovae. However, while Type Ia supernovae are thought to be responsible for enriching the interstellar medium with iron, core-collapse supernovae are though to be the most important source of elements from carbon to magnesium. Core-collapse supernovae may also be responsible for many of the elements heavier than iron, through a mechanism called the r-process. To what extent the r-process actually takes place in core-collapse supernovae is still one of the great unknowns of astronomy.

I am old enough to remember fallout shelters, Civil Defense drills such as the "This is a test" broadcasts on television, and 48-hour kits full of yucky candy. The Cold War is one of those historical periods that are better looked back on than lived through.

In 1963, the United States, Russia, and Great Britain concluded a Limited Test Ban Treaty that prohibited nuclear tests in the atmosphere, under the ocean, or in outer space. Like most successful treaties, it worked because it was in everyone's interest. The three powers had already learned most of what they wanted to know about weapons effects, and the dangers from radioactive fallout had become clear. Underground tests would suffice for further weapons development while avoiding the fallout.

The other element necessary for the success of the treaty was a means of verification. For the United States, this took the form of the Vela satellites, which were launched from autumn 1963 onwards. These satellites carried various detectors, which included gamma-ray counters designed to detect an exoatmospheric nuclear detonation.

There was considerable consternation when the satellites began to detect intense bursts of gamma radiation. Because the program was highly classified, this was not revealed to the public until 1973. By then, it was established that the sources of the gamma ray pulses were located far outside the Solar System. Cosmic gamma ray bursts are detected about once or twice a day, and last from a few milliseconds to several minutes. The more intense bursts also produce a faint optical signature, and in 1997 the Keck Telescope was able to measure the redshift in the optical spectrum of a burst and determine that the source of the burst lay billions of light-years away. This made the burst a million times more luminous than a Type Ia supernova. Such bursts are the most powerful bangs the universe has seen since the Big One.

Several mechanisms have been proposed for gamma ray bursts, and it is likely that more than one kind of burst takes place. The most powerful probably result from the collapse of a stellar core that is so massive that a black hole is formed, rather than a neutron star. This collapsar model is supported by observations showing that silicon-group elements were ejected from one gamma-ray burst at a tenth the speed of light.

So powerful are these bursts that James Annis of Fermilab has suggested that they explain the Fermi Paradox: "Where are they?" The heart of this paradox is the argument that intelligent life capable of interstellar travel ought to be common in the Universe. So why haven't they contacted us yet? Truth be told, there are many possible resolutions to the paradox. But Annis argues that gamma-ray bursts emit so much deadly radiation that they are capable of destroying all intelligent life in their host galaxy. Hence, the reason why no one has contacted us is because intelligent species are destroyed faster as they can evolve.

A collapsar at the heart of our own Galaxy would bathe one hemisphere of the Earth in deadly radiation. It would also play havoc with the chemistry of the upper atmosphere, destroying the ozone layer and exposing the other hemisphere to ultraviolet radiation from the Sun. Though humans could protect themselves from this radiation, their crops would be destroyed, along with phytoplankton in the oceans, unravelling the food chain and dooming our species to starvation.

Not all gamma-ray bursts are as powerful as the ones resulting from collapsars. A nonlethal, but nonetheless spectacular, gamma-ray burst took place in our Galaxy last December:

Scientists have detected a flash of light from across the Galaxy so powerful that it bounced off the Moon and lit up the Earth's upper atmosphere. The flash was brighter than anything ever detected from beyond our Solar System and lasted over a tenth of a second. NASA and European satellites and many radio telescopes detected the flash and its aftermath on December 27, 2004.

I'm a little annoyed that the article fails to distinguish between apparent luminosity and absolute luminosity, or between bolometric and visible luminosity. The burst on December 27 had a greater apparent bolometric luminosity than anything ever witnessed by astronomers. It appeared this bright only in the X-ray and gamma-ray portions of the electromagnetic spectrum, and only because it was within our own galaxy. Had it had an absolute luminosity as great as a collapsar, we would all be toast.

Not that this burst was a mere firecracker:

"The next biggest flare ever seen from any soft gamma repeater was peanuts compared to this incredible December 27 event," said Gaensler. "Had this happened within 10 light years of us, it would have severely damaged our atmosphere. Fortunately, all the magnetars we know of are much farther away than this."

That's a relief.

The source of this gamma-ray burst is believed to have been a magnetar, which is a highly magnetized neutron star. Magnetars are thought to be the source of soft gamma-ray repeaters, which are something like dwarf novae in the gamma-ray region of the spectrum. Unlike an ordinary neutron star, whose primary reservoirs of energy are its gravitational binding energy and rotational energy, a magnetar stores a significant fraction of its total energy as magnetic energy.

All neutron stars cool rapidly after their formation and develop a solid crust of super-dense iron. This seems incredible, given that the surface temperature of a neutron star is on the order of a million degrees, but the gravitational field is so intense (on the order of a hundred billion G) that the pressure is sufficient to force the iron into the solid state. Under the crust, the interior of a neutron star is a superfluid composed of neutrons. This superfluid contains trace numbers of protons and electrons that also make it a superconductor, and the effects of the neutron star's rotation on its superconducting interior produce an intense magnetic field.

As a neutron star continues to cool and shrink, strains develop in the crust that results in faulting and starquakes. In an ordinary neutron star, the starquake reduces the moment of inertia of the star, and astronomers observe a "glitch" in the rotational period of the star. Normally the rotational period slowly increases over time due to loss of energy to the magnetic field, but in a "glitch" the period abruptly decreases slightly.

In a magnetar, the starquake is not so gentle. I'm no expert in magnetohydrodynamics (MHD), which is a very specialized field, but I can imagine that the abrupt change in the rotational period of the magnetar would result in a considerable disturbance in the magnetic field. The resulting magnetized relativistic shock would be an efficient generator of both gamma rays and cosmic rays.

Scientists continue to follow the event with interest:

Scientists around the world have been following the December 27 event. RHESSI detected gamma rays and X-rays from the flare. Drs. Kevin Hurley and Steven Boggs of the University of California, Berkeley, are leading the effort to analyze these data. Dr. Robert Duncan of the University of Texas at Austin and Dr. Christopher Thompson at the Canadian Institute for Theoretical Astrophysics (University of Toronto) are the leading experts on magnetars, and they are investigating the "short duration" gamma-ray burst relationship.

Brian Cameron, a graduate student at Caltech under the tutorage of Prof. Shri Kulkarni, leads a second scientific paper based on VLA data. Amateur astronomers detected the disturbance in the Earth's ionosphere and relayed this information through the American Association of Variable Star Observers (http://www.aavso.org).

On a personal note, Shri Kulkarni was an assistant professor at Caltech when I was a graduate student there, and he was a favorite of many of the students. If I recall correctly, at that time he was working on neutron star binaries, another proposed mechanism for gamma-ray bursts, and on millisecond pulsars. His wife became pregnant with twins while I was there, and the graduate students began a "Name the Twins" contest that Kulkarni wisely ignored.

Suggestions included Phobos and Deimos, Alpha and Beta, and Castor and Pollux. Caltech is an international center of ubergeekdom.