“For years, scientists have debated how cosmic dust forms in space,” said Mikako Matsuura of Cardiff University. “But now, with the help of the powerful James Webb Space Telescope, we may finally have a clearer picture.”
The Butterfly Nebula, NGC 6302, is approximately 3,400 light-years from Earth in the constellation Scorpius, its twin lobes of bright gas sweeping out from a thick, dark waist. That waist is not an insect body, but a torus of dust surrounding the scorching remnant of a star like the Sun. The central core, a white dwarf at the remarkable 220,000 degrees Celsius,, is hot enough to power strong chemical reactions in the gas and dust surrounding it.
Astronomers used the Mid-Infrared Instrument (MIRI) on JWST to find unusually large crystalline silicate grains within that torus. Interstellar dust is usually around 0.1 microns in size, but these grains are more like a millionth of a meter across still tiny, but much larger than the average in the galaxy and closer in size to dust in planet-forming disks. In such disks, grains bump into and stick together, building pebbles that can accrete into planetesimals and, eventually, planets. The existence of large grains within a planetary nebula a formation born of star death means that the first stages of planet building can start even in the wake of a star’s death.
The discovery of quartz crystals introduces further complexity. Quartz, or crystalline silicon dioxide, occurs under certain thermal and chemical conditions. In the Butterfly Nebula, all these conditions are found in the torus, where stable, cooler regions permit slow growth of crystals, while adjacent turbulent areas generate what Matsuura called “fiery grime created in violent, fast-moving parts of space.” The presence of both types of environment within one nebula provides a unique laboratory for testing dust grain growth in wildly different conditions.
The physics of such growth have puzzled astronomers for many years. In the young solar system, grains moved within the protoplanetary disk, undergoing cycles of cooling and heating, evaporation and condensation. Careful analyses of old meteorites, e.g., calcium-aluminum-rich inclusions of the Allende meteorite, have demonstrated that the grains may migrate through enormous radial distances, entraining in their crystal structure the thermal history of their migration. In the Butterfly Nebula, a similar interaction of motion and environment powered by the white dwarf’s radiation and star winds could be sculpting the dust.
Adding to the mystery, JWST found polycyclic aromatic hydrocarbons (PAHs) in the nebula. These carbon-containing molecules, ubiquitous in interstellar space, are intriguing due to their potential for prebiotic chemistry. On Earth, PAHs are found in everyday situations such as burnt toast or car exhaust, but in space they can become involved in reactions producing more intricate organic molecules. PAHs are found in flat, disk-like forms in the Butterfly Nebula, perhaps created when particle-laden outflows from the white dwarf interact with ambient gas. Shock-generated chemistry in this way might be reminiscent of chemistry in star-forming regions, connecting stellar demise to the seeds of life.
The white dwarf in the middle itself is an incredible motor. White dwarfs are extremely dense, Earth-mass objects consisting primarily of carbon and oxygen. When cooling, their interiors can crystallize, a process that could power convection in their liquid interiors. This convection, coupled with fast rotation and extremely high electrical conductivity, can produce magnetic fields a million times more intense than Earth’s. In the Butterfly Nebula, the high radiation and possible magnetic activity of the white dwarf must affect dust dynamics, ranging from grain alignment to the extent of charged particles that enable molecular formation.
The Butterfly Nebula will become extinct over tens of thousands of years, its lobes merging into the interstellar medium. The quartz grains, PAHs, and other molecules created here will flow into molecular clouds, where they will eventually become part of a new star system. This way, the JWST’s observations record not only an instant in the demise of a star, but a phase in the cosmic cycle of material where the remains of a sun turn into raw materials for worlds to be made.
