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Toward a New Model of Dying Stars

When our Sun dies, it will briefly surround itself with a veil of exquisite beauty: an expanding shell of gas and dust known as a planetary nebula.

NGC 7009

A Hubble Space Telescope view of NGC 7009, the Saturn Nebula. [NASA/Bruce Balick et al.]


It is the final act of a star that has stopped producing energy in its core. The star’s outer layers blow off into space. Energized by ultraviolet radiation from the hot, dense core, they glow red, green, yellow, and other colors, with each color produced by different chemical elements.

Seyhun Hwang is using VIRUS-P to study these dying stars in a project tentatively known as VIRUS-P Investigation of Planetary Nebulae (VIP). His investigation may yield a new model of the final stages of life of a Sun-like star.

Hwang is looking at a discrepancy between two types of observations. The techniques should yield identical results, but they don’t. The difference may indicate that the structure of a planetary nebula is more complex than astronomers had thought.

Both techniques involve spectroscopy, which splits the light from an astronomical object into its individual wavelengths or colors. One technique, known in astronomical jargon as collisionally excited lines (CEL), produces strong spectral lines, which provide the “fingerprints” of individual chemical elements. The other, known as optical recombination lines (ORL), produce weak lines, so they are more difficult to detect.

The two techniques should show the same abundances of the different chemical elements in a planetary nebula, such as oxygen and carbon, but they don’t.

“One theoretical explanation is that there’s some hidden, cool component inside the hot nebula,” says Hwang, a Korean native who is working on his doctorate at Texas. “We believe the CEL lines are tracing a very hot component of the nebula, while the ORL lines are tracing a very cool region.”

To find out, Hwang is using VIRUS-P and the 107-inch (2.7-meter) Harlan J. Smith Telescope at McDonald Observatory to study seven planetary nebula. The first target was the Saturn Nebula, NGC 7009, which was named for its resemblance to the ringed planet.

Other spectrographs look at a thin slice of a nebula, providing a spectrum of the entire slice. It’s like looking at a smoothie after the ingredients have been blended together: you get the overall texture, but you don’t see the individual strawberries, bananas, and other ingredients.

VIRUS-P, on the other hand, allows Hwang to sample many small regions, while the instrument’s wide field of view allows him to see the entire nebula. That provides a three-dimensional map of the nebula’s composition, motion, and temperature, allowing him to see all of the “ingredients” that make up the nebula. If the nebula contains regions of colder material, they will show up in observations of the faint ORL lines.

 “That would give us a new model of a planetary nebula’s structure,” says Hwang. “Previously, the ‘big picture’ never imagined the existence of cold materials — just a cloud of diffuse gas with a smooth structure. But the new picture we suggest is that it’s not smooth — there are two different components in the nebula. One is the very well known hot gas, and the other is colder blobs that are metal-rich.”

If Hwang’s observations confirm that picture of a planetary nebula, the next step is to explain why the cooler regions exist.

They could be the remains of planets that were destroyed as the expanding nebula engulfed them, or they could even show that the internal workings of stars are uneven, with the process of nucleosynthesis, which “fuses” together lighter-weight elements to make heavier ones, concentrated in small regions.

The explanation Hwang favors, however, says that a disk of solid dust grains encircles the aging star. (The grains form as atoms carried away from the star’s surface by strong “winds” cool and join together.) As the star expels its outer layers into space, they overtake the disk and vaporize the dust grains. This region would contain more of the heavier elements that show up in the ORL lines.

Hwang’s early observations have not yet answered the questions, although he says that VIRUS-P is providing good information. “It’s delivering artistic-quality data,” he says. “It’s absolutely beautiful. Wow.”


A gallery of beautiful planetaries. From left, the Helix, Dumbbell, Cat's Eye, and Retina nebulae. [ESO (top), NASA (bottom)]