Stars like our Sun don't die through a supernova. Instead, after they exhaust their usable nuclear fuel, they shed their outer layers. This forms the beautiful clouds known as planetary nebulae, while the remaining core becomes a white dwarf. The mechanism by which stars jettison most of their mass has proven difficult to measure, since obtaining good data on the region close to stars is technically very challenging.

Using the Very Large Telescope (VLT) in Chile, astronomers have now measured the light scattered off the ejected matter from three stars nearing the end of their lives. As they describe in Nature, Barnaby R. M. Norris et al. found that huge, late-life stars known as asymptotic giants form large dust grains close to their surfaces, which are then driven outward by photon scattering. This finding has interesting implications for the seeding of interstellar space with the raw ingredients for forming new star systems.

When a star like our Sun nears the end of its life, the nuclear fusion that makes it shine runs at an accelerated rate. This excess energy inflates the outer layers of the star—called the envelope—so that it becomes huge, even as the surface temperature drops. While in this asymptotic giant (sometimes known as red supergiant) phase, stars are unstable: their surfaces pulse in and out. When the star is at its largest, standard astronomical models indicate that the temperature is low enough that heavier atoms (notably silicon) in the envelope can cluster to form dust grains.

Lower-mass stars (including the Sun) aren't able to create elements more massive than oxygen in their cores. However, the envelopes of typical stars contain heavier elements including magnesium, silicon, calcium, and iron, that were produced in earlier, much more massive stars that then exploded. But there aren't enough supernovas going off to create the distribution of stuff we see, so mass shed by lower-mass stars must be responsible for the remainder.

The dust clouds produced by these stars are an important part of the continuing cycle of star birth: cold dust clouds form the environment in which new stars can form. However, the mechanism that liberates the dust from the dying star that produced it isn't clear. Dust grains containing iron don't move much when they absorb photons, meaning they stick close to the star rather than spreading out into interstellar space.

The new observations by Norris et al. provide a way out of this difficulty. They measured light from three asymptotic giant stars (W Hydrae, R Doradus, and R Leonis). The polarization of the light enabled them to separate the light from the star itself (which is unpolarized) from photons scattered off dust grains near the surfaces (which is polarized). They determined that there are shells of dust very close to the surface, with grains of relatively large size: approximately 600 nanometers in diameter. Since these grains aren't obscuring the star's light, they must be mostly transparent in visible wavelengths, which limits the possibilities for their chemical composition.

Putting all this information together, the astronomers concluded that the grains must be iron-free silicates such as forsterite (Mg 2 SiO 4 ) and enstatite (Mg 2 SiO 4 ), found in interstellar dust clouds. Even though dust formed from these molecules is transparent to visible light, photons still scatter off them, giving them a bit of momentum, which they pass along to lighter gas molecules.

This phenomenon provides a good understanding of the flow of particles away from the surface of stars. Since mass loss by stars like our Sun contributes to the environment of interstellar space, knowing how asymptotic giants shed their envelopes is an important part of comprehending the cycle of star birth and death. The new observations by Norris et al. help enhance this comprehension.

Nature, 2012. DOI: 10.1038/nature10935 (About DOIs).