Evolved Stars

The lives of massive stars end dramatically with powerful supernovae explosions, with core remnants as neutron stars or black holes. Low mass stars like our Sun, on the other hand, die relatively peacefully and over a much longer timescale. When hydrogen is completely burned, the core of such a star contracts until it becomes hot enough to initiate helium burning. In this stage, a thin layer of hydrogen burning may continue around the helium core. The envelope of the star begins expanding and the star becomes a red giant. As the envelope cools, molecules begin to form in it, eventually leading to the formation of dust grains by processes which are still not well understood.

The stars in this stage are typically found to vary tremendously in brightness with periods of hundreds of days (see link on Mira's below). During this stage of evolution, as the envelope pulsates it also loses a substantial fraction of its mass to the surrounding interstellar medium. The final stage is the creation of a planetary nebula with a white dwarf at its center. One of the most well-known planetary nebulae, the Helix (NGC 7293), is shown in the figure at right; it lies about 650 light years away in the constellation Aquarius. The top panel is a composite image showing ionized hydrogen (green) and oxygen (blue) and molecular hydrogen (red). The green circle marks a bright region observed recently by RG astronomers in carbon monoxide (CO) emission with the Submillimeter Array. These observations are summarized in the lower panel as a series of maps of CO emission at different velocities, and show that the molecular gas is distributed in a large number of dense clumps and filaments extending over a wide range of velocities.

Among the areas of research on evolved stars that are being pursued by RG astronomers are studies of atoms and molecules in the circumstellar envelopes of evolved stars, circumstellar chemistry, mass-loss in the asymptotic giant branch stage of evolution, and the formation of proto-planetary nebulae.