Planetary Nebulae are some of the most beautiful astronomical objects.
When first observed with ground-based telescopes, they looked like a
stars surrounded by glowing rings of gas, and astronomers first
postulated that this gas would eventually condense into planets - thus
the name "planetary nebulae". However, as telescopes became more
powerful and our knowledge of the evolution of stars grew, it became
apparent that these objects did not represent infant solar systems,
but rather, a phenomena that occurs quite late in the star's lifetime.
Low and intermediate mass stars, like our Sun, spend the majority of
their life fusing hydrogen into helium in their cores. Eventually, the
nuclear reactions that generate the star's energy begin to die out.
As this happens, the star becomes unstable and it's outer layers are
ejected into space in a series of episodes, leaving behind the hot
core of the star. This hot core, which goes on to evolve into an
object called a White Dwarf, ionizes some of the ejected gas, causing
it to glow. The image shown is of a planetary nebulae known as the
"Cat's Eye". It is a composite of images taken with the Hubble Space
Telescope and Ground based telescopes (see here for
more info). The central star of this
planetary nebulae (CSPN) can be seen surrounded by regions of
different colored gas. These regions were ejected at different phases
in the star's evolution, and studying the gas's physical
characteristics thus gives us insight into what the star was like
during these ejection periods.
Although our understanding of these objects has grown enormously since they were first observed, there is much about them to understand. We would like to completely understand all the different phases in the life of such stars: how long they glow stably (like our Sun has done for four billion years), how their chemical composition and other physical characteristics change throughout their lives, how they shed their outer layers, how their gasses and radiation interact and influence their surroundings, and finally, how they die. Doing so gives us not only insight into the life our own Sun, but also into how such objects effect the evolution of the galaxies they inhabit.
On factor believed to greatly influence how a star evolves is what fraction of the star is made up of the heavier elements (it's metallicity). In order to explore this issue, we are studying planetary nebulae systems not only in our Galaxy, the Milky Way, but also in two smaller companion galaxies, called the Large and Small Magellanic Clouds (the LMC and the SMC). These other galaxies are metal-poor compared with our Galaxy, so differences between Milky Way PN and PN from the other two galaxies will give us clues on how the star's metallicity influences it's evolution.
Understanding a PN system requires an understanding of it's central star (CS). CSPN are the hot cores of the progenitor star's they evolved from, and the hotter a star is, the more of it's light is emitted in the ultraviolet and far-ultraviolet. In fact, it is not uncommon for the star to be completely masked by the light from it's glowing nebula when viewed at wavelengths our eyes can see. To get a clear view of these objects, we make use of telescopes sensitive to ultraviolet (UV) light. Since the Earth's atmosphere blocks most UV light, these telescopes are satellites, orbiting the Earth above the atmosphere.
We primarily use data from a telescope called FUSE - the Far Ultraviolet Spectroscopic Explorer . FUSE was conceived by Johns Hopkins University and is a NASA supported experiment. FUSE does not take pictures, but rather, it is a spectrograph. A spectrograph separates light out into its different wavelengths, the same way a prism divides sunlight into the colors of the rainbow. A spectrum is a plot of how much energy is emitted at each wavelength (color). The spectra shown below is that of a Galactic central star. Each element on the periodic table emits or absorbs light in a unique pattern of colors. These patterns are like fingerprints of the elements. Identifying and measuring a part of one of these patterns not only reveals the presence of the responsible element in the emitting or absorbing material, but also may reveal how much of that element is there, and other aspects of the material (like it's temperature).
The figure shows a FUSE spectrum of Abell 35 (in orange), a Galactic PN system. We use stellar atmospheres codes - computer programs which simulate how light changes as it traverses the star's atmosphere - to derive the physical characteristics of the star (eg, its temperature, chemical composition, and mass). Our computer model for Abell 35's central star is shown in dark green, and features caused by different ions in the star's photosphere are marked on top. Some of the starlight is intercepted by the nebula itself. The FUSE telescope covers a wavelength range in which the fingerprint of molecular hydrogen appears as a series of absorption features (these are the narrow dips in the energy spectrum which are marked along the bottom by numerous tick marks). We have also modeled this molecular hydrogen gas to determine its temperature and quantity. Our stellar and molecular hydrogen models combined are shown in light green, and well-match the observations (some absorption features by interstellar gas are marked directly below the spectrum). The good agreement between our synthetic spectrum and the actual observations give us confidence that our models are correct.
Surprisingly, the molecular hydrogen turns out to be very hot - around 1250 degrees Kelvin (about 1000 degrees Celsius). Instead of being destroyed by the intense radiation of the hot star, the molecular hydrogen survives, probably shielded by a unknown mechanism. This result and others (see here for another example from FUSE) indicate that it is apparently not uncommon for planetary nebula to harbor significant quantities of very hot molecular hydrogen gas.
Taken together with other measurements of the nebula, we can add together our derived mass of the central star (0.5 Solar masses) with that of its nebula (about 2.7 Solar masses) to estimate the mass of the progenitor star (3.2 Solar masses) - the star that this system evolved from. Prior to such work, estimates of the progenitor mass of PN systems have come from theoretical evolutionary models. Our observation-based method provides an important check to these theoretical predictions. (You can see the published details of our study of Abell 35 here, but you'll need a subscription to the Astrophysical Journal.)
This is an example of work we are conducting on both our sample of Galactic and Magellanic Cloud PN. Such work not only gives us a better understanding of the evolution of the individual PN systems, but when taken together, will broaden our understanding of the evolution of these stars as a whole.