Planetary Nebulae
Planetary Nebulae
| Group Name | pn |
| Reference | Strasbourg-ESO Catalogue of Galactic Planetary Nebulae (Acker+ 1992) |
| Prepared by | Brian Abbott (AMNH/Hayden) |
| Labels | Yes |
| Files | planneb.speck, planneb.label |
| Dependencies | none |
| Census | 778 nebulae and labels |
A planetary nebula is an expanding shell of gas ejected from a star late in its life cycle. Appearing like greenish disks to a telescopic observer, planetary nebulae received their name from their resemblance to the gaseous planets of our Solar System. In no way are they related to planets. Rather, they are products of dying stars.
As an intermediate-mass star exhausts its core hydrogen fuel, its helium core contracts and heats to meet the energy needs of the star. The core contraction releases gravitational energy, which has two effects. First, hydrogen just outside the core begins to burn, producing a more massive helium core over time. Second, the expansion of the star's envelope, or its outer layers, occurs. The star becomes a red giant.
For stars of less than about two solar masses, the core continues to condense until the temperature and density become sufficient to burn helium into carbon. The ignition of helium occurs rapidly, producing a flash of light, and the star's outer shells expand, leaving a bright core that soon becomes a white dwarf star. These expanding shells become the planetary nebula.
Planetaries are often spherical. As the gas from the star expands, it sweeps up the cooler gas like a snowplow. The gas glows because of the ultraviolet light from the stellar remains at the center, and often it appears green because of an abundance of ionized oxygen (OIII), which radiates in the green part of the visible spectrum.
Planetary Nebulae in the Galaxy
The Milky Way consists of two major star populations: the older halo population and the younger disk population. Because the planetary nebula phase of a star's evolution is relatively short, we observe only those that have occurred recently in the younger stellar population. Therefore, we expect to see planetary nebulae in the disk of the Galaxy.
Moreover, the inner disk of the Galaxy has a higher star density and a higher ratio of young stars to old. For these reasons, we expect to see planetaries in or near the Galactic disk and we expect to see an increased number toward the Galactic center.
The stars that will evolve into planetary nebulae also typically have relatively eccentric orbits around the Galaxy and, therefore, a wider range of distances above and below the Galactic disk. Thus we also expect to see increased numbers of planetaries above and below the Galactic plane.
In the Digital Universe, we see all of these trends. While the data are observationally biased, we do in fact see the planetaries trending toward the disk, with less correlation to the plane than, say, the HII regions. We also see more planetaries toward the Galactic center, as well as an increased amount above and below the plane at Galactic center.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott