Theories of stellar evolution successfully explain most of the properties of stars (like the Sun). A sketch follows. (For a more complete version of the story, see www.astro.washington.edu/balick/plneb.) An isolated star will go through a series of nuclear reactions in its compressed, hot, dense core which provide its heat and light. For stars that ultimately form planetary nebulae, a dense, Earth-size carbon core with a mass about 2/3 that of our Sun is left behind after the nuclear fires run out of fuel. Fresh material from the star's outer layers falls onto the hot core, heats up, ignites, and starts a reverberation process that, after a few thousand years, drives much (if not most) of the star's former outer layers into space. This generates a wind at speeds which start modestly (about 10 miles per second) and ramp up to 10,000 miles per second. (Imagine the effects on any orbiting planets!).
However, our understanding of the formation and shapes of planetary nebulae is far from complete. Since stars are round, the winds that they eject are expected to create round bubble-like planetary nebulae. However, only 15-20% of observed planetaries are round. Indeed, the shapes are far more spectacular. Images from the Hubble Space Telescope have led to an explosion of activity in the study of planetary nebulae. Ever since the first post-refurbishment images of the famous Cat's Eye Nebula (http://www.astro.washington.edu/balick/WFPC2/catseye.jpeg) astronomers and the public alike have gazed at these remnants of stellar ejection, mesmerized by their beauty, and puzzled by the formation of the the lovely and intricate patterns of gas that are seen.
The twin jets of M2-9 present highlight the need for a richer model to explain the formation and shapes of planetaries. The central star in M2-9 is known to be a very close pair which orbit one another at perilously close distances. It is even possible that one star is being engulfed by the other. The gravity of one pulls of weakly bound gas from the surface of the other and flings it into a thin, dense disk which surrounds both stars and extends well into space.
The disk can actually be seen in shorter exposure images obtained with the Hubble telescope. It measures approximately 10 times the diameter of Pluto's orbit. Models of the type that are used to design jet engines ("hydrodynamics") show that such a disk can successfully account for the jet-exhaust-like appearance of M2-9. The high-speed wind from the one of the stars rams into the surrounding disk, which serves as a nozzle. It is deflected in the perpendicular direction and forms the pair of jets that we see in the nebula's image. This is much the same process that takes place in a jet engine: the burning and expanding gases are deflected by the engine walls through a nozzle to form long, collimated jets of hot air at high speeds.
Image Factoids for M2-9
Nickname: Siamese Squid or the Twinjet Nebula
observed by HST: Aug 2 1997
distance: 0.65 kpc (2100 l.y.)
constellation: Ophiucus
HST instrument: WFPC2 (2 orbits) with filters F631N (neutral oxygen, shown
in red), F658N (once-ionized nitrogen, shown in green), F656N
(singly-ionized hydrogen, shown in blue)
Credits for the image of M2-9
Bruce Balick, University of Washington
Vincent Icke, Leiden University (The Netherlands)
Garrelt Mellema, Stockholm University
NASA
LINKS TO RELATED WWW PAGES:
infrared image of M 2-9