The end of a sun-like star's life was once thought to be simple: the star gracefully casting off a shell of glowing gas and then settling into a long retirement as a burned-out white dwarf.
Now, a dazzling collection of detailed views released today by several teams of astronomers using NASA's Hubble Space Telescope reveals surprisingly intricate glowing patterns spun into space by aging stars: pinwheels, lawn sprinkler style jets, elegant goblet shapes, and even some that look like a rocket engine's exhaust.
These eerie fireworks offer a preview of the final stage of our own Sun's life," says Bruce Balick of the University of Washington in Seattle. More than simply a stellar "light-show", these outbursts provide a way for heavier elements predominantly carbon cooked in the star's core, to be ejected into interstellar space as raw material for successive generations of stars, planets and, potentially, life.
The astronomers say the incandescent sculptures are forcing a re-thinking of stellar evolution. In particular, the patterns may be woven by an aging star's interaction with unseen companions: planets, brown dwarfs, or smaller stars.
"The first time we looked at the Hubble's breathtaking pictures, we knew that our older and simpler ideas of how these objects are formed had to be overhauled," says Howard Bond of the Space Telescope Science Institute (STScI), Baltimore, MD. "The basic question is: how do these nebulae shape themselves?"
"Hubble's colorful views are a feast for the eyes," says Mario Livio, also of the STScI. "Their beauty is matched only by the mystery."
Surprising new details revealed by the Hubble pictures include:
"We're still reveling in the quality of the data and the wealth of new details. In the longer term we're going to have to confront these strikingly symmetric structures with some fundamentally revised ideas about the final stages of a star's life," says Balick. "The lovely patterns of gas argue that some highly ordered and powerful process orchestrates the ways stars lose their mass, completely unlike an explosion."
A long-standing puzzle is how these nebulae acquire their complex shapes and symmetries. The red giant stars which preceded their formation should have ejected simple, spherical shells of gas. "Hubble's ability to see very fine structural details usually blurred beyond recognition in ground-based images enables us to look for clues to this puzzle," says Balick.
Several teams of astronomers will be observing planetary nebulae using new infrared instruments installed on the Hubble telescope last February. This way, astronomers can glimpse the ejection of material at a very early stage long before the expelled nebula starts to become visible optically. Given Hubble's high resolution, astronomers also hope to revisit the same nebula in a few years to actually see how the shell has further expanded into space. Their observations will be compared to predictions and either refine or dismiss current ideas on the mass ejection mechanisms of dying stars.
"These nebulae observed by Hubble give us a preview of our own Sun's fate. Some 5 billion years from now, after the Sun has become a red giant and burned the Earth to a cinder, it will eject its own beautiful nebula and then fade away as a white dwarf star," warns Bond.
NASA Headquarters, Washington, DC
Goddard Space Flight Center, Greenbelt, MD
Space Telescope Science Institute, Baltimore, MD