Download a high resolution image of the Helix Nebula or of Robert O’Dell.
NASHVILLE, Tenn. ñ In a process comparable to that of an artist who
turns a two-dimensional canvas into a three-dimensional work of art,
astronomers use the two- dimensional images that they capture in their
high-powered telescopes to reconstruct the three-dimensional structures
of celestial objects.
The latest example of this reconstructive artistry is a new model of
the Helix Nebula ñ one of the nearest and brightest of the planetary
nebulae, which are the Technicolor clouds of dust and glowing gas
produced by exploding stars. Efforts of this sort are providing
important new insights into the process that stars like the sun go
through just before their fiery deaths.
The analysis, published in the November issue of the Astronomical
Journal, was conducted by a team of astronomers led by C. Robert O’Dell
of Vanderbilt University. Combining sharp new images from the Hubble
Space Telescope with the best ground-based optical and radio images and
spectra, the astronomers have determined that the Helix Nebula is not,
in fact, shaped in a snake-like coil as some earlier analyses had
concluded. Instead of a helical shape, they have found that the nebula
consists of inner and outer shells of dust and gas that are oriented at
nearly 90 degrees from one another.
This new information
has allowed the researchers to determine not only the relative
positions of the nebula’s major features, but also the speed and
direction that the expanding dust and gas are moving. For example, they
figured out why the larger disk is brighter on one side than on the
other: It is because the nebula is moving through the interstellar
medium, something like a boat plowing through water. In this case,
however, the encounter compresses the colliding gases and causes them
to glow more brightly than they do in other parts of the ring.
"Our
new observations show that the previous model of the Helix was much too
simple," O’Dell said. "About a year ago, we believed the Helix was a
bagel shape, filled in the middle. Now we see that this filled bagel is
just the inside of the object. A much larger disk, shaped like a
washer, surrounds the filled bagel. This disk is oriented almost
perpendicular to the bagel."
Team member Peter McCullough
added, "To visualize the Helix’s geometry, imagine a lens from a pair
of glasses that was tipped at an angle to the frame’s rim. That would
be an odd-looking pair of glasses. Well, in the case of the Helix,
finding a disk inclined at an angle to a ring would be a surprise. But
that is, in fact, what we found." He and Margaret Meixner, both of the
Space Telescope Science Institute, contributed to the study.
Astronomers suspect that these complex patterns hold important
information about the conditions that existed in their progenitor stars
before they exploded. "We still don’t understand how you get such a
shape," O’Dell said. "If we could explain how this shape was created,
then we could explain the late stages of certain types of stars."
Currently, scientists believe that several of a star’s properties
may influence the way in which dust and gas is ejected when it
explodes. These include the star’s speed and axis of rotation, the
strength and axis of its magnetic field, and the influence of a close
companion star if it has one.
One group of astronomers argues that the gravitational influence of
companion stars alone can produce these patterns and that a star’s
rotation and magnetic field are not important. Other scientists,
however, contend that rotation, magnetic field and the influence of
companion stars all play a role.
One way that astronomers classify planetary nebulae is by the number
of axes that they contain. A non-polar nebula is one that has no axes:
material is sloughed off the star uniformly to form a spherical cloud
of dust and gas. A bipolar nebula is one that is created by ejecting
material primarily in a flat disk perpendicular to a single axis of
symmetry. Finally, a quadra-polar nebula possesses material expanding
outward in two disks, each with a different orientation. The new study
finds that the Helix Nebula is quadra-polar.
Space-based x-ray observations suggest that the Helix Nebula was
produced by a binary star system with the two stars so close that they
appear as a single image in optical telescopes. This suggests that the
orientation of one disk may have been influenced by the orbit of the
companion star and that the orientation of the other disk was
determined by the dying star’s spin axis or the axis of its magnetic
field.
"The new model strengthens the argument that the star’s rotation and
magnetic field axes play a role because the proponents of the companion
star-only model can’t explain quadra-polar patterns like this," said
O’Dell.
Another discovery that surprised the researchers is that the two
disks appear to have been formed at different times. The nebula’s inner
disk is expanding slightly faster than the outer disk, leading the
astronomers to estimate that the inner disk was formed about 6,600
years ago while the outer ring is about 12,000 years old.
Why did the star expel matter at two different epochs, leaving a gap
of 6,000 years? Right now, only the Helix Nebula knows the answer, the
astronomers said.
For more news about Vanderbilt research, visit Exploration, Vanderbilt’s online research magazine, at www.exploration.vanderbilt.edu.
Media contact: David Salisbury, (615) 343-6803
David.salisbury@vanderbilt.edu