Out of this Universe

AUIT's Bi-Monthly Astronomical Publication

Mysteries of the Helix

While the Helix nebula is not the only stellar object to have a complex form (e.g. the mysterious filaments in the supernova remnant M1 or the Rings of Supernova 1987A) it certainly harbors it's share of mysteries. The Cambridge University Press noted this in a report presented on the cloudy modeling of the central zone of the Helix Nebula entitled "CLOUDY modeling of weird Far-IR emission in the central zone of the Helix Nebula".
the helix nebula in infrared
Image: APOD

A 2007 image of the Helix Nebula by the Spitzer space telescope showed dust at a distance of 35 to 150 AU from it's central white dwarf star for which has been described by the observers as "comet debris":   [6]

"We were surprised to see so much dust around this star," said Dr. Kate Su of the University of Arizona, Tucson, lead author of a paper on the results appearing in the March 1 issue of Astrophysical Journal Letters. "The dust must be coming from comets that survived the death of their sun."

central helix nebula close-up   central helix nebula, Spitzer
Close-up of the central region showing star (gray arrow) and surrounding envelope of dust (yellow arrow)
Spitzer image of the central Helix Nebula. The second image is displayed in a false-color logarithmic scale.   [8]

Dust makes this cosmic eye look red. The eerie Spitzer Space Telescope image shows infrared radiation from the well-studied Helix Nebula (NGC 7293) a mere 700 light-years away in the constellation Aquarius. The two light-year diameter shroud of dust and gas around a central white dwarf has long been considered an excellent example of a planetary nebula, representing the final stages in the evolution of a sun-like star. But the Spitzer data show the nebula's central star itself is immersed in a surprisingly bright infrared glow. Models suggest the glow is produced by a dust debris disk. Even though the nebular material was ejected from the star many thousands of years ago, the close-in dust could be generated by collisions in a reservoir of objects analogous to our own solar system's Kuiper Belt or cometary Oort cloud. Formed in the distant planetary system, the comet-like bodies have otherwise survived even the dramatic late stages of the star's evolution. Credit: NASA, JPL-Caltech, Kate Su (Steward Obs, U. Arizona) et al.   [9]

A Comet-Like Mystery

close-up of helix cometary-knots
Pictured above is a close-up section from the original image located at the APOD website.

"What causes unusual knots of gas and dust in planetary nebulas? Seen also in the Ring Nebula, the Dumbbell Nebula and the Eskimo Nebula, the knots' existence was not initially predicted and their origins are still not well understood ...The above cometary knots have masses similar to the Earth but have radii typically several times the orbit of Pluto. One hypothesis for the fragmentation and evolution of the knots includes existing gas being driven out by a less dense but highly energetic stellar wind of the central evolving star. The Helix Nebula is the closest example of a planetary nebula created at the end of the life of a Sun-like star." Credit: C. R. O'Dell and K. Handron (Rice University), NASA   [10]

2nd close-up of helix cometary-knots
Image: Original at APOD Website.

"While exploring the inner edge of the Helix Nebula with the Hubble Space Telescope's Wide Field Planetary Camera 2, astronomers were able to produce this striking image - rich in details of an exotic environment. This planetary nebula, created near the final phase of a sun-like star's life, is composed of tenuous shells of gas ejected by the hot central star. The atoms of gas, stripped of electrons by ultraviolet radiation from the central star, radiate light at characteristic energies allowing specific chemical elements to be identified. In this image, emission from nitrogen is represented as red, hydrogen emission as green, and oxygen as blue. The inner edge of the Helix Nebula, also known as NGC 7293, is in the direction toward the central star, which is toward the upper right. Clearly visible near the inner edge are finger shaped cometary knots." Credit: R. O'Dell and K. Handron (Rice University)   [11]

Filaments in the Helix -- Cool and Stable

Spitzer image of Helix Nebula
Image from the Spitzer Space telescope

"A striking feature of the Helix, first revealed by ground-based images, is its collection of thousands of filamentary structures, or strands of gas. In this image the filaments can be seen under the transparent blue gas as red lines radiating out from the center. Astronomers believe that the molecules in these filaments are able to stay cooler and more stable because dense clumps of materials are shielding them from ultraviolet radiation. This image is a composite showing ionized H-alpha (green) and O III (blue) gases from the Hubble Space Telescope, and molecular hydrogen (red) from the Spitzer Space Telescope observations at 4.5 and 8.0 microns." Credit: NASA/JPL-Caltech/ESA/J. Hora (Harvard-Smithsonian CfA), C.R. O'Dell (Vanderbilt University) [13]

What caused so much excitement in the science community regarding the "comet-like knots" was the fact that they have never seen so many in a single nebula prior to the above photo. Though many of the features found in the nebula were known, having previously been resolved through the use of ground-based telescopes, it wasn't until the Hubble was pointed at the Helix that the detail and sheer amount (thousands) were revealed. Seen below is a ground-based image showing the area of the nebula where the smaller Hubble image (above) was taken.

ground-based image of helix
Image: ESA Science & Technology: Colliding Gas in the Helix Nebula
Copyright: Robert O'Dell (Rice Univ.), NASA