page_title: Cometary knot
hierarchical_section_title: Cometary knot
caption_attribution_description: These gigantic, tadpole-shaped objects are probably the result of a dying star's last gasps. Dubbed "cometary knots" because their glowing heads and gossamer tails resemble comets, the gaseous objects probably were formed during a star's final stages of life. Hubble astronomer C. Robert O'Dell and graduate student Kerry P. Handron of Rice University in Houston, Texas discovered thousands of these knots with the Hubble Space Telescope while exploring the Helix nebula, the closest planetary nebula to Earth at 450 light-years away in the constellation Aquarius. Although ground-based telescopes have revealed such objects, astronomers have never seen so many of them. The most visible knots all lie along the inner edge of the doomed star's ring, trillions of miles away from the star's nucleus. Although these gaseous knots appear small, they're actually huge. Each gaseous head is at least twice the size of our solar system; each tail stretches for 100 billion miles, about 1,000 times the distance between the Earth and the Sun. Astronomers theorize that the doomed star spews hot, lower-density gas from its surface, which collides with cooler, higher-density gas that had been ejected 10,000 years before. The crash fragments the smooth cloud surrounding the star into smaller, denser finger-like droplets, like dripping paint. This image was taken in August, 1994 with Hubble's Wide Field Planetary Camera 2. The red light depicts nitrogen emission ([NII] 6584A); green, hydrogen (H-alpha, 6563A); and blue, oxygen (5007A).
context_page_description: Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae, including the Helix Nebula, the Ring Nebula, the Dumbbell Nebula, the Eskimo Nebula, and the Retina Nebula. They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples. They are generally larger than the size of the Solar system, with masses of around 10⁻⁵ times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula.
  At optical wavelengths, the knots are seen as "the ionized skin of a dense, dusty molecular globule" forming a crescent-shaped head that is ionized and illuminated by the central star, with a trailing spoke or tail. In molecular hydrogen and carbon monoxide data, the tails of cometary knots are observed to be highly molecular. The central globule is at least 1000 times denser than the surrounding material that streams past it. The appearance is analogous to the tail of a comet that faces away from its star, but comets are solid bodies and much smaller in overall size and mass.
context_section_description: Cometary knots, also referred as globules, are structures observed in several nearby planetary nebulae (PNe), including the Helix Nebula (NGC 7293), the Ring Nebula (NGC 6720), the Dumbbell Nebula (NGC 6853), the Eskimo Nebula (NGC 2392), and the Retina Nebula (IC 4406).  They are believed to be a common feature of the evolution of planetary nebulae, but can only be resolved in the nearest examples.  They are generally larger than the size of the Solar system (i.e. the orbit of Pluto), with masses of around 10⁻⁵ times the mass of the Sun, which is comparable to the mass of the Earth. There are about 40,000 cometary knots in the Helix Nebula.
  At optical wavelengths, the knots are seen as "the ionized skin of a dense, dusty molecular globule" forming a crescent-shaped head that is ionized and illuminated by the central star, with a trailing spoke or tail.  In molecular hydrogen and carbon monoxide data, the tails of cometary knots are observed to be highly molecular. The central globule is at least 1000 times denser than the surrounding material that streams past it.  The appearance is analogous to the tail of a comet that faces away from its star, but comets are solid bodies and much smaller in overall size and mass.
  Globules located far and close to the central star present different characteristics. On the near side of the Helix Nebula, the central dusty globule of each cometary knot appears dark against the background as it absorbs the [O III] 5007 Angstrom light emitted in the nebular envelope.  Those on the far side do not obstruct this light source and so do not have this dark appearance. In addition, globules near the central star appear to have a distinct trailing tail, whereas those located farther do no exhibit such defined tails.
  The origin of cometary knots in planetary nebulae is still unknown and subject to active research. It is unclear whether they were created during the Asymptotic Giant Branch (AGB) phase and somehow managed to survive the AGB-PN transition, or if they were created when the star has already become a planetary nebula. The latter case would imply that the conditions in the planetary nebula host would have, at a certain point, triggered the formation of molecular clumps in its nebular envelope. Therefore, understanding the formation and evolution of cometary knots would not only give an insight into the physical properties of the planetary nebula host, but would also help draw a more detailed picture of the stellar evolution of low to intermediate mass stars.
caption: Cometary knots in the Helix Nebula
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