Resembling a bizarre setting from a science fiction movie, dramatic images sent back by NASA's Hubble Space Telescope have surprised astronomers by uncovering thousands of gigantic tadpole-shaped objects surrounding a dying star.
Dubbed "cometary knots" because their glowing heads and gossamer tails superficially resemble comets, they are probably the result of a dying star's final outbursts. Though ground-based telescopic observations have hinted at such objects, they have not previously been seen in such abundance, say researchers.
The knots were detected by Hubble astronomer C. Robert O'Dell and graduate student Kerry P. Handron of Rice University in Houston, Texas, while exploring the Helix nebula, a ring of glowing gases blown off the surface of a sunlike star late in its life.
O'Dell expects the gaseous knots, which are each several billion miles across, will eventually dissipate and vanish into the cold emptiness of interstellar space. However, he speculates that if the objects contract to form permanent solid bodies, they may contribute to a fraction (less than ten percent) of the missing mass of our galaxy, simply because of their sheer abundance around a typical dying star. (This so-called dark matter is a known source of gravity that affects the motions of stars in the galaxy).
The mysterious "space pods" came into view as O'Dell used Hubble's Wide Field Planetary Camera 2 to survey the Helix nebula, located 450 light-years away in the constellation Aquarius and the closest planetary nebula to Earth — so close that its angular size is almost half that of the full Moon.
The most visible cometary knots all lie along the inner edge of the ring, at a distance of trillions of miles from the central star. Their comet-like tails, each stretching a hundred billion miles, form a radial pattern around the star like the spokes on a wagon wheel. Though previous ground-based observations show a spoke pattern in the Helix, and some structure, O'Dell emphasizes that the Hubble images reveal an underlying population of many more smaller objects.
O'Dell made the observation because he was curious if these objects were the result of the star's final outburst which would bring comets out of "cold storage" by boiling off the icy, solid comet nuclei. This is how comets behave as they swing near our Sun.
The knots have just the right appearance and are at just the right distance from the dying star to be a long-sought comet cloud — much like the hypothesized Oort cloud encircling our solar system. However, each gaseous cometary "head" is at least twice the diameter of our solar system — far too large for the gaseous shell, called a coma, that surrounds an active comet as we know it.
The most likely explanation is the objects have been formed during the final years of a star's life when it ejects shells of gas into space. This "planetary nebula" formation happens in stages where, toward the end of the process, a faster moving shell of gas ejected off the doomed star collides with slower moving gas released ten thousand years before.
This collision of hot, lower density gas with cooler, higher density gas forms an unstable condition where the two gases intermix and fragment the previously smooth cloud. This process, called a Rayleigh-Taylor instability, breaks the cloud into smaller and denser finger-like droplets, like dripping paint.
Standard models predict that the knots should expand and dissipate within a few hundred thousand years. However, dust particles inside each gas ball might collide and stick together, snowballing to planet- sized bodies over time. The resulting objects would be like Earth- sized copies of the frigid, icy planet Pluto. These icy worlds would escape the dead star and presumably roam interstellar space forever.
If this phenomena is common among stars, then our galaxy could be littered with trillions of these objects, O'Dell concludes. "Planetary nebulae have been formed in our galaxy for billions of years and about one new one is created every year since this is the usual ending for the billions of sunlike stars inhabiting our Milky Way galaxy."
Hubble will be used to search more distant planetary nebulae for similar features. O'Dell hopes to revisit the Helix in a few years and take more images which might reveal the outward motion of the knots.
Public Information Manager
Space Telescope Science Institute, Baltimore, MD
News Editor, News Office
Rice University, Houston, TX