NGC6826

NGC 6826's eye-like appearance is marred by two sets of blood-red
"FLIERS" that lie horizontally across the image. The surrounding faint
green "white" of the eye is believed to be gas that made up almost
half of the star's mass for most of its life. This gas was the first
to be "coughed" outwards a few thousand years ago as the stellar
surface pulsated, whipping material at the star's former surface out
into space, and exposing hotter inner layers of the star to view. Now
hot remnant star drives a fast wind into the older material, forming a
hot interior bubble which pushes the older gas ahead of it to form a
bright rim, much as a snowplow forms a wall of snow ahead of it. The
all-but-invisible hot bubble fills the inside the rim, forming the
eye's "cornea" around the central star. (The star is one of the
brightest stars in any planetary.)

The small red FLIERs are a mysterious byproduct of this process. Some
of their observed characteristics suggest that they are like sparks
flung outward from the central star late in the very recent past (a
thousand years ago). Yet their shapes revealed in these Hubble
Telescope images seem to suggest that they are stationary, and that
material ejected from the star flows past them, scraping gas from
their surfaces. Future Hubble observations will monitor any changes in
the positions of FLIERs to resolve this issue. In either case, the
formation of FLIERs cannot be easily explained by any models of
stellar evolution.

Image Factoids for NGC 6826
Nickname: The Blinking Planetary Nebula
observed by Hubble: Jan 27 1996
distance: 0.7 kpc (2200 l.y.)
constellation: Cygnus
HST instrument: WFPC2 (2 orbits) with filters F658N (once-ionized nitrogen, shown in red), F502N (twice-ionized
oxygen, shown in green), and F469N (starlight filter, shown in blue).

Credits for the image of NGC 6826

Bruce Balick, University of Washington
Jason Alexander, University of Washington
Arsen Hajian, U.S. Naval Observatory
Yervant Terzian, Cornell University
Mario Perinotto, University of Florence (Italy)
Patrizio Patriarchi, Arcetri Observatory (Italy)
NASA