Eyeing the retina nebula
By David F. Salisbury
June 13, 2002
Planetary nebulae are the multicolored remnants of dead stars. When a star about the
size of the Sun runs out of nuclear fuel, the core collapses to form a much smaller dwarf star and
the outer layers are ejected to form an expanding cloud of dust and gas. Intense radiation from
the collapsed star ionizes the surrounding gases, producing the glowing colors. IC4406,
nicknamed the Retina Nebula, is one of the closest planetary nebulae to Earth.
The Hubble Space Telescope image provides us with a side view of the nebula. The
central core is shaped like a plump donut. The axis of the donut extends from the upper left to
lower right in the plane of the picture. The twin plumes that extend outward along the axis make
this a “bipolar” nebula.
The dark lines that form an irregular lattice overlaying the central region – giving it the
appearance of a human retina – represent a newly discovered phenomenon in the evolution of
planetary nebula.
C. Robert O’Dell – professor of physics at Vanderbilt – and his colleagues report the
discovery of this new feature in the June issue of the Astronomical Journal. They identified it in
the Retina Nebula and five other nearby planetary nebulae.
According to the scientists, the dark lines are lanes of compressed dust and gas caused
by an unknown type of hydrodynamic instability. (A different but well-known type of hydrodynamic
instability causes clouds to appear on a sunny day.) They calculate that the density of the dust
and gas in the dark lanes is much greater than the rest of the nebula, by perhaps a thousand
times. As a result, the lanes trap a large fraction of the dust and gas ejected from the star.
IC4406 is the smallest and youngest planetary nebula in the researchers’ study.
The Dumbell Nebula is slightly older and larger. It has lanes similar to those in the Retina
Nebula. However, the radiation from the central star has reached the lanes and begun to sculpt
them into a different shape.
The famous Ring Nebula is also bipolar, but it is oriented with its central axis facing
Earth, giving us a top-down perspective. The blue region at the center is the area that has been
cleaned out by the central star’s radiation. The edge of the blue area is the ionization front where
the radiation impinges on the expanding dust cloud. It is older and larger than the Retina and
Dumbbell Nebulae and the sculpting is further advanced.
In the Eskimo Nebula, the sculpting process has proceeded even further, reforming the
dense lanes into circular knots with cometary tails streaming outward away from the central star.
The Helix Nebula is an example of the final stage in this evolution.
A close-up view of the Helix Nebula shows the individual cometary knots that are formed
by eons of interaction between the intense light from the central star and the dense clumps of
dust and gas formed in the aftermath of the original explosion.
Understanding the nature of these features is important because they have an impact on
the stellar cycle of death and rebirth. Stars are fueled by hydrogen and helium. They create most
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Eyeing the retina nebula
of the heavier elements, including carbon, silicon and iron, in the nuclear fires that burn in their
core.
Planetary nebulae play a key role in recycling these materials throughout the universe.
Without them rocky planets like the Earth and carbon-based life forms like us would not exist.
The image of the Retina Nebula has been enhanced to dramatize its beauty. The
difference in brightness between the central region and the plumes extending to the upper left
and lower right is actually much greater than shown here. It has been selected as image of the
month by the Hubble Heritage Project, which sees the space telescope as an instrument for
extending human vision and bridging the gap between scientific endeavors and the public.
- VU Intern Ashley Dozier contributed to this report.
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