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Egg
Nebula:
ACS polarized image of the
central region of the nebula.
Image Credit:
NASA and The Hubble Heritage
Team (STScI/AURA) |
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Egg Nebula in Polarized
Light
by Raghvendra Sahai (JPL)
This
image of the Egg Nebula shows a dense expanding
cloud of gas and dust around a dying star.
The many different features seen in this
image, such as the multitude of roughly
circular arcs and a pair of searchlight
beams emanating from the center of the nebula
continue to enthrall and puzzle scientists
who study the life cycles of stars like
our Sun.
In this taken with the Hubble
Space Telescope's Advanced Camera for Surveys
(ACS), the dying central star is hidden
from our direct line-of-sight inside a very
dense, flattened, cocoon of dust. But the
starlight does escape preferentially in
other directions and is then scattered towards
the Earth from dust particles in the surrounding
nebula. The image shown here utilizes polarized
light in order to help us investigate the
properties of these dust particles (for
example, their sizes, which can then tell
us about how they were formed).
The polarization filters are
much like the ones used in ordinary sunglasses
which allow one to avoid the bright glare
of sunlight reflecting off a road or water
surfaces. Ordinary starlight consists of
light waves in which the vibrations of the
electromagnetic energy which make up these
waves are transverse to the direction in
which the light is traveling. For unpolarized
light, these vibrations are randomly oriented
at all angles. But when light waves are
reflected or scattered off something, only
waves with vibrations along a preferred
orientation reach ones eyes, and the light
is said to be polarized along that orientation.
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NICMOS
Outline:
ACS image of the Egg Nebula
with outline of NICMOS observations
(see below).
Image Credit:
NASA and The Hubble Heritage
Team (STScI/AURA) |
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By studying such polarized light
from the Egg Nebula, scientists can tell a
lot about the physical properties of the material
responsible for the scattering, as well as
the precise location of the central (hidden)
star which provides the illumination. The
image is a false-color composite of red light
images taken through three different polarization
filters, each one of which allows light polarized
in a specific direction to pass through it.
Each of the polarized light images is rendered
in either red, green or blue and then combined
to produce the image shown. In contrast to
the multitude of stars in the image which
appear white because they produce unpolarized
light, the Egg Nebula appears highly colorized
(i.e. not white) because most of the light
from it is polarized. The color changes gradually
as one goes around the nebula, indicating
that the polarization direction is also changing
gradually. Such a polarization pattern shows
that our ideas about how the nebula is illuminated
are basically correct. Some of the inner regions
of the nebula appear whitish because these
are very dense and the light is scattered
may times in random directions before reaching
us, causing a reduction in the overall polarization
level. Detailed quantitative analysis of this
polarized light image will give us valuable
information about the dust particles in the
Egg Nebula.
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Egg
Nebula in Infrared Light
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NICMOS
Infrared Observations:
(left) NICMOS infrared non-polarized
image, (middle) ACS visual polarized
image for comparision, and (right)
NICMOS infrared polarized image.
Image Credit: (left) NASA, R.
Thompson, M. Rieke, G. Schneider,
D. Hines (U. Arizona); R. Sahai
(JPL); and NICMOS Instrument Definition
Team; (middle)
NASA and The Hubble Heritage Team
(STScI/AURA);
(right) Z. Levay (STScI) |
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Hubble's has also taken infrared images
of the Egg Nebula with its Near Infrared
Camera and Multi-Object Spectrometer (NICMOS)
detector.
The image
on the left shows a NICMOS infrared non-polarized
image of the central portion of the Egg
Nebula. The middle image is a close-up of
the ACS polarized filters rotated to the
same orientation as the NICMOS images. The
image at right is a composite of three different
NICMOS polarization filters each taken at
a polarization angles of 120°
with respect to each other.
What
do the NICMOS images tell us?
The NICMOS non-polarized
image shows two spindle-like bubbles of
molecular hydrogen and dust along the long
axis of the nebula. The red tips of the
bubbles directly trace the shock front where
the high-speed outflow (expanding at more
than 62 miles per second or 100 kilometers
per second) collides with the denser and
slower-moving (at 12 miles per second or
20 kilometers per second) material of the
"arcs" seen in the ACS image. The bubbles
are seen to be closed at their ends by bright
caps of dense material, directly showing
that high density gas in the nebula is blocking
the flow of high velocity material escaping
from the top and bottom of the obscuring
dust cocoon.
These features confirm that the dark region
between the searchlight beams seen in the
ACS image does not result from a lack of
matter but from a lack of illumination.
The bright walls of the bubbles lie just
inside the outer edges of the searchlight
beams. This observation is consistent with
the hypothesis that the high-velocity outflow
is streaming out through the same holes
as the starlight. The NICMOS image also
shows emission from hot hydrogen molecules
in the regions that are dark in the ACS
image. With the far superior sensitivity
and detail of Hubble, we had expected to
see this ring-like region of glowing molecular
hydrogen to extend inwards, like the spindle-shaped
lobes, into the center of the nebula. Surprisingly,
this region remains very dark in the infrared.
It is possible that the dust in this region
is extremely thick and blocks the infrared
light from molecular hydrogen, which is
produced on the far side of the dust from
us.
More on
Polarization:
More on
the Egg Nebula:
Egg Nebula NICMOS Release STScI-PRC97-11
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