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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)

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.


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)

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.


Video Animations on Polarization Aspects of the Egg Nebula

Egg Nebula in Infrared Light

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)

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:

Zolt Levay's Polarization Demo

University of Colorado: Physics 2000- Polarization Demo

More on the Egg Nebula:

Egg Nebula WFPC2 Release STScI-PRC96-03

Egg Nebula NICMOS Release STScI-PRC97-11