Starship Asterisk*

The Egg Nebula in Polarized Light

Explanation: Where is the center of the Egg Nebula? Emerging from a cosmic egg, the star in the center of the Egg Nebula is casting away shells of gas and dust as it slowly transforms itself into a white dwarf star. The Egg Nebula is a rapidly evolving pre- planetary nebula spanning about one light year. It lies some 3,000 light-years away toward the northern constellation Cygnus. Thick dust blocks the center star from view, while the dust shells farther out reflect light from this star. Light vibrating in the plane defined by each dust grain, the central star, and the observer is preferentially reflected, causing an effect known as polarization. Measuring the orientation of the polarized light for the Egg Nebula gives clues to location of the hidden source. Taken by Hubble's Advanced Camera for Surveys</a> in 2002, this image is rendered in artifical "Easter-Egg" colors coded to highlight the orientation of polarization.

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are X-like bright lines a conical surface seen at 90° to its axis?
I tried to fit IR images of a disk (shown in red) and two lobes (shown in cyan) from here

...

Light reflected by a single dust grain is preferentially polarized. Light reflected by a WMO ("whole mess of") dust grains would be polarized only if the grains themselves were aligned by something. Reaching back into my "Gaseous Nebulae" class in grad school, I'm guessing that means the star is influencing and aligning the grains via its magnetic field. Does that sound right? (I've been retired too long, and haven't kept up like I should have).

—Keith

—
Keith Johnson
Retired Celestial Wizard and Keeper of the Cosmic Keys
Chemistry and biology are merely by-products of stellar nucleosynthesis.
—Anonymous Rowan University Adjunct

khjalmarj wrote: ↑Sun Apr 09, 2023 1:47 pm Light reflected by a single dust grain is preferentially polarized. Light reflected by a WMO ("whole mess of") dust grains would be polarized only if the grains themselves were aligned by something. Reaching back into my "Gaseous Nebulae" class in grad school, I'm guessing that means the star is influencing and aligning the grains via its magnetic field. Does that sound right? (I've been retired too long, and haven't kept up like I should have).

—Keith

—
Keith Johnson
Retired Celestial Wizard and Keeper of the Cosmic Keys
Chemistry and biology are merely by-products of stellar nucleosynthesis.
—Anonymous Rowan University Adjunct

the iron fraction of the dust would do that…
But if the rings of light in this APOD are in fact a light echo from a series of flashes and all the reflectors are near our line of sight, then the grazing ray reflection would polarize a photon by the plane that touches a dust particle's surface and is perpendicular to the bisector of the incident and reflected paths of the photon.

khjalmarj wrote: ↑Sun Apr 09, 2023 1:47 pm Light reflected by a single dust grain is preferentially polarized. Light reflected by a WMO ("whole mess of") dust grains would be polarized only if the grains themselves were aligned by something. Reaching back into my "Gaseous Nebulae" class in grad school, I'm guessing that means the star is influencing and aligning the grains via its magnetic field. Does that sound right? (I've been retired too long, and haven't kept up like I should have).

—Keith
...

Yeah, I'm not understanding the preferential polarization effect either. But I have a "dim" understanding of optics in general to begin with.

johnnydeep wrote: ↑Sun Apr 09, 2023 8:22 pm

khjalmarj wrote: ↑Sun Apr 09, 2023 1:47 pm Light reflected by a single dust grain is preferentially polarized. Light reflected by a WMO ("whole mess of") dust grains would be polarized only if the grains themselves were aligned by something. Reaching back into my "Gaseous Nebulae" class in grad school, I'm guessing that means the star is influencing and aligning the grains via its magnetic field. Does that sound right? (I've been retired too long, and haven't kept up like I should have).

—Keith
...

Yeah, I'm not understanding the preferential polarization effect either. But I have a "dim" understanding of optics in general to begin with.

Let's suppose that this APOD shows concentric spherical dust layers, illuminated from the star (hidden from us by a small dense accretion disk) and brightest where our line of sight is tangential to those spheres.
Then a reflected photon's path bends by 90°, and such reflection does polarize the photon somewhat in the plane, normal to the bisector of the photon's bended path

I wonder how they make this image. I tried links but they won't open.
If you take a snapshot through a linear polarisation filter, you would dim the light polarised perpendicular to your filter and highlight the places where a radius from the star to a dust sphere is close to the plane of your filter's polarisation.
Now suppose the two lobes cast shadows in two opposite radii, and that the X-shaped bright paths in this APOD are effect of a polarisation filter's plane fit to the lobes… Makes sense, does it?

I have, as you might imagine, absolutely nothing to add on the subject of polarization.

But let me do what I do best, namely, compare today's APOD subject, the Egg Nebula, with its sort of namesake, the Rotten Egg (or Calabash) Nebula!


The two nebulas don't look too similar, but you must remember that the Rotten Egg Nebula has not been photographed in polarized light. Note, however, that both nebulas have thick dust lanes along their "waists", which would be their equators. The Rotten Egg Nebula is clearly expelling a lot of material from its poles, whereas the Egg Nebula seems to have stopped pushing out so much matter, and it has a spherical distribution of matter around itself. The "bright arcs and circles" (as Wikipedia calls them) are perhaps remnants of distinct "pushes" as the star expelled its atmosphere "push by push"? The two "searchlight beams" in both directions are (I guess) - perhaps holes in the thick dusty torus around the central star, where light can escape?

Wikipedia wrote about the Egg Nebula:

The Egg Nebula (also known as RAFGL 2688 and CRL 2688) is a bipolar protoplanetary nebula approximately 3,000 light-years away from Earth...

The Egg Nebula's defining feature is the series of bright arcs and circles surrounding the central star. A dense layer of gas and dusts enshrouds the central star, blocking its direct light from our view. However, the light from the central star penetrates the thinner regions of this dusty enclosure, illuminating the outer layers of gas to create the arcs seen in this resplendent image (Hubble Site). Spectra of the starlight scattered by the dust reveal that the central star has a spectral type of F5. The photosphere of an F5 star is about 900 K hotter than that of the Sun, but it is still not hot enough to have begun ionizing the nebula. Therefore the Egg Nebula is at a slightly earlier evolutionary stage than the Westbrook Nebula whose spectral type B0 central star has just recently begun to ionize the nebula.

Wikipedia wrote about the Rotten Egg Nebula:

The Calabash Nebula, also known as the Rotten Egg Nebula or by its technical name OH 231.84 +4.22, is a protoplanetary nebula (PPN) 1.4 light years (13 Pm) long and located some 5,000 light years (47 Em) from Earth in the constellation Puppis. (...) The central star is QX Puppis, a binary composed of a very cool mira variable and an A-type main-sequence star.

The object is sometimes called the Rotten Egg Nebula as it contains a relatively large amount of sulphur. The densest parts of the nebula are composed of material ejected recently by the central star and accelerated in opposite directions. This material, shown as yellow in the image, is zooming away at speeds up to one and a half million kilometers per hour (one million miles per hour). Most of the star's original mass is now contained in these bipolar gas structures.
...
Much of the gas flow observed today seems to stem from a sudden acceleration that took place about 800 years ago. Astronomers believe that 1,000 years from now, the Calabash Nebula will become a fully developed planetary nebula.

And I can't keep this info about another protoplanetary nebula, Westbrook Nebula, away from you:

Wikipedia wrote about Westbrook Nebula:

Westbrook Nebula (CRL 618) is a bipolar protoplanetary nebula which is located in the constellation Auriga. It is being formed by a star that has passed through the red giant phase and has ceased nuclear fusion at its core. This star is concealed at the center of the nebula, and is ejecting gas and dust at velocities of up to 200 km/s.[2] The nebula is named after William E. Westbrook, who died in 1975.
This nebula began to form about 200 years ago, and primarily consists of molecular gas.
...
The central star is believed to be of spectral class B0 and has 12,200 times the solar luminosity...

The photosphere of this star is now hot enough to have begun ionizing the nebula, and the ionization region is expanding rapidly. The size and rate of growth indicates that ionization began around the year 1971.

Once a sizeable portion of the nebula has been ionized, it will have become a planetary nebula. This means the Westbrook Nebula is at a somewhat more advanced evolutionary stage than the Egg Nebula, whose spectral class F5 star has not yet begun to ionize the nebula material.

Sorry for including smileys in Wikipedia quotes. You are not allowed to, I know. But I couldn't help myself. Like, you know, what did you do in 1971? I went to school. And the central star of Westbrook Nebula began ionizing a nebula.

Ann

Ann wrote: ↑Mon Apr 10, 2023 4:58 am I have, as you might imagine, absolutely nothing to add on the subject of polarization.
Ann

I have finally got myself to actually read the description that wiki has for this APOD.

1) The X shaped bright lines a real. That's the star's light blocked by the accretion disk from outside and by the lobes from inside. Therefore the bright parts are a conic surface half a ly long rather than an artefact of a polarizing filter

2) This APOD is a merge of 3 exposures, where they used 3 different orientation of the polarizing filter, with 60° step between the 3 planes used.
So each of the 3 exposure dimmed the part of dusty spheres where radii were almost perpendicular to the filter's polarisation plane and highlighted the parts where radii were close to the filter's polarisation plane.
Now they colour-coded the 3 exposures and thus the merge has red, green and blue sectors; as a polarising filter is a weak dimmer the merge has in-between hues too