APOD: The Outburst Clouds of Star AG Car (2021 May 26)

[neufer]

How come this ring is no circle?

There must have been some strange things in the star's equator at the time this ring was teared off.
Like a dwarf companion. Or a rogue dwarf that roamed into AG Car.

• There is no clear explanation of how the central star could produce the nebula's shape :

Images de la NASA de la nébuleuse du Carré rouge

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<<The Red Square Nebula is a celestial object located in the area of the sky occupied by star MWC 922 in the constellation Serpens. The first images of this bipolar nebula, taken using the Mt. Palomar Hale telescope in California, were released in April 2007. It is notable for its square shape, which according to Sydney University astrophysicist Peter Tuthill, makes it one of the most nearly discrete-symmetrical celestial objects ever imaged.

The explanation proposed by Tuthill and his collaborator James Lloyd of Cornell University is that the square shape arises from two cone shapes placed tip-to-tip, as seen from the side. This also explains the "double-ring" structure seen in SN 1987A.

A series of faint spokes radiate from the center of the structure. One possible explanation is that these spokes are shadows cast by periodic ripples or waves on the surface of an inner disk close to the central star.

• There is no clear explanation of how the central star could produce the nebula's shape:
  
  Towards the end of their lives, many low-mass stars, like the Sun, slough off their outer layers to produce striking 'planetary' nebulae. But the hot star at the heart of the Red Square nebula, called MWC 922, appears to be relatively massive, suggesting another process formed its signature shape. "How did all this beautiful, crisp structure form?" asks Peter Tuthill of the University of Sydney in Australia. "This is the million dollar question.">>

[Chris Peterson]

Click to view full size image

How come this ring is no circle?
There must have been some strange things in the star's equator at the time this ring was teared off.
Like a dwarf companion. Or a rogue dwarf that roamed into AG Car.

See the reference I provided earlier that argues for the shape being a combination of a spherical shell and a bipolar shell.

[VictorBorun]

How come this ring is no circle?
See the reference I provided earlier that argues for the shape being a combination of a spherical shell and a bipolar shell.

Do you mean The Herschel view of the nebula around the luminous blue variable star AG Carina ?
There is a low resolution "Fig. 2. Left: View of the nebula in the optical. The bright Hα+[N ii ] ring nebula is illustrated in red (also shown in Fig. 1 at the same scale) while the fainter Hα+[N ii ] emission is shown in blue, differently scaled in surface brightness, revealing the northern extension (Weis 2008)."
It's more simple to fit a non-circle to:

Click to view full size image

The result fitting can be reapplied to the high resolution pic posted as APOD:

Click to view full size image