The Cats Eye Nebula (2009 Dec 27)

[APOD Robot]

The Cats Eye Nebula

Explanation: Three thousand light-years away, a dying star throws off shells of glowing gas. This image from the Hubble Space Telescope reveals the Cat's Eye Nebula to be one of the most complex planetary nebulae known. In fact, the features seen in the Cat's Eye are so complex that astronomers suspect the bright central object may actually be a binary star system. The term planetary nebula, used to describe this general class of objects, is misleading. Although these objects may appear round and planet-like in small telescopes, high resolution images reveal them to be stars surrounded by cocoons of gas blown off in the late stages of stellar evolution.

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[grahamluckhurst]

What causes the concentric circles around the central star?

[Case]

What causes the concentric circles around the central star?

If you mean the circles with the greenish edges, then try to look at it in 3D perspective, with one in front of the star and one behind the star. The ejected material expands along two cone shapes in both directions.

Edit: My bad, I now see what you meant. The large circles I described are not concentric (from our perspective).

[Chris Peterson]

What causes the concentric circles around the central star?

Diffraction. There are actually two diffraction effects visible here: the more commonly seen diffraction spikes caused by the supports holding the secondary mirror, and the Airy pattern of concentric maxima and minima caused by the telescope aperture. The latter is usually not seen on ground-based images because the pattern is jumbled up by atmospheric motion in long exposures, but is often seen on high resolution HST images like this one.

[neufer]

. Kinematics and morphology

<<The Cat's Eye Nebula is structurally a very complex nebula, and the mechanism or mechanisms, which have given rise to its complicated morphology, are not well understood. The central bright part of the nebular consists of the inner elongated bubble (inner ellipse) filled with hot gas. It in turn is nested into a pair of larger spherical bubbles conjoined together along their waist. The waist is observed as the second larger ellipse lying perpendicular to the bubble with hot gas.

The structure of the bright portion of the nebula is primarily caused by the interaction of a fast stellar wind being emitted by the central star with material ejected during the formation of the nebula. This interaction causes the emission of X-rays discussed above. The stellar wind, blowing with the velocity as high as 1900 km/s, has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends.

It is also suspected that the central star of the nebula may be a binary star. The existence of an accretion disk caused by mass transfer between the two components of the system may give rise to polar jets, which would interact with previously ejected material. Over time, the direction of the polar jets would vary due to precession.

Outside the bright inner portion of the nebula, there are a series of concentric rings, thought to have been ejected before the formation of the planetary nebula, while the star was on the asymptotic giant branch of the Hertzsprung-Russell diagram. These rings are very evenly spaced, suggesting that the mechanism responsible for their formation ejected them at very regular intervals and at very similar speeds. The total mass of the rings is about 0.1 solar masses. The pulsations that formed the rings probably started 15,000 years ago and ceased about 1,000 years ago, when the formation of the bright central part began. Further out, a large faint halo extends to large distances from the star. The halo again predates the formation of the main nebula. The mass of the halo is estimated as 0.26–0.92 solar masses.

. Open questions

Despite intensive study, the Cat's Eye Nebula still holds many mysteries. The concentric rings surrounding the inner nebula seem to have been ejected at intervals of from a few hundred to a few thousand years, a timescale which is rather difficult to explain. Thermal pulsations, which cause formation of planetary nebulae, are believed to take place at intervals of tens of thousands of years, while smaller surface pulsations are thought to occur at intervals of years to decades. A mechanism which would eject material over the timescales required to form the concentric rings in the Cat's Eye Nebula is not known yet.

The spectra of planetary nebulae consist of emission lines superimposed on a continuum. The emission lines may be formed either by collisional excitation of ions in the nebula, or by recombination of electrons with ions. Collisionally excited lines are generally much stronger than recombination lines, and so have historically been used to determine abundances. However, recent studies have found that abundances derived from recombination lines seen in the spectrum of NGC 6543 are some three times higher than those derived from collisionally excited lines. The cause of this discrepancy is probably related to spatial temperature fluctuations inside the nebula.>>

[zbvhs]

When I look at the Hubble photograph, I see first an inner ellipsoidal shell perhaps pushed out by radiation and/or mass ejection from the central star. Next out is a planar - perhaps equatorial - disk. The yellow-green stuff appears to be local interstellar material illuminated by radiation from the poles of the central star. Case mentioned a cone or conical structure. Would this be something mapped out by precession of the rotating star's magnetic axis?

Could the concentric rings in the Wikipedia photo be an Airy pattern? Do telescopes with different apertures show the same general structure?

[Chris Peterson]

Could the concentric rings in the Wikipedia photo be an Airy pattern? Do telescopes with different apertures show the same general structure?

Diffraction is only visible around bright stars. A non-point source is affected by diffraction- its theoretical resolution is lowered and local zones will show slightly higher or lower contrast- but the diffraction itself isn't directly apparent. I think the concentric rings in the Wikipedia image are successively ejected shells of gas. Whatever they are, however, they have some physical reality beyond a simple optical artifact.

All telescopes with circular apertures show the same general diffraction structure. The pattern is more complex for non-circular apertures or for multiple apertures (essentially what some multiple mirror telescopes have).

[DavidLeodis]

When using the "stars surrounded" link in the explanation I get a message on the YouTube website stating "This video has been removed due to terms of use violation". How very Intriguing!

[zbvhs]

I understand the math for an Airy pattern that you get when the star is a point source. I'm asking if anything similar has been worked out for an extended source? The thing doesn't look right to me so that's why I'm asking if the image is repeatable with different telescopes.

The extended image indicates that some big boom occurred. Is there anything in there to suggest that it might have been something like a Type 1a supernova?

[Chris Peterson]

I understand the math for an Airy pattern that you get when the star is a point source. I'm asking if anything similar has been worked out for an extended source? The thing doesn't look right to me so that's why I'm asking if the image is repeatable with different telescopes.

Yes, the results with an extended source are what I said: a slight variation in contrast depending on the local detail and a decrease in resolution. These things are essentially the result of all the overlapping Airy patterns from every point in the image. You will not see any structure from diffraction in extended images unless they are nearly as small as point sources themselves.

Since this image is showing something structural in the object, I'm sure it's repeatable with different telescopes. Indeed, the concentric rings seen in the image do show up in other images as well.

The extended image indicates that some big boom occurred. Is there anything in there to suggest that it might have been something like a Type 1a supernova?

It definitely wasn't a supernova. The progenitor star isn't (or wasn't) massive enough to generate a supernova. Planetary nebulas like this are produced when lower mass stars eject shells of material one or more times.

[The Code]

binary slow, nova. where 2 or more stars rip them selves to bits . I am sure there is another name for it.