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{{short description|Planetary nebula in the constellation Draco}}
] and X-ray data from the ]]]
{{Distinguish|Cat's Eye Galaxy}}

{{Infobox nebula
The '''Cat's Eye Nebula''' ('''NGC 6543''') is a ] in the constellation of ]. Structurally, it is one of the most complex nebulae known, with high resolution ] observations revealing remarkable structures such as knots, jets and sinewy arc-like features.
| image = NGC6543.jpg

| alt = An object resembling a red eye, with a blue pupil, red-blue iris and a green brow. Another green "brow" is placed under the eye, symmetrically versus the pupil.
It was discovered by ] on ], ], and was the first planetary nebula whose ] was investigated, by the English ] ] in ].
| caption = Composite image using optical images from the ] and X-ray data from the ].

| name = Cat's Eye Nebula
Modern studies reveal several mysteries. The intricacy of the structure may be caused in part by material ejected from a ] central star, but as yet there is no direct evidence that the central star has a companion. Also, measurements of chemical abundances reveal a large discrepancy between two measurement methods, the cause of which is uncertain.
| type = Planetary
| epoch = ]
| ra = {{RA|17|58|33.423}}<ref name="simbad"/>
| dec = {{DEC|+66|37|59.52}}<ref name="simbad"/>
| dist_ly = {{val|3.3|0.9|ul=kly}} ({{val|1.0|0.3|ul=kpc}})<ref name="Reedetal1999">{{Harv|Reed|Balick|Hajian|Klayton|1999}}</ref>
| appmag_v = 9.8B<ref name="simbad"/>
| size_v = Core: 20″<ref name="Reedetal1999"/>
| constellation = ]
| radius_ly = Core: 0.2
]<ref group=note>Distance × sin(diameter_angle / 2 ) = 0.2 ly. radius</ref>
| absmag_v = {{val|-0.2|+0.8|-0.6}}B<ref group=note>9.8B apparent magnitude – 5×{log(1.0 ± 0.3 kpc distance) − 1} = −{{val|0.2|+0.8|-0.6}}B absolute magnitude</ref>
| notes = complex structure
| names = ] 6543,<ref name="simbad"/> Snail&nbsp;Nebula,<ref name="simbad"/> Sunflower&nbsp;Nebula,<ref name="simbad"/> (includes ]),<ref name="simbad"/> Caldwell 6
}}
The '''Cat's Eye Nebula''' (also known as '''NGC 6543''' and '''] 6''') is a ] in the northern constellation of ], discovered by ] on February 15, 1786. It was the first planetary nebula whose ] was investigated by the English ] ], demonstrating that planetary nebulae were gaseous and not stellar in nature. Structurally, the object has had high-resolution images by the ] revealing knots, jets, bubbles and complex arcs, being illuminated by the central hot ] nucleus (PNN).<ref name="shaw">{{Cite journal
| last = Shaw
| first = R. A.
| title = The evolution of Planetary Nebula Nuclei (PNN)
| journal = Ph.D. Thesis, Illinois Univ., Urbana-Champaign
| date = 1985
| bibcode = 1985PhDT........13S}}</ref>
It is a well-studied object that has been observed from ] to ] ]. At the centre of the Cat's Eye Nebula is a dying Wolf Rayet star, the sort of which can be seen in the Webb Telescope's image of ]. The Cat's Eye Nebula's central star shines at ] +11.4. ] images show a sort of dart board pattern of concentric rings emanating outwards from the centre.


==General information== ==General information==
NGC 6543 is a high ] ] deep-sky object. It has the combined ] of 8.1, with high ]. Its small bright inner nebula subtends an average of 16.1 ], with the outer prominent condensations about 25 arcsec.<ref name=Reedetal1999-3>{{Harv|Reed|Balick|Hajian|Klayton|1999|p=2433}}</ref> Deep images reveal an extended ] about 300 arcsec or 5 ]s across,<ref name=Balick2001-1/> that was once ejected by the central ] star during its ] phase.


NGC 6543 is 4.4&nbsp;] from the current position of the ], less than {{frac|1|10}} of the 45&nbsp;]s between ] and the current location of the Earth's northern ]. It is a convenient and accurate marker for the axis of rotation of the Earth's ], around which the ] rotates. It is also a good marker for the nearby ] of the solar system, which is the center of the circles which ''every'' planet's north pole, and the north pole of every planet's orbit, make in the sky. Since motion in the sky of the ecliptic pole is very slow compared to the motion of the Earth's north pole, its position as an ecliptic pole station marker is essentially permanent on the time-scale of human history, as opposed to the ], which changes every few thousand years.
NGC 6543 is a very well studied planetary nebula. It is relatively bright at ] 8.1, and also has a high ]. It is situated at a high ], which means it is easily observable from the ], where historically most large ]s have been situated. NGC 6543 is situated almost exactly in the direction of the ].


Observations show the bright nebulosity has temperatures between {{val|7000}} and {{val|9000|ul=K}}, whose densities average of about {{val|5000}} particles per cubic centimetre.<ref name=Wesson1>{{Harv|Wesson|Liu|2004|pp=1026, 1028}}</ref> Its outer halo has the higher temperature around {{val|15000|u=K|fmt=commas}}, but is of much lower density.<ref name=Wesson2>{{Harv|Wesson|Liu|2004|p=1029}}</ref> Velocity of the fast ] is about {{val|1900|u=km/s}}, where ] analysis shows the current rate of mass loss averages {{val|3.2|e=-7}} solar masses per year, equivalent to twenty trillion ]s per second (20&nbsp;Eg/s).<ref name=Wesson1/>
While the bright inner nebula is rather small at 20 ]s in diameter, it has an extended ] of matter that the progenitor star ejected during its ] phase. This halo extends over a diameter of about 386 arcseconds (6.4 ]).


]
Observations show that the main body of the nebula has a density of about 5,000 particles/cm&sup3; and a temperature of about 8,000].<sup>]</sup> The outer halo has a somewhat higher temperature of about 15,000K and a much lower density.
Surface temperature for the central PNN is about {{val|80000|u=K|fmt=commas}}, being 10,000 times as luminous as the sun. ] is O7&nbsp;+&nbsp;]]-type star.<ref name=Wesson1/> Calculations suggest the PNN is over one ], from a theoretical initial 5 solar masses.<ref>{{Harv|Bianchi|Cerrato|Grewing|1986}}</ref> The central ] star has a radius of {{solar radius|0.65}} (452,000&nbsp;km).<ref></ref> The Cat's Eye Nebula, given in some sources, lies about three thousand light-years from Earth.<ref>{{Cite APOD|title=The Cat's Eye Nebula From Hubble|date=13 May 2007|access-date=October 26, 2011}}</ref>

The central star of NGC 6543 is an ], with a temperature of approximately 80,000]. It is approximately 10,000 times as luminous as the ], and its radius is about 0.65 times the solar value. ] shows that the star is currently losing mass in a fast ] at a rate of about 3.2&times;10<sup>-7</sup> solar masses per year - about 20 trillion ]s per second. The velocity of this wind is about 1900kms<sup>-1</sup>. Calculations indicate that the central star currently weighs just over one ], but theoretical evolutionary calculations imply that it had an initial mass of about 5 solar masses.<sup>]</sup>


==Observations== ==Observations==
The Cat's Eye was the first planetary nebula to be observed with a ] by ] on August 29, 1864.<ref>{{cite journal |last1=Huggins |first1=William |last2=Miller |first2=W.A. |title=On the spectra of some of the nebulae |journal=Philosophical Transactions of the Royal Society of London |date=1864 |volume=154 |pages=437–444 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000049721522;view=1up;seq=503 |doi=10.1098/rstl.1864.0013|doi-access=free |bibcode=1864RSPT..154..437H }} See p. 438, "No. 4373".</ref><ref name=Kwok1>{{harv|Kwok|2000|p=1}}</ref> Huggins' observations revealed that the nebula's spectrum was non-continuous and made of a few bright emission lines, first indication that planetary nebulae consist of tenuous ionised gas. Spectroscopic observations at these wavelengths are used in abundance determinations,<ref name=Wesson3>{{Harv|Wesson|Liu|2004|pp=1026–1027, 1040–1041}}</ref> while images at these wavelengths have been used to reveal the intricate structure of the nebula.<ref name=Balick1987-1/>

The nebula was discovered by ] on ], ], and was the first planetary nebula to be observed with a ], by ] in ]. Huggins' observations were the first indication that planetary nebulae consist of extremely ] gases. Since those early observations, NGC 6543 has been observed right across the ].


===Infrared observations=== ===Infrared observations===
Observations of NGC 6543 at ] wavelengths (about 60&nbsp;μm) reveal the presence of ] at low temperatures. The dust is believed to have formed during the last phases of the progenitor star's life. It absorbs light from the central star and re-radiates it at ] wavelengths. The spectrum of the infrared dust emission implies that the dust temperature is about 85&nbsp;K, while the mass of the dust is estimated at {{val|6.4|e=-4}} solar masses.<ref name=Klaas>{{Harv|Klaas|Walker|Müller|Richards|2006|p=523}}</ref>


Infrared emission also reveals the presence of ] material such as ] (H<sub>2</sub>) and ]. In many planetary nebulae, molecular emission is greatest at larger distances from the star, where more material is un-ionised, but molecular hydrogen emission in NGC 6543 seems to be bright at the inner edge of its outer halo. This may be due to ] exciting the H<sub>2</sub> as ejecta moving at different speeds collide. The overall appearance of the Cat's Eye Nebula in infrared (wavelengths 2–8&nbsp;μm) is similar in visible light.<ref name=Hora>{{Harv|Hora|Latter|Allen|Marengo|2004|p=299}}</ref>
].]]

Observations of NGC 6543 at ] wavelengths reveal the presence of ] at low temperatures. The dust is believed to have formed during the last phases of the progenitor star's life. It absorbs light from the central star and re-radiates it at infrared wavelengths. The spectrum of the infrared dust emission implies that the dust temperature is about 70K.

Infrared emission also reveals the presence of ] material such as ] (H<sub>2</sub>). In many planetary nebulae, molecular emission is greatest at larger distances from the star, where more material is un-ionised, but molecular hydrogen emission in NGC 6543 seems to be bright at the inner edge of its outer halo. This may be due to ] exciting the H<sub>2</sub> as ejecta moving at different speeds collide.<sup>]</sup>


===Optical and ultraviolet observations=== ===Optical and ultraviolet observations===
The Hubble Space Telescope image produced here is in false colour, designed to highlight regions of high and low ]. Three images were taken, in filters isolating the light emitted by singly ionised ] at 656.3&nbsp;], singly ionised ] at 658.4&nbsp;nm and doubly ionised ] at 500.7&nbsp;nm. The images were combined as red, green and blue channels respectively, although their true colours are red, red and green. The image reveals two "caps" of less ionised material at the edge of the nebula.<ref name=Wesson4>{{Harv|Wesson|Liu|2004|pp=1027–1031}}</ref>
].]]

NGC 6543 has been extensively observed at ] and ] ]. Spectroscopic observations at these wavelengths are used in abundance determinations, while images at these wavelengths have been used to reveal the intricate structure of the nebula.

The Hubble Space Telescope image produced here is in false colour, designed to highlight regions of high and low ]. Three images were taken, in filters isolating the light emitting by singly ionised ] at 656.3], singly ionised ] at 658.4nm and doubly ionised ] at 500.7nm. The images were combined as red, green and blue channels respectively, although their true colours are red, red and green. The image reveals two 'caps' of less ionised material at the edge of the nebula.


===X-ray observations=== ===X-ray observations===
]

In 2001, observations at ] wavelengths by the ] revealed the presence of extremely hot gas within NGC 6543 with the temperature of {{val|1.7|e=6|u=K}}.<ref name=Chu>{{Harv|Chu|Guerrero|Gruendl|Williams|2001}}</ref> It is thought that the very hot gas results from the violent interaction of a fast stellar wind with material previously ejected. This interaction has hollowed out the inner bubble of the nebula.<ref name=Balick1987-1/> Chandra observations have also revealed a ] at the position of the central star. The spectrum of this source extends to the hard part of the X-ray spectrum, to 0.5–{{val|1.0|ul=keV}}. A star with the ] temperature of about {{val|100000|u=K|fmt=commas}} would not be expected to emit strongly in hard X-rays, and so their presence is something of a mystery. It may suggest the presence of a high temperature ] within a ] system.<ref name=Guerrero>{{Harv|Guerrero|Chu|Gruendl|Williams|2001}}</ref> The hard X-ray data remain intriguing more than ten years later: the Cat's Eye was included in a 2012 Chandra survey of 21 central stars of planetary nebulae (CSPNe) in the ], which found: "All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot (~{{val|100000|u=K|fmt=commas}}) central star photospheres, possibly indicating a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds or PN mass fallback."<ref name=Kastner>{{Harv|Kastner et al.|2012}}</ref>
Recent observations at ] wavelengths by the ] have revealed the presence of extremely hot gas within NGC 6543. The image at the top of this article is a combination of optical images from the Hubble Space Telescope with the Chandra X-ray images. It is thought that the very hot gas results from the violent interaction of a fast stellar wind with material previously ejected. This interaction has hollowed out the inner bubble of the nebula.

Chandra observations have also revealed a ] at the position of the central star. The star would not be expected to emit strongly in X-rays, and so their presence is something of a mystery. It may suggest the presence of a high temperature ] within a ] system.<sup>]</sup>


==Distance== ==Distance==
Planetary nebulae distances like NGC 6543 are generally very inaccurate and not well known.<ref name=Reedetal1999-1>{{Harv|Reed|Balick|Hajian|Klayton|1999|p=2430}}</ref> Some recent Hubble Space Telescope observations of NGC 6543 taken several years apart determine its distance from the angular expansion rate of 3.457 milliarcseconds per year. Assuming a line of sight expansion velocity of 16.4&nbsp;km·s<sup>−1</sup>, this implies that NGC 6543's distance is {{val|1001|269}}&nbsp;]s ({{val|3|e=19|u=k}} or {{val|3300}}&nbsp;]s) away from Earth.<ref name=Reedetal1999-2>{{Harv|Reed|Balick|Hajian|Klayton|1999|pp=2433–2438}}</ref> Several other distance references, like what is quoted in ] in 2014 based on Stanghellini, L., et al. (2008) suggest the distance is {{val|1623}} parsecs ({{val|5300}} light-years).<ref>{{cite journal | last1 = Stanghellini | first1 = L | last2 = Shaw | first2 = RA | last3 = Villaver | first3 = E | year = 2008 | title = The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale | arxiv=0807.1129 | journal = The Astrophysical Journal | volume = 689 | issue = 1 | pages = 194–202 | doi = 10.1086/592395 | bibcode = 2008ApJ...689..194S | s2cid = 119257242 }}</ref>

]

A long standing problem in the study of planetary nebulae is that their distances are generally not well known. Many methods for estimating distances to planetary nebulae rely on making general assumptions, which may be very inaccurate for the object concerned.

In recent years, however, observations made using the Hubble Space Telescope have allowed a new method of determining distances. All planetary nebulae are expanding, and observations several years apart and with high enough ] will reveal the growth of the nebula in the plane of the sky. This is typically very small&mdash;only a few milliarcseconds a year or less. ] observations can reveal the velocity of expansion of the nebula along the line of sight using the ]. Then, comparing the angular expansion with the known expansion velocity, the distance to the nebula can be calculated.

Hubble Space Telescope observations of NGC 6543 several years apart have been used to calculate its distance. Its angular expansion rate is approximately 10 milliarcseconds per year, while its expansion velocity along the line of sight has been found to be 16.4kms<sup>-1</sup>. Combining these two results implies that NGC 6543 is about 1000 ]s away from Earth.<sup>]</sup>


==Age== ==Age==
The angular expansion of the nebula can also be used to estimate its age. If it has been expanding at a constant rate of 10 milliarcseconds a year, then it would take {{val|1000|260|u=years}} to reach a diameter of 20 arcseconds. This may be an upper limit to the age, because ejected material will be slowed when it encounters material ejected from the star at earlier stages of its evolution, and the ].<ref name=Reedetal1999-2/>

The angular expansion of the nebula can also be used to estimate its age. If it has been expanding at a constant rate, then to have reached a diameter of 20 arcseconds at 10 milliarcseconds a year would have taken about 1000 years.<sup>]</sup> This may be an upper limit to the age, as ejected material will be slowed as it encounters material ejected from the star at earlier stages of its evolution, as well as the ].


==Composition== ==Composition==
]


Like most astronomical objects, NGC 6543 consists mostly of ] and ], with heavier elements present in small quantities. The exact composition may be determined by spectroscopic studies. Abundances are generally expressed relative to hydrogen, the most abundant element.<ref name=Wesson2/>
]


Different studies generally find varying values for elemental abundances. This is often because ]s attached to telescopes do not collect all the light from objects being observed, instead gathering light from a slit or small ]. Therefore, different observations may sample different parts of the nebula.
Like most astronomical objects, NGC 6543 consists mostly of ] and ], with heavier elements present in small quantities. The exact composition may be determined by spectroscopic studies. Abundances are generally expressed relative to hydrogen, the most abundant element.


However, results for NGC 6543 broadly agree that, relative to hydrogen, the helium abundance is about 0.12, ] and ] abundances are both about {{val|3|e=-4}}, and the ] abundance is about {{val|7|e=-4}}.<ref name=Wesson3/> These are fairly typical abundances for planetary nebulae, with the carbon, nitrogen and oxygen abundances all larger than the values found for the sun, due to the effects of ] enriching the star's atmosphere in heavy elements before it is ejected as a planetary nebula.<ref>{{Harv|Hyung|Aller|Feibelman|Lee|2000}}</ref>
Different studies generally find varying values for elemental abundances. This is often because ]s attached to telescopes do not collect all the light from objects being observed, instead gathering light from a slit or small ]. Therefore, different observations may sample different parts of the nebula.


Deep spectroscopic analysis of NGC 6543 may indicate that the nebula contains a small amount of material which is highly enriched in heavy elements; this is discussed below.<ref name=Wesson3/>
However, results for NGC 6543 broadly agree that, relative to hydrogen, the helium abundance is about 0.12, ] and ] abundances are both about 3&times;10<sup>-4</sup>, and the ] abundance is about 7&times;10<sup>-4</sup>. These are fairly typical abundances for planetary nebulae, with the carbon, nitrogen and oxygen abundances all larger than the values found for the ], due to the effects of ] enriching the star's atmosphere in heavy elements before it is ejected as a planetary nebula.<sup>],]</sup>

Deep spectroscopic analysis of NGC 6543 may indicate that the nebula contains a small amount of material which is highly enriched in heavy elements; this is discussed further below.


==Kinematics and morphology== ==Kinematics and morphology==
The Cat's Eye Nebula is structurally a very complex nebula, and the mechanism or mechanisms that have given rise to its complicated morphology are not well understood.<ref name=Balick1987-1/> The central bright part of the nebula 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.<ref name=Reedetal1999-4>{{Harv|Reed|Balick|Hajian|Klayton|1999|pp=2438–2440}}</ref>


The structure of the bright portion of the nebula is primarily caused by the interaction of a fast ] being emitted by the central PNN with the visible 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 {{val|1900|u=km/s}}, has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends.<ref name=Balick1987-1>{{Harv|Balick|Preston|1987|pp=958, 961–963}}</ref>
]


It is also suspected that the central WR:+O7 spectral class PNN star, HD 164963 / BD +66 1066 / PPM 20679<ref name="simbad">{{Harv|SIMBAD|2006}}</ref> of the nebula may be generated by a ].<ref name="simbad"/> The existence of an ] caused by mass transfer between the two components of the system may give rise to ]s, which would interact with previously ejected material. Over time, the direction of the jets would vary due to ].<ref>{{Harv|Balick|Preston|1987}}</ref><ref>{{Harv|Miranda|Solf|1992}}</ref>
NGC 6543 is structurally a very complex nebula, and the mechanism or mechanisms which have given rise to its complicated morphology are not well understood.


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 ] of the ]. 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.<ref name=Balick2001-1>{{Harv|Balick|Wilson|Hajian|2001|p=354}}</ref> The total mass of the rings is about 0.1 solar masses.<ref name=Balick2001-2/> The pulsations that formed the rings probably started 15,000 years ago and ceased about {{val|1000}} years ago, when the formation of the bright central part began (see above).<ref name=Balick2001-3>{{Harv|Balick|Wilson|Hajian|2001|pp=359–360}}</ref>
The structure of the bright portion of the nebula is primarily caused by the interaction of a fast ] 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 has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends.<sup>]</sup>


Further, 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.<ref name=Balick2001-2>{{Harv|Balick|Wilson|Hajian|2001|p=358}}</ref>
It is also suspected that the central star of the nebula may be a ]. The existence of an ] caused by mass transfer between the two components of the system may give rise to ]s, which would interact with previously ejected material. Over time, the direction of the polar jets would vary due to ].<sup>]</sup>


==See also==
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 ] of the ]. 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.<sup>]</sup>
*]


==Notes==
Further out, a large faint halo extends to large distances from the star. The halo again predates the formation of the main nebula.
{{reflist|group=note}}


==Open questions== ==References==
{{Reflist}}


==Cited sources==
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 a few hundred years, a timescale which is rather difficult to explain. Thermal pulsations which cause planetary nebulae to be formed in the first place 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 yet known.
{{Refbegin}}

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The spectra of planetary nebulae consist of ]s superimposed on a ]. The emission lines may be formed either by ] of ] in the nebula, or by ] of ]s 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.<sup>]</sup> The cause of this discrepancy is disputed&mdash;suggestions include the presence of some material highly enriched in heavy elements, or sizable temperature fluctuations within the nebula.
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| doi = 10.1088/0004-6256/144/2/58
| arxiv = 1204.6055
| ref = {{sfnRef|Kastner et al.|2012}}
| hdl = 10486/661899
| s2cid = 53709567
}}
* {{citation
| last1 = Klaas
| first1 = U.
| last2 = Walker
| display-authors = 4
| first2 = S. J.
| last3 = Müller
| first3 = T. G.
| last4 = Richards
| first4 = P. J.
| last5 = Schreiber
| first5 = J.
| title = Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
| journal = Astronomy and Astrophysics
| date = 2006
| volume = 452
| issue = 2
| pages = 523–535
| bibcode = 2006A&A...452..523K
| doi = 10.1051/0004-6361:20053245
| doi-access = free
}}
* {{citation
| last1 = Kwok
| first1 = Sun
| author1-link = Sun Kwok
| title = The origin and evolution of planetary nebulae
| chapter = Chapter 1: History and overview
| date = 2000
| publisher = Cambridge University Press
| pages = 1–7
| isbn = 978-0-521-62313-1
| doi = 10.1017/CBO9780511529504
}}
* {{citation
| last1 = Miranda
| first1 = L. F.
| last2 = Solf
| first2 = J.
| title = Long-slit spectroscopy of the planetary nebula NGC 6543—Collimated bipolar ejections from a precessing central source?
| journal = Astronomy and Astrophysics
| date = 1992
| volume = 260
| issue = 1–2
| pages = 397–410
| bibcode = 1992A&A...260..397M
}}
* {{citation
| last1 = Moore
| first1 = S. L.
| title = Observing the Cat's Eye Nebula
| journal = Journal of the British Astronomical Association
| date = 2007
| volume = 117
| issue = 5
| pages = 279–280
| bibcode = 2007JBAA..117R.279M
}}
* {{citation
| last1 = Reed
| first1 = Darren S.
| last2 = Balick
| display-authors = 4
| first2 = Bruce
| last3 = Hajian
| first3 = Arsen R.
| last4 = Klayton
| first4 = Tracy L.
| last5 = Giovanardi
| first5 = Stefano
| last6 = Casertano
| first6 = Stefano
| last7 = Panagia
| first7 = Nino
| last8 = Terzian
| first8 = Yervant
| title = Hubble Space Telescope Measurements of the Expansion of NGC 6543: Parallax Distance and Nebular Evolution
| journal = Astronomical Journal
| date = 1999
| volume = 118
| issue = 5
| pages = 2430–2441
| bibcode = 1999AJ....118.2430R
| doi = 10.1086/301091
|arxiv = astro-ph/9907313 | s2cid = 14746840
}}
* {{citation
| last1 = SIMBAD
| title = Results for Cat's Eye Nebula
| author-link = SIMBAD
| date = December 22, 2006
| url = http://simbad.u-strasbg.fr/simbad/sim-id?protocol=html&Ident=Cat's+Eye+Nebula
}}
* {{citation
| last1 = Wesson
| first1 = R.
| last2 = Liu
| first2 = X.-W.
| title = Physical conditions in the planetary nebula NGC 6543
| journal = Monthly Notices of the Royal Astronomical Society
| date = 2004
| volume = 351
| issue = 3
| pages = 1026–1042
| bibcode = 2004MNRAS.351.1026W
| doi = 10.1111/j.1365-2966.2004.07856.x| doi-access = free
}}
{{Refend}}


==External links== ==External links==
{{Commons category|Cat's Eye Nebula}}
* {{Webarchive|url=https://web.archive.org/web/20090311002952/http://www.spacetelescope.org/news/html/heic0414.html |date=2009-03-11 }}
*
*
* ]
** October 31, 1999
** 2010 May 9
** 2016 July 3
* —HubbleSite article about the Cat's Eye Nebula.
*
* —article showing image composite process used to produce an image of the nebula
* {{WikiSky}}
*
{{Sky|17|58|33.423|+|66|37|59.52|3300}}
{{featured article}}
* – NGC 6543


{{Catalogs|IC=4677|C=6|NGC=6543}}
*
{{Caldwell catalogue}}
*
{{Ngc70}}
* - HubbleSite article about the Cat's Eye Nebula.
{{Stars of Draco}}
* - article showing the image composite process used by scientists to produce an image of the Cat's Eye Nebula.
{{Portal bar|Stars|Spaceflight|Outer space|Solar System}}

{{Authority control}}
==References==

# Wesson R., Liu X.-W. (2004), ''Physical conditions in the planetary nebula NGC 6543'', ''Monthly Notices of the Royal Astronomical Society'', vol. 351, p.1026
# Bianchi L., Cerrato S., Grewing M. (1986), ''Mass loss from central stars of planetary nebulae - The nucleus of NGC 6543'', ''Astronomy and Astrophysics'', vol. 169, p.227
# Hora J.L., Latter W.B., Allen L.E. ''et al'' (2004), ''Infrared Array Camera (IRAC) Observations of Planetary Nebulae'', ''Astrophysical Journal Supplement Series'', vol. 154, p.296
# Guerrero M.A., Chu Y-H., Gruendl R.A., (2001), ''The Enigmatic X-Ray Point Sources at the Central Stars of NGC 6543 and NGC 7293'', ''Astrophysical Journal'', vol. 553, p.55
# Reed D.S., Balick B., Hajian A.R. ''et al'' (1999), ''Hubble Space Telescope Measurements of the Expansion of NGC 6543: Parallax Distance and Nebular Evolution''. ''Astronomical Journal'', vol. 118, p.2430
# Hyung S., Aller L.H., Feibelman W.A. ''et al'' (2000), ''The optical spectrum of the planetary nebula NGC 6543'', ''Monthly Notices of the Royal Astronomical Society'', vol. 318, p.77
# Balick B., Preston H.L. (1987), ''A wind-blown bubble model for NGC 6543''. ''Astronomical Journal'', vol. 94, p.958
# Miranda L.F., Solf J. (1992), ''Long-slit spectroscopy of the planetary nebula NGC 6543 - Collimated bipolar ejections from a precessing central source?''. ''Astronomy and Astrophysics'', vol. 260, p.397
# Balick B., Wilson J., Hajian A.R. (2001), ''NGC 6543: The Rings Around the Cat's Eye'', ''Astronomical Journal'', vol. 121, p.354



{{DEFAULTSORT:Cat's Eye Nebula}}
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Latest revision as of 18:35, 2 December 2024

Planetary nebula in the constellation Draco Not to be confused with Cat's Eye Galaxy.
Cat's Eye Nebula
Emission nebula
Planetary nebula
An object resembling a red eye, with a blue pupil, red-blue iris and a green brow. Another green "brow" is placed under the eye, symmetrically versus the pupil.Composite image using optical images from the HST and X-ray data from the Chandra X-ray Observatory.
Observation data: J2000 epoch
Right ascension17 58 33.423
Declination+66° 37′ 59.52″
Distance3.3±0.9 kly (1.0±0.3 kpc) ly
Apparent magnitude (V)9.8B
Apparent dimensions (V)Core: 20″
ConstellationDraco
Physical characteristics
RadiusCore: 0.2 ly ly
Absolute magnitude (V)−0.2+0.8
−0.6B
Notable featurescomplex structure
DesignationsNGC 6543, Snail Nebula, Sunflower Nebula, (includes IC 4677), Caldwell 6
See also: Lists of nebulae

The Cat's Eye Nebula (also known as NGC 6543 and Caldwell 6) is a planetary nebula in the northern constellation of Draco, discovered by William Herschel on February 15, 1786. It was the first planetary nebula whose spectrum was investigated by the English amateur astronomer William Huggins, demonstrating that planetary nebulae were gaseous and not stellar in nature. Structurally, the object has had high-resolution images by the Hubble Space Telescope revealing knots, jets, bubbles and complex arcs, being illuminated by the central hot planetary nebula nucleus (PNN). It is a well-studied object that has been observed from radio to X-ray wavelengths. At the centre of the Cat's Eye Nebula is a dying Wolf Rayet star, the sort of which can be seen in the Webb Telescope's image of WR 124. The Cat's Eye Nebula's central star shines at magnitude +11.4. Hubble Space Telescope images show a sort of dart board pattern of concentric rings emanating outwards from the centre.

General information

NGC 6543 is a high northern declination deep-sky object. It has the combined magnitude of 8.1, with high surface brightness. Its small bright inner nebula subtends an average of 16.1 arcsec, with the outer prominent condensations about 25 arcsec. Deep images reveal an extended halo about 300 arcsec or 5 arcminutes across, that was once ejected by the central progenitor star during its red giant phase.

NGC 6543 is 4.4 minutes of arc from the current position of the north ecliptic pole, less than 1⁄10 of the 45 arcminutes between Polaris and the current location of the Earth's northern axis of rotation. It is a convenient and accurate marker for the axis of rotation of the Earth's ecliptic, around which the celestial North Pole rotates. It is also a good marker for the nearby "invariable" axis of the solar system, which is the center of the circles which every planet's north pole, and the north pole of every planet's orbit, make in the sky. Since motion in the sky of the ecliptic pole is very slow compared to the motion of the Earth's north pole, its position as an ecliptic pole station marker is essentially permanent on the time-scale of human history, as opposed to the pole star, which changes every few thousand years.

Observations show the bright nebulosity has temperatures between 7000 and 9000 K, whose densities average of about 5000 particles per cubic centimetre. Its outer halo has the higher temperature around 15,000 K, but is of much lower density. Velocity of the fast stellar wind is about 1900 km/s, where spectroscopic analysis shows the current rate of mass loss averages 3.2×10 solar masses per year, equivalent to twenty trillion tons per second (20 Eg/s).

An optical image of the nebula's surrounding halo

Surface temperature for the central PNN is about 80,000 K, being 10,000 times as luminous as the sun. Stellar classification is O7 + -type star. Calculations suggest the PNN is over one solar mass, from a theoretical initial 5 solar masses. The central Wolf–Rayet star has a radius of 0.65 R☉ (452,000 km). The Cat's Eye Nebula, given in some sources, lies about three thousand light-years from Earth.

Observations

The Cat's Eye was the first planetary nebula to be observed with a spectroscope by William Huggins on August 29, 1864. Huggins' observations revealed that the nebula's spectrum was non-continuous and made of a few bright emission lines, first indication that planetary nebulae consist of tenuous ionised gas. Spectroscopic observations at these wavelengths are used in abundance determinations, while images at these wavelengths have been used to reveal the intricate structure of the nebula.

Infrared observations

Observations of NGC 6543 at far-infrared wavelengths (about 60 μm) reveal the presence of stellar dust at low temperatures. The dust is believed to have formed during the last phases of the progenitor star's life. It absorbs light from the central star and re-radiates it at infrared wavelengths. The spectrum of the infrared dust emission implies that the dust temperature is about 85 K, while the mass of the dust is estimated at 6.4×10 solar masses.

Infrared emission also reveals the presence of un-ionised material such as molecular hydrogen (H2) and argon. In many planetary nebulae, molecular emission is greatest at larger distances from the star, where more material is un-ionised, but molecular hydrogen emission in NGC 6543 seems to be bright at the inner edge of its outer halo. This may be due to shock waves exciting the H2 as ejecta moving at different speeds collide. The overall appearance of the Cat's Eye Nebula in infrared (wavelengths 2–8 μm) is similar in visible light.

Optical and ultraviolet observations

The Hubble Space Telescope image produced here is in false colour, designed to highlight regions of high and low ionisation. Three images were taken, in filters isolating the light emitted by singly ionised hydrogen at 656.3 nm, singly ionised nitrogen at 658.4 nm and doubly ionised oxygen at 500.7 nm. The images were combined as red, green and blue channels respectively, although their true colours are red, red and green. The image reveals two "caps" of less ionised material at the edge of the nebula.

X-ray observations

X-ray image of Nebula.

In 2001, observations at X-ray wavelengths by the Chandra X-ray Observatory revealed the presence of extremely hot gas within NGC 6543 with the temperature of 1.7×10 K. It is thought that the very hot gas results from the violent interaction of a fast stellar wind with material previously ejected. This interaction has hollowed out the inner bubble of the nebula. Chandra observations have also revealed a point source at the position of the central star. The spectrum of this source extends to the hard part of the X-ray spectrum, to 0.5–1.0 keV. A star with the photospheric temperature of about 100,000 K would not be expected to emit strongly in hard X-rays, and so their presence is something of a mystery. It may suggest the presence of a high temperature accretion disk within a binary star system. The hard X-ray data remain intriguing more than ten years later: the Cat's Eye was included in a 2012 Chandra survey of 21 central stars of planetary nebulae (CSPNe) in the solar neighborhood, which found: "All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot (~100,000 K) central star photospheres, possibly indicating a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds or PN mass fallback."

Distance

Planetary nebulae distances like NGC 6543 are generally very inaccurate and not well known. Some recent Hubble Space Telescope observations of NGC 6543 taken several years apart determine its distance from the angular expansion rate of 3.457 milliarcseconds per year. Assuming a line of sight expansion velocity of 16.4 km·s, this implies that NGC 6543's distance is 1001±269 parsecs (3×10 k or 3300 light-years) away from Earth. Several other distance references, like what is quoted in SIMBAD in 2014 based on Stanghellini, L., et al. (2008) suggest the distance is 1623 parsecs (5300 light-years).

Age

The angular expansion of the nebula can also be used to estimate its age. If it has been expanding at a constant rate of 10 milliarcseconds a year, then it would take 1000±260 years to reach a diameter of 20 arcseconds. This may be an upper limit to the age, because ejected material will be slowed when it encounters material ejected from the star at earlier stages of its evolution, and the interstellar medium.

Composition

Blue-green diffuse disk with complex circular structure in its center. The disk is crossed by s-shaped brown curve.
Image of NGC 6543 processed to reveal the concentric rings surrounding the inner core. Also visible are the linear structures, possibly caused by precessing jets from a binary central star system.

Like most astronomical objects, NGC 6543 consists mostly of hydrogen and helium, with heavier elements present in small quantities. The exact composition may be determined by spectroscopic studies. Abundances are generally expressed relative to hydrogen, the most abundant element.

Different studies generally find varying values for elemental abundances. This is often because spectrographs attached to telescopes do not collect all the light from objects being observed, instead gathering light from a slit or small aperture. Therefore, different observations may sample different parts of the nebula.

However, results for NGC 6543 broadly agree that, relative to hydrogen, the helium abundance is about 0.12, carbon and nitrogen abundances are both about 3×10, and the oxygen abundance is about 7×10. These are fairly typical abundances for planetary nebulae, with the carbon, nitrogen and oxygen abundances all larger than the values found for the sun, due to the effects of nucleosynthesis enriching the star's atmosphere in heavy elements before it is ejected as a planetary nebula.

Deep spectroscopic analysis of NGC 6543 may indicate that the nebula contains a small amount of material which is highly enriched in heavy elements; this is discussed below.

Kinematics and morphology

The Cat's Eye Nebula is structurally a very complex nebula, and the mechanism or mechanisms that have given rise to its complicated morphology are not well understood. The central bright part of the nebula 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 PNN with the visible 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 WR:+O7 spectral class PNN star, HD 164963 / BD +66 1066 / PPM 20679 of the nebula may be generated by a binary star. The existence of an accretion disk caused by mass transfer between the two components of the system may give rise to astronomical jets, which would interact with previously ejected material. Over time, the direction of the 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 1000 years ago, when the formation of the bright central part began (see above).

Further, 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.

See also

Notes

  1. Distance × sin(diameter_angle / 2 ) = 0.2 ly. radius
  2. 9.8B apparent magnitude – 5×{log(1.0 ± 0.3 kpc distance) − 1} = −0.2+0.8
    −0.6B absolute magnitude

References

  1. ^ (SIMBAD 2006)
  2. ^ (Reed et al. 1999)
  3. Shaw, R. A. (1985). "The evolution of Planetary Nebula Nuclei (PNN)". Ph.D. Thesis, Illinois Univ., Urbana-Champaign. Bibcode:1985PhDT........13S.
  4. (Reed et al. 1999, p. 2433)
  5. ^ (Balick, Wilson & Hajian 2001, p. 354)
  6. ^ (Wesson & Liu 2004, pp. 1026, 1028)
  7. ^ (Wesson & Liu 2004, p. 1029)
  8. (Bianchi, Cerrato & Grewing 1986)
  9. Constellation Guide
  10. Nemiroff, R.; Bonnell, J., eds. (13 May 2007). "The Cat's Eye Nebula From Hubble". Astronomy Picture of the Day. NASA. Retrieved October 26, 2011.
  11. Huggins, William; Miller, W.A. (1864). "On the spectra of some of the nebulae". Philosophical Transactions of the Royal Society of London. 154: 437–444. Bibcode:1864RSPT..154..437H. doi:10.1098/rstl.1864.0013. See p. 438, "No. 4373".
  12. (Kwok 2000, p. 1)
  13. ^ (Wesson & Liu 2004, pp. 1026–1027, 1040–1041)
  14. ^ (Balick & Preston 1987, pp. 958, 961–963)
  15. (Klaas et al. 2006, p. 523)
  16. (Hora et al. 2004, p. 299)
  17. (Wesson & Liu 2004, pp. 1027–1031)
  18. (Chu et al. 2001)
  19. (Guerrero et al. 2001)
  20. (Kastner et al. 2012)
  21. (Reed et al. 1999, p. 2430)
  22. ^ (Reed et al. 1999, pp. 2433–2438)
  23. Stanghellini, L; Shaw, RA; Villaver, E (2008). "The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale". The Astrophysical Journal. 689 (1): 194–202. arXiv:0807.1129. Bibcode:2008ApJ...689..194S. doi:10.1086/592395. S2CID 119257242.
  24. (Hyung et al. 2000)
  25. (Reed et al. 1999, pp. 2438–2440)
  26. (Balick & Preston 1987)
  27. (Miranda & Solf 1992)
  28. ^ (Balick, Wilson & Hajian 2001, p. 358)
  29. (Balick, Wilson & Hajian 2001, pp. 359–360)

Cited sources

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