Cat's Eye Nebula
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| Cat's Eye Nebula | |
 Composite image using optical images from the HST and X-ray data from the Chandra X-ray Observatory |
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| Observation data ( Epoch J2000) |
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|---|---|
| Right ascension | 17h 58m 33.423s |
| Declination | +66° 37′ 59.52″ |
| Distance | 3.3 ± 0.9 kly (1.0 ± 0.3 kpc) |
| Apparent magnitude (V) | 9.8B |
| Apparent dimensions (V) | Core: 20″ Halo: 5′.8 |
| Constellation | Draco |
| Physical characteristics | |
| Radius | Core: 0.2 ly |
| Absolute magnitude (V) | -0.2+0.8−0.6B |
| Notable features | complex structure |
| Other designations | NGC 6543, Snail Nebula, Sunflower Nebula, (includes IC 4677) |
| See also: Planetary nebula, Lists of nebulae | |
The Cat's Eye Nebula ( NGC 6543) is a planetary nebula in the constellation of Draco. Structurally, it is one of the most complex nebulae known, with high-resolution Hubble Space Telescope observations revealing remarkable structures such as knots, jets and sinewy arc-like features.
It was discovered by William Herschel on February 15, 1786, and was the first planetary nebula whose spectrum was investigated by the English amateur astronomer William Huggins in 1864.
Modern studies reveal several mysteries. The intricacy of the structure may be caused in part by material ejected from a binary 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 measurements done by two different methods, the cause of which is uncertain.
General information
NGC 6543 is a very well-studied planetary nebula. It is relatively bright at magnitude 8.1, and also has a high surface brightness. It is situated at right ascension 17h 58.6 m and declination +66°38'. Its high declination means it is easily observable from the northern hemisphere, where historically most large telescopes have been situated. NGC 6543 is situated almost exactly in the direction of the North Ecliptic Pole.
While the bright inner nebula is rather small at 20 arcseconds in diameter (Reed et al. 1999), it has an extended halo of matter that the progenitor star ejected during its red giant phase. This halo extends over a diameter of about 386 arcseconds (6.4 arcminutes).
Observations show that the main body of the nebula has a density of about 5,000 particles/cm³ and a temperature of about 8,000 K. (Wesson & Liu 2004) The outer halo has a somewhat higher temperature of about 15,000 K and a much lower density.
The central star of NGC 6543 is an O-type star, with a temperature of approximately 80,000 K. It is approximately 10,000 times as luminous as the sun, and its radius is about 0.65 times the solar value. Spectroscopic analysis shows that the star is currently losing mass in a fast stellar wind at a rate of about 3.2×10−7 solar masses per year - about 20 trillion tons per second. The velocity of this wind is about 1900 km/s. Calculations indicate that the central star currently weighs just over one solar mass, but theoretical evolutionary calculations imply that it had an initial mass of about 5 solar masses. (Bianchi, Cerrato & Grewing 1986)
Distance
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 angular resolution will reveal the growth of the nebula in the plane of the sky. This is typically very small—only a few milliarcseconds a year or less. Spectroscopic observations can reveal the velocity of expansion of the nebula along the line of sight using the Doppler Effect. 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 16.4 km/s. Combining these two results implies that NGC 6543 is about 1000 parsecs (3×1019 m), or about 3300 light-years away from Earth. (Reed et al. 1999)
Age
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 1000 ± 260 years. (Reed et al. 1999) 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 interstellar medium.
Composition
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 3×10−4, and the oxygen abundance is about 7×10−4. 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. (Wesson & Liu 2004) (Hyung et al. 2000)
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
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 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 has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends. (Balick & Preston 1987)
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. (Miranda & Solf 1992)
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. (Balick, Wilson & Hajian 2001)
Further out, a large faint halo extends to large distances from the star. The halo again predates the formation of the main nebula.
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 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.
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. (Wesson & Liu 2004) The cause of this discrepancy is disputed.