| Constellation: | Draco |
| Right ascension: | 17h58m33.42s |
| Declination: | +66°37'59.5" |
| Apparent magnitude: | 9 |
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The Cat's Eye Nebula or 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, bubbles and sinewy arc-like features. In the center of the Cat's Eye there is a bright and hot star; around 1000 years ago this star lost its outer envelope, producing the nebula.
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. The results of the latter investigation demonstrated for the first time that planetary nebulae consist of hot gases, but not stars. Currently the nebula has been observed across the full electromagnetic spectrum, from far-infrared to X-rays.
Modern studies reveal several mysteries. The intricacy of the structure may be caused in part by material ejected from a binarycentral 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. Hubble Telescope observations revealed a number of faint rings around the Eye, which are spherical shells ejected by the central star in the distant past. The exact mechanism of those ejections, however, is unclear.
Information :
NGC 6543 is a 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 m 33.4 s and declination +66°37'59″.[4] Its high declination means it is easily observable from thenorthern hemisphere, where historically most large telescopes have been situated. NGC 6543 is situated almost exactly in the direction of the North Ecliptic Pole.
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| Composite image using optical images from the HSTand X-ray data from the Chandra X-ray Observatory |
While the bright inner nebula is rather small—the major axis of the inner ellipse is 16.1 arcseconds, while the distance between the condensations is 24.7 arcseconds—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 300 arcseconds (5 arcminutes). The cat's Eye nebula lies three thousand light-years from Earth.
Observations show that the main body of the nebula has a density of about 5,000 particles/cm³ and a temperature of about 7,000–9,000 K.[7] The outer halo has a higher temperature of about 15,000 K and a much lower density.
In 1994, Hubble first revealed NGC 6543's surprisingly intricate structures, including concentric gas shells, jets of high-speed gas, and unusual shock-induced knots of gas.The central star of NGC 6543 is an O7 + [WR]–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.
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| An optical Image of the Nebula's Surrounding Halo |
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| An optical Image of the Nebula's Surrounding Halo |
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 bycollisional 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.
