Cat's eye nebula
|Cat's eye nebula|
|Designation|| NGC 6543|
|Right ascension||17h 58m 33.423s|
|Declination||+66° 37′ 59.52″|
|Type of object||Planetary nebula|
|Magnitude|| Apparent Mag: +8.1|
Absolute Mag: -0.2
|Distance from Earth||3,300 ly|
|Radial velocity||-65.7±0.9 km/s|
|Proper motion|| RA: 3.6 mas/yr|
Dec.: 6.0 mas/yr
The Cat's eye nebula (NGC 6543) is a well known planetary nebula in the constellation of Draco. It is one of the most structurally complex nebulae and perhaps the most spectacular. Discovered in 1786, the nebula comprises a small but striking core surrounded by a vast, though much fainter, envelope of gas. E.E. Bernard classified the brightest knot in the nebula in 1900. It is classified in the Index Catalogues as IC 4677 and is also sometimes referred to as either teh Sunflower nebula or the Snail nebula.
The nebula was first observed by William Herschel on February 15, 1786. He compared the shape of the nebula to the disks of outer planets, leading to this type of nebula being called "planetary nebula." Later, William Huggins' investigation of the spectra of the Cat's eye nebula in 1864 revealed it was composed primarily of hot gases and no stars. Since then, the nebula has been studied in a variety of different wavelengths, from x-ray, to ultraviolet and radio. In 1994, the Hubble Space Telescope imaged the nebula, exposing unseen detail and remarkable intricate structures. Images of the nebula taken by Hubble are some of the most breath taking images ever taken.
Properties and Structure
The nebula has an extremely complex structure, containing gas shells, shock-induced knots of gas and gas jets. It can be divided into two components, the core and the much larger surrounding envelope.
The core is located at the centre of the nebula. The inner core can be described as an ellipse with a major axis of 16.1 arcseconds, and an outer ellipse covering the whole core of major axis 24.7 arcseconds. These corresponds to physical sizes of 14.8 ly and 22.6 ly respectively. The core is thought to have a temperature of 7,000-9,000 k. The nebula is expanding at 16.4 km/s or 3.457 milliarcsecond/year (mas/yr).
In the core is an type O7 Wolf-Rayet star. Although it is though to have a radius only 65% that of the Sun, its great temperature of 80,000 k means it is 10,000 times more luminous than the Sun. The star has a strong stellar wind of 1,900 km/s, and is ejecting around 3.2×10-7 solar masses of material each year. Surrounding the core region (but not as far as the halo) are eleven concentric rings of material. These rings are centred on the central star and are thought to consist of material ejected by the star.
The envelope of gas surrounding the core is significantly larger than the core, with an apparent diameter of 386 arcseconds (337 ly) or 16 times the size of the core. The envelope is significantly fainter than the core and only visible through large telescopes. This extended halo is believed to have a temperature of the order of 15,000 k. The halo has a mass of 0.26-0.92 solar masses.