Cat's Eye Nebula

The Cat's Eye Nebula (NGC 6543) is a planetary nebula in the constellation Draco. He is structurally one of the most complex nebulae known. High-resolution images from the Hubble Space Telescope revealed unusual structures such as knots, jets and arc-like features. Visually, it resembles the cat's eye and was named accordingly.

It was discovered on 15 February 1786 by William Herschel, and was the first planetary nebula whose spectrum was investigated ( by the amateur astronomer William Huggins 1864).

General Information

NGC 6543 is a very well -studied planetary nebula. It has a magnitude of 8.1 and is therefore not visible to the naked eye. But he has a very high surface brightness. Its right ascension is 17h 58.6m and its declination 66 ° 38 '. Its high declination means that it can be easily seen from the northern hemisphere, on which there are most large telescopes. NGC 6543 is located almost at the north pole of the Earth's orbit and is therefore also known by the older name Ekliptiknordpol nebula known.

While the diameter of the bright inner part with about 20 seconds of arc occupies a very small part, exists around the fog, a 6.4 arcmin large halo that was ejected from his old star, as this was a red giant.

Said main body has has a density of about 5,000 particles per cubic centimeter and a temperature of about 8,000 K. The outer Halo a temperature of 15,000 K and a much lower density.

The central star is an O- type star and has a temperature of 80,000 K. It seems about 10,000 times brighter than our sun, but has, in comparison with her, only 0.65 times the radius.

Spectroscopic analyzes showed that it loses its fast stellar wind currently mass. There are about 3.2 × 10-7 solar masses per year - 20 trillion tons per second. The wind speed is 1900 km / s Calculations showed that the stellar mass is only slightly more than one solar mass, but you go in the calculations assume that he originally had once 5 solar masses.

Observation

The nebula was discovered on 15 February 1786 by William Herschel, and was by the amateur astronomer William Huggins in 1864, the first planetary nebulae, in which one has studied the spectrum. Huggins ' observations were the first, which indicated that it consists of extremely rarefied gas. Since these early observations NGC 6543 was studied across the entire electromagnetic spectrum.

Infrared observations

From observations of NGC 6543 in the infrared region shows that there is interstellar dust in low temperature regions. It is suspected that he has formed in the last phase of the original central star. The dust absorbs light from the star, and converts it into infrared light. From the infrared spectrum of the dust has been estimated that he has a temperature of about 70 K.

Infrared emissions showed the existence of unionized material such as molecular hydrogen (H2). In many planetary nebulae is the emission by molecules at the edge of the nebula is greatest, however, appears in NGC 6543, the strongest emission by molecular hydrogen around the inner part of the outer halo to be. One reason could be that different fast shock waves that contain the H2 collide.

Optical and ultraviolet observations

NGC 6543 has been extensively studied in the ultraviolet and optical wavelength range. The images of these wavelengths are used to make the complicated structure of the nebula visible. However spectroscopic observations serve to determine the chemical composition.

The image of the Hubble Space Telescope does not show the colors of the nebula, as they are in reality. It was created in order to show the distribution of high and low Ionisationsgehalt. For this purpose, three images were superimposed, showing the singly ionized hydrogen of wavelength 656.3 nm, 658.3 nm and ionized nitrogen at wavelength 500.7 nm of ionized oxygen. As the colors of the individual wavelengths are red, red and green, has red, green and blue assigned them to better differentiate the three channels. It can be seen in the picture at the corners of two areas of slightly ionized material.

X-ray observation

Through observations of X-rays by the Chandra X-ray Observatory, the existence of extremely hot gas in NGC 6543 was detected in 2001. The picture at the beginning of the article is a combination of optical images from the Hubble Space Telescope and Chandra X-ray data of the telescope. It is believed that the very hot gas is generated, because there is strong interactions between the star and the wind ejected material. This also meant that the interior of the nebula has been eroded.

Through the observations with Chandra a point source could be identified at the position of the central star. A star encounters but usually not as strong X- rays, so that their presence can not explain. This could indicate the presence of a hot accretion disk in a binary system.

Distance

A long- standing problem in planetary nebulae was the distance determination. Many best practices were based on assumptions that may not be applicable to planetary nebulae.

In recent years there has been by the Hubble Space Telescope, a new method for distance determination. Since expand all planetary nebulae, you can also do this with high-resolution telescopes that have made images over several years to determine. The change is usually very small ( a few thousandths of seconds of arc or even less). Spectroscopic analysis can be defined by means of the Doppler effect, the expansion velocity of the nebula in the direction observation line. Comparing this expansion with the change of the angle of the fog occupies in the sky, one can thus determine the distance.

The a couple of years old recordings from the Hubble Space Telescope of NGC 6543 were used to determine the distance. Its expansion in the sky increases annually by about 10 thousandths of seconds of arc, while its expansion direction observation line is 16.4 km / s. It was concluded that NGC 6543 about 1000 parsecs (3 × 1019 m) ​​is removed from the earth.

Age

The size change can be used to determine the age. Suppose the fog been expanded always with today's rate of 10 thousandths of seconds of arc per year. Since he has now reached a diameter of 20 arc seconds, his age is 1000 years. However, this is only an upper limit, since the expansion is constantly slowed. This is done by the collision with ejected material from an earlier phase of life of the star, or by collision with the interstellar medium.

Composition

NGC 6543 consists mostly of hydrogen and helium and very little heavy elements ( metals). However, the results of different analyzes of the deposits can vary. The reason lies in the very small aperture of the spectroscope in telescopes. They only take a very small part true and mostly observe therefore not the same parts of the nebula.

The ratio of helium to hydrogen is 0.12 to carbon and nitrogen, each 3 × 10-4 and 7 × 10-4 to oxygen. These data are typical for planetary nebulae. Your amount of carbon, nitrogen and oxygen are larger than that of the sun, because the helium fusion had enriched the stellar atmosphere with these heavy elements before they were ejected as a planetary nebula.

Through precise spectroscopic analysis of NGC 6543 could be shown that the fog has some small regions that are highly enriched in heavy elements. This will be discussed below.

Movement and shape

NGC 6543 is a very complex fog and the mechanisms that led to its complicated shape, are not yet well understood.

The interaction of the stellar wind with the ejected material are the main reason for the bright areas of the nebula. This leads to the emission of X-rays. The stellar wind has eroded the inner bladder of the mist and out to break the bubble at both ends.

It is also believed that the central star is part of a binary system. The accretion disk may have been caused by the mass exchange between the two stars. This could have come to the formation of the polar jets, then affect the previously ejected material. Due to the precession gradually changes the direction of the jets.

Outside of the brighter inner region can be seen up to eleven concentric rings that were rejected before the planetary nebula has formed. The star was then in an asymptotic Gigantic secondary branch of the Hertzsprung- Russell diagram. The uniform distribution of the Rings suggests mechanisms that have ejected the material of the rings at very regular intervals and very similar speeds.

Even further is outside in very large distance from the star around him a large weak halo. He, too, was, before the main fog has formed.

Current research subject

While the Cat's Eye Nebula was studied very carefully, many questions still remain open. The concentric rings around the inner nebula was ejected at intervals of several hundred years. This duration is difficult to explain. Generated by thermal pulsations that exist at the beginning of planetary nebulae, have a time interval of several thousand years, while smaller pulsations occur at the surface only once every few decades. We know the mechanism does not, which could form the concentric rings around the Cat's Eye Nebula in this period.

The spectrum of planetary nebulae consists of emission lines. They are formed either by collision of excited ions or by recombination of the electrons with the ions. Collision lines are substantially stronger than recombination and have therefore been used previously to determine the chemical composition. According to recent studies the occurrence, was determined by the recombination lines in the spectrum of NGC 6543, about three times higher than that which was determined by the collision lines. The reason for these differences is debatable. The explanations range from a few areas that contain a lot of material from heavy elements, to resizable temperature fluctuations in the fog.