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Cat's Eye Nebula

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   CAPTION: NGC 6543

      Planetary nebula           Lists of nebulae

   The Cat's Eye Nebula.
   Composite image using optical images
   from the HST and X-ray data
   from the Chandra X-ray Observatory
                    Observation data
   ( Epoch J2000.0)
   Right ascension         17^h 58^m 33.4^s
   Declination             +66° 37′ 59″
   Distance                3,300±900 ly
   Apparent magnitude (V)  +8.9
   Apparent dimensions (V) Core: 20″ Halo: 5′.8 arcmins
   Constellation           Draco
                Physical characteristics
   Radius                  Core: 0.2 ly
   Absolute magnitude (V)  -
   Notable features        complex structure
   Other designations      NGC 6543

   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 58.6m 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 , 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.^1 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.^2

Observations

   The nebula was discovered by William Herschel on February 15, 1786, and
   was the first planetary nebula to be observed with a spectroscope, by
   William Huggins in 1864. Huggins' observations were the first
   indication that planetary nebulae consist of extremely rarefied gases.
   Since those early observations, NGC 6543 has been observed right across
   the electromagnetic spectrum.

Infrared observations

   Observations of NGC 6543 at infrared wavelengths 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 70 K.

   Infrared emission also reveals the presence of un-ionised material such
   as molecular hydrogen (H[2]). 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 H[2] as ejecta moving at different speeds
   collide.^3

Optical and ultraviolet observations

   NGC 6543 has been extensively observed at ultraviolet and optical
   wavelengths. 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 ionisation. Three images
   were taken, in filters isolating the light emitting 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

   Recent observations at X-ray wavelengths by the Chandra X-ray
   Observatory 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 point source 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 accretion disk within a binary star
   system.^4

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 (3×10^19 m)
   away from Earth.

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 years. 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

   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.
   Enlarge
   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^−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.^1,6

   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.^7

   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.^8

   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.^9

   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.^1 The cause of this discrepancy is disputed—suggestions include
   the presence of some material highly enriched in heavy elements, or
   sizable temperature fluctuations within the nebula.

   Retrieved from " http://en.wikipedia.org/wiki/Cat%27s_Eye_Nebula"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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