Cat's Eye Nebula

Last updated
Cat's Eye Nebula
Emission nebula
Planetary nebula
NGC6543.jpg
Composite image using optical images from the HST and X-ray data from the Chandra X-ray Observatory.
Observation data: J2000 epoch
Right ascension 17h 58m 33.423s [1]
Declination +66° 37′ 59.52″ [1]
Distance3.3±0.9  kly (1.0±0.3  kpc ) [2]   ly
Apparent magnitude (V)9.8B [1]
Apparent dimensions (V)Core: 20″ [2]
Constellation Draco
Physical characteristics
Radius Core: 0.2 ly [note 1]  ly
Absolute magnitude (V)−0.2+0.8
−0.6B [note 2]
Notable featurescomplex structure
Designations NGC 6543, [1] Snail Nebula, [1] Sunflower Nebula, [1] (includes IC 4677), [1] Caldwell 6
See also: Lists of nebulae

The Cat's Eye Nebula (also known as NGC 6543 and Caldwell 6) is a planetary nebula in the northern constellation of Draco, discovered by William Herschel on February 15, 1786. It was the first planetary nebula whose spectrum was investigated by the English amateur astronomer William Huggins, demonstrating that planetary nebulae were gaseous and not stellar in nature. Structurally, the object has had high-resolution images by the Hubble Space Telescope revealing knots, jets, bubbles and complex arcs, being illuminated by the central hot planetary nebula nucleus (PNN). [3] It is a well-studied object that has been observed from radio to X-ray wavelengths.

Contents

General information

NGC 6543 is a high northern declination deep-sky object. It has the combined magnitude of 8.1, with high surface brightness. Its small bright inner nebula subtends an average of 16.1 arcsec, with the outer prominent condensations about 25 arcsec. [4] Deep images reveal an extended halo about 300 arcsec or 5 arcminutes across, [5] that was once ejected by the central progenitor star during its red giant phase.

NGC 6543 is 4.4  minutes of arc from the current position of the north ecliptic pole, less than 1⁄10 of the 45  arcminutes between Polaris and the current location of the Earth's northern axis of rotation. It is a convenient and accurate marker for the axis of rotation of the Earth's ecliptic, around which the celestial North Pole rotates. It is also a good marker for the nearby "invariable" axis of the solar system, which is the center of the circles which every planet's north pole, and the north pole of every planet's orbit, make in the sky. Since motion in the sky of the ecliptic pole is very slow compared to the motion of the Earth's north pole, its position as an ecliptic pole station marker is essentially permanent on the time-scale of human history, as opposed to the pole star, which changes every few thousand years.

Observations show the bright nebulosity has temperatures between 7000 and 9000  K , whose densities average of about 5000 particles per cubic centimetre. [6] Its outer halo has the higher temperature around 15,000 K, but is of much lower density. [7] Velocity of the fast stellar wind is about 1900 km/s, where spectroscopic analysis shows the current rate of mass loss averages 3.2×10−7 solar masses per year, equivalent to twenty trillion tons per second (20 Eg/s). [6]

An optical image of the nebula's surrounding halo Heic0414b.jpg
An optical image of the nebula's surrounding halo

Surface temperature for the central PNN is about 80,000 K, being 10,000 times as luminous as the sun. Stellar classification is O7 +  [WR]-type star. [6] Calculations suggest the PNN is over one solar mass, from a theoretical initial 5 solar masses. [8] The central Wolf–Rayet star has a radius of 0.65 R☉ (452,000 km). [9] The Cat's Eye Nebula, given in some sources, lies about three thousand light-years from Earth. [10]

Observations

The Cat's Eye was the first planetary nebula to be observed with a spectroscope by William Huggins on August 29, 1864. [11] [12] Huggins' observations revealed that the nebula's spectrum was non-continuous and made of a few bright emission lines, first indication that planetary nebulae consist of tenuous ionised gas. Spectroscopic observations at these wavelengths are used in abundance determinations, [13] while images at these wavelengths have been used to reveal the intricate structure of the nebula. [14]

Infrared observations

Observations of NGC 6543 at far-infrared wavelengths (about 60 μm) 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 85 K, while the mass of the dust is estimated at 6.4×10−4 solar masses. [15]

Infrared emission also reveals the presence of un-ionised material such as molecular hydrogen (H2) and argon. 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 H2 as ejecta moving at different speeds collide. The overall appearance of the Cat's Eye Nebula in infrared (wavelengths 2–8 μm) is similar in visible light. [16]

Optical and ultraviolet observations

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 emitted 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. [17]

X-ray observations

X-ray image of Nebula. Cat's Eye Nebula.X-ray image.jpg
X-ray image of Nebula.

In 2001, observations at X-ray wavelengths by the Chandra X-ray Observatory revealed the presence of extremely hot gas within NGC 6543 with the temperature of 1.7×106 K. [18] 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. [14] Chandra observations have also revealed a point source at the position of the central star. The spectrum of this source extends to the hard part of the X-ray spectrum, to 0.5–1.0  keV . A star with the photospheric temperature of about 100,000 K would not be expected to emit strongly in hard 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. [19] The hard X-ray data remain intriguing more than ten years later: the Cat's Eye was included in a 2012 Chandra survey of 21 central stars of planetary nebulae (CSPNe) in the solar neighborhood, which found: "All but one of the X-ray point sources detected at CSPNe display X-ray spectra that are harder than expected from hot (~100,000 K) central star photospheres, possibly indicating a high frequency of binary companions to CSPNe. Other potential explanations include self-shocking winds or PN mass fallback." [20]

Distance

Planetary nebulae distances like NGC 6543 are generally very inaccurate and not well known. [21] Some recent Hubble Space Telescope observations of NGC 6543 taken several years apart determine its distance from the angular expansion rate of 3.457 milliarcseconds per year. Assuming a line of sight expansion velocity of 16.4 km·s−1, this implies that NGC 6543's distance is 1001±269  parsecs (3×1019 k or 3300  light-years) away from Earth. [22] Several other distance references, like what is quoted in SIMBAD in 2014 based on Stanghellini, L., et al. (2008) suggest the distance is 1623 parsecs (5300 light-years). [23]

Age

The angular expansion of the nebula can also be used to estimate its age. If it has been expanding at a constant rate of 10 milliarcseconds a year, then it would take 1000±260 years to reach a diameter of 20 arcseconds. This may be an upper limit to the age, because ejected material will be slowed when it encounters material ejected from the star at earlier stages of its evolution, and the interstellar medium. [22]

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. Catseye-big.jpg
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. [7]

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. [13] 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. [24]

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 below. [13]

Kinematics and morphology

The Cat's Eye Nebula is structurally a very complex nebula, and the mechanism or mechanisms that have given rise to its complicated morphology are not well understood. [14] The central bright part of the nebula consists of the inner elongated bubble (inner ellipse) filled with hot gas. It, in turn, is nested into a pair of larger spherical bubbles conjoined together along their waist. The waist is observed as the second larger ellipse lying perpendicular to the bubble with hot gas. [25]

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 PNN with the visible material ejected during the formation of the nebula. This interaction causes the emission of X-rays discussed above. The stellar wind, blowing with the velocity as high as 1900 km/s, has 'hollowed out' the inner bubble of the nebula, and appears to have burst the bubble at both ends. [14]

It is also suspected that the central WR:+O7 spectral class PNN star, HD 164963 / BD +66 1066 / PPM 20679 [1] of the nebula may be generated by a binary star. [1] 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. [26] [27]

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. [5] The total mass of the rings is about 0.1 solar masses. [28] The pulsations that formed the rings probably started 15,000 years ago and ceased about 1000 years ago, when the formation of the bright central part began (see above). [29]

Further, a large faint halo extends to large distances from the star. The halo again predates the formation of the main nebula. The mass of the halo is estimated as 0.26–0.92 solar masses. [28]

See also

Notes

  1. ↑ Distance × sin(diameter_angle / 2 ) = 0.2 ly. radius
  2. ↑ 9.8B apparent magnitude – 5×{log(1.0 ± 0.3 kpc distance) − 1} = −0.2+0.8
    −0.6    B absolute magnitude

Related Research Articles

<span class="mw-page-title-main">Nebula</span> Body of interstellar clouds

A nebula is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions, the formations of gas, dust, and other materials "clump" together to form denser regions, which attract further matter and eventually become dense enough to form stars. The remaining material is then thought to form planets and other planetary system objects.

<span class="mw-page-title-main">Planetary nebula</span> Type of emission nebula created by dying red giants

A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.

<span class="mw-page-title-main">Helix Nebula</span> Planetary nebula in the constellation Aquarius

The Helix Nebula is a planetary nebula (PN) located in the constellation Aquarius. Discovered by Karl Ludwig Harding, most likely before 1824, this object is one of the closest of all the bright planetary nebulae to Earth. The distance, measured by the Gaia mission, is 655±13 light-years. It is similar in appearance to the Cat's Eye Nebula and the Ring Nebula, whose size, age, and physical characteristics are similar to the Dumbbell Nebula, varying only in its relative proximity and the appearance from the equatorial viewing angle. The Helix Nebula has sometimes been referred to as the "Eye of God" in pop culture, as well as the "Eye of Sauron".

<span class="mw-page-title-main">H II region</span> Large, low-density interstellar cloud of partially ionized gas

An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds of light years, and density from a few to about a million particles per cubic centimetre. The Orion Nebula, now known to be an H II region, was observed in 1610 by Nicolas-Claude Fabri de Peiresc by telescope, the first such object discovered.

<span class="mw-page-title-main">Dumbbell Nebula</span> Planetary nebula in the constellation Vulpecula

The Dumbbell Nebula is a planetary nebula in the constellation Vulpecula, at a distance of about 1360 light-years. It was the first such nebula to be discovered, by Charles Messier in 1764. At its brightness of visual magnitude 7.5 and diameter of about 8 arcminutes, it is easily visible in binoculars and is a popular observing target in amateur telescopes.

<span class="mw-page-title-main">North America Nebula</span> Emission nebula in the constellation Cygnus

The North America Nebula is an emission nebula in the constellation Cygnus, close to Deneb. It is named because its shape resembles North America.

<span class="mw-page-title-main">Eskimo Nebula</span> Planetary nebula in the constellation Gemini

The Eskimo Nebula, also known as the Clown-faced Nebula, Lion Nebula, or Caldwell 39, is a bipolar double-shell planetary nebula (PN). It was discovered by astronomer William Herschel in 1787. The formation resembles a person's head surrounded by a parka hood. It is surrounded by gas that composed the outer layers of a Sun-like star. The visible inner filaments are ejected by a strong wind of particles from the central star. The outer disk contains unusual, light-year-long filaments.

<span class="mw-page-title-main">Sombrero Galaxy</span> Galaxy in the constellation Virgo

The Sombrero Galaxy is a peculiar galaxy of unclear classification in the constellation borders of Virgo and Corvus, being about 9.55 megaparsecs from the Milky Way galaxy. It is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster. It has an isophotal diameter of approximately 29.09 to 32.32 kiloparsecs, making it slightly bigger in size than the Milky Way.

<span class="mw-page-title-main">Saturn Nebula</span> Planetary nebula in the constellation Aquarius

The Saturn Nebula is a planetary nebula in the constellation Aquarius. It appears as a greenish-yellowish hue in a small amateur telescope. It was discovered by William Herschel on September 7, 1782, using a telescope of his own design in the garden at his home in Datchet, England, and was one of his earliest discoveries in his sky survey. The nebula was originally a low-mass star that ejected its layers into space, forming the nebula. The central star is now a bright white dwarf star of apparent magnitude 11.5. The Saturn Nebula gets its name from its superficial resemblance to the planet Saturn with its rings nearly edge-on to the observer. It was so named by Lord Rosse in the 1840s, when telescopes had improved to the point that its Saturn-like shape could be discerned. William Henry Smyth said that the Saturn Nebula was one of Struve's nine "Rare Celestial Objects".

<span class="mw-page-title-main">Little Dumbbell Nebula</span> Planetary nebula in the constellation Perseus

The Little Dumbbell Nebula, also known as Messier 76, NGC 650/651, the Barbell Nebula, or the Cork Nebula, is a planetary nebula in the northern constellation of Perseus. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects as number 76. It was first classified as a planetary nebula in 1918 by the astronomer Heber Doust Curtis. However, others might have previously recognized it as a planetary nebula; for example, William Huggins found its spectrum indicated it was a nebula ; and Isaac Roberts in 1891 suggested that M76 might be similar to the Ring Nebula (M57), as seen instead from the side view.

<span class="mw-page-title-main">Owl Nebula</span> Planetary nebula in the constellation Ursa Major

The Owl Nebula is a planetary nebula approximately 2,030 light years away in the constellation Ursa Major. Estimated to be about 8,000 years old, it is approximately circular in cross-section with a faint internal structure. It was formed from the outflow of material from the stellar wind of the central star as it evolved along the asymptotic giant branch. The nebula is arranged in three concentric shells, with the outermost shell being about 20–30% larger than the inner shell. The owl-like appearance of the nebula is the result of an inner shell that is not circularly symmetric, but instead forms a barrel-like structure aligned at an angle of 45° to the line of sight.

<span class="mw-page-title-main">NGC 2438</span> Planetary nebula in the constellation Puppis

NGC 2438 is a planetary nebula in the southern constellation of Puppis. Parallax measurements by Gaia put the central star at a distance of roughly 1,370 light years. It was discovered by William Herschel on March 19, 1786. NGC 2438 appears to lie within the cluster M46, but it is most likely unrelated since it does not share the cluster's radial velocity. The case is yet another example of a superposed pair, joining the famed case of NGC 2818.

<span class="mw-page-title-main">NGC 7027</span> Planetary nebula in the constellation Cygnus

NGC 7027, also known as the Jewel Bug Nebula, is a very young and dense planetary nebula located around 3,000 light-years from Earth in the constellation Cygnus. Discovered in 1878 by Édouard Stephan using the 800 mm (31 in) reflector at Marseille Observatory, it is one of the smallest planetary nebulae and by far the most extensively studied.

<span class="mw-page-title-main">NGC 6302</span> Bipolar planetary nebula in the constellation Scorpius

NGC 6302 is a bipolar planetary nebula in the constellation Scorpius. The structure in the nebula is among the most complex ever observed in planetary nebulae. The spectrum of NGC 6302 shows that its central star is one of the hottest stars known, with a surface temperature in excess of 250,000 degrees Celsius, implying that the star from which it formed must have been very large.

<span class="mw-page-title-main">NGC 6751</span> Planetary nebula in the constellation Aquila

NGC 6751, also known as the Glowing Eye Nebula, is a planetary nebula in the constellation Aquila. It is estimated to be about 6,500 light-years away.

<span class="mw-page-title-main">NGC 2022</span> Planetary nebula in the constellation Orion

NGC 2022 is a planetary nebula in the equatorial constellation of Orion, located at a distance of 8.21 kilolight-years from the Sun. It was first observed by William Herschel on December 28, 1785, who described it as: considerably bright, nearly round, like a star with a large diameter, like an ill-defined planetary nebula. In medium-sized amateur telescopes it looks like a small grayish patch of light. It is not very bright but it is still easy to spot it in the eyepiece. Even in a telescope as small as 80mm it can just be seen using a narrowband filter such as an OIII filter as a 'fuzzy' star. The object has the shape of a prolate spheroid with a major to minor axis ratio of 1.2, an apparent size of 28″, and a halo extending out to 40″, which is about the angular diameter of Jupiter as seen from Earth.

<span class="mw-page-title-main">NGC 602</span> Open cluster in the constrellation Hydrus

NGC 602 is a young, bright open cluster of stars located in the Small Magellanic Cloud (SMC), a satellite galaxy to the Milky Way. It is embedded in a nebula known as N90.

<span class="mw-page-title-main">NGC 5307</span> Planetary nebula in the constellation Centaurus

NGC 5307 is a planetary nebula in the southern constellation of Centaurus, positioned less than 3° to the northeast of the star Epsilon Centauri. It was discovered by English astronomer John Herschel on April 15, 1836. The nebula is located at a distance of approximately 10.6 kilolight-years from the Sun. The central star, designated PNG 312.3+10.5, is a weak emission-line star, superficially similar to the WC subtype of Wolf–Rayet stars. It has a spectral class of O(H)3.5 V.

<span class="mw-page-title-main">NGC 7662</span> Planetary nebula in the constellation Andromeda

NGC 7662 is a planetary nebula located in the northern constellation Andromeda. It is known as the Blue Snowball Nebula, Snowball Nebula, and Caldwell 22. This nebula was discovered October 6, 1784 by the German-born English astronomer William Herschel. In the New General Catalogue it is described as a "magnificent planetary or annular nebula, very bright, pretty small in angular size, round, blue, variable nucleus". The object has an apparent visual magnitude of 8.3 and spans an angular size of 32″ × 28″. Parallax measurements give a distance estimate of 5,730 ± 340 ly (1,757 ± 103 pc).

<span class="mw-page-title-main">NGC 2867</span> Planetary nebula in the constellation Carina

NGC 2867 is an elliptical Type II planetary nebula in the southern constellation of Carina, just over a degree to the NNW of the star Iota Carinae. It was discovered by John Herschel on April 1, 1834. Herschel initially thought he might have found a new planet, but on the following night he checked again and discovered it had not moved. The nebula is located at a distance of 7,270 light-years from the Sun.

References

  1. 1 2 3 4 5 6 7 8 9 ( SIMBAD 2006 )
  2. 1 2 (Reed et al. 1999)
  3. ↑ Shaw, R. A. (1985). "The evolution of Planetary Nebula Nuclei (PNN)". Ph.D. Thesis, Illinois Univ., Urbana-Champaign. Bibcode:1985PhDT........13S.
  4. ↑ ( Reed et al. 1999 , p. 2433)
  5. 1 2 ( Balick, Wilson & Hajian 2001 , p. 354)
  6. 1 2 3 ( Wesson & Liu 2004 , pp. 1026, 1028)
  7. 1 2 ( Wesson & Liu 2004 , p. 1029)
  8. ↑ ( Bianchi, Cerrato & Grewing 1986 )
  9. ↑ Constellation Guide
  10. ↑ Nemiroff, R.; Bonnell, J., eds. (13 May 2007). "The Cat's Eye Nebula From Hubble". Astronomy Picture of the Day . NASA . Retrieved October 26, 2011.
  11. ↑ Huggins, William; Miller, W.A. (1864). "On the spectra of some of the nebulae". Philosophical Transactions of the Royal Society of London. 154: 437–444. Bibcode:1864RSPT..154..437H. doi: 10.1098/rstl.1864.0013 . See p. 438, "No. 4373".
  12. ↑ ( Kwok 2000 , p. 1)
  13. 1 2 3 ( Wesson & Liu 2004 , pp. 1026–1027, 1040–1041)
  14. 1 2 3 4 ( Balick & Preston 1987 , pp. 958, 961–963)
  15. ↑ ( Klaas et al. 2006 , p. 523)
  16. ↑ ( Hora et al. 2004 , p. 299)
  17. ↑ ( Wesson & Liu 2004 , pp. 1027–1031)
  18. ↑ ( Chu et al. 2001 )
  19. ↑ ( Guerrero et al. 2001 )
  20. ↑ ( Kastner et al. 2012 )
  21. ↑ ( Reed et al. 1999 , p. 2430)
  22. 1 2 ( Reed et al. 1999 , pp. 2433–2438)
  23. ↑ Stanghellini, L; Shaw, RA; Villaver, E (2008). "The Magellanic Cloud Calibration of the Galactic Planetary Nebula Distance Scale". The Astrophysical Journal. 689 (1): 194–202. arXiv: 0807.1129 . Bibcode:2008ApJ...689..194S. doi:10.1086/592395. S2CID   119257242.
  24. ↑ ( Hyung et al. 2000 )
  25. ↑ ( Reed et al. 1999 , pp. 2438–2440)
  26. ↑ ( Balick & Preston 1987 )
  27. ↑ ( Miranda & Solf 1992 )
  28. 1 2 ( Balick, Wilson & Hajian 2001 , p. 358)
  29. ↑ ( Balick, Wilson & Hajian 2001 , pp. 359–360)

Cited sources

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.