NGC 6578, ESO 590-12, PK 010-01 1, PN VV' 376, PN VV 163, PN Sa 2-322, PN G010.8-01.8, Hen 2-381
Type: Planetary Nebula
Con: Sagittarius
RA: 18h 16m 16.5s
Dec: ?20° 27? 0.0?
Mag(v): 12.9
Size: 0.17' x 0.16'
NGC 6578 was discovered on 18 August 1882 by the American astronomer Edward Charles Pickering using the 15-inch Equatorial at Harvard College Observatory. Pickering noted his discovery (HN 47) as "a planetary nebula, stellar, equal to 13th magnitude star."
Among the more obscure of brighter planetary nebulae, NGC 6578 is buried deeply within the star clouds of northern Sagittarius somewhat under a degree north-northeast of Mu Sagittarii. Far away, 6000 light years with an uncertainty of about 2000 light years, and young, it is angularly small, just 8.5 seconds of arc in diameter, or about a quarter of a light year across. H. D. Curtis (left image) says little: "Disk nearly round...no ansae or structural details visible...Rather faint." Hubble on the other hand (at right) reveals a classic double shell, the inner one filled with expanding threads of gas and caused by a fast wind from the central star hammering the outer envelope. While Curtis could not see the fainter outer halo, he was right, there are no ansae or jets as in NGC 7009 or NGC 6543. What may seem to be jets in his drawing are (from the Hubble image) clearly just extensions of the inner shell. Why some nebulae have ansae and others do not is a mystery. Part of the object's faintness is due to some three magnitudes of absorption of light by intervening dust in the thick mid-line of the Milky Way, the nebula a mere 10 degrees from the center of the Galaxy.
n6578bw by HDCurtis Lick Observatory.jpeg n6578 Hubble image by HBond STScI.jpeg
Curtis was close in his assessment of the central star, estimating it at magnitude 15, which modern methods show to be 15.7. Combination of nebular and stellar brightness gives a cool (for such objects) temperature of 63,000 Kelvin (consistent with little or no ionized helium radiation) and, from the distance, a luminosity of about 4000 times that of the Sun. Theory then indicates a core mass (that of the old nuclear burning remnant of the original star) of 0.57 times that of the Sun, which in turn suggests an initial mass (before the star lost its outer envelope) of around 1.3 times that of the Sun, all this discussion very uncertain. Modest mass, though, goes along with no evidence for chemical enrichment of the nebula. The star is still in a state of heating with more or less constant luminosity, and when it hits about 100,000 Kelvin, it will begin to cool and dim, while the nebula, expanding at about 20 kilometers per second, will dissipate into space, leaving a modest white dwarf behind.
I found very few amateur observation reports of NGC 6578, perhaps supporting Jim Kaler's comment that this is a rather obscure planetary nebula. Here is one undated report by Steve Coe in Arizona. Observing with a 13" f/5.6, he notes: "Faint, small, round, no central star even at 320X. This greenish planetary was recognized at 100X, it is about three times the size of the Airy disk. This observation was on a night I rated 8/10 for seeing and 10/10 for transparency." I'm quite interested to hear from our Deep Sky Forum readers with their observations of this planetary nebula.
u5hc0601r_c0f.jpeg
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Some content of this OOTW was abstracted from work by the late Dr. James Bailey "Jim" Kaler, an American astronomer (University of Michigan, United States Naval Observatory, University of Illinois).
Sidebar
Morpho-kinematic Properties of Wolf–Rayet Planetary Nebulae
SHAPE mesh model of NGC 6578 published in The Astrophysical Journal Supplement Series, Volume 260, Number 1 (2022).
Author: Ashkbiz Danehkar, Department of Astronomy, University of Michigan, 1085 S. University Ave, Ann Arbor, MI 48109
The majority of planetary nebulae show axisymmetric morphologies, whose causes are not well understood. In this work, we present spatially resolved kinematic observations of 14 Galactic PNs surrounding Wolf–Rayet ([WR]) and weak emission-line stars (wels), based on the H? and [N ii] emission taken with the Wide Field Spectrograph on the Australian National University 2.3 m telescope. Velocity-resolved channel maps and position–velocity diagrams, together with archival Hubble Space Telescope (HST) and ground-based images, are employed to construct three-dimensional morpho-kinematic models of 12 objects using the program shape. Our results indicate that these 12 PNs mostly have elliptical morphologies, with either open or closed outer ends. The kinematic maps show the on-sky orientations of the interior shells in NGC 6578 and NGC 6629, as well as the compact (?6'') PNs Pe 1-1, M 3-15, M 1-25, Hen 2-142, and NGC 6567, in agreement with the elliptically symmetric morphologies seen in high-resolution HST images.
3D model of the planetary nebula NGC 6578
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