NGC 6578

In Sagittarius

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6578 NGC 6578
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 (from a combination of inaccurate methods perhaps 6500 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. Curtis (left-hand image) says little: "Disk nearly 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.

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.

Left: Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: Hubble image by H. Bond (STScI) and NASA.