NGC 6741

In Aquila

From Jim Kaler's STARS; Return to Planetary Nebulae

NGC 6741 NGC 6741
The central stars of planetary nebulae span a huge range of temperature, within which NGC 6741 is, at 180,000 Kelvin, one of the hotter (one study taking it to 220,000), comparable to NGC 2440 and NGC 7027. The nebula, just 8 seconds of arc across, is small, rendering it hard to discern features, as witness Curtis's drawing. He says of it: "No central star...small bright oval...indistinct ring structure...scarcely perceptible protuberance at the western end." Hubble dramatically expands on this description by showing an interior filled with threads of hot gas (with a high temperature approaching 18,000 Kelvin, much higher than the average of 10,000 K) that terminates in a sharp boundary with no visible outer shell, the central star remaining invisible. NGC 6741 goes also goes by the odd popular name (not much used) of the "Phantom Streak Nebula."

Located within the Milky Way in southwestern Aquila (not quite five degrees north of Lambda Aquilae), the distance is best estimated through its two or so magnitudes of light-absorption by interstellar dust, which gives a range between 4900 and 8700 light years, which in turn gives a range in diameter for the object of two-tenths to a third of a light year.

While the central star is not seen here, it has been detected at magnitude 20.0, and thus is easily lost within the bright nebulosity, its high temperature causing it to glow mostly in the far ultraviolet part of the spectrum where we cannot see it. The stellar luminosity, which depends on distance, is hard to constrain, but is probably of the order of 1000 Suns, perhaps a bit greater. The star has apparently recently hit maximum temperature for a roughly-0.6 or so solar mass core and has begun to cool. As the nebula expands at 22 kilometers per second, the star's radiation will shift ever-more toward the visual, eventually rendering it visible to the eye within a larger, fainter nebula. NGC 6741 appears rich in helium, while (unlike NGC 2440 and 7027) normal in carbon content, and confused in nitrogen (for which enrichment is unclear).

Left: Image by H. D. Curtis from Publications of the Lick Observatory, Volume 13, Part III, 1918. Right: ESA/Hubble and NASA.