| Emission nebula | |
|---|---|
| Planetary nebula | |
 NGC 6778 (center) | |
| Observation data:J2000epoch | |
| Right ascension | 19h 18m 25.0s[1] |
| Declination | −01° 35′ 47″[1] |
| Distance | 10,300 ly (3,150[2] pc) |
| Apparent magnitude (V) | 11.9[3] |
| Apparent diameter | 8.5″[4] |
| Constellation | Aquila |
| Designations | NGC 6778 or 6785,[5]HD 180871 |
| See also:Lists of nebulae | |
NGC 6778 is aplanetary nebula (PN) located about 10,300[1] light years away from the Sun in theequatorialconstellation ofAquila. It is positioned 5° to theSSW of the prominent starDelta Aquilae. This nebula was discovered by German astronomerAlbert Marth during the period 1863–1865. English astronomerJohn Herschel may have mistakenly catalogued it as NGC 6785, as nothing can be found now at the coordinates he gave for it. In theNew General Catalogue it was described as a "small, elongated, ill-defined disc".[3]
This planetary nebula displays a disrupted morphology, with a roughly S-shaped feature projecting out of an irregular shell, along with linearjet-like forms. The inner part contains a highly distortedelliptical ring oriented along an east-west direction.[6] The nebula and ring have anangular size of8.5″ and they are about 4,400 years old, while the jets are around 1,700 years old.[4]
At the center of the nebula is a closebinary star system with anorbital period of just 3.68 hours, one of the shortest known among the binary central stars that form a planetary nebulae. Their proximity to each other provides strong evidence that they have passed through acommon-envelope (CE) phase earlier in theirevolution.[7] The pair may now consist of a primary star with0.6 M☉, ared dwarf secondary companion having0.3 M☉, and asemimajor axis of only 0.005 AU (1.1 R☉).[6]
The surrounding nebula is inclined about 85° to the line of sight from the Earth. The torus of this nebula is notable for its high expansion velocity, measured at up to 60–70 km/s. It displays collimated outflows, possibly as a result of the CE behavior of the central stars or else a short-livedaccretion disk. The pair of jets are located along aposition angle of 15°,[7] showing fast outflow withradial velocities of around 100 km/s.[6] The nebula is notable for the rich amount of filamentary "low-ionization structures", or LIS,[7] resembling the wispy structures ofnova explosions.[6] These too may be the result of the CE interaction.[7]
In addition to a higher than normalhelium abundance, the nebula displays unusually strong opticalrecombinationlines fromheavier elements; one of the highest known among PNe. These may be the result of a very latethermal pulse, or "final flash", following the initial ejection of the stellar envelope that formed the nebula. The pulse would have ejected chemically-enriched material into the old nebula, changing its abundances.[8]
Media related toNGC 6778 at Wikimedia Commons