| Reflection nebula | |
|---|---|
| Protoplanetary nebula | |
 RAFGL 2688, the Egg Nebula | |
| Observation data:J2000epoch | |
| Right ascension | 21h 02m 18.75s[1] |
| Declination | +36° 41′ 37.8″[1] |
| Distance | approx[2]3000 ly (920 pc) |
| Apparent magnitude (V) | 14.0[1] |
| Apparent dimensions (V) | 30″ × 15″ |
| Constellation | Cygnus |
| Physical characteristics | |
| Radius | 0.2[a] ly |
| Absolute magnitude (V) | 4.2[b] |
| Designations | RAFGL 2688,[1] The Egg,[1] Cygnus Egg[1]V1610 Cyg |
| See also:Lists of nebulae | |
TheEgg Nebula (also known asRAFGL 2688 andCRL 2688) is abipolarprotoplanetary nebula approximately 3,000light-years away fromEarth. Its peculiar properties were first described in 1975 using data from the 11 μm survey obtained with sounding rocket by Air Force Geophysical Laboratory (AFGL) in 1971 to 1974. (Previously, the object was catalogued byFritz Zwicky as a pair of galaxies.)
The Egg Nebula's defining feature is the series of bright arcs and circles surrounding the central star. A dense layer of gas and dusts enshrouds the centralstar, blocking its direct light from our view. However, the light from the central star penetrates the thinner regions of this dusty enclosure, illuminating the outer layers of gas to create the arcs seen in this resplendent image (Hubble Site). Spectra of the starlight scattered by the dust reveal that the central star has aspectral type of F5.[4] Thephotosphere of an F5 star is about 900 K hotter than that of the Sun, but it is still not hot enough to have begun ionizing the nebula. Therefore the Egg Nebula is at a slightly earlier evolutionary stage than theWestbrook Nebula whose spectral type B0 central star has just recently begun to ionize the nebula.
The dusty enclosure around the central star is very likely a disc. The bipolar outflows in the image indicate that the system has angular momentum, which is very likely generated by anaccretion disc. In addition, a disc geometry would account for the varying thickness of the enclosure that allows light to escape along the disc's axis and illuminate the outer layers of gas, but still blocks it from our direct view along the disc edge. Although dusty discs have been confirmed around severalpost-AGB objects (S. De Ruyter et al., 2006), a disc around the Egg Nebula is yet to be confirmed.
The Egg Nebula shows strong microwave emission from rotational transitions ofcarbon monoxide (CO) andhydrogen cyanide (HCN).[5][6] The presence of strong HCN emission indicates that the progenitor AGB star was acarbon star.Millimeter wavespectral lines from 38 molecular species have been detected in the outflow.[7] The CO and HCN spectra have a strong blue-shiftedP Cygni absorption feature, and show the presence of a ~100 km/sec high velocity wind inside of the remnant AGB wind (which is expanding at 18 km/sec).[8]
The Egg Nebula was photographed by theWide Field and Planetary Camera 2 of theNASA/ESA Hubble Space Telescope.
The Egg Nebula emits polarized light[9] which can also be detected visually by a medium-sized telescope.[10]
![Image produced from exposures in visible and infrared light from Hubble’s Wide Field Camera 3.[2]](/mt/?noimg=&dark=on&url=http%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aEgg_Nebula_HST.jpg)
