The Egg Nebula (also known as RAFGL 2688 and CRL 2688) is a bipolar protoplanetary nebula approximately 3,000

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s away from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

. 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 by

Fritz Zwicky Fritz Zwicky (; ; February 14, 1898 – February 8, 1974) was a Swiss astronomer. He worked most of his life at the California Institute of Technology in the United States of America, where he made many important contributions in theoretical and ...

as a pair of galaxies.) V1610CygLightCurve

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 central

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

, 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 a

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting th ...

of F5. The

photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

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 the Westbrook 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 an

accretion disc An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other ...

. 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 several post-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 of

carbon monoxide Carbon monoxide ( chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

(CO) and

hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound with the formula HCN and structure . It is a colorless, extremely poisonous, and flammable liquid that boils slightly above room temperature, at . HCN is produced on a ...

(HCN). The presence of strong HCN emission indicates that the progenitor AGB star was a

carbon star A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous red giant, whose atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monoxide, which consumes mo ...

Millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union (ITU) designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It lies between the super high frequency band and the ...

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s from 38 molecular species have been detected in the outflow. The CO and HCN spectra have a strong blue-shifted P 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). The Egg Nebula was photographed by the Wide Field and Planetary Camera 2 of the NASA/ESA Hubble Space Telescope. The Egg Nebula emits polarized light which can also be detected visually by a medium-sized telescope.

Gallery

File:Egg Nebula HST.jpg, Image produced from exposures in visible and infrared light from Hubble’s Wide Field Camera 3.

Notes

distance × sin ( diameter_angle / 2 ) = 0.2 ly. radius
14.0 apparent magnitude - 5 * (log₁₀(920 pc distance) - 1) = 4.2 absolute magnitude

References

Hubble Finds Searchlight Beams and Multiple Arcs around a Dying Star
STScI Press release: ''Space Telescope Science Institute''. January 16, 1996. * De Ruyter et al. Keplerian discs around post-AGB objects: a common phenomenon? Astronomy & Astrophysics 448, 641–653 (2006).

External links

* {{Cygnus (constellation) Protoplanetary nebulae Cygnus (constellation) Cygni, V1610