The Egg Nebula (also known as RAFGL 2688 and CRL 2688) is a bipolar

protoplanetary nebula A protoplanetary nebula or preplanetary nebula (PPN, plural PPNe) is an astronomical object which is at the short-lived episode during a star's rapid evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary ne ...

approximately 3,000

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s away from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

. 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 an ...

as a pair of galaxies.)

Description

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 a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

, 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 the ...

of F5. The

photosphere The photosphere is a star's outer shell from which light is radiated. It extends into a star's surface until the plasma becomes opaque, equivalent to an optical depth of approximately , or equivalently, a depth from which 50% of light will esc ...

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 Westbrook Nebula (CRL 618) is a bipolar nebula, bipolar protoplanetary nebula which is located in the constellation Auriga (constellation), Auriga. It is being formed by a star that has passed through the red giant phase and has ceased nuclear fusi ...

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 most frequently a star. Friction, uneven irradiance, magnetohydrodynamic effects, and ...

. 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 A post-AGB star (pAGB, abbreviation of post-asymptotic giant branch) is a type of luminous supergiant star of intermediate mass in a very late phase of stellar evolution. The post-AGB stage occurs after the asymptotic giant branch (AGB or second-as ...

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 poisonous, flammable gas that is colorless, odorless, tasteless, and 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 (formerly known as prussic acid) is a chemical compound with the chemical formula, formula HCN and structural formula . It is a highly toxic and flammable liquid that boiling, boils slightly above room temperature, at . HCN is ...

(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 Stellar atmosphere, atmosphere contains more carbon than oxygen. The two elements combine in the upper layers of the star, forming carbon monox ...

Millimeter wave Extremely high frequency (EHF) is the International Telecommunication Union designation for the band of radio frequencies in the electromagnetic spectrum from 30 to 300 gigahertz (GHz). It is in the microwave part of the radio spectrum, between t ...

spectral line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency ...

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 The Wide Field and Planetary Camera 2 (WFPC2) is a camera formerly installed on the Hubble Space Telescope. The camera was built by the Jet Propulsion Laboratory and is roughly the size of a baby grand piano. It was installed by servicing mission ...

of the NASA/ESA Hubble Space Telescope. The Egg Nebula emits polarized light which can also be detected visually by a medium-sized telescope. Egg Nebula HST

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 IRAS catalogue objects