Westbrook Nebula (CRL 618) is a

bipolar Bipolar may refer to: Astronomy * Bipolar nebula, a distinctive nebular formation * Bipolar outflow, two continuous flows of gas from the poles of a star Mathematics * Bipolar coordinates, a two-dimensional orthogonal coordinate system * Bipolar ...

protoplanetary nebula which is located in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Auriga AURIGA (''Antenna Ultracriogenica Risonante per l'Indagine Gravitazionale Astronomica'') is an ultracryogenic resonant bar gravitational wave detector in Italy. It is at the Laboratori Nazionali di Legnaro of the Istituto Nazionale di Fisica Nuclea ...

. It is being formed by a star that has passed through the

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

phase and has ceased nuclear fusion at its core. This star is concealed at the center of the nebula, and is ejecting gas and dust at velocities of up to 200 km/s. The nebula is named after William E. Westbrook, who died in 1975. CRL618FastWind2Colored

This nebula began to form about 200 years ago, and primarily consists of molecular gas. The outer part of the nebula is the result of interaction between rapid bipolar outflow and the gas that was ejected when the star was passing through its

asymptotic giant branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

phase. The lobes are inclined about 24° to the line of sight. The energy being radiated from the nebula consists of scattered light from the star at the core, light being emitted from a compact

HII region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

surrounding the star, and energy from the shock-excited gas in the lobes. The dynamics of the molecular gas envelope can be studied by examining microwave

emission-line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency range, ...

spectra from carbon monoxide rotational transitions. These spectra show two distinct velocity components. The narrow cores of the spectral lines show the familiar parabolic profile of a slow (20 km/sec), high optical depth stellar wind from an

Asymptotic Giant Branch The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...

(AGB) star. This material was expelled before the object became a protoplanetary nebula and constitutes the bulk of the nebula's mass. A second component arising from the far faster post-AGB wind is also visible. The fast (>190 km/sec) wind component becomes more prominent in higher frequency, higher energy spectral lines, because the fast wind has a higher temperature than the slow AGB wind. The central star is believed to be of spectral class B0 and has 12,200 times the solar luminosity. The photosphere of this star is now hot enough to have begun ionizing the nebula, and the ionization region is expanding rapidly. The size and rate of growth indicates that ionization began around the year 1971. Once a sizeable portion of the nebula has been ionized, it will have become a planetary nebula. This means the Westbrook Nebula is at a somewhat more advanced evolutionary stage than the

Egg Nebula The Egg Nebula (also known as RAFGL 2688 and CRL 2688) is a bipolar protoplanetary nebula approximately 3,000 light-years away from Earth. Its peculiar properties were first described in 1975 using data from the 11 µm survey obtained with ...

, whose spectral class F5 star has not yet begun to ionize the nebula material.

References

{{reflist Protoplanetary nebulae Auriga (constellation)