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.
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 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 spectra from 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 ...
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
In physics, optical depth or optical thickness is the natural logarithm of the ratio of incident to ''transmitted'' radiant power through a material.
Thus, the larger the optical depth, the smaller the amount of transmitted radiant power throug ...
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
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 ...
B0 and has 12,200 times the solar luminosity
The solar luminosity (), is a unit of radiant flux ( power emitted in the form of photons) conventionally used by astronomers to measure the luminosity of stars, galaxies and other celestial objects in terms of the output of the Sun.
One nomin ...
. 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
A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives.
The term "planetary nebula" is a misnomer because they are unrelate ...
. 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)