Tuesday, June 8, 2021

The Westbrook nebula

The Westbrook Nebula (also known as CRL 618 and PK166-06) is a bipolar protoplanetary nebula, located at a distance of around 11,500 light years in the constellation of Auriga. The nebula was named after William Westbrook.

It was formed when nuclear fusion in a red giant ceased and the star ejected its outer envelope. It is believed that the nebula was formed around 200 years ago, thus its core is still very hot. It is a B0-type star that is more than 12,000 times brighter than the Sun. Due to this, in the near future, the central star will be able to ionize its surrounding environment.

The current observed emission in the nebula is due to scattered light from the central star and the gas that is excited, due to the shock-waves in the lobes.

The outer part of the nebula resulted from the interaction between the material that was ejected towards the end of the star’s life, and the material that was ejected more recently in the form of a rapid bipolar outflow.

Observations have shown two velocity components in the expanding gas. The first one is “slow” moving gas at 20 km/sec that is due to material that was ejected in the form of a stellar wind from the progenitor star. This is the main mass component of the nebula and it was ejected prior to the protoplanetary nebula phase. The second component is the fast one (> 190 km/sec) and it’s from material that was ejected at the beginning of the protoplanetary phase.

Image: Composite optical image of the Westbrook Nebula from the Hubble Space Telescope. The image was created using broadband filters that are centred at 547 nm (V-band, blue), 606 nm (V-band, green), together with narrow band filters that are focused on the emission of ionized nitrogen ([N II] 658 nm, red) and ionized sulfur ([S II] 673 nm and 953 nm, red),

Image Credit: ESA/Hubble & NASA

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