Anne’s Image of the Day: The Westbrook Nebula

December 6, 2013

The Westbrook Nebula, a protoplanetary nebula in Auriga

Image Credit: ESA/Hubble & NASA

The Westbrook Nebula (also known as CRL 618, PK 166-06 1 and AFGL 618) is a bi-polar, finger-shaped protoplanetary nebula that began to form about 200 years ago. The nebula, which is named after William E. Westbrook, who died in June 1975 at the age of 26, lies some 5,000 light-years away from Earth in the northern constellation of Auriga (the Charioteer).

Despite their name, protoplanetary nebulae (like planetary nebulae) have nothing to do with planets: they are clouds of dust and gas formed from material shed by an aging central star with a mass up to eight times that of the Sun. For such a star death is a long process. After billions of years, the hydrogen fuel that powers the star begins to run out. The star balloons to great size and becomes a red giant. Eventually, however, the star collapses back on itself. This increases the temperature at its core and most of the stars material is catapulted into space, enveloping itself in expanding clouds of gas, but the core is not yet hot enough to make the gas itself glow on its own. Instead, the gas is merely reflecting the light from the star.

But as the star continues to evolve, it becomes hot enough to emit strong ultraviolet light. At that point it will have the power needed to make the gas glow, and will become a real full-fledged planetary nebula. But before the nebula begins to shine, fierce winds of material ejected from the star will continue to shape the surrounding gas into intricate patterns. Then the star cools down and all that is left after this process is the exposed, hot and dead core, known as a white dwarf.

A protoplanetary nebula is a relatively short-lived phenomenon, what means there are relatively few of them in existence at any one time. Moreover, they are very dim, requiring powerful telescopes to be seen. This combination of rarity and faintness means they were only discovered comparatively recently. It is a rare opportunity for astronomers to find one and so learn more about them and observe the beginning of the formation of planetary nebulae, as well as the short and poorly understood phase of stellar evolution when a red giant collapses into a white dwarf.

The Westbrook Nebula is being formed by a star that has passed through the red giant phase and has ceased nuclear fusion at its core. The star, with 12,200 times the luminosity of our Sun, is hidden at the center of the nebula, while its light escapes at the poles of a thick, dark, equatorial lane of obscuring dust. Here the dust lane is thinner, so symmetric nebulous regions can form above and below the equatorial plane.

This strange and irregular nebula primarily consists of molecular gas, including toxic gases such as carbon monoxide and hydrogen cyanide. The complex finger-like features are protruding at different angles from the poles – telltale signs of massive outflows of material spewing out at velocities of up to 200 kilometers per second. Smoke-ring features are seen in the “fingers”, which may arise from powerful shocks generated as gas is ejected out at irregular intervals and pushes out into the surroundings.

The Westbrook Nebula is evolving so rapidly that we can literally watch its changes: its “fingertips” have shown strong brightness variations during the past decade.

This image was prepared from many separate exposures taken using Hubble’s newest camera, the Wide Field Camera 3. Exposures through a green filter (F547M) were coloured blue, those through a yellow/orange filter (F606W) were coloured green and exposures through a filter that isolates the glow from ionised nitrogen (F658N) have been coloured red. Images through filters that capture the glows from singly and doubly ionised sulphur (F673N and F953N) are also shown in red. The total exposure times were about nine minutes through each filter.