Swan song for stars and cameras

The swirling, paint-like clouds in the darkness of space in this stunning image seem surreal, like a portal to another world opening up before us. In fact, the subject of this ESA/Hubble Picture of the Week is very real. We are seeing vast clouds of ionised atoms and molecules, thrown into space by a dying star. This is a planetary nebula named Kohoutek 4-55, a member of the Milky Way galaxy situated just 4600 light-years away in the constellation Cygnus (the Swan).

Planetary nebulae are the spectacular final display at the end of a giant star’s life. Once a red giant star has exhausted its available fuel and shed its last layers of gas, its compact core will contract further, enabling a final burst of nuclear fusion. The exposed core reaches extremely hot temperatures, radiating very energetic ultraviolet light that energises the enormous clouds of cast-off gas. Molecules in the gas are ionised and glow brightly; here, red and orange indicate nitrogen molecules, green is hydrogen and blue shows oxygen in the nebula. Kohoutek 4-55 has an uncommon, multi-layered form: a bright inner ring is surrounded by a fainter layer of gas, all wrapped in a broad halo of ionised nitrogen. The spectacle is bittersweet, as the brief phase of fusion in the core will end after mere tens of thousands of years, leaving a white dwarf that will never illuminate the clouds around it again.

This image itself is also a swan song, the final work of one of Hubble’s instruments: the Wide Field and Planetary Camera 2 (WFPC2). Installed in 1993 to replace the original Wide Field and Planetary Camera, WFPC2 was responsible for some of Hubble’s most enduring images and fascinating discoveries. It in turn was replaced by the Wide Field Camera 3 in 2009, during Hubble’s final servicing mission. The data for this image were taken a mere ten days before the instrument was removed from the telescope, as a fitting send-off for WFPC2 after 16 years’ work. The latest and most advanced processing techniques have been used to bring the data to life one more time, producing this breathtaking new view of Kohoutek 4-55.

[Image Description: A planetary nebula, a glowing shell of material thrown off by a star. A small central region of greenish clouds is encircled by a glowing, jagged ring, like a hole torn in fabric. A band of silvery-blue clouds outside this is again encircled by a larger, fainter yellow ring of gas. Puffy, smoky clouds of orange and red gas billow out from there into a large oval nebula, fading into the dark background of space.]