A Pacman in the night sky

A wealth of details makes this small gas nebula, which was discovered in 1881 by US astronomer Barnard, a popular target for deep sky lovers.

On 26 November 1881, the American astronomer Edward Emerson Barnard encountered a small gas nebula in the constellation of Cassiopeia. He was actually looking for comets whose discovery was rewarded with a prize of 200 dollars, which would be equivalent to 4,400 dollars today. He described his discovery as a faint, diffuse nebula, no more than ten arc minutes across. It was named NGC 281, its central star cluster was named IC 5090.

NGC 281 is a typical emission nebula, whereby the matter contained within – usually hydrogen – is excited by one or more very hot stars. At the centre of the nebula, which is around 9,500 light years away, is the multiple star HD 5005. It is the brightest (around mag. 7.8) and at the same time the hottest member of the cluster, so its high-energy UV radiation provides the majority of the ionization of the hydrogen in the surrounding region.

Such a region usually appears spherical, with the energy source in the centre. With NGC 281, however, about a quarter of the nebula is hidden by dust clouds that are in the foreground. This creates its unusual appearance that is both photographically and visually striking, which reminded astronomers in the 1980s of a famous computer game character, giving this small object the commonly-used nickname the Pacman Nebula. And it really does look like the greedy nebula is snapping at the dust cloud.

NGC 281 is easy to find, close to Cassiopeia's brightest star, α Cas. Under a dark sky, it can be seen with a small aperture telescope or even with binoculars as a faint nebula around a bright central star. HD 5005 is a challenging multiple star system of four suns; the most difficult component is just 1.4" distant. The typical Pacman shape is visible in a medium aperture telescope using a nebula filter: besides the eye-catching nebula, a fainter area becomes visible, revealing the dust cloud’s wedge-shaped incision in the foreground.

A small and compact dark cloud directly to the north of the central star is difficult but rewarding. It’s worth trying to observe this with larger optics from 8 inches upwards.

Author: Nico Schmidt / Licence: Oculum-Verlag GmbH