November 12, 2013
The Pacman Nebula (NGC 281) in Cassiopeia
Image Credit & Copyright: J-P Metsävainio
The Pacman Nebula (NGC 281), described by Nasa as ‘a busy workshop of star formation’ is an emission nebula and H II region of a little over 100 light-years across that lies in the Perseus Spiral Arm, one of two major spiral arms of the Milky Way galaxy, about 9,500 light-years away from Earth in the northern constellation Cassiopeia (the Seated Queen).
The nebula is known informally as the “Pacman Nebula” for its resemblance, in optical images, to the video game character from early 80′s. In optical images the “mouth” of the Pacman character appears dark because of obscuration by dust and gas, but in the infrared the dust in this region glows brightly.
At the Pacman’s center lies IC 1590, an open star cluster with several young high-mass stars, that contain 8 times the Sun’s mass or more, which generate the nebula’s glow. They have only formed in the last few million years. The brightest member of the cluster is HD 5005, actually a multiple-star system that consists of an 8th-magnitude primary with four companions.
Dense knots of gas and dust are absorbing light in the center of the Pacman Nebula. These knots are so-called Bok globules, named after astronomer Bart Bok, who proposed their existence in the 1940′s. Bok hypothesized that giant molecular clouds, on the order of hundreds of light-years in size, can become perturbed and form small pockets where the dust and gas are highly concentrated. If they capture enough mass, they most commonly result in the formation of single, double or multiple star systems in their cores.
The Pacman Nebula is divided into two subregions: the region in the upper middle of the image (above the “mouth”), which is surrounded by 10-million-degree gas, and contains significant amounts of magnesium, sulfur and silicon. The presence of these elements suggests that a supernova recently went off in that area. In the lower part of the image is a younger region with cooler gas where new stars are being born.
This image is a natural color composition from the emission of ionized elements, R=80%Hydrogen+20%Sulfur, G=100%Oxygen and B=85%Oxygen+15%Hydrogen to compensate otherwise missing H-beta emission. This composition is very close to a visual spectrum.