NGC 281 · Pacman Nebula — how young stars eat their own birth cloud

At first glance the story is quickly told: a red disk, a dark notch for a mouth — there is the Pacman Nebula. The nickname arrived only with the character from the 1980s video game. The far better twist, though, is that the impression of eating fits the physics remarkably well: inside NGC 281, young massive stars really are destroying the cloud they came from.

Barnard saw no Pacman yet. Edward Emerson Barnard found the large, very diffuse nebulosity in the early 1880s with a small refractor and published it in 1884; John Dreyer entered it as NGC 281 in the New General Catalogue in 1888. On visual observations like those, the later video-game figure was barely recognizable. Only photography turned the dark bay in front of the bright emission nebula into that memorable profile. The catalog name Sh2-184 denotes the same H II region, while IC 1590 is the young open cluster at its center.

The lamp sits in the middle of the cloud. The hottest stars of IC 1590 emit so much ultraviolet radiation that they ionize hydrogen and make the nebula glow. Their winds and radiation simultaneously push outward against the surrounding gas. That creates the bright shell; denser parts of the molecular cloud resist longer and remain standing as dark knots, spikes and pillars. The apparent mouth is therefore not empty. A good part of it consists of foreground dust swallowing the light behind it.

The black islands are possible stellar nurseries. Right at the bright center, compact Bok globules stand as silhouettes against the ionized gas. Some such cold dust knots can collapse under their own gravity and form new stars. Others are eroded by the nearby O stars faster than they can collapse. The image thus holds two opposite processes side by side: stars form out of the cloud — and the same stars then dissolve it again.

NGC 281W is more than a file name. The “W” denotes the western molecular cloud of the complex, in which water masers have been observed as very precise distance markers. Their trigonometric parallax gives 2.82 ± 0.24 kiloparsecs, about 9,200 light-years. At that distance NGC 281 lies roughly 300 parsecs, almost 1,000 light-years, outside the thin galactic disk. The complex may be a fragment of a much older, expanding superbubble. The possible causal chain therefore spans several generations: earlier supernovae compressed the large cloud, the massive stars of IC 1590 formed from it, and their radiation is today triggering the next round of star formation at the cloud edges.

What this image shows. Red is Hα, blue-green O III — the two windows of the dual-band filter, which shuts out the brightened sky background. With only a quarter of an hour of exposure, the image is a sharp portrait of the bright center, not a deep survey of the whole molecular cloud — in the faint outer regions it stays correspondingly grainy.