The Cosmic Checkmark

This long, gently curved streak crossing the field is LBN 534, an isolated molecular cloud in the constellation Andromeda, nicknamed the Cosmic Checkmark for its shape. It lies about 1,400 light years away. At its southern tip glows a small blue patch, the reflection nebula vdB 158: there, dust grains scatter the light of a nearby hot star, HD 222142. That scattered light looks blue for the same reason the daytime sky does, scattering favours short wavelengths.

The rest of the cloud is a filament of cold dust. Where it is lit obliquely it appears as faint bluish nebulosity; where it is dense it shows as shadow, blocking the stars behind it. This interplay of reflection and absorption traces the object's long, characteristic curve.

That comet-like outline is a genuine scientific puzzle. The most commonly favoured explanation is a collision between two interstellar clouds, whose compression zone would have forged the elongated shape. But there is another lead: the cloud points toward the massive-star association in Lacerta (Lac OB1), and it could equally have been sculpted by supernova shock waves or by radiation from those stars. The question remains open.

A small intruder hides in the field: the planetary nebula PK 110-12.1, a tiny blue-green disc. Despite its apparent closeness, it sits far beyond the cloud, some 15,000 light years away, roughly ten times more distant. Its presence is only a line-of-sight coincidence. The image totals 8 h 30 min of broadband exposure, acquired remotely from Texas.