Lacerta OB1

Lore & Background

The Lacerta OB1 association region extends across the southern part of Lacerta, partially spilling into Andromeda, in an area little-known to amateurs due to its faintness. Its stars range from fourth to ninth magnitude; the brightest, 6 Lacertae (magnitude 4.51) and 10 Lacertae (magnitude 4.88), are visible to the naked eye on dark nights, appearing bluish and scattered.

Binoculars reveal more bluish components and a slight concentration of sixth- and seventh-magnitude stars around 10 Lacertae, but no nebulosity is visible with binoculars or small telescopes. The nebular filaments, dark or faintly luminous, appear only in long-exposure photographs, concentrated around 10 Lacertae; some, like vdB 158, are illuminated by nearby stars. The region's strong northern declination favors observation from north of the equator, with best evening viewing in autumn (August to January).

Characteristics and structure

The region contains two clouds with evidence of recent star formation: LBN 437, near the most massive star, and GAL 110-13, a cometary cloud to the northeast. LBN 437 lies on the southeastern edge of a long CO-emitting cloud (Kh 149) and near the ionized nebula Sh2-126 (LBN 428), illuminated by 10 Lacertae. Its densest part, section A, hosts the young Herbig Ae/Be star LkHα 233 (V375 Lacertae), which excites several bipolar Herbig-Haro objects (HH 398, HH 808–814). The area is rich in pre-main-sequence stars, including five T Tauri stars between the cloud and 10 Lacertae, forming a small association spanning 24 arcminutes (about 2.6 parsecs).

Their formation was triggered by compression from the ultraviolet radiation of 10 Lacertae. GAL 110-13, at about 440 parsecs, is illuminated by massive stars HD 222142, HD 222046, and HD 222086, which also light the reflection nebula vdB 158. Its cometary shape, elongated away from the association's center, suggests star formation may have been influenced by a supernova explosion from a massive member, combined with stellar winds; the supernova is supported by the presence of runaway stars.

Stellar components and evolution

The association is divided into two subgroups: Lac OB1a (eastern, age 16–25 million years, 51 members) and Lac OB1b (southwestern, age 12–16 million years, 45 members). Overall, it hosts one O-class star (10 Lacertae), 35 B-class, 46 A-class, one F-class, 8 K-class, 3 M-class, and the carbon star HD 222241. Star formation occurred at distinct times, with the most recent a few million years ago; initial events likely took place in an isolated giant molecular cloud, with the first massive stars generating an ionization front that triggered secondary processes. Some researchers suspect Lac OB1a may not be a true stellar group, as it should contain pre-main-sequence stars if real; young stars in its direction are found toward GAL 110-13.

Reader's Guide

Lacerta OB1 is significant as one of the closest OB associations to the Solar System, offering a nearby laboratory for studying the later stages of star formation in a young stellar group. Its proximity and location far from the galactic plane reduce line-of-sight confusion, making it an easily studied research field. The region illustrates how massive stars can shape their environment: the ultraviolet radiation of 10 Lacertae compressed a molecular cloud, triggering the formation of a small group of pre-main-sequence stars, while stellar winds and a possible supernova shaped the cometary cloud GAL 110-13 and may have produced runaway stars. The presence of two distinct subgroups with different ages and motions provides insight into sequential star formation, where the birth of the first massive stars initiated secondary processes in distant cloud fragments.

The region's legacy lies in its demonstration of how star formation exhausts its natal gas, leaving only faint nebular remnants and a dispersed association of stars. For amateur observers, the association is notable mainly for its stellar components—a handful of bluish stars visible to the naked eye—while the subtle nebular filaments require long-exposure astrophotography to reveal. The ongoing debate over its distance (ranging from 370 to 600 parsecs) underscores the challenges in measuring such systems, but the region remains a key object for understanding the Gould Belt and the evolution of OB associations.