Sh2-1

Sh2-1 is an emission and reflection nebula in the constellation Scorpius.

It is observed in the northwestern part of the constellation, associated with the group of stars that make up the "head" of Scorpius; invisible with small amateur instruments, it becomes easily visible in long-exposure photos that take the star field around the star π Scorpii, one of the southernmost of the group. Its position in the southern hemisphere means that from northern latitudes it appears relatively low on the southern horizon, with the exception of the tropical belt. The most suitable period for its observation in the evening sky falls between May and September.

Sh2-1 is a large nebula that reflects the blue radiation of its exciting star, π Scorpii; This star also produces an ionization front that makes the cloud shine even independently. The cloud shows an elongated and filamentous appearance and is part of what remains of the large molecular cloud from which the Antares Association (SCO OB2) was formed, a bright OB association comprising the blue stars located north of Scorpius, which in turn is part of the large Scorpius-Centaurus Association, the closest OB association to the Solar System. The part most in contact with the star, with a uniform appearance, is known as vdB 99. The mass of molecular hydrogen contained in Sh2-1 is about 50 M⊙, while the distance, consistent with the average distance of the SCO OB2 association, is estimated at around 200 parsecs (about 650 light years).