This peculiar belted nebula is ionised by two giant stars: the O9.5 III
HD 37737 and an anonymous B0 II class star according to Avedisova.
HD 37737 may be a runaway star and is a close binary with a period of 7.84 days.
Sh 2-231 to Sh 2-235 form a complex of HII regions located together in the Perseus arm in the outer galaxy. This complex may be associated with an old (330 thousand years) supernova remnant,
FVW 172.8+1.5.
There is a giant molecular cloud,
G174+2.5, containing Sh 2-231, Sh 2-232, Sh 2-233, and Sh 2-235 that is at a distance of about 1800 pc.
Ionised by the O9.5III giant HD 37737 and an anonymous B0 II giant. [
1]
Sh 2-231 to Sh 2-235 form a complex of HII regions located together in the Perseus arm in the outer galaxy. This complex may be associated with an old (330 thousand years) supernova remnant,
FVW 172.8+1.5. [
2]
There is a giant molecular cloud containing Sh 2-231, Sh 2-232, Sh 2-233, and Sh 2-235 that is at a distance of about 1800 pc.[
3]
Notes
1. ^ Avedisova, V. S. & Kondratenko, G. I. (1984). "Exciting stars and the
distances of the diffuse nebulae", Nauchnye Informatsii, Vol. 56, 59.
[1984NInfo..56...59A]
2. ^ Kang, Ji-hyun, Koo, Bon-Chul, & Salter, Chris (2012). "An Old Supernova
Remnant within an H II Complex at l ≈ 173°: FVW 172.8+1.5", The
Astronomical Journal, Vol. 143, 75. [2012AJ....143...75K]
3. ^ Shepherd, D. S. & Watson, A. M. (2002). "A Detailed Study of G173.58+2.45:
an Intermediate-Mass Star-forming Region", The Astrophysical Journal, Vol.
566, 966-973. [2002ApJ...566..966S]