Orion OB1 (Ori OB1) is a contingent group of several dozen hot giant stars of
spectral types O and B in
Orion. Associated are thousands of lower-mass stars, and a (smaller but significant) number of
protostar
A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostellar phase is the earliest one in the process of stellar evolution. For a low-mass star (i.e. that of the Sun or lower), it lasts about 5 ...
s. It is part of the larger
Orion molecular cloud complex
The Orion molecular cloud complex (or, simply, the Orion complex) is a star-forming region with stellar ages ranging up to 12 Myr. Two giant molecular clouds are a part of it, Orion A and Orion B. The stars currently forming within the complex a ...
. Owing to its relative closeness and complexity it is the most closely studied OB association.
The Orion OB1 consists of the following subgroups:
*Orion OB1a - the group of stars northwest of the Orion Belt stars with an average age of about 12 million years. Within this grouping is another subgroup known as the 25 Orionis group.
They are located near the star of
Bellatrix
Bellatrix is the third-brightest star in the constellation of Orion, positioned 5° west of the red supergiant Betelgeuse (Alpha Orionis). It has the Bayer designation γ Orionis, which is Latinized to Gamma Orionis. With a slightly va ...
.
*Orion OB1b - the three bright stars
ζ Ori (Alnitak),
ε Ori (Alnilam), and
δ Ori (Mintaka) which make up the
asterism known as "Orion's Belt", and minor stars. This group has an average age of approximately 8 million years and is further subdivided into three subgroups.
*Orion OB1c - the stars in
Orion's Sword Orion's Sword is a compact asterism in the constellation Orion. It comprises three stars (42 Orionis, Theta Orionis, and Iota Orionis) and M42, the Orion Nebula, which together are thought to resemble a sword or its scabbard. This group is so ...
that are created by
42 Orionis,
θ Orionis, and
ι Orionis. These stars are about 3-6 million years old.
*Orion OB1d - the stars of the
Orion Nebula
The Orion Nebula (also known as Messier 42, M42, or NGC 1976) is a diffuse nebula situated in the Milky Way, being south of Orion's Belt in the constellation of Orion. It is one of the brightest nebulae and is visible to the naked eye in the nig ...
and
M43 (the youngest stars)
Signatures of stellar
debris disk
A debris disk (American English), or debris disc (Commonwealth English), is a circumstellar disk of dust and debris in orbit around a star. Sometimes these disks contain prominent rings, as seen in the image of Fomalhaut on the right. Debris dis ...
evolution have been detected in the 1a and 1b subgroups.
[J. Hernandez et al., Astrophys. J. 652:472-481 (2006)]
References
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Orion molecular cloud complex
Stellar associations