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
protostars. It is part of the larger
Orion molecular cloud complex. 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
25 Orionis, less commonly known by its Bayer designation Psi1 Orionis (ψ1 Orionis, ψ1 Ori) is a fifth-magnitude star in the constellation Orion (constellation), Orion. It lies among a dense star cluster, cluster of low-mass pre-main-se ...
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
42 Orionis is a class B1V (blue main-sequence) star in the constellation Orion. Its apparent magnitude is 4.59 and it is approximately 900 light years away based on parallax.
The primary star, Aa, has one spectroscopic companion Ab of m ...
,
θ Orionis The Bayer designation θ Orionis (Theta Orionis) is shared by several astronomical objects, located near RA DEC :
* θ1 Orionis (41 Orionis), the Trapezium Cluster, an open star cluster, the ''Orion OB Association 1d''
** θ1 Orionis A (41 Ori ...
, and
ι Orionis
Iota Orionis (ι Orionis, abbreviated ι Ori) is a multiple star system in the equatorial constellation of Orion the hunter. It is the eighth-brightest member of Orion with an apparent visual magnitude of 2.77 and also the brightest m ...
. 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 evolution have been detected in the 1a and 1b subgroups.
[J. Hernandez et al., Astrophys. J. 652:472-481 (2006)]
See also
*
Orion's belt
References
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Orion molecular cloud complex
Stellar associations