Orion OB1

{{Short description|Hot giant star group}}

{{Infobox stellar association

| name = Orion OB1

| image = 250px

| caption =

| credit =

| constellation = Orion

| epoch = J2000

| ra = {{RA|05|36}}{{cite journal| title=The Orion OB1 association. 1: Stellar content| author=Brown, A. G. A. | author2=de Geus, E. J. | author3=de Zeeuw, P. T.| journal=Astronomy and Astrophysics| date=September 1994| volume=289| issue=1| pages=101–120| issn=0004-6361| bibcode=1994A&A...289..101B|arxiv = astro-ph/9403051 }}

| dec = {{DEC|-01|12}}

| dist_ly = 1040-1630 ly

| dist_pc = 320-500 pc

| span_ly =

| span_pc =

| uvw =

| age_myr = ~10 {{Cite journal |last1=Briceño |first1=César |last2=Calvet |first2=Nuria |author2-link=Nuria Calvet|last3=Hernández |first3=Jesús |last4=Vivas |first4=A. Katherina |last5=Mateu |first5=Cecilia |last6=Downes |first6=Juan José |last7=Loerincs |first7=Jaqueline |last8=Pérez-Blanco |first8=Alice |last9=Berlind |first9=Perry |last10=Espaillat |first10=Catherine|author10-link=Catherine Espaillat |last11=Allen |first11=Lori|author11-link=Lori Allen (astronomer) |last12=Hartmann |first12=Lee|last13=Mateo|first13=Mario|last14=Bailey|first14=John I. III|date=30 January 2019 |title=The CIDA Variability Survey of Orion OB1. II Demographics of the Young, Low-mass Stellar Populations |journal=The Astronomical Journal |volume=157 |issue=2 |page=85 |arxiv=1805.01008 |bibcode=2019AJ....157...85B |doi=10.3847/1538-3881/aaf79b |issn=1538-3881 |s2cid=119085444 |doi-access=free }}

| mass_msol = >{{val|8.0|e=3}}

| num_members =

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}}

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:{{cite journal

| title=The O Associations in the Solar Neighborhood

| author=Blaauw, Adriaan

| journal=Annual Review of Astronomy and Astrophysics

| date=1964

| volume=2

| issue=1

| pages=213–246

| doi=10.1146/annurev.aa.02.090164.001241

| bibcode=1964ARA&A...2..213B}}

• 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.{{cite journal

| author=Briceño, César

| author2=Hartmann, Lee

| author3=Hernández, Jesús

| author4=Calvet, Núria|author4-link=Nuria Calvet

| display-authors=etal

| title=25 Orionis: A Kinematically Distinct 10 Myr Old Group in Orion OB1a

| journal=The Astronomical Journal

|date=June 2007

| volume=661

| issue=2

| pages=1119–1128

| bibcode=2007ApJ...661.1119B

| doi=10.1086/513087|arxiv = astro-ph/0701710 | s2cid=118950935

}}{{cite journal

| author=Harper, Graham M.

| author2=Brown, Alexander

| author3=Guinan, Edward F.

| title=A New VLA-Hipparcos Distance to Betelgeuse and its Implications

| journal=The Astronomical Journal

|date=April 2008

| volume=135

| issue=4

| pages=1430–40

| bibcode=2008AJ....135.1430H

| doi=10.1088/0004-6256/135/4/1430| doi-access=free

}} They are located near the star of Bellatrix.

• 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.{{cite journal

| title=A photometric study of the Orion OB 1 association. I - Observational data

| author=Warren, W. H. Jr. | author2=Hesser, J. E.

| journal=Astrophysical Journal Supplement Series

| date=June 1977

| volume=34

| pages=115–206

| bibcode=1977ApJS...34..115W

| doi=10.1086/190446}}{{cite journal

| title=A photometric study of the Orion OB 1 association. III - Subgroup analyses

| author=Warren, W. H. Jr. | author2=Hesser, J. E.

| journal=Astrophysical Journal Supplement Series

| date=April 1978

| volume=36

| pages=497–572

| bibcode=1978ApJS...36..497W

| doi=10.1086/190510}}

• Orion OB1c - the stars in Orion's Sword 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 and M43 (the youngest stars)

Signatures of stellar debris disk evolution have been detected in the 1a and 1b subgroups.N. Calvet et al., The Astronomical Journal 129:935–946 (2005)J. Hernandez et al., Astrophys. J. 652:472-481 (2006)

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