The Orion

Orion (constellation)

Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...

 OB1 stellar association is a contingent group of several dozen hot giant stars of spectral types

Stellar classification

In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...

 O and B. Associated are thousands of lower-mass stars, and a (smaller but significant) number of protostars

Protostar

A protostar is a large mass that forms by contraction out of the gas of a giant molecular cloud in the interstellar medium. The protostellar phase is an early stage in the process of star formation. For a one solar-mass star it lasts about 100,000 years...

. It is part of the larger Orion Molecular Cloud Complex

Orion Molecular Cloud Complex

The Orion Molecular Cloud Complex refers to a large group of bright nebula, dark clouds, and young stars located in the constellation of Orion. The cloud itself is between 1,500 and 1,600 light-years away and is hundreds of light-years across...

. Owing to its relative closeness and complexity it is the most closely studied OB association.

The Orion OB1 association 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.
• Orion OB1b - the three bright stars ζ Ori (Alnitak), ε Ori
  Epsilon Orionis
  Alnilam is a large blue supergiant star in the constellation of Orion. Its Flamsteed designation is 46 Orionis....
  
   (Alnilam), and δ Ori
  Delta Orionis
  Delta Orionis , traditionally known as Mintaka , is a star some 900 light years distant in the constellation Orion. Together with Zeta Orionis and Epsilon Orionis , the three stars make up the belt of Orion, known by many names across many ancient cultures...
  
   (Mintaka) which make up the asterism
  Asterism (astronomy)
  In astronomy, an asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one. Like constellations, asterisms are in most cases composed of stars which, while they are visible in the same general direction,...
  
   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
  The Orion's Sword is an astronomical asterism in the constellation Orion. It comprises three stars under the prominent asterism, Orion's Belt. M42, the Orion Nebula is located in the center...
  
  , 3-6 million years old
• Orion OB1d - the stars of the Orion Nebula
  Orion Nebula
  The Orion Nebula is a diffuse nebula situated south of Orion's Belt. It is one of the brightest nebulae, and is visible to the naked eye in the night sky. M42 is located at a distance of and is the closest region of massive star formation to Earth. The M42 nebula is estimated to be 24 light...
  
   and M43 (the youngest stars)

Signatures of stellar debris disk

Debris disk

A debris disk 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 disks have been found around both evolved and young stars, as well as at least one debris disk in orbit around a...

evolution have been detected in the 1a and 1b subgroups.