Orion the Hunter is a familiar constellation for us. Orion is visible from December to early March from most part of the earth. The constellation get its name from the ancient greeks Orion, the hunter. Orion Constellation can be recognised by “Orion Belt”.
Orion Belt is the most recogeriesable part of Orion Constellation in night sky. Near to Orion’s belt there is a row fainter stars (below the orion belt). These group of stars represents Orion’s sword and the stars are present in M42, Orion Nebula.
This area is an extraordinary active site of star formation also. Over the years, there is no other region has received such intense astronomical scrutiny, nor been studied with such a variety of observational tools. There is a giant molecular cloud complex present in this region. We call it Orion Molecular Cloud Complex.
These clouds are mainly made up of molecular hydrogen. But other elements are also present such as CO, HII, Dust grains etc. total mass of cloud complex is around 10⁵ Solar mass.
Regions of Cloud complex and Orion nebula
The cloud complex is mainly divided into two regions. Orion A and Orion B. But it does not over here, there in other branches also.
Regal is the seven brightest star in the night sky are the brightest star in orion Nebula. It is 200 thousand times luminous than our Sun. There is a bright central region which is the home of happiest stars in the Nebula. These stars are called trapezium( because of its shape). There are two binary star system in the trapezoid discovered by Hubble Telescope ( so trapezium contains 4 to 6 stars). Each one of these stars is 15 to 30 times massive than sun and at a distance of about 1 parsec from one another).
The region- Orion nebula (M42) – which is an HII region is visible for us. This region is formed because of Trapezium stars. These are O and B type stars. So, the photons from these stars can dissociate the hydrogen molecule and ionize to form HII (H+). So, this region is an HII region. The HII region extends in some 10s of parsec. This is because of that the electrons and protons are highly energetic. But recombinations are possible at some point in time. But it will dissociate again soon. At a large distance from the star, the intensity of ionizing photons is low. So, there dissociation and recombination are possible. Here the amount of HII is less. The photons can be emitted from this region.
Orion Nebula can be recognised by Orion belt. It lies roughly 1,300 – 1500 light-years from us. At some 12 to 16 light-years in radius. By the help of binocular or small telescope we cannot is that the centre stars are hazy clearly define the luminosity or shine of the great Orion Nebula, M42.
We cannot map the molecular hydrogen directly. It is because that Hydrogen does not have an electric or magnetic dipole moment. So, Molecular hydrogen does not give rotational spectra. So, we use some other molecules for this. The next abundant molecule after H2 is CO. So, we use CO to map H2 clouds. We mainly use 2.6 mm line of ¹²CO for this. It is the radiation emitting because of rotational transition J=1 -> J=0 of {}^12CO molecule. If there is one CO molecule it means there will be 10^4 Hydrogen molecules.
Star formation is happening in the dense regions of molecular clouds. We map the dense cores by using 1.3 mm line of NH3 molecule.
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Written by:
Varghese Reji
Integrated Ph.D
Department of Astronomy and Astrophysics
TIFR, Mumbai
Chinnumol Sebastian
M. Sc. Physics
Assumption College, Changanasherry
Poetry of Reality 