Chemical Composition of Orion Nebula

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rion Nebula (Figure 1), also known as Messier 42¹ (or just M42, and also NGC 1976), is a diffuse nebula² situated in the Milky Way, roughly at 1,500 light years from us (planet Earth), being the closest region of massive star formation. According to data collected by astronomers the Nebula has a mass 2,000 times greater than the Sun and is 24 light-years across. The Orion Nebula is easily detected in the night sky with the naked eye (in an environment with low light pollution), being south of Orion’s Belt³. The Orion Nebula has been the most observed H_II  region due to 3 factors: its apparent size, low reddening and high emission measure. The main goal of this article is to give a chemical perspective of the M42. This cloud of gas is being heated by the intense radiation of the Trapezium stars⁴ embedded within it.

Figure 1: Image taken by SAC (Section of Astronomy, Astrophysics and
 Aeronautics of Academic Section of Coimbra)

1- From the Messier catalogue; 2- A diffuse nebula is H II region, which is a region of interstellar atomic hydrogen that is ionized. 3- Orion’s Belt is a group of 3 stars in the Orion constellation that form the belt of Orion; 4- The trapezium stars are a tight open cluster of stars in the heart of the Orion Nebula.

1-H_II regions

  An HII region is a region in space, that consists of a cloud of partially ionized gas. This region is a nursery of stars, that is, a place where the formation of stars takes place. These regions have a size ranging from one to hundreds of light years. Their density can go from a few to about a million particles per cubic cm. The Orion Nebula was the first object of this type to be discovered.

  These regions have any shape (irregular), that’s because of the irregular distribution of stars inside the nebula. HII regions can give birth to thousands of stars over a period of several million years. With time, that stars will die in a form of a supernova explosion, and strong stellar winds, give irregular form to the HII regions. 

2- The abundance of certain elements

  Most of the dust surrounding the stars that belong, to the M42 is ionized hydrogen. Although most of 

the gas is ionized hydrogen, there are small amounts of other elements. All the abundances presented here, are relative to the abundance of hydrogen (that is, for example, C(carbon)/H(hydrogen)) and in mols. Those elements are Helium (He) with an abundance of 0.1; Carbon (C) with 3.4×10^(-4) abundance; Nitrogen (N) with an abundance of    6.8×10^(-5); Oxygen (O) with an abundance of 3.8×10^(-4) and Silicon (Si) with an abundance of 3.0×10^(-6).

Element	Solar System	M42
He/H	0.1	0.1
C/H	3.6×10-4	3.4×10-4
N/H	1.1×10-4	6.8×10-5
O/H	8.5×10-4	3.8×10-4
Si/H	3.6×10-5	3.0×10-6

 Table 1: Elemental abundances in the solar system and M42. 

3- The origin of those elements 

  What precedes an HII region is a giant molecular cloud (GMC). A GMC is a cold (10-20 K¹) and dense cloud that is mostly composed of molecular hydrogen. As the stars are born inside of a GMC, the most massive ones can have enough energy to ionize the surrounding gas. In this way, a field of ionizing radiation is created around the star that heats up the gas and expands it. As the stars explode and die, more heavy elements are released into the cloud². That is, during the lifetime of the star heavier elements are formed on the star's core by nuclear nuclear fusion. As heavier elements are being formed in the star, its mass increase as well and the star start to collapse in its own height, until it reaches a point that collapses in the form of a supernova, that’s the case for heavy stars. If the star is a low-mass star, will slowly eject its atmosphere via stellar winds, forming a planetary nebula. Also, as the gas is heated and accelerated by the star, some collisions begin to happen, and heavier elements start to form. Like a giant accelerator of particles with a diameter of billion or trillion kilometres. 

  The nuclear fusion inside the star happens in two ways. Hydrogen fusion and Helium fusion. Hydrogen fusion, also called “hydrogen burning”, should not be misunderstood as the chemical combustion of hydrogen in an oxidizing atmosphere. Stellar hydrogen fusion occurs through two, main, processes: the proton-proton chain and the carbon-nitrogen-oxygen (CNO) cycle. With the exception of white dwarfs, most of the stars, are fusing hydrogen by these two processes. A Proton-Proton chain reaction occurs in the core of lower-mass stars, like our sun. 

  Through this process, a helium-4 nucleus is created by a sequence of reactions. The first two protons merge to form a deuterium nucleus (one proton plus one neutron) along with an ejected positron and neutrino. Every time the proton–proton chain reaction happens, roughly 26.73 MeV of energy is released. This process realises energy in the form of gamma rays that will interact with electrons and protons heating the interior of the star. The proton-proton chain reaction is insensitive to temperature since a 10% increase in temperature would increase energy production by 46%. In stars with a higher mass, the dominant energy production process is the CNO cycle. This process consists of a catalytic cycle that uses nuclei of carbon, nitrogen and oxygen as intermediaries to, in the end, produce helium nuclei like the proton-proton chain. A complete CNO cycle releases, approximately 25.0 MeV of energy. As we can see a proton-proton chain releases more energy, why? In the CNO cycle, some energy is lost through neutrino emission. Unlike the proton-proton chain, the CNO cycle is highly sensitive to temperature, since a 10% rise in temperature would increase energy production by 350%.

Left: 'CNO-I cycle. The helium nucleus is released at the top-left step.' Right: ' Proton-Proton chain reaction'

  As the result of hydrogen fusion, stars, accumulate helium in their cores, although their cores don’t become hot enough to initiate helium fusion. When the star leaves the red giant branch³ it starts to fuse helium, after accumulating enough helium in its core to ignite it. In stars with a mass similar to the sun, this begins at the tip of the red giant with a helium flash from a degenerate helium core. Stars are more massive, do it without a flash and execute a blue loop⁴ before reaching the asymptotic giant branch. Although it is called a blue loop, stars on a blue loop are typically yellow giants, possibly Cepheid variables⁵. These stars fuse helium until the core is largely carbon and oxygen. In all cases, helium is fused to carbon by a process called, the triple-alpha process⁶. Then, by the alpha process, oxygen, neon and heavier elements are formed. The alpha process produces, preferentially, elements with even numbers of protons by the capture of helium nuclei. Elements with odd numbers of protons are formed by other fusion pathways. 

1- The kelvin is the base unit of temperature in the International System of Units (SI), having the unit symbol K. 2- Stellar nucleosynthesis. 3- Hertzsprung–Russell diagram. 4- The blue loop is a stage in the life of an evolved star where it changes from a cool star to a hotter one before cooling again. 5-A Cepheid variable is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude. 6-The triple-alpha process is a set of nuclear fusion reactions by which three helium-4 nuclei (alpha particles) are transformed into carbon. 

4- Interstellar Molecules 

  To date, over 100 molecules have been identified, ranging from the simplest diatomic species to long chains like the cyanopolyyne HC₁₁N. Dense cores contain many of the more complex species found so far, like the shock-heated regions of Orion. Investigations into the hot cores of the H_II regions show that there is an abundance of hydrogenated species, such as H₂O, NH₃, CH₃OH and H₂S. However, this raise some questions, that posed a theoretical challenge. Let’s consider the collision of two atoms. They will approach each other with positive total energy. Unless energy can, somehow, be given to a third body, the atoms will simply rebound after their encounter. Here, on Earth, the collision between three atoms can occur with appreciable frequency, but in the vastly more rarefied interior of an H_II cloud is unlikely to happen that phenomenon. It is also possible for the energy sink to be a photon, i.e., for the two atoms to form an excited molecule which radiatively decays to the ground state before it can dissociate.

  Most of the molecules observed contain one or more carbon atoms. While no inorganic species found in space contain more atoms than NH₃, organic exist in complex rings and chains. Since the carbon bonds play such a dominant role on Earth, we can predict that the same should happen in the interstellar environment. Also, in the interstellar environment, the cosmic abundance of oxygen exceeds that of carbon, so it’s not a surprise that the relatively tightly bound CO is the most abundant species after H₂ itself.  

Top left: NH₃ known as ammonia; Top right: H₂S known as Hydrogen sulphide; Bottom: CH₃OH known as methanol or methyl alcohol.

5- References 

Appenzeller; Harwit; Kippenhahn; Strittmatter; Trimble, eds. (1998). Astrophysics Library (3rd ed.).       New York: Springer.

Clayton, D. D. (1968). Principles of Stellar Evolution and Nucleosynthesis. University of  Chicago   Press.

Garner, R. (2019). Messier 42 (The Orion Nebula). Accessed on: 10, June, 2020, in:   https://www.nasa.gov/feature/goddard/2017/messier-42-the-orion-nebula.com

Ian Ridpath. (2012). A Dictionary of Astronomy: H II region (2nd rev. ed.). Oxford University Press.

Peimbert, M. and Peimbert, S. T. (1976). Chemical Composition of the Orion. Nebula. Department of   Physics and Astronomy, University College, London and Instituto de Astronomía UNAM, \Apartado   Postal 70-264, Mexico 20, D. F., Mexico.

Schuler, S. C.; King, J. R.; The, L.-S. (2009), "Stellar Nucleosynthesis in the Hyades nOpen Cluster",   The Astrophysical Journal, 701 (1): 837–849, arXiv:0906.4812, Bibcode:2009ApJ...701..837S,   doi:10.1088/0004-637X/701/1/837

Stahler, S. W. and Palla, F. (2004). The Formation of Stars. WILEY-VCH Verlag. GmbH & Co. KGaA.   Weinh.