The Orion Nebula – M42

A joint contribution by Werner Wöhrmann, Dr. Gerold Holtkamp, Thomas Grunge and

Dr. Achim Tegeler, February 23, 2024

The Orion Nebula or M42 (also NGC 1976) is probably one of the most striking and thus also well-known objects in the night winter sky. It was first discovered at the beginning of the 17th century by Nicolas Claude Fabri de Peiresc in 1610 and described in his records in 1611 [1]. Galileo Galilei also pointed his telescope at the Orion Nebula and was the first to describe the star arrangement of the Trapezium. He mentioned three Trapezoid stars, but not the nebula.

Christiaan Huygens published the first drawing of the Orion Nebula in 1659. It was not until 1769 that Charles Messier added the Orion Nebula with the number 42 to his catalog that was published 1771 [2]. [2].

Henry Draper took the first photo of the nebula on his photographic plates in 1880 [3].

In the middle of the 19th century, spectroscopic investigations revealed the gaseous nature of the nebula [4]. Hydrogen had the largest share. The composition of the nebula is now well known; in addition to hydrogen and oxygen, helium makes up 10% and other elements are present in trace amounts [5].

M42 is an emission nebula, meaning it emits light itself. It is located in the so-called Orion Arm of our Milky Way. This is the name given to the spiral arm of our spiral galaxy, in which we and our sun also lie. The Orion Nebula can be seen with the naked eye south of the belt stars of the constellation Orion. It is part of the Orion molecular cloud complex, a collection of many objects distributed throughout the constellation. All of these objects are not only apparently adjacent but also physically adjacent by astronomical standards.

The distance of M42 is now given as 1,350 light years at an extent of 24 light years.

The hydrogen in the nebula is excited to glow by the star Theta 1 Orionis C (HD 37022), a member of the open cluster Trapezium- especially in the wavelength around 656 nm, the H-alpha line of hydrogen. Within this open star cluster are the four brightest stars (A-D), which visually form the trapezoid shape (from the geometrical point of view a trapezoid always shows two parallel sides, which is not the case here) and can be seen with small telescopes; With the naked eye, the trapezoid is only perceived as a single star. However, these four stars are each multiple systems made up of up to six individual stars, so that the cluster represents a complex structure made up of stars, some of which are huge giants with up to 45 solar masses. [6].

The Trapezium (with the stars Theta Orionis A-E) at the center of the Orion Nebula – Photo: Werner Wöhrmann

The above-mentioned brightest star of the Trapezium, Theta 1 Orionis C, has 34 solar masses, belongs to the spectral class O7, has a surface temperature of over 45,500 K and a luminosity of 250,000 suns. It emits a stellar wind of extremely hot plasma that travels at over 1,000 km/s, exciting the surrounding hydrogen and also producing X-rays - a blue giant on the main sequence of the Hertzsprung Russel diagram [7] and therefore a real representative of superlatives [8].

The Trapezium Cluster is a subgroup of the Large Orion Nebula Cluster and currently contains approximately 3,500 stars less than 3 million years old. By astronomical standards, these stars are still very young. In fact, new stars are still constantly being formed there from the material from the Orion Nebula. The Orion Nebula is therefore considered one of the most active star-forming regions in the immediate vicinity of our solar system. Investigations in 2001 were able to detect a large number of brown dwarfs and objects with planetary masses down to 3 Jupiter masses [9] [10].

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[1] Guillaume Bigourdan: La decouverte de la nebuleuse d’Orion par Peiresc. In: Comptes rendus de l’Académie des Sciences. Band 162, 1916, S. 489–490

[2] Messier Catalogue https://archive.is/Rh9Y3

[3] https://commons.wikimedia.org/wiki/File:Orion_1880.jpg

[4] https://royalsocietypublishing.org/doi/pdf/10.1098/rspl.1865.0011

[5] https://arxiv.org/pdf/astro-ph/0408249v1.pdf

[6] S. Simón-Díaz et al.: Detailed spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. 2006, doi:10.1051/0004-6361:20053066

[7] Hertzsprung-Russel-Diagram https://en.wikipedia.org/wiki/HertzsprungRussell_diagram

[8] https://www.mpia.de/news/wissenschaft/2007-10-orion-nebel

[9] https://www.eso.org/public/news/eso0124/#3

[10] https://arxiv.org/pdf/2004.13915.pdf