To better fit with the overall arc of the ME3 story the Ontarom mission now occurs around the time of the coup (when Cerberus is at the peak of its powers, and would be interested in capturing Alliance communication hubs) and Noveria takes place as part of the run up to Cronos station, as the Alliance moves to take back the Horsehead Nebula and finish Cerberus.

The Horsehead Nebula (also known as Barnard 33) is a small dark nebula in the constellation Orion.^[2] The nebula is located just to the south of Alnitak, the easternmost star of Orion's Belt, and is part of the much larger Orion Molecular Cloud Complex. It appears within the southern region of the dense dust cloud known as Lynds 1630, along the edge of the much larger, active star-forming H II region called IC 434.^[3]

The Horsehead Nebula is approximately 422 parsecs or 1375 light years from Earth.^[3][1] It is one of the most identifiable nebulae because of its resemblance to a horse's head.^[4]

History[edit]

The nebula was first recorded in 1888 by Scottish astronomer Williamina Fleming on a photographic plate taken at the Harvard College Observatory.^[5] One of the first descriptions was made by E.E. Barnard, describing it as: 'Dark mass, diam. 4′, on nebulous strip extendingsouth from ζ Orionis.', cataloguing the dark nebula as Barnard 33.^[6]

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Structure[edit]

The dark cloud of dust and gas is a region in the Orion Molecular Cloud Complex where star formation is taking place. It is located in the constellation of Orion, which is prominent in the winter evening sky in the Northern Hemisphere and the summer evening sky in the Southern Hemisphere.

Colour images reveal a deep-red colour that originates from ionised hydrogen gas (Hα) predominantly behind the nebula, and caused by the nearby bright star Sigma Orionis. Magnetic fields channel the gases, leaving the nebula into streams, shown as foreground streaks against the background glow.^[7] A glowing strip of hydrogen gas marks the edge of the enormous cloud, and the densities of nearby stars are noticeably different on either side.

Heavy concentrations of dust in the Horsehead Nebula region and neighbouring Orion Nebula are localized into interstellar clouds, resulting in alternating sections of nearly complete opacity and transparency.^[8] The darkness of the Horsehead is caused mostly by thick dust blocking the light of stars behind it.^[9] The lower part of the Horsehead's neck casts a shadow to the left.^[10] The visible dark nebula emerging from the gaseous complex is an active site of the formation of 'low-mass' stars. Bright spots in the Horsehead Nebula's base are young stars just in the process of forming.

Image gallery[edit]

• Horse Head Nebula and Flame Nebula showing their surrounding nebulosity in Hydrogen alpha as seen in amateur telescopes.

• Horsehead and flame Nebulae 384mm scope Ha-RGB imaged with amateur equipment.

• Interstellar dust of the Horsehead Nebula as revealed by the Hubble Space Telescope.

• The Horsehead Nebula seen by SPECULOOS's Callisto telescope

See also[edit]

Notes[edit]

1. ^Based on the parallax of 2MASS J05405172-0226489, a young stellar object embedded in the 'head' of the nebula.

References[edit]

1. ^ ^abcBrown, A. G. A.; et al. (Gaia collaboration) (August 2018). 'Gaia Data Release 2: Summary of the contents and survey properties'. Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A..616A..1G. doi:10.1051/0004-6361/201833051.
2. ^Arnett, Bill (2000). 'Horsehead Nebula'. Retrieved July 21, 2014.
3. ^ ^ab'VLT Images the Horsehead Nebula'. European Southern Observatory. European Southern Observatory. 25 January 2002. Retrieved 1 March 2019.
4. ^Sharp, Nigel (2014). 'The Horsehead Nebula'. National Optical Astronomy Observatory. Association of Universities for Research in Astronomy. Retrieved July 21, 2014.
5. ^Cannon, Annie J. (June 1911). 'Williamina Paton Fleming'. Science (published June 30, 1911). 33 (861): 987–988. Bibcode:1911Sci..33.987C. doi:10.1126/science.33.861.987. PMID17799863.
6. ^Barnard, E. E. (1919). 'On the dark markings of the sky, with a catalogue of 182 such objects'. Astrophysical Journal. 49: 1–24. Bibcode:1919ApJ..49..1B. doi:10.1086/142439.
7. ^'The Horsehead Nebula/IC434'. National Optical Astronomy Observatory. NOAO. Retrieved 12 May 2014.
8. ^Morgan, W.W.; Lodén, Kerstin (1966). 'Some Characteristics of the Orion Association'. Vistas in Astronomy. 8 (1): 83–88. Bibcode:1966VA...8..83M. doi:10.1016/0083-6656(66)90023-7. ISSN0083-6656.
9. ^Mayo Greenberg, J (2002). 'Cosmic dust and our origins'. Surface Science. 500 (1–3): 793–822. Bibcode:2002SurSc.500.793M. doi:10.1016/S0039-6028(01)01555-2. ISSN0039-6028.
10. ^Nemiroff, R.; Bonnell, J., eds. (21 July 2009). 'The Horsehead Nebula'. Astronomy Picture of the Day. NASA. Retrieved 12 May 2014.

External links[edit]