The Northern Hemisphere during the winter months.
The Orion Nebula contains one of the brightest star clusters in the night sky. With a magnitude of 4, this nebula is easily visible from the Northern Hemisphere during the winter months. It is surprising, therefore, that this region was not documented until 1610 by a French lawyer named Nicholas-Claude Fabri de Peiresc. On March 4, 1769, Charles Messier inducted the Orion Nebula, M42, into his list of stellar objects. Then, in 1771, Messier released his list of objects for its first publication in Memoires de lAcademie.1
The Orion Nebula is one of the closest stellar regions to the Earth. Using parallax measurements, it has been estimated that this nebula is only 1,500 light years away. In addition, the Orion Nebula is a relatively young star cluster, with an approximate age of less than one million years. It has even been speculated that some of the younger stars within the cluster are only 300,000 years old.
The Orion Nebula is an emission nebula because of the O-type and B-type stars contained within it. These high-temperature stars emit ultraviolet (UV) light that ionizes the surrounding hydrogen atoms into protons (H+) and electrons (e-). When the protons and electrons recombine, the electrons enter a higher energy level (n=3). Then, when the electron drops from the n=3 level to the n=2 level, an H??photon is emitted. 2 This photon has a wavelength of 6563 , and therefore corresponds to the red portion of the visible spectrum. It is these H? photons which give the nebula the distinctive red color which we see.
The extreme brightness of the O-type and B-type stars, coupled with the Earths atmosphere, has always made high-resolution imaging of the star-forming region difficult. But recent advances in adaptive optics and the repair of the Hubble Space Telescope have allowed for incredible detail into the center of the dust cloud. 3 The technological advances have also helped reveal several faint stars within the center of the nebula.
The Orion Nebula is a spectacular sight. Consequently, it has been a preferred target of the Hubble Space Telescope (HST) over recent years. The HST has provided a great deal of insight into the complicated process of star formation. In June of 1994, C. Robert ODell, a Rice University
astronomer, discovered the presence of protoplanetary disks around some stars of the Orion Nebula. After surveying 110 M42 stars, ODell found that 56 of them had disks around them. It has been speculated that the disks identified in the Hubble survey are a missing link in the understanding of how planets, like those in our planetary system, form. 4 According to current theories, the dust contained within the protoplanetary disks eventually condenses to form planets. Furthermore, the abundance of the protoplanetary disks reinforces the assumption that planetary systems are common throughout the universe.
The suggestion that the Orion Nebula may eventually lead to planetary formation has become the basis for much discussion. More specifically, Doug Johnstone, an NSERC Post-Doctorate Fellow at the University of Toronto, developed an opposing perspective. At a meeting of the American Astronomical Society, on January 14, 1997, Johnstone suggested that the disks around young cluster stars may not survive long enough for planets to form within them. 5 Furthermore, he concluded that certain favorable conditions must exist in order to promote planetary formation, and that the hostile environment of the Orion Nebula may actually inhibit the creation of planets. With the present limited knowledge of nebulae, no conclusive evidence exists to support either argument.
On April 9, 1998, Cornell University astrophysicist Martin Harwit published his discovery of the presence of massive amounts of water in the Orion Nebula. This was the first time that water has been found in a star-forming region. The find demonstrates that water plays a vital role in star formation. In addition, this discovery implies that water is prevalent in space. Harwit speculates that the water acts as a coolant, by carrying heat away from the condensing clouds. It is believed that this process is necessary to slow down the particles in order to allow the compression of the particles into new stars. 6
The discovery of water in the Orion Nebula will undoubtedly provide the basis for further study. More specifically, it will prompt scientists to search for water in other regions of space at different stages of star formation. Then, if water is present in each, it may suggest that the oceans of Earth are older than even the planet that now contains them. 7
Several unresolved problems remain concerning the Orion Nebula. The fate of the protoplanetary disks, for example, is presently impossible to predict. Without a more detailed understanding of how planets actually form, it cannot be assumed that the events within the Orion Nebula are analogous to the events that led to the formation of the planets in the solar system. Furthermore, the detection of water in the nebula has revealed the need to revise the theory of star formation to include water as a major component.
Despite the fact that great progress is being made in terms of observational techniques and investigation, a great deal of information about the universe remains a mystery. Further analysis of the Orion Nebula, however, may help unravel some of the mysteries, including the origin of the solar system.
Bless, R.C. Discovering the Cosmos. University Science Books. Sausalito, California. 1996.
Manning, Elizabeth. Water Among the Stars. United Press International. ABCNews. April 9, 1998.
Press Release. Destruction of Protoplanetary Disks in Orions Trapezium Explained. January 14, 1997.
Sky & Telescope. Protoplanetary Possibilities in the Trapezium. Sky Publishing Corporation. October, 1994.
University of Cambridge. Cambridge Astronomy Dictionary. Cambridge: Cambridge
University Press. 1995.