Eyes on the Great Nebula


Photographs of the Orion Nebula have become vastly more detailed in recent years.


In my previous post we visited a zone of winter evening sky centered on the three stars of Orion’s Belt. This asterism’s central location in the constellation of the hunter, as well as its exceptional symmetry, provides an eye-catching starting place for exploring surrounding features.

Among adjacent celestial highlights visible even from urban locations are supergiant stars Rigel and Betelgeuse diagonally flanking Orion’s Belt; Sirius, the brightest star in the night sky; twin stars Castor and Pollux; and brilliant Capella, which passes near the zenith on February evenings when viewed from mid-northern latitudes.

Now we’ll focus on the middle of this amazing array, at another special sight. Starting with the triad of Orion’s Belt, look about five arc degrees southward (half a fist length seen at arm’s length) toward a zone between the Belt and two notable stars marking the base of Orion’s rectangular frame (second magnitude Saiph and brilliant Rigel). In those stars’ midst on clear sparkling nights we may spot third magnitude Iota Orionis at a patch of light celebrated in celestial lore as the sword of Orion.

The primary feature of the sword is a spectacular ensemble variously known as The Great Nebula in Orion, Messier 42, and NGC 1976.  It’s the most visible portion of the much larger Orion Molecular Cloud complex pervading the constellation.

The Great Nebula is a captivating sight at all viewing levels—naked eye to Hubble Space Telescope. Since Galileo’s time the Great Nebula has provided one of the first objects of interest when successive advances in observational technology became available.

For example, in a research paper recently submitted to the Monthly Notices of the Royal Astronomical Society an international team headed by Anna F. McLeod of the European Southern Observatory described analyses of the central portion of M42. Their study used the Multi Unit Spectroscopic Explorer (MUSE) instrument aboard Yepun (UT4), the fourth Unit Telescope of the Very Large Telescope array at the ESO’s Paranal Observatory in the high Atacama Desert of northern Chile.

Regarding Orion’s Great Nebula, the ESO team noted: “Because of its vicinity … [to Earth] in combination with the fact that it is associated with recent star formation and a comparatively high surface brightness, it is the perfect object to study elemental abundances and therefore help understand not only the chemical evolution of the interstellar medium, but also the process of nucleosynthesis.” 

Easily visible with unaided eyes at sites far from outdoor lights, and from urban sites with the help of binoculars, the Orion Nebula has the appearance of a fuzzy star. Our attention is easily drawn to this nearest region of star and planet birth activity. At a distance of 1,370 light years, what eyes reveal as a hazy patch of light is actually a vast and complex system about 12 light years across, comprising a variety of fascinating components.

The heart of M42 is illuminated by energy radiating from mutually orbiting massive stars in a four star set known as the Trapezium. Its primary components are easily discerned with small telescopes and represent the most prominent features of the nebula other than its swirls of glowing gas and dark dust patches.

The Trapezium includes the brightest of approximately 3,500 members of the Orion Nebula (star) Cluster (ONC). Most of those stars have relatively low luminosities, significantly dimmed by intervening clouds of interstellar dust. Therefore, the Orion Nebula Cluster’s stars are best revealed via observations of dust penetrating infrared radiation. The ONC is one of the youngest star clusters known, with an age estimated at about one million years.

Vanderbilt University astrophysicist C. Robert O'Dell described The Great Nebula in Orion as a “thin blister of ionized gas on the front of a giant molecular cloud, and the extremely dense associated (star) cluster.”

The Great Nebula's extent covers a sky area about equal to that of the Full Moon’s disk; however, most of the Nebula’s light comes from its innermost fraction surrounding the Trapezium. This area is known as the Huygens region, after Christian Huygens, the 17th century Dutch astronomer who first described its telescopic appearance. Within that brighter inner zone, several distinct features are apparent, including the “Dark Bay” just northeast of the Trapezium, a slanting Bright Bar stationed southeast of Trapezium stars, and from the Bright Bar, a Dark Lane extending toward the northwest.

The inner region of the Orion Nebula.

Credit: NASA, C.R. O'Dell and S.K. Wong

Additionally the Orion Nebula contains many Herbig-Haro (HH) objects, which have bright jets extending outward along spin axes of embryonic stars. The structures appear as puffy chains of bright nebulosity formed when superheated gas blasts in opposite directions, penetrating and illuminating central regions of circumstellar disks. HH objects are named after University of Hawaii astronomer George Herbig and Mexican astronomer Guillermo Haro, who did initial research on these objects during the mid-twentieth century.

The Orion Nebula contains about 200 structures called proplyds which feature circumstellar disks that eventually may spawn exoplanets. Proplyd is a contraction of “protoplanetary disk,” referring to structures responsible for these protostars’ appearance. Proplyds are also seen in other nebulae such as the iconic “Pillars of Creation” in the Eagle Nebula, and stars such as HL Tauri. Interesting cusp-like structures observed in many of these objects was first noted during a 1979 study of M42.  

Proplyds exhibiting the distinctive cusps were initially spotted near the most luminous Trapezium star, Theta 1 Orionis C. They are generally found in nebula regions, such as M42, which are flooded with intense ultraviolet radiation and that also contain rich concentrations of stars. The brightest proplyds contain convex cusps that tend to face toward nearby sources of intense stellar winds and photon pressure. Proplyds located further from strong sources of stellar light appear as dark silhouettes against luminous backgrounds. Studies of these remarkable objects are an important part of the quest to explain how planets form, and hopefully to discover how life might begin on distant worlds.

Now with (or without) binoculars in hand or telescope on the ground, let’s bundle up and head out for a personal view of the Orion Nebula.

A way-station for seeing M42, as it transits the celestial meridian, will be about 7:00 pm, Sunday, February 28. This marks its highest elevation of the evening. For observers at mid-northern latitudes, the Great Nebula will be directly south at that time, about halfway between horizon and zenith.