Barnard’s Loop and the Orion-Eridanus Bubble

In 1895 the American astronomer E.E. Barnard of Yerkes Observatory, discovered a faint ring of gas surrounding the whole of the constellation Orion. Revealed by his long exposure photographs with a portrait camera, the ring is now called Barnard's Loop. It is in the form of an ellipse, measuring some 14° by 10°, elongated in the same direction as the constellation. Barnard's Loop is brightest in the east, and to the south of the star Betelgeuse. A photo of the giant loop is shown as Figure 1 below.

The Loop consists of hot ionized hydrogen. Dust was found in photographs taken by Gemini astronauts Charles Conrad and Richard Gordon in 1966. The Earth's atmosphere blocks UV radiation from reaching our ground based telescopes however the astronauts were able to take the photographs above the atmosphere from the spacecraft. The UV photos showed Barnard's Loop as a 19° by 14° ellipse, enclosing the optical Loop. This ionized hydrogen has had its electrons stripped away by the intense radiation of the very hot O, B type stars in Orion's central region. As the radiation loses its energy as one travels further from the source (the hot O, B stars), the ionization process finally ceases, and a visible "sphere" appears. This is called by astronomers as "Stromgren Spheres" in honor of the German astronomer B. Stromgren that originally worked out the theory to explain the sharp boundaries surrounding hot O, B type stars. This apparently what Barnard's Loop is - a giant Stromgren Sphere.

Houston astronomers C.R. O'Dell, Karl Heinze and D.G. York first postulated that the Loop is a cosmic bubble blown by the radiation of the OB1association stars of Orion. Pushing outward by the pressure of their radiation, the stars have bulldozed the interstellar material into its above normal density immediately outside the optical loop. They estimated the
time needed to blow a bubble this size is about 3 million years, which appears to be the age of some of the older stars in Orion. Orion is a very active region of star formation, and photographs taken years apart show motions in many of the central stars and filamentary structures.

Could Barnard's Loop be the result of an ancient supernova explosion(s)? Possibly so. The well known Runaway Stars AE Aur, 53 Ari and μ Col seem to speeding away from the central part of Orion as a result of an event or several events between 2.5 and 5 million years ago. It is believed that these stars were thrown away by either losing their binary members from a supernova event, or suffered a much perturbed orbit by the close passing of
a massive star.

On a much larger scale is the Orion-Eridanus Bubble. (Figure 2) This gigantic bubble which has a diameter of some 36° (projected) and a linear diameter of some 240 parsecs, It  is also believed to be the result of the same ionizing radiation of the OB1 association stars. A detailed study by A.G. A. Brown, D. Hartmann, and W.B. Burton reporting in Astronomy and Astrophysics, (Vol. 300, page 903, 1995) using a neutral hydrogen survey, showed an expansion velocity of the shell at about 40 km/sec. Their age estimates place this bubble at 1.8 - 5.4 million years, distance to the center of the bubble - 380 pc, mass of its shell: 2.3 ±0.7 x 10⁵ M_¤.  (M_¤ = Mass of Sun).   

The origin of this cosmic bubble in the 1970's was thought to be by a series of supernovae about 2 million years ago. The OB1 association stars are the most probable location of the suggested supernovae. The recent studies indicate that the size and expansion velocity of the bubble is due the combined result of stellar winds and supernovae.

Additional evidence showing the age of the Loop and Bubble comes from the Orion OB1 associations mentioned earlier. These associations are not clusters of stars, they simply formed in a region of space. The intense hot environment plus enormous gravitational "kicks" at work began to dissolve these associations and they began to fly apart. New studies of the space motions of these stars, derived from their proper motions and radial velocities, give in many instances the rates of expansion of these associations. Then knowing their distances, we can determine their ages. The Orion complex shows an expansion age of the belt of 4 million years, and the trapezium stars are younger than 1 million years. This is why the Orion region has generated much interest from professional astronomers !!

Barnard's Loop would be an excellent photographic experiment with a 35mm camera and 50 mm lens mounted piggy back on a motor driven telescope. Try some guided exposures for at least 30 minutes in a dark site. Please share your efforts with us. Figure 1 shows Barnard's Loop and Figure 2 shows the Orion-Eridanus bubble.

                          Figure 1. Barnard's Loop surrounding Orion’s central region

                          Figure 2. The Orion-Eridanus Bubble. Notice how it 4 times the size of the Orion Constellation.

REFERENCES:

Brown, A.G.A., Hartmann, D., Burton, W.B., 1995, The Orion OB1 Association II. The Orion-Eridanus Bubble, Astronomy and Astrophysics, 300, pp. 903-922.

Burnham, R. Jr., 1978, Burham's Celestial Handbook, Dover Publications, New York, p 1329.

Murdin, P., Allen, D., 1979, Catalogue of the Universe, Cambridge University Press, p. 105.