A Quintuple Star in Orion
Perhaps the finest multiple star in the sky and visible to both northern and southern observers, sigma Orionis is a system of five stars, of which four are visible in a small telescope. The brightest star of this group is one of the most luminous stars known and will one day expire, like many stars in Orion, in a spectacular supernova explosion.
• Sigma Orionis doesn’t have an easy-to-remember name, but it’s not hard to find. It’s just south of Alnitak, the eastermost star in Orion’s belt. What looks like one star to the naked eye is actually five stars. The total visual magnitude is 3.6, so it’s visible even in light-polluted city skies.
• The five stars in the sigma Orionis system are labeled A to E. A and B are the brightest and lie so close together they look like a single star in even the largest telescope. But in a backyard scope at 100-150x, you’ll easily resolve the C, D, and E components. Star C is the faintest, and at magnitude 10.0, it might require a 6-inch or larger scope.
• The A and B components are each tremendously massive and bright: more than 35,000 and 30,000 times brighter than our Sun with a total mass of almost 35 Suns. The A/B pair are only 90 astronomical units (AU) apart and orbit each other every 170 years.
• Component C of sigma Orionis is some 3900 AU away, and the D and E are 4600 and 15,000 AU (about 1/4 light year) away from the main A/B components. The system itself is some 1,150 light-years away from the Sun.
• The ultraviolet light from A/B excite surrounding gas and dust clouds. A and B will end their lives as spectacular supernova explosions, while the smaller C, D, and E components will end their lives as white dwarfs after briefly expelling their atmospheres as planetary nebula.
• Component E is a strange star, with a strong magnetic field and strange helium-rich patches in its atmosphere.
• Just northwest of sigma Orionis, you’ll find the triple star sytem Struve 761. Depending on your optics, you might see this system within the same field of view as sigma Orionis.
The C, D, and E components of sigma Orionis will eventually leave the orbit of A/B and travel the galaxy as lone stars. Sigma Orionis, Struve 761, the stars of Orion’s belt and the Orion Nebula are all part of the magnificent Orion OB1 association, a recently-formed aggregation of gas, dust, and new stars that’s only now starting to disperse into the Orion arm of the Milky Way.
This region of Orion’s belt is an astrophotographer’s dream. Within the same field as sigma Orionis lies the Horsehead and Flame Nebulae along with other beautiful emission and reflection nebulae. A visual observer will only see stars in this region of the sky, although you may see a bit of nebulosity if you have a big telescope, pitch-black sky, and an H-Beta filter.
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