Double and multiple stars in Orion's Sword
12/22/2013 3:21 PM
An easy constellation for most anyone to find in the night sky is Orion. The three bright stars of nearly equal brightness and distance from each other, in an almost straight line, and surrounded by the quadrangle of Betelgeuse, Bellatrix, Saiph and Rigel are pretty hard to miss. And being located partly on either side of the celestial equator, this shape is visible from most every populated area on Earth.
Ancient people in Greece saw this constellation as a hunter. This make sense, given the way the three stars in the middle look like a belt, and it isn't hard to imagine a hunter standing there with a club over his head, fighting Taurus to the west. Below the belt is Orion's sword, three stars that aren't as bright as the previous seven, but nonetheless still get our attention. Those stars are found here:
Those three stars are not all that far apart in angular distance; a mere 2 degree. That's about the width of your pinky and ring finger held together at arm's lenth. Yet, within that area are several multiple and double stars that are worth seeking out with binoculars or a small telescope. Here's what they are, and how to find them.
Theta Orionis: The Trapezium
There is little doubt that this is the best-known grouping of stars in this area. Nestled within the heart of M42, the Orion Nebula, these very young, hot stars have blown away much of the gas and dust in the region, and are both illuminating it and ionizing it, which helps us see that gas and dust quite clearly. In fact, that makes it appear that Theta Orionis looks like a "fuzzy star" even naked eye. It looks singular viewed that way, and even with binoculars. But with a telescope, it becomes obvious that much more is going on here.
In the graphic to the right, note how these four stars form a trapezoid shape. They are quite close together, and will require a fair bit of magnification to split them into their four components. Try 75x to 100x to split them. But even more is visible, though a larger telescope will be required. Beyond those four stars, 2 more may be revealed with a 6 inch / 150mm or larger telescope, and additional magnification. These are the "E" and "F" components. And even further, a "G" and "H" set of components is there too! Try this link for more information on that, which can be quite challenging and difficult to split and observe.
See the video below for more information on what to expect and observe.
Right Ascension: 05hr, 35.4m
Declination: -05 27
Distance: 490 pc / 1,600 light years
Apparent magnitude: 4.0
Apparent dimentions: 47 arc seconds
Iota Orionis, Struve 747 and Struve 745
There are three more multiple / double stars in Orion's Sword. The bottom "sword" star is Iota. This is a triple star, with the primary at 3rd magnitude, and the secondary at 7th, about 11 arc seconds away to the southeast. Unfortunately, Stellarium does not show these two stars; only the primary. There are several good drawings of this star however; here's an Iota Orionis Google search that can bring some up for you to peruse. The tertiary companion is much fainter, at 10th magnitude, so a larger scope or darker skies will be required to pick it up visually. However, if you can see them, the colors are fascinating. The two brighter stars are both blue white, but because one is much brighter than the other, a color contrast occurs, making one appear bluish, and the other white. Coupled with the red appearance of the dim, third star, it makes for a wonderful sight. Use 50x or more magnification to split these.
Just to the west of Iota is Struve 747. This is a wide pair that most any telescope can split (though likely not binoculars), as the components are a good 34 arc minutes apart. Low power can grab these stars along with Iota and the next double star pair listed below in one field of view.
Struve 745 is much dimmer. These stars are 8th and 9th magnitude, making them harder to see, but their proximity to both Iota and Struve 747 should make them a target that is sought out. With the primary at 280 light years distance and the secondary at 150, this is an interesting study in brightness and distance. There is more about all of these pairs in the video below.
And don't miss what you can see in Orion's Sword beyond double stars: The open cluster NGC1981 awaits, as do the nebulae of M42 and M43.