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Three double stars in Orion, open cluster NGC 1528, and spiral galaxy NGC 1532

December 23-30, 2010: Three double stars in Orion and open cluster NGC 1528 make nice targets for small-telescope owners, while spiral galaxy NGC 1532 awaits large-telescope owners.
This week’s initial targets are three sets of double stars within the constellation Orion the Hunter. You’ll split each of them easily through a 4-inch telescope.
Astronomy: Roen Kelly
Each week, Astronomy magazine Senior Editor Michael E. Bakich, a master at explaining how to observe, posts a podcast about three objects or events you can see in the sky.

Targets for December 22-29, 2010
Small telescope: Three double stars in Orion
Small telescope: Open cluster NGC 1528
8-inch or larger telescope: Spiral galaxy NGC 1532
A three-peat in the Hunter
This week’s initial targets are three sets of double stars within the constellation Orion the Hunter. You’ll split each of them easily through a 4-inch telescope.

The first, Mintaka (Delta [δ] Orionis), is one of the sky’s easiest double stars to find. Just locate Orion’s Belt. Mintaka is the northernmost of the three stars. The brighter component shines at magnitude 2.2 with a pure white light. Mintaka B, only 2 percent as bright as the primary but still registering a nice magnitude 6.3, glows with a deep-blue color. A distance of 53" separates the pair.

This star’s common name descends to us from the Arabic Al Mintakah, which means “the Belt.”

The second of our three doubles is Meissa (Lambda [λ] Orionis), which marks Orion’s head. This duo features a magnitude 3.6 blue star as the primary and a pale-white magnitude 5.5 companion. The separation, 4.4", is much closer than the other two binaries I mention here, but it’s still an easy split through a small scope. Start with a magnification around 100x to separate this pair, and increase the power if necessary.

In the classic 1899 book Star Names and Their Meanings, stellar nomenclature expert Richard Hinckley Allen states that this star got its name because of an error by the 14th-century lexicographer al-Firuzabadi in his extensive Arabic dictionary that served as the basis for subsequent European Arabic dictionaries. According to Allen, the Arabic title Al Maisan, which means “the proudly marching one,” originally applied to one of the names for the star Gamma (γ) Geminorum, Almeisan. But al-Firuzabadi misapplied it to Lambda Orionis.

My third selection, Sigma (σ) Orionis (sorry, it doesn’t have a common name) isn’t just a binary star; it’s a nice multiple star system of four luminaries. You’ll find Sigma 0.8° southwest of magnitude 1.7 Alnitak (Zeta [ζ] Orionis).

Because their separation is a scant 0.25", components A and B will appear as a single star until you view the pair at high power through a 12-inch telescope. A shines at magnitude 4.2 and B isn’t far behind at magnitude 5.1. Both appear bluish-white.

The C component is the system’s faintest, glowing at magnitude 8.8, but it’s not a tough catch. It lies 2.4" from the A-B pair.

Finally, you’ll spot the D component 58" from the A-B pair. It shines at magnitude 6.6.

Nice and bright in Perseus
This week’s second small-telescope object is open cluster NGC 1528 in Perseus. It shines at magnitude 6.4 and measures 18' across. That gives it an area nearly 36 percent as large as the Full Moon.

You’ll find NGC 1528 1.6 east-northeast of magnitude 4.3 Lambda (λ) Persei. Sharp-eyed observers may just detect it as a hazy star from a dark observing site. Through a 4-inch telescope, use a magnification of 150x to spot 50 member stars.

Many stars group into whirls and other patterns. An 8-inch scope will show nearly 100 stars. The brightest star in the cluster, magnitude 8.8 SAO 24496, sits just west of center.

Two for the price of one
This week’s large-telescope target is spiral galaxy NGC 1532 in Eridanus. Actually, this object is a double galaxy. It combines the magnificent magnitude 9.9 edge-on spiral NGC 1532 with magnitude 11.7 elliptical NGC 1531, which sits less than 2' to the northwest. But NGC 1532 is the real treat here.

Through most scopes, it appears as a cosmic needle nearly 6 times as long as it is wide. Its true measurements are 11.2' by 3.2', but the oblong haze around the core is tough to see, so the galaxy looks thinner than it really is.

Through a 16-inch or larger telescope, you’ll see the spiral arms extend in a north-northeast to south-southwest orientation. With magnifications of 200x or more, the brilliant core appears surrounded by the aforementioned haze. And as long as you have a 16-inch scope at your disposal, look for the magnitude 15.2 irregular galaxy PGC 14664. It lies 12.5' southeast of NGC 1532.

You’ll find NGC 1532 and NGC 1531 just 1.5 northwest of magnitude 3.6 Upsilon4 (υ4) Eridani. Star charts also identify that star as 41 Eridani.
Expand your observing with these online tools from Astronomy magazine

Star Dome
Check out's interactive star dome to see an accurate map of your sky. This tool will help you locate this week's targets.

Observing Talk
After you listen to the podcast and try to find the objects, be sure to share your observing experience with us by leaving a comment at the blog or in the Reader Forums.


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