The Deepsky: Top-100

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Binocular Objects (2): Collinder 70, a big open cluster incl. Orions Belt

The winter constellation of Orion holds many treasures for deep sky observers, some of them among the best you can find in the night sky. But there is one object that most observers have “seen”, yet few are aware of its existence. The name of the object is Collinder 70, a large open cluster, located around the belt of Orion. Collinder 70 includes all three belt stars: Mintaka (the Girdle), Alnilam (The string of Pearls) and Alnitak (the Belt). The cluster has a diameter of almost 3 degrees and consists of 100 stars or more. Most of them are of the 10th magnitude or brighter.

This is an object for binoculars! In a telescope you would only see a part of this magnificent deep sky object. On a clear and moonless night, seeing Collinder 70 with all it’s stars grouped around the three bright belt stars, will give you the same WOW experience as looking at the Pleiades or the Alpha Persei cluster for the first time.

When observing, take your time. Follow the beautiful chains of stars. If you have 10x binoculars or more, look for some double or multiple's. Try to split the A and C component of Mintaka (Delta Orionis), the star at the western end of Orion’s belt. Mintaka is an eclipsing binary with a period of 5.7 days. The 6.7 magnitude C component is separated by 52.6” from the A component in position angle 359 degrees.

Find the 6th magnitude E component of the multiple star system Sigma Orionis, separated by 41.60” from the A component, in position angle 61 degrees. You can find Sigma Orionis just a degree southwest of Zeta Orionis (Alnitak), the easternmost of the belt stars. Look closely and see if you can find some stars that differ in color. Maybe you should try to make a sketch of the cluster. Whatever you do, enjoy it!

On the first image below you see an overview of the constellation Orion, the red circle representing the field of view (FOV) of 3.5 degrees binoculars centered on Collinder 70. As you can see, you definitely need binoculars to see the whole cluster in the same FOV. Binoculars with a 5 or 6-degree FOV are better. They show Collinder 70 nicely framed in its surroundings. The second image should give you an impression of Collinder 70, seen through binoculars with a FOV of again 3.5 degrees.

Collinder 70


Image generated with SkyTools 2 from Capella Soft




Collinder 70


Image generated with SkyTools 2 from Capella Soft





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