Binocular Open Clusters

Open Star Clusters
an activity for binoculars or a small telescope

Astronomers believe that stars are born as members of an open star 
cluster. Over many hundreds of millions of years, the cluster’s stellar 
components slowly drift apart as they orbit the center of the Milky Way 
galaxy. Eventually, the cluster dissipates. Our sun is thought to have been a member of 
such a cluster nearly 5 billion years ago.

In this activity, the binocular user scans the sky to find ten open clusters, some of which 
are just out of sight of the unaided eye. Other clusters, not on this list but still worthy of 
observation nonetheless, will likely be seen as well.

Common 10 x 50 binoculars will do the job nicely. Clear, dark skies are ideal, though. 
So try to avoid hunting for these celestial wonders when a bright moon is present and 
try to do so away from city lights and air pollution.

Suggest starting point: Before you begin your search, find two bright nebula. Both are 
fantastic binocular targets, and both are star forming regions which are currently 
creating open star clusters, ones that will be seen many tens to hundreds of thousands 
of years in the future.
1. M42. Look low in the west 90 minutes after sunset for the constellation Orion. 
Between its three “Belt Stars” of equal brightness and the constellation’s brightest 
star, Rigel, glows something that doesn’t look quite starlike with the unaided eye. 
Binoculars reveal this – the famous Orion Nebula – to be an utterly fascinating star 
forming nebula. M42 is a sight that people in the northern and southern 
hemispheres can enjoy.
2. Eta Carinae. One quarter between the Southern Cross and the sky’s second 
brightest star, Canopus, glows another star forming nebula, Eta Carinae, which 
some observers claim is more rewarding than the aforementioned M42. Plus, there 
is more. One half binocular field east-northeast lies a bright open cluster NGC 3532. 
And the Eta Carinae nebula is one binocular field north of one of our recommended 
open clusters IC 2602, the Southern Pleiades. One binocular field west of Eta 
Carinae shines NGC 3114, yet another cluster. This is a busy area!

Click here to view/download the PDF of this Observing Challenge.