Wait until Orion is reasonably high in the southeastern sky before looking at the cluster Collinder 70. This map is drawn for around 9 o’clock. Created with Stellarium
Have you seen Collinder 70 lately? I bet you have. It’s another designation for the Belt of Orion, and a genuine star cluster like the Seven Sisters or the Hyades. Per Collinder was a Swedish astronomy graduate student. For his 1931 doctoral dissertation he wrote a paper about the properties of star clusters. Just to refresh, a star cluster is a bunch of stars all born together from a cloud of gas and dust called a nebula. They "feel" one another’s gravity and hang together for millions of years as a group, as they orbit the center of the Milky Way galaxy.
There are thousands of star clusters in our galaxy, more than enough to satisfy everyone, from the novice to the professional. Some are loose and gangly, while others a packed so tight, you can hardly separate star from star. The best look like jewels spilled from a treasure box.
The border of Collinder 70 (Orion’s Belt) is outlined in the picture above. This is how the cluster appears in a pair of binoculars. I’ve outlined the "Seahorse" figure. Created with Stellarium
Orion’s Belt is the 70th entry in Collinder’s catalog of 471 clusters. Most of these were discovered by other astronomers, but as he examined their photographs of the sky, Collinder discovered a few of his own. Collinder 70 contains about 125 stars, the brightest of which are the Belt stars Alnitak (al-NYE-tak), Alnilam (al-NYE-lam) and Mintaka (min-TAK-kuh). From a dark sky, you’ll begin to see there’s more to the Belt than at first meets the eye. Look carefully and at least a dozen more stars will pop out.
To really appreciate this gem, look at it through binoculars. In my 10x50s, the contrast of the brilliant "three" against a rich background of fainter stars arranged in loops and chains is a most heavenly sight. One particular chain of stars reminds me of a seahorse. Can you see it?
The Belt of Orion is shown in great detail in this time exposure photograph. Just below Alnitak (bright star, lower left), you can see the dark stump of the Horsehead Nebula. Credit: Digitized Sky Survey, ESA/ESO/NASA FITS Liberator
In long time exposure photographs of the Belt, you’ll see that the cluster is swathed in hazy nebulosity. Just below the leftmost Belt star, is a dark patch in the shape of a horse’s head. Called the Horsehead Nebula, it’s almost as well known as its neighbor, the Orion Nebula. The Horsehead is called a dark nebula because it stands out in silhouette against the bright, nebulous background. Although it’s obvious in photos, the Horsehead requires at least an 8-inch telescope and a good map to find. Within the nebula’s dusky borders, new stars are emerging from dense pockets of gas and dust like butterflies from their chrysalises. In the far future, it’s likely that fresh-faced stars within the Horsehead will light it up from within, changing its shape altogether.
This closeup of the Horsehead Nebula shows its chunky texture as well as several stars emerging from inside the nebula. Credit: European Southern Observatory (ESO)
Nothing stays put in the sky. Over several thousands of years, nebulas and clusters look much the same, but in the vastness of time beyond human imagination, stars evolve, clusters break apart and new ones form. A nebula like the Horsehead is worked by forces from within and without, changing its shape like clay in a child’s hands.