Collinder 70

Magnitude A / B / C
Separation A-BC/ AC
Position angle A-BC / AC
Spectral class A / B / C
Colour A / B / C

Detail Sketch:
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: Orion
: 05 35.06 / -01.05
: 140
: 0.6
: 100
: ?
: 430

: 09/01/11 / 22.15
: Landgraaf
: 3 / 3
: 15x80 Vixen Binocular
: -
: 15 / 210
image Cr70

Observing report

This object is very easy to find. Collinder 70 is made up of the three belt-stars of Orion and the 80+ stars surrounding them. The first impression you get from Collinder 70 through big binoculars is overwhelming. You see the three very bright belt stars of Orion shining with a bright white-bluish light, surrounded by loads of fainter stars. An impressive object. The cluster is well detached from its surroundings. There is a large range of brightness, from the very bright belt-stars to very faint stars, just visible with averted vision. I count about 80 stars, which makes Collinder 70 a medium to rich cluster. The “spine” of the cluster is made up by the three white-bluish belt stars.

There are some long chains of stars visible, winding around the three brightest members of Collinder 70. There are also two areas that seem more or less empty. One in the south-western corner and one to the east-southeast of the central belt-star. I do not detect any glow of nebulosity or unresolved stars. There are two yellow-orange stars visible. One on the edge of the field of view in the west, the other slightly southwest of the central star. I really enjoyed sketching this wonderful object. Highly recommended if you have binoculars or a rich-field telescope!

Stacks Image 280

Image from Voyager by CapellaSoft


Let’s have a look at data of the 6 most striking stars in my sketch.


The spectral classes of the individual stars confirm the color of all six stars I observed visually, although I must admit that the blue-white stars look more white than blue to me. When paired with a yellow or orange star (like for example Albireo), the color blue seems to be more a little more obvious than when looking at an isolated O-star. However, with Albireo, it is just a trick of the eye-brain combination, caused by the contrast between the two colored stars.

O stars are very massive, hot and bright stars, and they are very rare. Only about 1 in 3,000,000 of the main sequence stars in the solar neighbourhood is a Class O star! And here you have three in a a singe field of view.


Which stars belong to Collinder 70?

I could not find any data that tells me which stars actually belong to collinder 70. In most maps the area around Alnitak and Sigma Orionis are not included.
The stars in Collinder 70 are also plotted as an OB association on some maps, the Orion 1b association. An OB association is a loose grouping of O and B stars, scattered across a region up to several hundred light-years across. The members of an OB association are all of roughly the same age and formed from the same interstellar cloud within the past few million years. The Orion OB 1b association is a part of a massive complex, the Orion OB1 association.

The Orion OB1 complex roughly can be divided in a few sub-associations:

Ori OB 1a: group of stars located northwest of the Belt of Orion;

Ori OB 1b: the stars centered on the Belt of Orion, including the three bright belt stars;

Ori OB 1c: stars in the Sword of Orion, directly in front of the Orion Nebula and in the area around the Horsehead nebula;

Ori OB 1d: also stars in the Sword of Orion, but now the stars in the Orion Nebula and M43.

Move the mouse pointer over the image to identify the four different regions.

This beautiful image was shot by Wei-Hao Wang, who granted me permission to use it for this article.


Voyager by CapellaSoft
A revisit to agglomerates of early type Hipparcos stars, J.A. Caballero and L. Dinis
The Backyard astronomers guide, Terence Dickinson and Alan Dyer
The Internet Encyclopedia of Science
Handbook of Star Forming Regions Volume 1, The Northern Sky, edited by Bo Reipurth
Overview of the Orion Complex by John Bally
Stars and their Spectra by James Kaler