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Canis Major

Submitted by Kevin Jardine on 26 March, 2005 - 21:16

Sirius, the most famous star in Canis Major, has an absolute bolometric magnitude of 1.23, according to this 1978 paper, which makes it about 27 times brighter than the Sun, but still far too dim to appear on the galaxy map. (The dimmest star to appear on the galaxy map is about 1200 times brighter than the Sun.) Sirius appears to be bright only because it is close (2.65 pc according to the same paper).

However, there are many truly bright stars in the direction of the constellation Canis Major with widely varying distance estimates.

First, let's take a look at the stars associated with Sharpless 310.

Sharpless 310

Sharpless 310 is the largest HII region on the galaxy map. (Sharpless 109 appears to be larger but it is actually a complex of many overlapping HII regions.)

Sharpless 310 is ionised by two multiple star systems (Tau Canis Majoris and UW Canis Majoris) each made up of several O-class stars.

According to a 1997 paper there are at least 5 visually identifiable stars in the Tau Canis Majoris system. Moreover, one of the two stars at the heart of the system turns out to be on close inspection itself a triple star, including two extremely close O-class stars that orbit each other every 1.282122 days - the closest O-class orbit known. (Such close stars must be so distorted by their mutual gravitational attraction that they would look more like eggs than spheres.)

Amateur astronomers have nicknamed Tau Canis Majoris the Mexican Jumping Star - apparently because it appears to jump around when examined through some telescopes.

According to the same paper, UW Canis Majoris consists of an O7.5Iabf and and O9.7Ib star.

The paper says that Tau Canis Majoris is likely part of the cluster NGC 2362 and gives widely varying estimates for the distance to this cluster as 900, 1420 and 1620 pc.

The distance to NGC 2362 currently used on the galaxy map is 1389 pc. The distance to the NGC 2362 OB association including both Tau Canis Majoris and UW Canis Majoris is 1200 pc. Through an error, UW Canis Majoris appears on the galaxy map twice, with a second distance estimate of 1514 pc. The HII region Sharpless 310 is estimated to be at a distance of 1500 pc. This is compatible with a distance to NGC 2362 of 1514 pc, but is not close to the other estimates of 900, 1200, 1389, 1420 or 1620 pc.

To add to the confusion, this 1996 paper estimates a distance modulus of 10.97 for NGC 2362, or 1493 pc, and a 2001 paper estimates a distance of 1480 pc.

So which should it be?

After checking with the star cluster database, it appears that its distance estimate for NGC 2362 may be based on old information (there are no new sources listed). So I'm inclined to go with the 2001 estimate of 1480 pc. This is compatible with a distance of 1500 pc for Sharpless 310, and a distance of 1514 pc for UW Canis Majoris and the NGC 2362 OB association.

However, this leaves a related mystery unsettled. The smaller ring nebula, Sharpless 308, is currently estimated to be at 1600 pc on the galaxy map, leaving it just at the boundary of the Sharpless 310 HII region. This 2000 paper points out that this ring nebula is generally considered to be created by the Wolf Rayet star WR 6, also called EZ Canis Majoris. The paper says that this star is believed to have a distance of 1800 pc.

Both of these distance estimates are contradicted by the current distance estimate for WR 6 used on the galaxy map - 970 pc.

So which figure is correct: 970, 1600 or 1800 pc?

A look at the Wolf-Rayet star database reveals a reference to this 1996 paper which is a thorough analysis of the vicinity of WR 6 and Sharpless 308. So it does appear that the 970 pc estimate is based on recent research, and thus Sharpless 308 should also be at a distance of 970 pc and is not associated with the Mexican Jumping Star complex.

But there's more ...

Collinder 121

The OB association Collinder 121 has a current distant estimate of 552 pc on the galaxy map. However, a paper mentioned above concludes that WR 6 is likely associated with Collinder 121. WR 6 has a distance estimate of 970 pc. How to reconcile these estimates?

One way to at least reduce the discrepency is through this 2003 paper, which concludes that there is a new OB association CMa OB2, at 592 pc, and a star cluster (not an OB association) called Collinder 121, at 1050 pc.

This would significantly reduce the gap between WR 6 and the Collinder 121 star cluster to 80 pc, but this is still fairly large. Moreover, the above mentioned paper points out that the Hipparcos parallax for WR 6 suggests a much closer distance of 575 pc and therefore suggests that WR 6 is associated with CMa OB2 at 592 pc.

The star cluster database has a Collinder 121 entry, but no distance estimate, so a star cluster with this name does not appear on the galaxy map.


It seems that there are three major structures in the direction of Canis Major. First, at a distance of about 1500 pc, is the NGC 2362 OB association, which includes the NGC 2362 star cluster with the complex Tau Canis Majoris system at its heart, and is surrounded by the large Sharpless 310 HII region. Second is the CMa OB2 association, which includes Sharpless 308 and the star WR 6 and probably Sharpless 303 and 304 as well. CMa OB2 probably lies at a distance of 592 pc.

In between at a distance of 1050 pc lies the Collinder 121 star cluster.

Unrelated to all this is the CMa OB1 association. I haven't mentioned CMa OB1 in this review because it is in a slightly different direction than the other two structures and the distance to this association and its associated clusters and HII regions seems more assured at 1000 - 1150 pc.