TAU CMA (Tau Canis Majoris). Looking like any other modest star of a bright constellation, here Canis Major, the Larger Dog, a close examination of Tau Canis Majoris (known by no proper name) yields a set of amazing surprises. Almost fifth magnitude (4.40), and needing reasonably dark conditions to see, it is one of the sky's rare very hot and blue class O (O9) supergiants, its modest apparent brightness the result of its huge distance of 4800 light years, the light you see having left the star around 2800 BC! It is so far away that we would not have a good handle on its distance at all except for the fact that Tau is the premier member (or so most believe) of a star cluster called NGC (for New General Catalogue) 2362. Since we can easily compare the brightnesses of stars within such a cluster with those of clusters whose distances we do know, we can find NGC 2362's distance quite accurately. More important, Tau CMa is a massive multiple star, having at least five components. We first see it as a triple, the bright star we know as Tau resolved into a pair only 0.15 seconds of arc apart (with magnitudes of 4.9 and 5.3) with a more distant 10th magnitude companion 8 seconds of arc away. Two more may lie even farther away.) The spectrograph shows that the brighter star of the inner pair resolves again into a yet-closer double that whirl around each other with a 155 day period, while one of these stars is AGAIN double, an eclipser in the mold of Sheliak with a VERY short period of 1.28 days. The brilliance of these stars reveals their huge masses, the inner four together shining with the light of half a million Suns, which suggests average masses around 20 times solar. The star therefore is an amazing record holder, as among stars of class O it contains both the longest-known period among doubles detected with the spectrograph and the shortest-known period for eclipsers. The combined light is so confused that little can be determined about the individuals. The tight inner pair must be only around a tenth of an Astronomical Unit apart, however, a quarter Mercury's distance from the Sun, close enough that the stars severely tidally distort each other. The next member out orbits this pair at a distance near Jupiter's from the Sun, the fourth at a distance of at least 223 AU, taking over a year to make the circuit around the others. The far outer tenth magnitude component is at least 13,000 AU out, and in the dense environment of the cluster cannot last long as a companion to the inner quartet. We speculate that the stars were not born in this configuration, but that the bright quartet was formed when two doubles within the cluster passed too close together and merged to create this remarkable system. Yet more remarkable is the system's youth. The cluster, and its component stars, are estimated to be a mere five million years old, only four percent the age of the sky's most famed cluster, the Pleiades. Not far down the line, in astronomical terms anyway, all the big stars of Tau CMa will explode.
Written by Jim Kaler. Return to STARS.