Sigma Orionis

Located in the 2 million year old Orion OB1b association subgroup, sigma Orionis is an interesting multiple star which could possibly be called a very poor open cluster. Its claim to fame is that it's the star which illuminates the very dim emission nebula IC 434, against which is silhouetted the famous Horsehead dark nebula. This region of recent and even ongoing star formation is about 1,500 light-years distant.

Sigma is an unresolvably close binary consisting of a 4th magnitude O9.5 and a 5th magnitude B star separated by a mere 0.3 arcseconds. To the east, at 12 arcseconds' distance, is a 6.8 magnitude companion, and 41 arcseconds to the northeast is a 6.3 magnitude star, both of which should be visible at a magnification of 15-20X.

Any binocular will reveal a wide double consisting of equally bright stars separated by 1 arcminute and located 3 arcminutes to the northwest of sigma. A small scope, or even a giant binocular, will split the southern star of this pair into a duo of 8.5 magnitude stars 8 arcseconds apart. After sigma, the other five stars are all of spectral type B, and the colour of all six is an icy bluish-white.

The group, if indeed its stars all lie at the same distance from us, has a projected diameter of 1.5 light-years, which is 1/3 the distance between the Sun and alpha Centauri. The projected distance between sigma and the nearest part of the molecular cloud whose illuminated edge is catalogued as IC 434 is about 15 light-years.

Lambda Orionis

Marking Orion's head, lambda Orionis is the brightest star in the sparse open cluster Collinder 69. Perhaps 20 stars belong to this one-degree-plus diameter group, which is estimated to be about 1,400 light-years away--about the same distance as the younger Orion OB1 subgroups of the Belt and Sword. Cr 69 is best observed with binoculars or rich field telescopes.

While it would be called a young cluster, at an estimated age of 11 million years this group is old enough that one of its former more massive members has already exploded as a supernova some 350,000 years ago. The evidence for this is a 7-degree diameter bubble of ionized gas centered on Cr 69 and blowing out into the surrounding remnants of the molecular clouds from which the cluster formed. Moreover, the probable remnant of the supernova, the runaway neutron star called Geminga, is flying directly away from Cr 69.

Lambda is a relatively easy double star, the 3.5 magnitude primary being an evolved O8 giant with a 5.5 magnitude type B main-sequence companion 4 arcseconds distant to the northeast. Within perhaps a few hundred thousand years lambda A will be the next star in this group to go supernova. The five next brightest stars are B-types, whose brightnesses range from magnitude 4.4 to 7.5.

A slightly more De binary is located 18 arcminutes to the north of lambda (the brightest star just above lambda in the inset chart). HR1883 is a north-south oriented pair, comprised of a 5.7 magnitude evolved B9 giant and a 9.7 magnitude companion 2.8 arcseconds away. The 4 magnitude difference in brightness makes this pair a little bit more difficult than the separation would suggest.

For an even greater challenge, try tackling a fairly tight binary which may or may not be an outlier of Cr 69. It's located one degree due west of HR1883 (the northern star of the equally bright north-south pair near the right edge of the inset chart). This east-west pair has 7.9 and 8.6 magnitude stars separated by 1.2 arcseconds, theoretically resolvable in a 100mm aperture 'scope.

Open cluster NGC 2362 is observationally very interesting, but not what one would call a challenge to find. At magnitude 4.1, it's one of the brighter clusters in the sky, although most of this light is contributed by just one "star." The challenge, if any, is to have a clear view of the low southern sky, and hopefully to luck into a night of reasonably steady seeing. With a declination of -25 degrees, at the latitude of Ottawa it climbs no higher than 20 degrees above the horizon.

This one of the youngest clusters, a mere 5 million years old. Despite its youth, there seems to be an absence of nebulosity or other leftover material in the immediate vicinity. Perhaps the stellar winds from the brighter stars has indeed had enough time to sweep away any residual natal gas.

The dominant luminary is tau Canis Majoris, a multiple star consisting of five components, but you'll only ever see this system as an unresolvable quadruple and its easily-seen distant companion. The primary is an O9 supergiant of magnitude 4.9, with a magnitude 5.3 companion a mere 0.15 arcseconds distant that takes over a year to orbit the primary (at a distance of at least five times the Sun-Pluto separation). The brighter star of this pair is itself an even tighter double that completes an orbit in only 155 days (at about the Sun-Jupiter distance), while one of these stars is again double, whizzing around each other every 1.28 days (1/4 the Sun-Mercury separation)! The average size of each of these four stars is around 20 solar masses, and together they emit as much light as half a million Suns. In the not-too-distant future they all will explode as supernovae.

The fifth star of this quintuple system that you will easily see in a small 'scope shines more feebly at 10th magnitude and is located 8 arcseconds to the east (a projected separation of at least 13,000 AU). But a star orbiting this far out will not remain bound for long due to the gravitational nudges imparted by other stars in the cluster. Indeed, it's quite likely that this remarkable system resulted from the merging of two binaries which then formed the inner quartet, the fifth member possibly being a loosely captured interloper. Looking ahead, one can imagine that many thousands of years hence one or more stars of this system may be ejected from the cluster at hundreds of kilometers per second as a runaway.

After tau, and like its most-distant companion, the dozen or so brightest cluster members are all around magnitude 10, each being no more than 1/100 the brightness of tau's O9 primary. Some 3 to 4 dozen fainter stars are thought to be members as well. To appreciate just how intrinsically bright the stars in this cluster are, consider that a star like our Sun located within the cluster would be seen as a dim, 16th magnitude speck, barely visible in a 16 or 18-inch telescope.

Even though the distance to NGC 2362 is a fairly considerable 4800 light-years (3 times the distance to the Orion nebula), in this direction there is remarkably little obscuration from intervening interstellar dust. The reddening is just 0.1 magnitude in B-V, and hence the visual extinction is 0.3 magnitude. Many open clusters at similar distances in other directions around the band of the Milky Way are dimmed by a full magnitude or more.

NGC 2362 is a fairly compact 6 arcminutes in diameter (1/5 the apparent diameter of the Moon), which equates to a true diameter of 8 light-years. Compare this with the average separation of 7 light-years between stars in our part of the Galaxy. Because of its compactness, it's recommended that moderate magnifications be used, and perhaps even higher power if the seeing permits. The minimum instrument which will begin to show the cluster stars surrounding tau is a 60mm aperture binocular.

Because of the great difference in the brightness between tau and the other cluster members, a curious effect manifests itself if your telescope jiggles. Brighter tau seems to lag behind the other stars due to the persistence of its image on the retina, resulting in an out-of-sync oscillation. This has inspired the nickname, "the Mexican jumping star." Try nudging or tapping your 'scope to see this for yourself!