D e s e r t E x p o s u r e
January 2012
Columba, the Dove
Plus the planet for January.
Around 10:30 p.m. on these cold January evenings, the first-magnitude star Canopus can be seen skimming westward above our southern horizon.
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About 15 degrees above Canopus, you can find the small constellation of Columba, the Dove. This southern constellation is another creation of Petrus Plancius from the stars that were below Canis Major's (the Big Dog) hind legs. These stars were not really part of Canis Major's constellation figure, so in 1592, Plancius grouped them into a new constellation he called Columba Noachi ("Noah's Dove"). By the time the International Astronomical Union formalized the constellations in 1930, this name had already been shortened to Columba, the Dove.
The brightest star in this rather inconspicuous constellation is Alpha Columbae, also known as Phact. This 2.7 magnitude star's name is from the Arabic Al-Fakhita, which translates as "The Dove." The next brightest star in this constellation, at magnitude 3.12, is Wezn ("the Weight"). The rest of Columba's stars are unnamed, except for their Bayer Designation.
In the spectral classification sequence, the type O stars are the hottest and brightest along the main sequence. These bluish stars, at the extreme end of the main sequence classifications, are rather rare with only one in three million being a type O star. Most are far away, and even though they are very bright, they appear faint from our perspective due to the distance.
Columba contains one of the few type O stars visible to the naked eye, Mu Columbae. Even so, it is only magnitude 5.15; classified as an O9.5, it is at the smaller end of the type O stars, only 4.5 times the diameter of our Sun. But can you imagine a sun in our sky that is not only 4.5 times the diameter of the sun we see, but with a surface over six times hotter? This would be blinding and make life impossible.
Not only is Mu Columbae a huge star, it rotates very rapidly, completing one turn in just 1.5 days. Compare that to our Sun's rather lazy 25.4 day rotation rate. This means that the Sun's equator is spinning at 1.3 miles per second, while Mu Columbae's equator zips along at 152 miles per second!
Mu Columbae is also moving rapidly through space. Since these huge type O stars burn out very quickly, it cannot have been traveling too long. Astronomers have traced Mu Columbae's motion backward toward the north-northeast into Orion. They also traced another star, AE Aurigae, another O9.5 star just a little fainter than Mu Columbae, back into Orion. Both stars were in the area of the Trapezium, the cluster at the center of the Great Orion Nebula, about the same time.
The thinking is that these two stars were originally part of a pair of multiple star systems in or near the Orion Nebula some two and a half million years ago. Since the Trapezium Cluster is only a million years old, it cannot be involved with this episode. Looking around this area, astronomers discovered that Iota Orionis (Na'ir al Saif), at the southern tip of Orion's sword, is a multiple star system with the brightest component a very close double having an unusually eccentric orbit — a sign of gravitational interactions with other stars.
It appears that another multiple star system approached Iota Orionis two and a half million years ago, and they passed through each other. The mutual gravity among these stars caused the two multiple star systems to exchange two members. In the process, AE Aurigae and Mu Columbae were ejected in opposite directions, separating at a rate of over 120 miles per second. While this sounds like a rare event, 10% to 15% of type O and B stars in the sky are "runaway stars," like AE Aurigae and Mu Columbae.
The Planets for January 2012
Venus will grace our evening sky not only in January, but for the first half of 2012. It is the first object visible in the sky (except for the Moon) after sunset. Venus is still rather full with a 79% illuminated disc, slowly becoming less full. At midmonth its diameter is 13.8 seconds-of-arc across and Venus gleams brilliantly at magnitude -4.0. The Goddess of Love sets around 8:15 p.m. It starts the month in central Capricornus, moving into Aquarius, where it ends the month just before entering into Pisces.
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(times MST)
Jan. 5, 6 p.m. — Earth closest to the Sun this year (91,402,046 miles) Jan. 9, 12:30 a.m. — Full Moon Jan. 16, 2:08 a.m. — Last Quarter Moon Jan. 23, 12:39 a.m. — New Moon Jan. 24, 6 p.m. — Mars stationary Jan. 30, 9:10 p.m. — First Quarter Moon |
Jupiter is the second object to become visible as it gets dark, almost 70 degrees up in the south-southeast. Sparkling at magnitude -2.5, Jupiter starts the month in Pisces and moves quickly into Aries on Jan. 9, where it ends the month. Jupiter's disc is 41.2 seconds-of-arc across; it sets around 1 a.m.
Mars starts the month in Leo, drifting eastward into Virgo on Jan. 13, where it becomes stationary on Jan. 24. Glowing redly at magnitude -0.1, Mars comes up in the east around 9:45 p.m. with a disc 10.3 seconds-of-arc across and 93% illuminated at midmonth.
Saturn glows at magnitude +0.6 as it rises around 12:30 a.m. Saturn moves slowly eastward in eastern Virgo. Saturn's disc is 17.0 seconds-of-arc across while the Rings are 38.7 seconds-of-arc across and tipped down 15.0 degrees with the northern face showing.
Mercury is still visible in the morning sky shortly before sunrise in the east-southeast during the first part of the month. The Messenger of the Gods has "turned around the bend" and is heading around the back side of the Sun. At the beginning of the month, Mercury rises at 5:40 a.m., rising later each day. Its disc is 5.6 seconds-of-arc across and 81% illuminated, becoming fuller each day. Mercury starts the month in Ophiuchus, moving eastward into northern Sagittarius. Not stopping there, it continues into Capricornus, where it ends the month approaching the Sun.
The brilliant stars of winter along with Venus and Jupiter invite you out to gaze upon their splendor. So bundle up to stay warm and "keep watching the sky"!
An amateur astronomer for more than 40 years, Bert Stevens
is co-director of Desert Moon Observatory in Las Cruces.