February will be the best month for planetary observers in a long while, particularly for those prepared to get up early. Jupiter and Mars continue their dalliance in the pre-dawn sky. At the start of the month the former reaches its highest point in the south before sunrise, although in the southerly constellation of Libra it never rises much more than 20 degrees above the horizon. The Moon is near by on the 7th and 8th before passing a few degrees above Mars on the morning of the 9th.
A few days later Mars make its closest approach to Antares, the brightest star in the constellation of Scorpius. Antares — the “rival” of Mars — owes its name to its orange colour. The two objects will be, thanks to the planet’s present distance from Earth, almost exactly the same brightness that should allow for a proper comparison of their colours, a task normally made more difficult by the superior brightness of Venus.
The most spectacular planetary apparition of February, though, belongs to the evening sky, as Venus recovers from superior conjunction on February 9. It is not high, so a clear western horizon during twilight will be needed, but the planet’s brightness at nearly magnitude minus 4 will ensure that it is easily seen. Over the next few months it will climb in the sky, but there is an extra reason to seek it out this month. On the 28th Venus makes its closest approach to Mercury.
The innermost planet also goes through superior conjunction on the 17th, but by the end of month should be visible and bright enough to be seen even during twilight. Look two and a half degrees below and to the right of Venus to find this most elusive world.
The stellar sky in February is covered by some of the most distinctive and easily recognised constellations, led by Orion, the hunter, low in the southwest once darkness falls. The three stars of Orion’s belt point up to Aldebaran in Taurus, another orange/red star to compare with Antares and Mars, and down to Sirius, marked on this month’s map.
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Sirius is the brightest star in the sky and its low position as seen from the UK often results in spectacular scintillation as the Earth’s atmosphere distorts the star’s light, causing it to appear to flicker between many colours. It is actually a binary system, with a faint white dwarf orbiting a bright white main sequence star, and recent observations with the Hubble Space Telescope have suggested a complex history of interaction between the two components.
The details are still unclear, and the fact that the two stars still have a decidedly elliptical orbit suggests that any interaction must have been slight. We can be sure, though, that Sirius owes its prominence to its proximity rather than to any astrophysical oddity. The system is just eight and a half light years away, and its brightness has meant that it has been used as a calendar marker by many cultures; its heliacal rising, the time when it could first be seen in the sky, marked the season of flooding of the Nile for ancient Egyptians, and the onset of winter for the Mori. Ancient astronomers paid attention too, and Ptolemy’s measurements of the star’s position in the Almagest were accurate enough that Edmund Halley in the 18th century could use them to calculate Sirius’s motion.
Sirius has also been key to more recent debates about the cartography of the local neighbourhood. Astronomers have learnt to identify clusters of stars by pinpointing those that move together through the galaxy, but this is not an easy task. At one point, Sirius was believed to be part of the cluster of stars associated with the brightest stars in the Plough, visible nearly overhead in the evening sky during this season of the year.
More recent work makes it instead the most prominent member of the Sirius supercluster, the third of three clusters of stars within a few hundred light years of our position. The other two, the Pleiades and Hyades in Taurus, are compact enough and far enough away to appear on the sky as compact clusters, but Sirius’s cohort is scattered across the sky. Intriguingly, unlike a classical cluster whose members are expected to share a common place and time of origin, the Sirius supercluster’s members are a diverse bunch, although most are young. Further results from the European Space Agency’s Gaia satellite, charged with mapping the movements of the stars with unprecedented care, should help to clarify the picture.
It is wonderful that even the brightest star in the sky is still the subject of research into its mysteries, and the thought provides plenty to ponder while enjoying a crisp cold February sky. Sirius and its retinue have plenty to teach us yet.