February’s night sky.

February is recognised as the final month of the winter in the northern hemisphere and conversely, south of the Equator, the last month of the summer. While the weather often remains decidedly chilly at this time, the shortest days are past and the evenings are well and truly stretching out. At the latitude of London (approximately 50°N), sunset occurs around 4.50pm on the 1st February, yet by the end of the month, the Sun remains above the horizon until 5.40pm.

Among the first stars to reveal themselves in the February twilight sky are those towards the south-east, with the appearance of Betelguese, Rigel and Sirius. The latter of these three happens to be the brightest in the sky, other than our own Sun. On a steady night, it appears a beautiful crisp steely color due to its high temperature of about 10000°C. It literally shines white hot! Sirius is an interesting object from an astrophysical perspective. It lies within 10 light years of the Sun and appears to move through space like the stars of Ursa Major (aka. the Big Dipper or Plough), suggesting they may all have been born from the same cloud of interstellar gas. However, according to one recent study by a team of astronomers led by Howard Bond of the Space Telescope Science Institute, Sirius is only 240 million years old, around half the age of these other stars. They conclude that this discrepancy might be reconciled if Sirius has experienced a rather complicated life involving interacting in the past with its companion star, Sirius B, which is now a faint white dwarf.

Figure 1. A map of the stars visible towards the south and east during February evenings. Several rich, prominent and interesting star clusters are highlighted.

This month stargazers have access to a number of beautiful star clusters that are amongst the closest to the Earth (see Figure 1). This includes the Pleiades, also known as the Seven Sisters, which lies at a distance of only 400 light years. This stellar grouping has attracted much interest throughout history and continues to be intensely studied by astronomers today. That is because it is both young (100 million years old) and relatively rich and provides opportunities for scientists to study in detail stars with a broad range of sizes andin different stages of their lives and test the reliability and accuracy of their computer models. While obvious to the naked-eye, the Pleiades, which span a few times with width of the Moon on the sky, are better seen with some optical aid. Indeed, the cluster is a truly awesome sight in a telescope at low magnification (about 20×).

Some 15° to the south-east of the Pleiades lies the 600 million year old Hyades (Figure 1). This is the second closest cluster to the solar system at around 150 light years. As the Hyades is so close, it appears distributed over a large swathe of the night sky, an area about 50×50 times the width of the full Moon. Considering this large spatial extend, the Hyades are best observed through a standard pair of binoculars such as 10×50. Counter-intuitively, the brightest star appearing in the midst of the cluster, the orange colored Aldebaran, isn’t actually part of this population. It lies in front of the Hyades at a distance of only 65 light years!

Figure 2a. A schematic map of the sky centred on the Praesepe star cluster. The circular outline represents the 5.5 degree field of view of standard 10 x 50 binoculars. The smaller gray circle illustrates the size of the full moon.

A further 60° to the east, and climbing up into the eastern sky later in evening, lies Praesepe (M44). At only 600 light-years, it too is one of the closest star clusters to the solar system. M44, which is sometimes referred to as the Beehive cluster, is approximately 600 million years old and because it is quite rich, contains several red giant stars as well as a number of white dwarfs. Both these classes of star are associated with the later phases of the stellar life-cycle. Consequently, this cluster also provides professional astronomers good opportunities to refine their understanding of astrophysics. For example, from studying Praesepe, and other star clusters, astrophysicists have determined that a star that is about 4 times the size of the Sun (referred to as a 4 solar mass star), will likely end its life as a white dwarf that is approximately 80% the size of the Sun, In other words, a 4 solar mass star returns the vast majority of the gas it is made from back into interstellar space when it expires, from where it can become part of the next generation of stars.

Figure 2b. A close in map of the night sky around the Praesepe. Here the circular outline, centred on the cluster, represents the field of view of a telescope at 40-50x magnification. The stars plotted here are those that are visible through a telescope of aperture 80-100mm.

M44 can be seen with the naked-eye (Figure 1), but only as an ensemble of stars – no individual members are visible without optical aid such as binoculars or a telescope. A standard pair of 10×50 binoculars will reveal around hundred or so stars contained within an area twice the width of the full Moon in diameter (Figure 2a), while a 80-100mm telescope, at modest magnification (40×), will show several hundred stars (Figure 2b). Either way, Praesepe is an inspiring sight viewed through either type of instrument.

Unfortunately in 2018 the February evening sky is lacking in observable planets. Mercury, Venus, Uranus and Neptune are simply too close to the Sun to be readily visible. While Mars, Jupiter and Saturn have also passed close to the Sun relatively recently, all three have now emerged into the early morning sky. By mid-month Jupiter is rising by 1am but given its southern declination it remains rather low in the sky even at sunrise. Mars and Saturn present a similar situation. By mid-month, the former rises by 3am but is still only low in the sky towards the south by sunrise. The latter rises after 4am and remains very low, achieving an elevation of only 10° by dawn.

As for the Moon this February – due to the Blue Moon in January, it is just past full as the month begins so February misses out on a full Moon. Waning quarter occurs on the 7th and the new phase is reached by the 15th. The Moon has grown back to the first quarter phase by the 23rd February and is 95% illuminated by the end of the month. This year, February has only 28 days since 2018 isn’t a leap year.