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The New Year is upon us, welcome it in with

The Australian Night Sky - January 2016

By Wayne Roberts

What's In The Sky This Month

The Planets

Our Monthly Feature

 

[Click here to show or hide the explanatory notes]
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What’s in the sky this month: January 2016

 2nd Last quarter Moon,
  Moon at apogee (furthest from Earth, 404,277 km).  3rd Earth at perihelion (closest to Sun, 147.1 million km/0.9833 a.u.).
 6th Pluto in conjunction with Sun.
 9th Mercury at perihelion (46.00 million km/0.3075 a.u.),
  Close conjunction of Venus and Saturn (see planetary section below),
  Jupiter stationary (begins retrograde motion).
10th New Moon.
14th Mercury in inferior conjunction (midnight 14th/15th).
15th Moon at perigee (closest to Earth, 369,619 km).
17th First quarter Moon.
24th Full Moon.
30th Moon at apogee (404,553 km).

Note that 'a.u.' as mentioned above stands for 'astronomical unit', where one a.u. equals the average Earth-Sun distance, currently defined as 149,507,870.7 km; note also that Mercury's perihelion refers to the physical separation of it and the Sun in space, and is unrelated to their angular separation in our sky.

Retrograde motion refers to a planet's east to west movement against the starry backdrop, as opposed to the 'normal' west to east motion, which is indicative of the planets progression along its orbital path and referred to as direct or prograde motion. Retrograde motion occurs in the months around opposition and is caused by the same effect as that witnessed when a car which is overtaken seems to momentarily move backwards relative to trees or other objects in the background.

The Quadrantid meteor shower peaks at 7 pm on the 4th (according to the International Meteor Organization, it should be noted that peak times are difficult to predict). This is a major shower, but unfortunately for us one with a radiant (the point in the sky from which the meteors appear to emanate) deep in the northern sky. While the ZHR – the Zenith Hourly Rate, the number of meteors which can be expected to be seen under a clear dark sky with the radiant at the zenith (straight up) – is 120, the location of the radiant in the constellation Bootes mandates that it only rises about 3° above our northern horizon at best. We are further disadvantaged, and mightily so, by the fact that at the predicted time of maximum, the radiant lies fully 58° below our horizon. Add to this the fact that evening twilight doesn't fully dissipate until around 10:30 pm and that the Quadrantids, unlike some showers, are known to have a quite sharp peak, only lasting for 2-3 hours, and you can appreciate that our viewing prospects are severely compromised. Those wishing to check out the shower nonetheless would be best advised to monitor the northern horizon for a few hours after sunset (8:46 pm).

The Quadrantids is one of only two major showers, the other being last month's Geminids, for which the parent body is an asteroid – catalogued as 2003EH1 – rather than a comet. Another unusual feature is its name; most showers are named after the constellation containing the radiant, but while this radiant is now in Bootes, it was in the now discontinued constellation of Quadrans Muralis when the shower was first recognised, and the name has stuck.

Another event of note this month which does not favour us is the occultation of Aldebaran (Alpha [α] Tauri) by the Moon on the 20th. Shining at magnitude 0.8, Aldebaran is the brightest of all stars which can be occulted by our satellite – Antares (Alpha Scorpii), Spica (Alpha Virginis) and Regulus (Alpha Leonis), magnitudes 1.0, 1.0 and 1.3 respectively, are the only other candidates which shine brighter than 2nd magnitude.

Aldebaran is almost 2° above the upper limb of the Moon as the pair rise around 5 pm, and in any event is never closer than a little over ½°, passing this close shortly after midday, when well below our horizon. The occultation favours viewers well north of the equator, in the USA, Greenland, Iceland, the western edge of Europe and the NW extreme of Africa.

Lunar occultations of Aldebaran are cyclic; this is the 13th of 49 between January 2015 and September 2018, 12 of which occur in 2016. The previous cycle ran from 1996 to 2000, and the next will be from 2033 to 2037.

After reaching a modest best in our morning skies last month, Comet Catalina (C/2013 US10) is now heading north, but still available for compromised viewing early in the month. On the first few days of the month, Arcturus (Alpha Bootis, magnitude -0.1) acts as a handy signpost, although the trade-off is that, as the fourth brightest star in the night sky, Arcturus may serve to further dim the view of the comet. On the 1st, Catalina lies 1¼° SW (upper right) of Arcturus; by the third it is 2¼° NE (lower left) of the star. The following chart shows the situation at 5:02 am, one hour before sunrise, on the intervening day of the 2nd, with just 40' (⅔°) separating the two. Having risen at 3:52:00 am and 3:52:37 am, comet and star respectively sit 11° above the horizon (as a rough guide, one finger held at arm’s length spans a little over 1°, a closed fist 10°, an open hand, tip of little finger to thumb tip, 20°). As morning twilight will first start to brighten the sky around 4:15 am, the viewing window, which requires a flat eastern horizon, is somewhat tentative. My software indicates that the comet will shine at magnitude 4.9 at this time, but readers should bear in mind both the unpredictable nature of the brightness of all comets, and the fact that this figure represents the total light output over the extended (albeit small) disk of the comet, whereby it would appear a little dimmer than a pin-point star rated at the same brightness.

As a matter of interest, Catalina, although it was at perihelion (its closest point to the Sun) in mid-November, doesn’t reach perigee (closest to Earth) until the 17th of this month, by which time it is unobservable from our southerly location, and is predicted to have dimmed a little, to magnitude 5.1.

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THE PLANETS

Mercury

On New Year's Day, Mercury resides in the evening sky, some two thirds of the way through an apparition which began in mid-November, and well within evening twilight. It is at an altitude of 13° when the Sun sets at 8:45 pm and itself sets just under 1¼ hours later, at 9:58 pm. Although it shines brightly, at magnitude -0.3 – on a par with Alpha Centauri, the third brightest star in the sky – it will not be easy to pick out with the naked eye, close to the horizon and bathed in twilight. The innermost planet's disk spans 7.4" and is 47% illuminated, making for an attractive, albeit small, sight through a telescope.

After reaching perihelion (its closest point to the Sun) on the 9th, Mercury swings in between Earth and the Sun on its inner orbit – when it is said to be in inferior conjunction – a couple of days before mid-month, at midnight on the night of the 14th/15th. In doing so, it transfers to our morning skies, beginning a more interesting apparition during which it will manage to break free of twilight, although not by a great deal, and will rise up towards (but never quite meet) brilliant Venus.

Mercury alone, among all the planets, grows dimmer in our skies as it approaches us and brighter as it moves away. This counter-intuitive behaviour is because its small orbit means that its distance from Earth varies by a relatively small amount, whereby the fuller phase we see when it’s on the far side of the Sun increases it’s brightness in our sky by a greater amount than the decrease caused by the larger distance.

As a consequence of this phenomenon, Mercury grows dimmer each day as it approaches us from the 1st through until inferior conjunction on the night of the 14th, when it registers a faint magnitude 4.7 – and is totally invisible of course, being in conjunction with the Sun. Subsequently, its visibility improves quite rapidly, both increasing in brightness as it moves away from us, and rising progressively earlier than the Sun and therefore in a darker sky.

As January draws to a close, Mercury is rising at 4:42 am, one hour and fifty minutes before sunrise (6:32 am, with the planet at an altitude of 20°), and thus just a little before twilight commences. Via the mechanism described above, Mercury's brightness has recovered to 0.15, its 7.8" disk being 46% illuminated.

On the face of it therefore, the end of the month is the best time in January to view Mercury, but the 63% illuminated waning gibbous Moon interferes. To avoid its influence, it would be necessary to view on the 23rd, with Luna setting at 5:24 am, ten minutes after Mercury rises. The problem this presents, however, is that Mercury rises, at 5:14 am, in twilight, as the Sun rises just one hour and nine minutes later (with Mercury a mere 12° above the horizon). Best to avoid this pincer movement, be patient, and view early next month, when the various parameters are more favourable.

Mercury crosses the border from Sagittarius to Capricornus on the 2nd, then reverses direction and loops back over the border on the 9th as it swings in towards the Sun from our perspective en-route to inferior conjunction; it then sees the month out in Sagittarius.

Venus

Venus rules the predawn sky throughout January, it's appearance being made all the more captivating by the fact that it shares the sky with another three of the five naked eye planets all month, (one of the three, Saturn, is close to the horizon early in the month), with Mercury completing the line up late in the month; the icing on the cake is a tight conjunction with beautiful Saturn on the 10th.

As January begins, Venus is rising at 3:37 am, and is almost 27° clear of the horizon when the Sun rises at 6:01 am. The brilliant orb blazes forth at magnitude -4.06 and displays a 14" disk, 77% illuminated.

Saturn lies 9° to its SE (below right) and Mars 33° NW (above left), with Jupiter 35° further on along the ecliptic (the path through our sky followed by the Sun and, very nearly, the planets). The scene is further enhanced by a close conjunction of Jupiter with the 64% illuminated waning gibbous Moon, less than 1° separating them. Here's the view at 4:31 am; as the chart shows, a number of prominent stars are also nearby. Many more than shown here act to complete the scene, with the sky ablaze with many of the night sky's luminaries – 13 of the 17 brightest in fact, including the brightest four (Sirius [Alpha Canis Majoris], Canopus [Alpha Carinae], Rigil Kentaurus [Alpha Centaurus] and Arcturus [Alpha Bootis]). This chart and the two to follow are configured for 1½ hours before their respective times of sunrise.

On the 7th, with the Venus-Saturn conjunction approaching, the Moon again gets into the act, this time as a slender and appealing 12% waning crescent – it’s 4½° from Venus and 6¾° from Saturn at 4:36 am, as shown below; Venus and Saturn are still 2¾° apart at this stage.

The Venus/Saturn conjunction two days later on the 9th is not to be missed, with both planets visible in the field of view of a low-medium power eyepiece, only 30' or ½° separating them. The following chart, exhibiting the same screen resolution as the previous one for comparison purposes, shows the scene at 4:38 am, with the pair 10° above the horizon. Venus rises at 3:39:32 am, Saturn at 3:41:04 am – Venus’ brightness has decreased imperceptibly to magnitude -4.04 and its phase has fattened marginally to 79%; the span of the disk remains at 14" (note that the view will be almost as riveting a day later, when Saturn sits just 38' above Venus).

See the notes to follow on Saturn for a magnification of the above chart, depicting some of the major Saturnian moons visible through amateur 'scopes.

On the 25th, Venus skirts the western edge of the open star cluster M21, and is only ½° or so from M20, the Trifid Nebula, meaning all three should fit comfortably within the field of view of a medium power eyepiece. Unfortunately the Full Moon will compromise the view – even Venus' own brilliance in the eyepiece will dampen the spectacle of the Trifid in particular.

Come the end of the month, Venus is rising at 4:01 am, just over 2½ hours after sunrise (6:32 am), at which time it is at an altitude of 27½°; the magnitude is down a touch more to -3.98, and the disk span to 12", while the phase is out to 85%.

Having been in Libra when the clocks chimed in the new year, Venus crosses into Scorpius late in the afternoon of New Year's Day, to Ophiuchus on the 5th, and then Sagittarius on the 21st, where it remains for the rest of the month.

Earth

Our home planet is at perihelion, its closest point to the Sun, on the 3rd, at a distance of 147.1 million km or 0.9833 a.u.

Mars

As shown in the earlier chart depicting the planetary line up, Mars is well placed for viewing in the morning sky. On the 1st, the Red Planet rises at 1:56 am, whereby it has already attained an altitude of some 27° before morning twilight starts to penetrate the darkness. Its disk, 91% illuminated and shining at magnitude 1.25, is still very small, 5.6", and as such cannot be expected to show detail through the eyepiece.

By the 31st, the situation has improved somewhat – the disk is out to 6.8", 90% illuminated, and it shines considerably brighter, at magnitude 0.85, on a par with Aldebaran (Alpha Tauri), the foreground star masquerading as the luminary of the open star cluster The Hyades.

Of more interest regarding a comparison of the appearance of Mars with that of the stars, is its proximity and wavering equivalence with Spica (Alpha Virginis). At magnitude 1.0, Spica outshines Mars until January’s third week, but is a little dimmer at the end of the month. Irrespective of the similarity in their brightness, the two appear distinct throughout, blue-white Spica contrasting with the orange hue of Mars.

Planet and star are quite close early, Mars being 5½° east (lower right) of the planet on the 1st, but by the 31st, the gap has widened to 21°.

Mars will continue to grow bigger and brighter in our sky as it approaches opposition (opposite the Sun in our sky, one rising as the other sets, and at its closest to Earth) in May, when it will shine brilliantly at magnitude -2.05 and span 18".

Mars begins the month in Virgo and transitions to Libra on the 17th.

Jupiter

Absent the Moon, mighty Jupiter rules the morning skies in the hours before incomparable Venus enters the stage. As the month begins, Jupiter breaches the eastern horizon at 12:27 am; its disk spans 39", shines fiercely at magnitude -2.16 and is technically short of fully lit at a phase of 99%. The King of the planets, which reaches opposition early in March, is stationary relative to the background stars on the 9th as it begins its period of retrograde motion.

The best viewing window commences around the end of January's first week – the Moon interferes at the start of the month, but by the 7th doesn't rise, as a 12% waning crescent, until 3:41 am, Jupiter having risen at 12:04 am. Thereafter the Moon ceases to be a factor until the 17th when, as a 45% waxing crescent, it remains above the western horizon until 12:39 am, over an hour after Jupiter's rise time of 11:25 pm (on the 16th). In the days to follow, it both grows in phase and moves easterly, closer to Jupiter.

By the 23rd the window has closed, with the nearly Full Moon remaining in the sky until after the commencement of twilight. The last couple of days of the month offer a further, albeit tentative, window, with the 73% waning gibbous Moon rising just over ¾ hour after Jupiter, 11:21 pm vs 10:32 pm, on the night of the 29th – the time difference stretches to one hour and twenty three minutes on the following night (11:51 pm as against 10:28 pm). By this stage, Jupiter has brightened to magnitude -2.35, is officially fully illuminated and spans an impressive 42".

Given that New Moon falls on Sunday 10th, this would seem to be an ideal time to view the planet, with a transit of both Io and its shadow on display to heighten the spectacle. Jupiter rises at 11:52 pm on Saturday 9th, spanning 40" and shining at magnitude -2.22. Io's shadow enters the eastern side of the disk at 1:54 am and exits the western limb at 4:10 pm, allowing the entire shadow transit to be viewed under a dark sky (sunrise is not until 6:09 am, so twilight will not commence until around 4:20 am). Io itself first enters the disk at 2:59 am and its transit will still be underway as twilight begins.

The following chart is configured for 3:15 am, with Io 16 minutes into its transit and its shadow near the centre of the disk; Io's dark shadow will show up against Jupiter’s bright surface more readily than the moon itself, which at magnitude 5.45, will be barely visible in the eyepiece.

The three remaining Galilean moons are strung out to the east (right) of their parent. Europa shines at magnitude 5.7, Ganymede at 5.05 and Callisto, which has been omitted from the chart so as to facilitate sufficient magnification within the size limits imposed by this presentation (it lies a little further out from Ganymede than Ganymede is from Io), at 6.1. Note that, as these magnification ratings indicate, the moons would themselves verge on naked eye visibility were it not for the brilliance of Jupiter; as such they are very prominent in binoculars (held steadily or mounted) or a finder 'scope (at magnitude 5.05, Ganymede would certainly be faintly visible under a dark sky if without the glare of its parent).

Jupiter stays within Leo all month, coming within ¼° of the border with Virgo before retrograde motion kicks in on the 9th, reversing its path; it will actually remain within Leo until August.

Saturn

Saturn, probably the favourite planet of the majority of casual sky observers, currently resides in our morning skies. On the first of the month it rises at 4:09 am, a little under two hours before the 6:01 am sunrise, by which time it has climbed to 20° above the horizon, sporting a 15" disk; the mesmerising ring system is inclined to our line of sight at an angle of 26.1° and spans 34.7".

As indicated in the notes on Venus, Saturn participates in a memorable conjunction with that dazzling orb on the 9th, by which time the above statistics read 3:41 am, nearly 2½ hours, 6:08 am, 27°, 15", 26.2° and 35.0". The chart to follow, configured for 4:38 am – 1½ hours before sunrise – is a magnification of the conjunction chart displayed earlier when discussing the event in relation to Venus:

As the chart shows, the two planets approach so closely that the distance between Saturn and its outer major moon, Iapetus, is an appreciable proportion, over 22%, of the distance between the planets (from our viewing perspective); were the conjunction to have occurred when Iapetus was at greatest separation from its parent, this figure would stand at in excess of 26%!

With only ½° (the apparent width of a Full Moon) between Venus and Saturn, they can be viewed together telescopically at low-medium power. At magnitude 6.25, HIP81632 is far and away the brightest star shown on the chart. The depiction of the stars by the software, while it normally does a good job of representing the brightness of stars by their depicted sizes, is actually somewhat misleading in this instance – the reddish star immediately above the HIP designate is the next brightest, at magnitude 9.5, approximately 19 times dimmer!

While viewing of the moons of Saturn is best reserved for when the planet sits higher in the sky, here, nevertheless, is a further magnification concentrating on the moons at the time of the conjunction, with the brightness of the stars marked with their magnitudes:

The moons shown shine at the following magnitudes: Rhea 10.3, Dione 11.0, Enceladus 12.3, Tethys 10.8, Titan 8.9 and Iapetus 10.9. While the first five brightness ratings are sourced from Starry Night software, the last is my estimate, but I am confident it won't be far off the mark. Neither Starry Night nor any other reference I have been able to secure takes into account the orbital position of Iapetus in relation to Saturn – this is most unfortunate, as Iapetus is a two toned body, with one hemisphere as dark as charcoal and the other very much brighter, this factor affects its brightness in our skies more than does Saturn's distance from us.

My estimate is based on an overall range of 10.2-11.9, with a brightness discount for the fact that Saturn is not long past solar conjunction (the end of November), when Saturn is at its greatest distance from us, and an upgrade for the fact that Iapetus lies to the west of Saturn on the 9th, whereby its bright hemisphere faces us.

While there are no nearby stars to confuse with the four moons close to Saturn, and Titan stands out because of its relative brightness, identifying Iapetus is a little less straight forward (troublesome little beggar that it is laughing). Nevertheless, as the chart shows, there is only one star, of magnitude 11.9, near it which approaches its brightness (nor are there any others just out of the field of view of the chart that do so) and this star can actually aid in detection – merely look in the appropriate location for two 'stars' which are a little brighter than the others, and the brighter and farthest removed from Saturn of these two is your target.

By the end of the month, prospects for viewing the Ringed Planet have improved somewhat, although it is still strictly a morning target: it rises at 2:22 am and is 47° clear of the horizon when the Sun rises over four hours later (6:32 am). The disk spans 16" and the rings 35.8"; rings and disk taken together shine at magnitude 0.5 all month (to be more precise, 0.51 at the beginning and 0.53 at the end).

Saturn is in the constellation of Ophiuchus (which should cause the astrologers some consternation wink ) and will remain so not only throughout January, but all year and then some.

Uranus

The so-called ice giant Uranus (a descriptor it shares with Neptune, as distinct from the gas giant tag bestowed upon Jupiter and Saturn, which orbit considerably closer to the Sun) is well advanced in this apparition, but still available for viewing, particularly early in the month.

On the 1st it transits (reaches its highest point above the horizon) at 7:40 pm, just over an hour before sunset, and sets at 1:24 am (on the morning of the 2nd); it displays a 3.6" disk shining at magnitude 5.80.

As New Moon falls on the 10th, the Saturday evening of the 9th would seem to be an appropriate and convenient time to target Uranus. On this night it transits a little earlier, at 7:09 pm, and sets at 12:52 am. The planet's disk is imperceptibly smaller, at 3.5", and its brightness has also decreased marginally, to 5.82.

Uranus sits in a rather bland portion of the sky at this date and time; the following wide field chart serves to indicate its position relative to two prominent stars, Betelgeuse (Alpha Orionis, mag 0.4) and Aldebaran (Alpha Tauri, mag 0.8), as well as M45, better known as the Pleaides or Seven Sisters, and Hamal (Alpha Arietis). Note that all the Uranus finder charts to follow are configured for 10:30 pm, around the time that the last traces of evening twilight have dissipated:

Knowing where to look in the sky, refer now to the magnified chart below; all the labelled stars are visible in the previous chart and the two charts can be related via the agency of Hamal:

The magnitudes of the labelled stars in this chart are as follows: Hamal 2.0, Sheratan (Beta [β] Arietis) 2.6, Eta [η] Piscium 3.6, Epsilon [ε] Psc 4.25 and Delta [δ] Psc 4.43. All are naked eye stars, albeit faint with regards to the last two.

Use the three brighter stars to identify Epsilon and Delta, then refer to the final chart, which labels stars with their magnitude ratings:

Knowing that Uranus shines at magnitude 5.8, this final chart makes short work of identifying the planet through your finder 'scope. Simply note that the angle formed by Delta-Epsilon-Uranus is just a little sharper than a right angle (90°), and that Uranus is the lower of a nearly straight line of three ‘stars’ – it's a little brighter than the closer of its two companions and a little dimmer than the far one. The yet slightly dimmer star of magnitude 6.12 may also be handy as a reference.

Leaving Delta (magnitude 4.43 at lower left on the chart) out of the equation, Uranus and the other four labelled stars fit very comfortably within the field of view of a finder 'scope, and it shouldn't be too difficult, by relating the chart to what you see through the finder, to determine which of the points of light is Uranus.

Identification may be further aided by Uranus' tendency to shine more steadily than the shimmering stars (stars are essentially point sources of light due to their extreme distance from us and are therefore more affected by our shifting bubbling atmosphere than are the planets, which span a measurable portion of the sky) and by its subtle blue-green hue, as opposed to the whites and subtle reds of the stars.

Once you think you’ve nailed the planet, switch to the main eyepiece at a magnification of at least 100x, preferably a little higher, to confirm your capture by resolving the planet's tiny disk – if you’re on target, you’ll immediately be struck by the beautifully vivid blue colouration of the disk at this higher magnification.

As the month ends, the planet, shining at magnitude 5.86 and still spanning 3.5", is becoming more difficult to target: by the time twilight has subsided, around 10:15 pm, it sits just 13° clear of the horizon, setting at 11:27 pm.

Uranus is in Pisces, and will remain there until 2018/19 (although it approaches to within ¼° of Aries in August 2017 before withdrawing back deeper into Pisces courtesy of retrograde motion).

Neptune

As indicated by its location low in the west in the overview chart for Uranus, Neptune is now too far advanced in this apparition for clear and sharp views to be had – as depicted on the chart, configured for 10:30 pm on the 9th, just as evening twilight is fully extinguished, Neptune is less than 9° above the western horizon.

Although the situation was a little more tenable on the 1st of the month, when it was around 14° high as twilight ended around 10:35 pm – spanning 2.2" and shining at magnitude 7.93 – even then it sets less than 1¼ hours later (at 11:49 pm), permitting only a minimal and compromised viewing opportunity.

The situation at the end of the month has deteriorated further, as Neptune moves toward conjunction with the Sun at the end of February (rising and setting with it) – as January ends, Neptune sets in twilight at 9:53 pm; its span and brightness stand at 2.2" and magnitude 7.95 respectively.

Neptune is in the constellation of Aquarius and will remain so until 2022/23.

Pluto

Pluto's situation, from an Earthly viewing perspective, is even less tenable, as it reaches conjunction with the Sun on the 6th of this month.

For the record, it sets 10 minutes after the Sun on the first, then, having moved into our morning skies after conjunction, it rises at 4:39 am on the 31st, less than 2 hours before sunrise, when it's only 20° clear of the eastern horizon. The tiny frozen orb's span and brightness vary from 0.093" and mag 14.42 to 0.094" and mag 14.26 throughout the month.

Pluto will remain within Sagittarius until 2023/24.

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Our Monthly Feature

What better celestial spectacle to feature on the first month of the year than the first object in the Messier catalogue, M1 (NGC 1952), the Crab Nebula?

The Crab is a supernova remnant, the expanding envelope resulting from the explosion of a massive star. The creation of the Crab has been linked to a brilliant 'guest star' visible in daylight during 1054; the most extensive records of its appearance were left by Chinese astronomers of the time, although some mention has been found in Japanese and, quite recently, Middle Eastern writings.

At the heart of the Crab Nebula lies the Crab Pulsar, an incredibly dense and rapidly rotating (thirty times per second, no less, breakneck speed for a body perhaps 20 km in diameter) neutron star, catalogued as PSR B0531+21, which, along with the expanding nebula, is all that remains of the progenitor star. In a nutshell, neutron stars are formed when a massive star collapses under its own gravitational attraction; the implosion and resultant compression is so great that electrons which previously orbited atomic nuclei in the star's core are in effect forced into the nuclei where they combine with the positively charged protons therein (electrons having a negative charge) to form neutrons, which are without charge. Thus instead of the previously existing electrons, protons and neutrons, neutrons alone come to dominate – hence the name neutron star.

The Crab Nebula was first spotted by John Bevis in 1731; twenty seven years later, in 1758, Charles Messier made it the first entry in what was to become his famous catalogue, after he initially mistook it for Halley's comet and determined that a list of bodies with an outwardly cometary appearance would be helpful to himself and others when searching for comets. The nebula got its name in 1844 when the 3rd Earl of Rosse made a sketch of it through a telescope and thought what he had drawn looked like – a crab.

The Crab's recognition as a supernova remnant can be traced back as far as 1921, when it was recognised that images taken several years apart showed subtle changes – it was Edwin Hubble, in 1928, who first proposed that the nebula was associated with the 'guest star' of many centuries earlier, and after initial reluctance by his peers, it eventually gained notoriety as the first supernova remnant to be identified.

The Crab's distance from us is a little uncertain, but has been given as 6500 light years, with an uncertainty of +/- 1600 light years. Using the figure of 6500 l.y. and given that the event was witnessed in our skies in 1054, we can deduce that the actual explosion happened around 5400 BC.

While the Crab is conspicuous as supernova remnants go, be aware that a dark sky is needed if the view is not to disappoint the first time viewer – it’s a misty object at best through amateur 'scopes (other than when imaged with an extended exposure); under a light polluted sky you’ll be met with an almost invisible haze, if you can see it at all.

This chart is configured for 11pm on the Saturday night of the 9th, under a moonless sky:

All star charts courtesy of StarryNight®ProTM Version 6.4.3/Simulation Curriculum Corp.

The Crab is actually quite easy to find – just use the prominent stars Aldebaran, Betelgeuse, Procyon (Alpha Canis Minoris), Alhena (Gamma [γ] Geminorum) and Elnath (Beta Tauri), of magnitudes 0.8, 0.4, 0.4, 1.9 and 1.6 respectively, along with the Pleiades, to identify magnitude 3.0 Zeta [ζ] Tauri and your task is almost complete. While fainter than the stars used to pinpoint it, Zeta is still a comfortable naked eye capture. Just fit a low power eyepiece and focus on Zeta, then nudge your 'scope to the north-west (lower left) and you’ll came across the nebula a little over 1° away – you can then experiment with different magnifications to find that which is most pleasing; you’ll probably prefer views at low power, as higher magnifications will 'wash out' the image, making it very dim.

We've all been spoilt by the beautiful and colourful images of the Crab returned by Hubble and other advanced telescopes – while the view through your 'scope will be bland by comparison, the knowledge that you're looking at the remains of an exploded star and that the photons of light imprinting on your pupils have been travelling for six and a half thousand years lends impact to what you see.

As always, any questions, comments or suggestions are welcome and may be directed to: waynerobertsau@yahoo.com.au

Until next month:

CLEAR SKIES and HAPPY HUNTING