Compiled by Wayne Roberts

th Thurs 5 New Moon. th Sat 7 Dark Sky Viewing Night, Benalla Sports and Equestrian Centre (adjacent to racecourse). Fri 13th First quarter Moon. th Mon 16 Moon at perigee (closest to Earth, 367,391 km). th Wed 18 Monthly meeting, Hockey clubrooms, Churchill Park, Waller St., 7:30 pm. th Thur 19 Full Moon. th Fri 20 Pluto stationary. rd Mon 23 September equinox. th Wed 25 Mercury at aphelion (furthest from , 69.8 million km) th Fri 27 Last quarter Moon. th Sat 28 Moon at apogee (furthest from Earth, 404,308 km). Astronomy Benalla’s experience with the weather as of late has been reminiscent of our horror run last ; we have now been clouded out on each of our last two monthly viewing nights. The silver lining on, literally, the cloud was the success of two outreach ventures during the month – a solar viewing at a Shepparton primary school and an abbreviated night viewing of the Moon, Saturn, Venus and Omega Centauri at which we played host to a group of visiting Japanese students. Both events were well received, with the youngsters in Shepparton eager to examine the telescopes and view the Sun, and the Japanese students bowled over by views of, in particular, the Moon and Saturn. The 23rd of this month marks the September equinox, when the sun rises due east and sets due west and days & nights are of equal length. The transition from winter to summer is a double edged sword from an astronomical observer’s viewpoint – the nights are becoming warmer and more hospitable while at the same time starting later and finishing earlier. Slides and roundabouts. Our monthly viewing nights are held on the Saturday closest to New Moon unless it falls on a Wednesday; on these occasions we view on the preceding Saturday to avoid a Moon setting late in the evening. This month, with New Moon falling on a Thursday, it sets almost two hours after the Sun, giving us a relatively rare opportunity (averaging two months in seven) to observe the lunar surface. The extremely thin 3% waxing crescent will make an interesting target early in the evening.

Set up 6:00 pm. Sunset 6:01 pm. (Twilight ends 7:24 pm). Moon sets 7:51 pm. Twilight begins 5:04 am. Sunrise 6:27 am. Moon rises 7:35 am.

As September begins, Mercury is in the very early stages of its best evening apparition of the year. On our viewing night it is 10° above the sunset horizon and sets just under an hour later at 6:56 pm, its 94% illuminated disk spanning 5" [" denotes arc-second = 1/60th of an arc-minute (symbol ') or 1/3600th of a degree (symbol °)] and piercing the twilight at magnitude -0.7. Watch the inner planet as it climbs ever higher with each passing day throughout September and into next month before diving back toward the western horizon in late October. At the end of September, as Mercury’s inner orbit causes it to begin catching up to Earth, its apparent size has increased to 5.9", phase and magnitude have decreased to 74% and -.05 respectively (remember from last month that Mercury is the only planet which shines brighter when on the far side of the Sun than when on the same side as Earth) and elevation at sunset has increased to 23°. It reaches greatest eastern elongation (24°) from the Sun on October 9th, a couple of days after rising up past Saturn which is sinking slowly toward the western horizon. This month and next represent your best opportunity this year to follow the speedy ‘Messenger of the Gods’ as its size, phase, magnitude and altitude change at a rate far faster than any other planet. Brilliant Venus continues to dominate the western sky, climbing higher and shining a little brighter on each successive September night. As the month begins it stands 37° above the sunset horizon, its 15" disk 74% illuminated and shining at magnitude -4.02. On our viewing night, these figures read 38°, 15", 72% and -4.05 respectively; look for Spica (Alpha [α] Virginis, mag 1.0) 2° SW (lower left) of Venus, and Saturn 12½° east (upper right). By month’s end the respective parameters are 42°, 18", 63% and -4.17. On the morning of the 9th, the 12% lit waxing crescent Moon’s northern limb passes just 6½' – less than a quarter the size of the lunar disk – from Venus while still beneath the eastern horizon; use the Moon and Venus as mutual tools to locate each other in broad daylight – the planet runs ahead of the Moon’s illuminated NW limb by 1½° (three lunar disks) at 9 am and 2° at 10 am (one finger at arm’s length is approx. 1°) Mars has just cleared morning twilight and continues to pull painfully slowly away from the eastern horizon throughout September, with the pace picking up slightly as the month progresses. On the morning of the 8th (following our viewing night) it rises at 4:44 am, just under 1¾ hours before the Sun and stands 17° above the sunrise horizon, its 4.1" disk 96% lit and shining at magnitude 1.64. Look for it 15° SE of Pollux (Beta [β] Geminorum, mag 1.15) and 22° east of brilliant Jupiter (thumb to little finger of an open hand at arm’s length is approx. 20°). The Red Planet’s ruddy hue and the fact that it shines far more vigorously than any star in its vicinity should make it fairly easy to locate. If you turn a telescope or binoculars on Mars at this time you will see that it stands only ½° west (upper left) of M44, the Beehive Cluster; on the following morning it is even closer, skimming the SW (upper) edge of the cluster. Under a dark sky, M44 is quite easy to see with the naked eye; can you still see it close to the horizon and with Mars shining nearby? By month’s end it rises at 4:00 am and is 19° above the horizon at sunrise (5:54 am), its size, phase and magnitude are 4.4", 95% and 1.61 respectively. Our neighbour’s leisurely climb above the eastern horizon continues to accelerate at a very modest rate throughout the year; by end it rises about ¼ hour after midnight. Mighty Jupiter is king of the morning sky. On the 8th it rises at 3:22 am, more than three hours before sunrise, at which time it is 26° above the horizon. Its 35" disk is 99% illuminated and blazes fiercely at magnitude -2.1. Scan Jupiter’s trailing edge with a telescope to catch its inner Galilean moon, Io, emerging from an occultation between 5:36:46 am and 5:40:22 am, having passed behind its parent body just before it cleared the eastern horizon; note

that this event occurs a little over ½ hour after twilight commences (but still more than ¾ hour before sunrise). Jupiter, which shares the morning sky with Mars, is pulling away from the eastern horizon at a considerably faster rate (due to Mars’ smaller orbit carrying it eastward against the starry backdrop of our sky at a greater rate than Jupiter) than its little brother. By month’s end it is rising at 2:07 am and is over 29° clear of the horizon at sunrise; it is a little bigger at 37" and brighter at -2.2. Saturn and its wondrous ring system are now well advanced in this apparition but still a captivating sight in the early evening sky. This beauty sets on our viewing night at just 10:13 pm, so target it as soon as the sky fully darkens (evening twilight nominally ends at 7:24 pm, but the slim crescent Moon will brighten the sky slightly until it sets just under ½ hour later) in order to minimise the blurring effects of Earth’s atmosphere. Saturn shines at magnitude 0.7, its disk spans 16" and it stands 49° clear of the horizon when the Sun goes down. The magnificent ring system, spanning more than twice the diameter of the planet’s disk, is now inclined at almost 18½°. To locate 2 Saturn with the naked eye early in the evening, note that it is about /3 as far above Venus as the latter is above the Moon. As Venus continues to pull away from the western horizon daily, it closes the gap with Saturn; on the 18th the pair stand side by side with Venus 3½° to Saturn’s south (left). How many of Saturn’s brightest moons can you see? Here are the circumstances at 8 pm on our viewing night (they won’t change appreciably before Saturn sets): Titan, the largest and brightest at magnitude 8.8, lies 2½' to Saturn’s east (upper right); Rhea (mag 10.2) is on the same side a little under half as far 2 from the planet and Dione (mag 10.9) is closer in again, slightly left of, and /3 of the way along, a line from Saturn to Rhea. On the opposite side of the planet to this trio, and about as far out as Dione, lies Tethys (mag 10.7); depending on your optics and seeing conditions, you may see Enceladus (mag 12.2) just 8" closer in than Tethys. Finally, look for Iapetus (mag 11.6) which lies 2' (80% as far out as Titan) NNW (lower right) of Saturn. Only one star (USNO J1423428-115250) brighter than magnitude 14 is as close to the planet as the above moons. It shines at a dim mag 13.9 and is away from the ‘action’, 1¼' to Titans lower right; so unless you are viewing through a very large instrument, all the points of light near Saturn are moons. Uranus reaches opposition (directly opposite the Sun in our sky and closest to Earth) early next month, so its prime viewing time lies in the coming months. Tonight it rises at 8:02 pm and transits (reaches its highest point above the horizon) at 1:52 am, its 3.7" disk shining at magnitude 5.7, rendering it just visible to the keen naked eye under good conditions. While views clear of the horizon at a reasonable hour are still a couple of months away, find it tonight 13½° south (to the right early evening, upper right later) of Algenib (Gamma [γ] Pegasi, mag 2.8), the southern- most (top right) star in the Great Square of Pegasus. Delta [δ] Piscium is the nearest comfortable naked eye star (mag 4.4) to Uranus and lies south or below Uranus, 80% of the way along Algenib → Uranus (“the line”) and about 3½° to the right; the next nearest is 20 Ceti, mag 4.45, which is 5½° past Uranus as an extension of “the line”. Don’t confuse the planet with two nearby stars of the same brightness – 96 Piscium 2° below Uranus and 51 Piscium, just left of the ¾ mark of “the line”; the planet will have a noticeable blue/green tinge. Confirm your sighting by bumping up the magnification to detect the planet’s disk. Note that last month’s viewing notes included finder charts for both Uranus and Neptune; with both planets having wide orbits and near the time of opposition, neither has moved very far. Neptune, the outermost planet-proper (poor old defrocked Pluto), reached opposition late last month and so is entering its prime viewing time. The two nearest naked eye stars are Ancha (Theta [θ] Aquarii, mag 4.15), 3½° north (below left before midnight) of the planet, and Sigma [σ] Aquarii, mag 4.8, 2° east (below right before midnight). Consult last month’s notes and/or a star chart; as with Uranus, the planet’s slight blue/grey hue will distinguish it from nearby

stars and high magnification (around 200x) will display the tiny 2.4" disk. Neptune shines at magnitude 7.8 and as such will be prominent in binoculars or a finder ‘scope. Pluto remains embedded in the star-rich of Sagittarius; having reached opposition early in July, this is a prime time to seek it out. On the 20th, it ceases retrograde motion and resumes its slow eastward movement relative to the starry backdrop.. Of course at magnitude 14.1 and with a disk far too small for amateur instruments, it represents a considerable challenge. Again, last month’s notes included a (wide field) finder chart. Tonight it transits at 7:48 pm and sets at 2:53 am; those wishing to attempt it are invited to send me an e-mail requesting a detailed finder chart.

This month we take a look at a feature of the night sky which is often overlooked – that of multiple star systems. It is not widely appreciated outside the ranks of amateur (and professional) astronomers that the majority of stars do not form in isolation, but rather as pairs, triples or even more populous groupings orbiting around one another. Some systems can be resolved into their individual stars with the naked eye, but most require magnification. Some have such tight orbits or are so far away or both that they cannot be ‘split’ even by professional telescopes; these systems, which are known to be multiple courtesy of spectroscopic analysis of their emitted light, are referred to, fittingly, as spectroscopic doubles (or triples etc.). Obviously the multiples featured below will be resolvable through amateur ‘scopes, although some systems will contain sub-systems which may be difficult or impossible to split. In addition, we will look at a few doubles which do not orbit one another but merely lie along the same line of sight and may be separated by huge distances; these are referred to as ‘optical doubles’. As all stars, whether in multiple systems or otherwise, are in constant motion, the angular distance between them from our perspective is also constantly changing, albeit very slowly; with this in mind, note that all angular separations in the following text are current. Note also that as we are dealing with huge distances which are measured in light years, discrepancies in stated particulars exist between competing sources; distances, magnitudes and angles given below are generally drawn from Starry Night software or Wikipedia. Let’s start our tour with one of the best, Rigel Kentaurus, otherwise known as Alpha Centauri, the brightest of the Pointers to the Southern Cross (AKA Crux) and the closest to our Sun. Although there is an appreciable difference in magnitude between the two components – the ‘primary’ is mag 0 and the ‘secondary’ mag 1.4 – this tends not to be too obvious through the eyepiece and the double, separated by 10", always strikes yours truly as resembling a pair of car headlights approaching from the distance. The stars take just under 80 years to complete an orbit separated by a distance varying from Sun → Saturn to Sun → Pluto. But the story does not end there; a whopping 2.2° away is Proxima Centauri, a red dwarf shining at a modest 11.0. Proxima (so named because it is closer to our solar system than the primary & secondary) is thought to be gravitationally bound to the other two, but there is an element of uncertainty about this. Its orbit, if it is part of the system, takes between 100,000 and 500,000 years. As charts 1 and 2 to follow will show, Proxima Cen is well removed from Alpha Cen in our sky, making it appear incredulous that the two are (thought to be) bound; the separation is caused jointly by the wide orbit of Proxima and the proximity of the system to Earth. Moving a short distance to Circinus, Alpha Cir – the brightest member of this faint constellation – is a binary whose primary and secondary shine at mag 3.8 and 8.5 respectively at a separation of 5.7". The system is 54 light years distant and has a period of 2600 years with an orbital separation of 260 au (1 au = the average Earth-Sun distance). Note that I have not previously viewed this double and one other respected, but dated, source gives the separation as 16"; computing an approximate angular separation

from the above figures yields some 15" if an assumption is made that the Earth-primary-secondary angle is a right angle, but I do not know that to be the case and it is unlikely to be so. What do you see? Not far away is Acrux (Alpha Crucis), the star at the head of the Southern Cross. This system is a real work of art in more ways than one. High magnification splits the mag 1.4 primary and mag 2.1 secondary, which are separated by 4.1", into a sight very similar to the Alpha Centauri pairing. They are thought to be separated by at least 430 au and take a minimum of 1500 years to complete an orbit. Additionally, TYC 8979-1072-1 (AKA Alpha-3 Crucis), lies all of 90" away shining much less vigorously at mag 4.8; this star shares the motion through space of the others and so is thought (but not positively known) to be gravitationally bound to them. Nor does this story end there – the primary, α1, is itself a spectroscopic double whose components orbit each other over a period of only 76 days at a separation of 1 au; there is also speculation that α3 may be a double. Thus what appears to the naked eye as a single star is actually at least four and possibly five stars! Two of the other four stars which form the Southern Cross are also doubles: Becrux (AKA Mimosa, Beta Crucis) is a spectroscopic double whose components orbit each other every five years at a separation of 5.4 – 12 au. Gacrux (Gamma Crucis) is an optical double (the system was previously thought to be gravitationally bound) with the secondary of magnitude 6.4, 2" from the primary and five times as distant; Gacrux may also have an as yet undetected white dwarf (the collapsed remains of a Sun-like star which has consumed all its nuclear fuel) in a true binary orbit. Still in Crux, Mu [μ] Crucis is a double whose components shine at 4.0 and 5.1 at a separation of 35". Hopping across the border into the constellation Musca, we find our next target, Theta Muscae. Telescopically, θ Mus is seen to have a magnitude 7.5 secondary 5.5" from the magnitude 5.5 primary. This system remains somewhat of a mystery; it is an optical double, with the secondary just in the line of sight of the primary but not physically associated with it, but the primary itself is at least a spectroscopic double with a period of 19.14 days, and irregularities in the data suggest that it may actually be a triple. While in Musca, target Eta [η] Muscae, another optical double; the primary, of magnitude 4.8 is just less than 60" away from magnitude 7.2 TYC9242-879-1. The primary may also be a spectroscopic double, although I am a little unsure on this point. If you’re up for a challenge, try Beta Muscae, whose components, of magnitudes 3 and 4 are separated by just 2". Moving now away from the far south , no treatment of double stars should overlook Epsilon [ε] Lyrae, known as the Double Double. While this is more conveniently situated in the sky for viewers in the northern hemisphere, it certainly rates both a mention and a look here. Readily located as the nearest naked eye star to brilliant Vega (Alpha Lyrae, mag 0), ε Lyr is easily split (even with binoculars or a 1 2 finder ‘scope) into two components, ε and ε . These two systems are 3½ ' apart and orbit each other over a period of hundreds of thousands of years. Both ε1 and ε2 are themselves doubles, but much closer and more difficult to split: The former’s components are 2.6" apart and of mag 4.7 & 6.2, the two orbiting one another over a period of approximately 1200 years; the other’s 2.3" apart, of mag 5.1 and 5.5 with an estimated 600 year period. Additionally a fifth star has been confirmed to orbit one of the ε2 stars over a period of some tens of years; the maximum separation of 0.2" is far too small for amateur instruments to detect. And if that doesn’t rock your boat, it is possible that other nearby stars are also gravitationally bound, resulting in, perhaps, a ten star system. Sargas (Theta Scorpii), the bright mag 1.8 star in the tail of the scorpion, is also a fine double. Sargas is the brightest star in the constellation apart from brilliant (Alpha Scorpii, mag 1.0) at one end and Shaula (Lambda [λ] Scorpii, mag 1.6) at the other. It is an optical double; the secondary is 6.5" away, shining at magnitude 5.4.

All the above systems are visible to the naked eye (Proxima Centauri is not, Theta Muscae, at mag 5.5, is very faint). Four finder charts follow: the first shows targets in Centaurus, Circinus, Musca and Crux; the second is a high magnification chart to help locate dim Proxima should you wish to do so; the third portrays Crux and Musca and the last Lyra and Scorpius. Note that Mu Crucis is marked Mu1 Crucis and Epsilon Lyrae as Epsilon2a Lyrae. Note also that Mu is poorly labelled; it is the large star under the ‘sa’ of Mimosa in chart 1, not the smaller star under the comma. Similarly, in chart 3, it is the large star under the ‘u’ of Becrux, not the smaller star next to the ‘M’ of Mu. The Pointers, Scorpius, Sagittarius and the Winter (or Summer) Triangle are also shown to aid in orientation.

Chart 1

Chart 2

Chart 3

Chart 4. All star charts courtesy of StarryNight®ProTM Version 6.4.3/Simulation Curriculum Corp.

This selection has barely scratched the surface folks; between the tendency of stars to form in multiple systems and the inevitable occurrence of chance alignments, doubles, triples etc. abound in the night sky. Almost everyone knows of Castor and Pollux, the ‘twin’ stars of Gemini (which are not currently visible in our night sky, apart from a brief window before dawn), but did you know that Castor can be telescopically seen to be a triple star system? Further, each of these stars is a spectroscopic binary, bringing the number of gravitationally bound stars to six, and there is at least one more optical (in the line of sight) component to the system. Another of my favourites, currently visible in the early morning sky, is brilliant Rigel (Beta Orionis), above (here in the south) the belt of Orion. Rigel has a companion that, while not far below the threshold of naked eye visibility at magnitude 6.7, requires considerable magnification and careful examination to detect because of its proximity to its primary; just to add spice to the equation, Rigel B, as it is known, is itself a spectroscopic double. As a matter of unrelated interest, Rigel is a true giant of a star; while even much larger behemoths exist throughout the heavens, if Rigel were to replace our Sun, it would span 35° of the sky (and fry our planet)! Even Sirius, the brightest star in the night sky (with about the same viewing window as Rigel) has a companion, Sirius B. Sirius B is a stellar remnant which was more massive and luminous than Sirius itself (‘Sirius A’) until it evolved into a white dwarf some 120 million years ago. The separation between the two varies between 3" and 11" over their 50 year orbit; because they were at their closest just nine years ago, Sirius B is currently a real challenge to detect next to its scintillating primary. As you view multiple star systems, I suggest you do so in a progressive manner, first examining the system with the naked eye, then with your finder ‘scope, followed by the lowest magnification available through the eyepiece and so on up to the highest mag. that equipment and seeing conditions will allow. This will ensure ‘capture’ of all stars in the system, it’s also a good idea to consult a reference beforehand to know what to expect. That’s all for this month folks; with warmer weather just around the corner we can look forward to more comfortable viewing conditions, albeit later in the evening after the summer sun has set. Any comments, questions or suggestions? Feel free to e-mail me at [email protected] Until next month: