September
2020
Updated: 15 September 2020
Welcome to the night skies of Spring, featuring Scorpius, Sagittarius, Aquila, Lyra, Cygnus, Mars, Jupiter and Saturn
Note: Some parts of this webpage may be formatted incorrectly by older browsers.
The Alluna RC-20 Ritchey Chrétien telescope was installed in March, 2016.
The 20-inch telescope is able to locate and track any sky object (including Earth satellites and the International Space Station) with software called TheSkyX Professional, into which is embedded a unique T-Point model created for our site with the telescope itself.
Explanatory Notes:
Rise and set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual horizon.
Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space
as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is
given first.
When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a planet, asteroid or comet from the Sun.
The zenith is the point in the sky directly overhead from the observer.
Eclipses always occur in pairs, a lunar and a solar but not necessarily in that
order, two weeks apart. The
meridian is a semicircle starting from a point on the horizon that is exactly due north from the observer, and arching up into the sky to the zenith and
continuing down to a point on the horizon that is exactly due south. On the way down it passes through the South Celestial Pole which is 26.6 degrees above the
horizon at Nambour. The elevation of the South Celestial Pole is exactly the same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the
horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this point in the sky in the southern hemisphere, and to an equivalent point in the
northern hemisphere, near the star Polaris, which from Australia is always below the northern horizon.
All astronomical objects rise until they reach the meridian, then they begin to set. The act of crossing or 'transitting' the meridian is called 'culmination'.
Objects closer to the South Celestial Pole than its altitude above the southern horizon do not rise or set, but are always above the horizon, constantly
circling once each sidereal day. They are called 'circumpolar'. A handspan at arm's length with fingers spread covers an angle of approximately 18 - 20 degrees.
Your closed fist at arm's length is 10 degrees across. The tip of your index finger at arm's length is 1 degree across. These figures are constant for most people, whatever
their age. The Southern Cross is 6 degrees high and 4 degrees wide, and Orion's Belt is 2.7 degrees long. The Sun and Moon average half-a-degree (30 arcminutes)
across.
mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and
magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude
8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as
magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and
Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the
overhead Sun is magnitude -26.5. Each
magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0
stars are exactly 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 =
The Four Minute Rule
How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross
the meridian on successive days. This 24 hours is a little longer than one complete rotation, as the curve in the Earth's
orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars,
clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to
the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one
rotation, and is called a 'sidereal day'.
As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute
difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:
1. The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the
course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a
year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.
2.
For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means
that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars
of autumn, such as Orion are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east. The stars
change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to
its theoretical starting point.
We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those
we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All
the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift with
the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones
(such as Saturn or Neptune) moving the slowest.
Solar System
Sun:
The Sun begins the month in the zodiacal constellation of Leo, the Lion. It leaves Leo and passes into Virgo, the Virgin on September 10.
M
October 31:
Moon at 8 days after New, as on September 25.
The photograph above shows the Moon when approximately eight days after New, just after First Quarter. A rotatable view of the Moon, with ability to zoom in close to the surface (including the far side), and giving detailed information on each feature, may be downloaded
here. A professional version of this freeware with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the Moon's surface) and many other useful features is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage.Click
here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool ! All these downloads are freeware, although the authors do accept donations if the user feels inclined to support their work.
Lunar Feature for this Month
Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting
attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it
is important. This month we will look at the network of grooves or 'rilles' adjacent to the ghost crater, Prinz and
near the bright crater Aristarchus.
Much older is the nearby crater Prinz, which was formed well before the Imbrium Event of 3.8 billion years ago, and was badly damaged by that Event,
and then almost swamped by the great lava flood that created the Mare Imbrium (Sea of Rains).
The southern wall of Prinz has been carried away, but the northern and eastern
portions of the wall are intact. Their bases are submerged beneath the lava
plain, but some peaks protrude up to 1000 metres above ground level.
To the north of Prinz are numereous rilles, together known as the Rimae Prinz. These are
groove-like valleys that often start in a small, more-or-less circular depression without a rim, much like a terrestrial sinkhole or subsidence.
Some extend for up to 80 kilometres. Similar features can be found on Earth, the subsidence being associated with volcanic activity, where a magma chamber below ground has been emptied, with the
molten lava running off through an underground tube, which becomes a tunnel when the lava has drained away and the tube cooled. These tubes generally become
narrower the further they go, finally petering out. The tubes are usually just below the ground surface. Some 'lava tubes' formed in this way can be found near
Ravenshoe in North Queensland - Google "Undara Lava Tubes".
If the roof of a lava tube collapses into the empty tube, a long, narrow, sinuous valley is formed, starting at the sinkhole and then becoming narrower
the further it extends. It is thought that the rilles seen above formed in this way. The mystery is not completely solved however, for one rille in the image
above is crossed almost at right-angles by a 7 kilometre long rille with straight sides, rounded ends and a perfectly flat floor. You can find this
feature, which hardly looks natural, midway between the centre of Prinz and the upper right-hand corner of the image.
See
image No. 9 of the 
Aristarchus
Prinz
Wilhelm Prinz (1857-1910) was a German-Belgian astronomer noted for his
early lunar photographs. He worked in the Belgian Royal Observatory at Uccle,
and his photographs reached such a high standard for his time, that he was given
access to the new 36-inch refractor at Lick Observatory in America, in order to
produce a lunar atlas. This he completed and self-published as 'Agrandissements
de Photographies Lunaires' in 1894. There was only one edition of this book,
for technology was advancing rapidly and better lunar photographs were soon
being produced by others around the world.
The Prinz rilles are located inside the rectangle above.
Click
Geocentric Events:
It should be remembered that close approaches of Moon, planets and stars are only perspective effects as seen from the Earth - that is why they are called
'geocentric or Earth-centred phenomena'. The Moon, planets and stars do not really approach and dance around each other as it appears to us from the vantage
point of our speeding planet.
September 3: Moon 3.3º south of Neptune at 10:32 hrs
October 3: Venus 5 arcminutes south of the star Regulus (Alpha Leonis, mv= 1.36) at 08:58 hrs
October 3: Limb of Moon 11 arcminutes south of Mars at 13:25 hrs
October 3: Mercury at aphelion at 16:10 hrs (diameter = 5.4")
October 4: Moon 2.9º south of Uranus at 19:19 hrs
October 4: Pluto at eastern stationary point at 17:26 hrs (diameter = 0.1")
October 9: Limb of Moon 7 arcminutes south of the star Mebsuta (Epsilon Geminorum (mv= 3.06) at 20:22 hrs
October 11: Jupiter at eastern quadrature at 23:14 hrs ( diameter = 39.2")
October 14: Mars at opposition at 09:04 hrs (diameter = 22.3")
October 14: Moon 4.1º north of Venus at 14:48 hrs
October 15: Pluto at eastern quadrature at 19:59 hrs (diameter = 0.1")
October 18: Moon 7º north of Mercury at 06:33 hrs
October 18: Saturn at eastern quadrature at 23:56 hrs (diameter = 16.6")
October 19: Limb of Moon 38 arcminutes north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 21:50 hrs
October 20: Mercury at Greatest Elongation East (24º 28') at10:43 hrs (diameter = 6.6")
October 22: Moon 1.5º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 04:07 hrs
October 22: Moon 1.6º north of the star Nunki (Sigma Sagittarii, mv=2.02) at 14:07 hrs
October 23: Limb of Moon 40 arcminutes south of Pluto at 07:36 hrs
October 23: Moon 2.4º south of Saturn at 13:14 hrs
October 27: Moon 3.8º south of Neptune at 19:27 hrs
October 30: Moon 1.9º south of Mars at 05:51 hrs
October 31: Venus at perihelion at 06:36 hrs (diameter = 13.2")
The Planets for this month:
M
ercury: During September, the innermost planet is in the evening twilight sky, setting soon after the Sun. It will become noticeable low in the west after sunset early in the month. Mercury will pass very close to the first magnitude star Spica on the evening of September 22. Mercury will reach its greatest angular distance from the Sun (Greatest Elongation East, 25º 49') on the evening of October 1, when it will set 1 hour 57 minutes after the Sun. On that date its phase will be 60.7%. Mercury will pass between the Earth and the Sun (inferior conjunction) on October 26, and will then return to the eastern pre-dawn sky, becoming visible before sunrise.
Venus: This, the brightest planet, was an 'evening star' in the first five months of 2020. It passed through inferior conjunction (when it overtook the Earth by passing between us and the Sun) on June 4, after which it moved to the eastern pre-dawn sky. It is now a 'morning star', and on September 1 it may be spotted high in the north-east before dawn breaks, in the constellation Gemini. During September it rises at about 3:30 am. Venus reached its greatest elongation west (43º 17') on August 14. This was when it was at its greatest angular distance from the Sun, and at magnitude -4.3 it remains very bright, bright enough to cast faint shadows. Venus will cross into Cancer on September 4, and will remain a 'morning star' until early next year. The waning crescent Moon will be close to Venus on the morning of September 14.
(The coloured fringes to the second, third and fifth images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus itself. The planet was closer to the horizon when these images were taken than it was for the second photograph, which was taken when Venus was at its greatest elongation from the Sun).
March 2020 mid-May 2020 early July 2020 mid-August 2020 December 2020
Click here for a photographic animation showing the Venusian phases. Venus is always far brighter than anything else in the sky except for the Sun and Moon. For all of 2019 up to July, Venus appeared as an 'Morning Star' in the eastern pre-dawn sky, but in August last year it moved to the evening sky to be an 'Evening Star'. Last June it moved back to the pre-dawn sky and it is presently a 'morning star' once again. It will become an 'evening star' again next March, although it will not be clearly visible until May.
Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
Mars: The red planet is becoming a better object for viewing this month, as the Earth is catching up to it. Mars is slowly increasing in angular size, and by September 1 it reached 19 arcseconds in diameter, with a brightness of -1.52 (brighter than the brightest star, Sirius). It passed through western quadrature (rising at midnight) on June 7. Mars begins the month in the constellation of Pisces, and is by far the brightest night object in that part of the sky, except for the Moon. On September 1 Mars will rise almost due-east at about 9:15 pm, and on September 30 at 7 pm. Mars will perform its retrograde S-bend in Pisces between September 10 and November 14. During this period it will be close to Earth and quite large and bright. At the midpoint between these two dates, Mars will be at its closest distance to Earth, an event called "opposition", as it will be directly opposite the Sun in the sky, rising in the east as the Sun sets in the west. The actual date of opposition will be October 14, when the angular diameter of Mars will be 22.3 arcseconds. The waning gibbous Moon will be in the vicinity of Mars on September 6 and October 3
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
Mars photographed from Starfield Observatory, Nambour on June 29 and July 9, 2016, showing two different sides of the planet. The north polar cap is prominent.
Brilliant Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374, and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure 5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905. He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were not there, the galactic centre would be so bright that it would turn night into day.
Mars near opposition, July 24, 2018
Mars, called the red planet but usually coloured orange, in mid-2018 took on a yellowish tint and brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena were caused by a great dust storm which completely encircled the planet, obscuring the surface features so that they were only seen faintly through the thick curtain of dust. Although planetary photographers were mostly disappointed, many observers were interested to see that the yellow colour and increased brightness meant that a weather event on a distant planet could actually be detected with the unaided eye - a very unusual thing in itself.
The three pictures above were taken on the evening of July 24, at 9:05, 9:51 and 11:34 pm. Although the fine details that are usually seen on Mars
were hidden by the dust storm,
some of the larger features can be discerned, revealing how much Mars rotates in two and a half hours. Mars' sidereal rotation period (the time taken for one complete
rotation or 'Martian day') is 24 hours 37 minutes 22 seconds - a little longer than an Earth day. The dust storm began in the Hellas Desert on May 31, and after two months
it still enshrouded the planet. In September it began to clear, but by then
the close approach had passed.
Central meridian: 295º.
The two pictures immediately above were taken on the evening of September 7, at 6:25 and 8:06 pm. The dust storm was finally abating, and some of the surface features were becoming visible once again. This pair of images also demonstrates the rotation of Mars in 1 hour 41 minutes (equal to 24.6 degrees of longitude), but this time the view is of the opposite side of the planet to the set of three above. As we were now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet had fallen to 20 arcseconds). Well past opposition, Mars on September 7 exhibited a phase effect of 92.65 %.
Central meridian: 180º.
Jupiter: This gas giant planet reached opposition on July 14, and now is suitable for viewing for most of the night. On September 1 it will be high in the east as soon as night falls, in the constellation of Sagittarius. The waxing gibbous Moon will be two degrees south of Jupiter as darkness falls on September 25, with Saturn seven degrees east of the Moon.
Jupiter as photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent, sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. All of these are features in the cloud tops of Jupiter's atmosphere.
Jupiter at opposition, May 9, 2018
Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was
taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc.
Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25
pm, with the Great Red Spot very close to Jupiter's central meridian. The third photograph was taken at 10:20 pm, when Europa was approaching
Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth
photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres.
It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º,
55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from
Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.
Jupiter at 11:34 pm on May 18, nine days later. Changes in the rotating cloud patterns are apparent, as some cloud bands rotate faster than others and
interact. Compare with the first photograph in the line of four taken on May 9. The Great Red Spot is ploughing a furrow through the clouds of the South
Tropical Belt, and is pushing up a turbulent bow wave.
Jupiter at opposition, June 11, 2019
The third photograph was taken at 10:41 pm, when Europa was
about a third of its way across Jupiter. Its dark shadow is trailing it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth
photograph at 10:54 pm shows Europa and its shadow about a quarter of the way across. This image is enlarged below. The fifth photograph shows Europa on Jupiter's central meridian at 11:24
pm, with the Great Red Spot on Jupiter's limb. The sixth photograph taken at 11:45 pm shows Europa about two-thirds of the way through its transit, and the
Great Red Spot almost out of sight. In this image, the satellite Callisto may be
seen to the lower right of its parent planet. Jupiter's elevation above the horizon for the six photographs in order was 66º,
70º, 75º, 78º, 84º and 86º. As the evening progressed, the 'seeing' proved quite variable.
There have been numerous alterations to Jupiter's belts and spots over the thirteen months since the 2018 opposition. In particular, there have
been major disturbances affecting the Great Red Spot, which appears to be slowly
changing in size or "unravelling".
It was very fortuitous that, during the evenings of the days when the 2018 and
2019 oppositions occurred, there was a transit of one of the satellites as well as the appearance of the Great Red Spot. It was also interesting in that the
same satellite, Europa, was involved both times.
Jupiter as it appeared at 7:29 pm on July 2, 2017. The Great Red Spot was in a similar position near Jupiter's eastern limb (edge) as in the fifth picture in the series above.
It will be seen that in the past two months the position of the Spot had drifted
when compared with the festoons in the Equatorial Belt, so must rotate around the planet at a slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the
planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less. This high rate of rotation has made the planet quite oblate. The prominent 'bay'
around the Red Spot in the five earlier images appeared to be disappearing, and a darker streak along the northern edge of the South Tropical Belt
was moving
south. In June this year the Spot began to shrink in size, losing about 20% of its
diameter. Two new white spots have developed in the South Temperate Belt, west of the Red Spot. The five upper images were taken near opposition, when the Sun was
directly behind the Earth and illuminating all of Jupiter's disc evenly. The July 2 image was taken just four days before Eastern Quadrature, when the angle
from the Sun to Jupiter and back to the Earth was at its maximum size. This angle means that we see a tiny amount of Jupiter's dark side, the shadow being
visible around the limb of the planet on the left-hand side, whereas the right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to
the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter, sitting in front of the North Tropical Belt, just to the left of its centre.
Saturn:
Neptune:
The photograph above was taken when Saturn was close to opposition, with the Earth between Saturn and the Sun. At that time, the shadow of Saturn's globe upon the
Ring system was directly behind the planet and hardly visible. The photograph below was taken at 7:14 pm on September 09, 2018, when Saturn was near eastern quadrature.
At such a time, the angle from the Sun to Saturn and back to the Earth is near its maximum, making the shadow fall at an angle across the Rings as seen from
Earth. It may be seen falling across the far side of the Ring to the left side of the globe.
Uranus:
Neptune, photographed from Nambour on October 31, 2008
Pluto: The erstwhile ninth and most distant planet reached opposition on July 16.
Almost overhead as darkness falls,
it will be visible in the constellation Sagittarius until midnight. The best time to search for Pluto this month will be in the hours around
8 pm. Pluto's angular diameter
is 0.13 arcseconds, less than one twentieth that of Neptune. Located between Jupiter and Saturn this month in the eastern end of Sagittarius, it is a faint 14.1 magnitude
object. On September 1, Jupiter and Pluto will be only 5.1º apart, and this separation will increase until September 13, when Jupiter reaches its eastern stationary
point. After that, Jupiter will reverse direction and head beck towards Pluto, and by September 30 they will be only 4.6º apart. Their closest approach (41 arcminutes)
will occur on the morning of November 13. A telescope with an aperture of 25 cm is capable of locating Pluto.
The movement of the dwarf planet Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field: 200 arcseconds
This is a stack of four images, showing the movement of Pluto over the period October 22 to 25, 2014. Pluto's image for each date appears as a star-like point at the upper right corner of the numerals. The four are equidistant points on an almost-straight line. Four eleventh magnitude field stars are identified. A is GSC 6292:20, mv = 11.6. B is GSC 6288:1587, mv = 11.9. C is GSC 6292:171, mv = 11.2.
D is GSC 6292:36, mv = 11.5. (GSC = Guide Star Catalogue). The position of Pluto on October 24 (centre of image) was at Right Ascension = 18 hours 48 minutes 13 seconds, Declination = -20º 39' 11". The planet moved 2' 51" with respect to the stellar background during the three days between the first and last images, or 57 arcseconds per day, or 1 arcsecond every 25¼ minutes.For the rest of this year, there will be a fine conjunction of the planets Jupiter and Saturn in the night sky. On September 25, the waxing gibbous Moon
will join them, adding additional interest. In March, Mars approached Jupiter and Saturn, and on March 4 and 5 Jupiter was almost exactly between Mars and Saturn. By March 18, Mars
and Jupiter were together, with the crescent Moon just above them. The following night, the Moon was below Saturn. On March 20, Mars and Jupiter
were at their closest, only 42 arcminutes apart. On March 21, Saturn crossed into Capricornus, and on March 30 Mars did likewise. On the
morning of April 1, Mars reached Saturn, and the two planets were less than a degree apart, with Jupiter 6.3º above them. Jupiter
will spend the rest of the year approaching
Saturn, and they will be within 7 arcminutes of each other on December 21. During this period Mars left Saturn, travelling east through Capricornus. It passed
into Aquarius on May 9 and into Pisces on June 25. On September 1 Mars was 91º (five handspans) east of Saturn, and had become six times
brighter than the ringed planet. Mars will come to a halt against the background
stars of Pisces on September 10, and will reverse direction for nine weeks. On
November 14 it will halt again, and then head east once more, entering Aries on January 5, 2021.
Midway between these two dates, Mars will be at opposition as the Earth
overtakes it on October 14. On that date, Mars will be at its closest to us, at
its biggest and brightest.
Meanwhile, Jupiter came to a halt against the stars of
Sagittarius on May 14 and began its retrograde loop towards its point of opposition, which it reached on July 14. Similarly, Saturn came to a stop
against the stars of Capricornus on May 11, and reversed direction, crossing back into Sagittarius on July 3. After it reached opposition on July 21, Saturn
will halt again on September 29 and then head east once more, crossing back into Capricornus on December 16. Jupiter followed Saturn into Capricornus on December 19.
Jupiter will pass by Saturn on December 22, their distance apart being a little less than a quarter of the width of the Moon. Jupiter and Pluto had a close approach on
July 1 and will have another on November 13 (0.7º or 41 arcminutes apart each time).
Meteor Showers:
Radiant: Near the bright star Capella
Use this
Although most meteors are found in swarms associated with debris from comets, there are numerous 'loners', meteors travelling on solitary
paths through space. When these enter our atmosphere, unannounced and at any time, they are known as 'sporadics'. On an average clear and dark evening, an
observer can expect to see about ten meteors per hour. They burn up to ash in their passage through our atmosphere. The ash slowly settles to the ground as
meteoric dust. The Earth gains about 80 tonnes of such dust every day, so a percentage of the soil we walk on is actually interplanetary in origin. If a
meteor survives its passage through the air and reaches the ground, it is called a 'meteorite'. In the past, large meteorites (possibly comet nuclei or small asteroids) collided with the Earth and produced huge craters which still exist
today. These craters are called 'astroblemes'. Two famous ones in Australia are Wolfe Creek Crater and Gosse's Bluff. The Moon and Mercury are covered with such
astroblemes, and craters are also found on Venus, Mars, planetary satellites, minor planets, asteroids and even comets.
Comets:
ZHR = zenithal hourly rate (number of meteors expected to be observed at the zenith in one hour). The
maximum phase of meteor showers usually occurs between 3 am and sunrise. The reason most meteors are observed in the pre-dawn hours is because at that time
we are on the front of the Earth as it rushes through space at 107 000 km per hour (30 km per second). We are meeting the meteors head-on, and the speed at
which they enter our atmosphere is the sum of their own speed plus ours. In the evenings, we are on the rear side of the Earth, and many meteors we see at that
time are actually having to catch us up. This means that the speed at which they enter our atmosphere is less than in the morning hours, and they burn up less
brilliantly.
Note: NEOWISE refers to the
Near-Earth Object Wide-field Infrared Survey Explorer satellite which
discovered this comet. This satellite was placed in orbit in December 2009
as the WISE (W
Comet
Comet
In
December 2018, Comet 46P/Wirtanen swept past Earth, making one of the ten closest approaches of a comet to our planet since 1960. It was faintly visible to the naked eye for two weeks. Although Wirtanen's nucleus is only 1.2 kilometres across, its green atmosphere became larger than the Full Moon, and was an easy target for binoculars and small telescopes. It reached its closest to the Sun (perihelion) on December 12, and then headed in our direction. It passed the Earth at a distance of 11.5 million kilometres (30 times as far away as the Moon) on December 16. In the week preceding it was at its brightest at magnitude 4, but this was a cloudy week at Nambour. It passed between the Pleiades and Hyades star clusters on the night of December 19-20, but the light of the almost Full Moon made it difficult to see. It then headed towards the star Capella in Auriga, which it passed on December 24-25. It is currently 470 million kilometres away in the constellation Virgo, between Spica and Arcturus, and close to the star Heze (Zeta Virginis), but at 18th magnitude it is too faint for most amateur telescopes. Click here for more information and chart
This comet, (C/2007 N3), discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.
Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.
The
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click here to generate these charts.
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Similar real-time charts can also be generated from another source, by following this second link:
The first, circular chart will show the full hemisphere of sky overhead. The zenith is at the centre of the circle, and the cardinal points are shown around the circumference, which marks the horizon. The chart also shows the positions of the Moon and planets at that time. As the chart is rather cluttered, click on a part of it to show that section of the sky in greater detail. Also, click on Update to make the screen concurrent with the ever-moving sky.
The stars and constellations around the horizon to an elevation of about 40o can be examined by clicking on
The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.
The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.
This month, the Eta Carinae Nebula is poorly placed for viewing, as it is very
close to or below the southern horizon for all of the night.
The Stars and Constellations for this month:
These descriptions of the night sky are for 9 pm on September 1 and 7 pm on September
30. Broadly speaking, the following description starts low in the west and
follows the horizon to the right, heading round to the east, then south, then
west, then overhead.
The Ring Nebula was ejected from the central Sun-sized star towards the end of its life, as powerful stellar winds blew away the outer layers of its atmosphere to form a ring.
Approaching due north is the constellation Cygnus, the Swan. Cygnus is also known as the Northern
Cross, but to us in the southern hemisphere it appears this month upside-down and tilted to the left. The star at the bottom of the Cross, or the
highest one above the horizon as we see it tonight, is a beautiful double star or binary, called Albireo (see below).
Whereas most binaries are a pair of similar stars, there are many in which the two stars are very different, such as brilliant Sirius the Dog Star with its tiny white dwarf companion known as 'The Pup'. Albireo's two components have a marked colour contrast, the brighter star being a golden yellow, and the fainter companion being a vivid electric blue. It is a wonderful object to view with a small telescope.
At the top of the Northern Cross (which is the star closest to the horizon as we see it) is the bright first magnitude star
Deneb, or Alpha Cygni.
Deneb is a white star, and is the nineteenth brightest in the sky. It will be due north at about 9.00pm at mid-month. Its name means 'The Tail', as it marks the tail
of the Swan. Deneb, although bright, is one of the most distant individual stars we can see with the unaided eye. It lies at a distance of about 2600 light years. This means
that the light entering your eye tonight actually left Deneb in the 6th century BC, about the time when the world's first 'scientist', Thales of Miletus, was studying the
night sky from ancient Ionia, part of today's Turkey. Everything you see in the night sky is a view into the past, due to the finite speed of light.
Vega is the brightest star at centre left, with the stars of Lyra to its right. Deneb is the brightest star near the bottom edge. Cygnus, or the 'Northern Cross', stretches up vertically from Deneb to Albireo, above centre. We see the Northern Cross upside-down. The three bright stars of Aquila form a line at upper right, with Altair, the brightest, being the middle one. The small diamond-shaped constellation of Delphinus, the Dolphin, is above centre-right.
In the north-east, the Great Square of Pegasus has cleared the horizon, and is standing on one corner. It is very large, each side being around
15 degrees long. It is about as large as a fist held at arm's length. The Great Square is remarkable for having few naked-eye stars within it.
The names of the four stars marking the corners of the Square (starting at the uppermost one and moving in a clockwise direction) are Markab, Algenib, Alpheratz and Scheat. Although these four stars are known as the Great Square of Pegasus, only three are actually in the constellation of Pegasus, the Winged Horse. In point of fact, Alpheratz is the brightest star of the constellation Andromeda, the Chained Maiden.
In the east, a mv 2.2 star is about a handspan above the horizon. This is Beta Ceti, the brightest ordinary star in the constellation Cetus, the Whale. Its common name is Diphda, and it has a yellowish-orange colour. By rights, the star Menkar or Alpha Ceti should be brighter, but Menkar is actually more than half a magnitude fainter than Diphda. Menkar does not rise at the beginning of September until about 11 pm.
Cetus is a large constellation, running around the eastern horizon tonight, and to the unaided eye it appears unremarkable. But it does contain a most interesting star, which even men of old noticed, naming it Mira, the Wonderful (see below). Between Cetus and Pegasus is the faint zodiacal constellation of Pisces, the Fishes, which contains a faint ring of stars known as the 'Circlet'.
Above Diphda is Fomalhaut, a bright, white first magnitude star in the faint constellation Piscis Austrinus, the Southern Fish. The first planet to be photographed circling another star is embedded in the dust ring surrounding Fomalhaut. Above Fomalhaut and to the right is a large, flattened triangle of stars, Grus, the Crane.
A little more than a handspan above the south-eastern horizon is
Achernar, which is the ninth brightest star. From locations south of
Newcastle, Achernar is circumpolar, i.e. it never dips below the horizon but is always in the sky. A hot blue-white star, Achernar is the main star in the
constellation Eridanus the River, which winds its way from Achernar towards the south-eastern horizon and then turns along the horizon to the east towards
Cetus. It then continues below the horizon all the way to Orion, which this month will not rise above the eastern horizon until a little after midnight.
Achernar's visual magnitude ( mv) is 0.45, and it is a hot blue-white star of B3 spectral type. The width of the field is 24 arcminutes and the faintest stars are mv 15.
To the left and higher than Achernar, the faint constellation of Phoenix may be seen.
Its brightest star is Ankaa or Alpha Phoenicis, a mv 2.39 star which is halfway between Diphda and Achernar, but slightly above.
One-and-a-half degrees to the right of Ankaa is a fourth magnitude star,
Kappa Phoenicis.
Due south, the Large Magellanic Cloud (LMC) is a faint, diffuse glowing patch low to the horizon, only eight degrees up from the latitude of Nambour. The Small Magellanic Cloud (SMC) is about a handspan above it, and to the right of Achernar. Both of these Clouds appear as faint smudges of light, but in reality they are dwarf galaxies containing millions of stars.
From Nambour's latitude, these two clouds never set. Each day they circle the South Celestial Pole, which is a point in our sky 26.6 degrees above the horizon's due south point. Objects in the sky that never set are called 'circumpolar'. The LMC and SMC are described below.
The Southern Cross (Crux) is setting low in the south-west, with the two Pointers Alpha and Beta Centauri vertically above it. Alternative names for these two Pointers are Rigel Kentaurus and Hadar. The two pointers are eight degrees apart. Alpha is the one further away from Crux. Whereas Alpha Centauri (see below) is the nearest star system to our Sun, only 4.2 light years distant, Beta is eighty times further away. Beta Centauri must have an absolute magnitude much greater than Alpha, in order to appear nearly as bright.
At top - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross) with the
dark cloud of dust known as the Coalsack above Alpha Crucis. Bottom - star clusters in the Milky Way with the Eta Carinae nebula near the lower edge.
Just above the second brightest star in the Cross (Beta Crucis) is a brilliant small
star cluster known as 'Herschel's Jewel Box'. In the centre of the cluster is a red supergiant star, which is just passing through.
Beta Crucis (left) and the Jewel Box cluster
If the night is dark and the skies are clear, a black dust cloud known as the Coalsack can be seen just above Acrux, the left-most and brightest star of the
Cross. Surrounding Crux on three sides is the large constellation Centaurus, its two brightest stars being the brilliant Alpha and Beta Centauri. The rest of the
constellation of Centaurus arches over Crux from above it, to its right-hand side, and then underneath it, where it adjoins Carina and Vela.
Adjoining Crux on its left-hand side is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. To the upper left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle.
Six of the zodiacal constellations are overhead tonight. The faint constellation of Libra is low in the west. Starting from Libra and looking above it, we see the bright constellation of Scorpius, the Scorpion, with the red-supergiant star Antares marking the Scorpion's heart. This famous zodiacal constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion. Antares is the brightest star in Scorpius, and is a red type M supergiant of magnitude 0.9. Antares is the fifteenth brightest star.
Another very distant star that is easily seen with the unaided eye is Zeta Scorpii, which, like Deneb, also lies at a distance of 2600 light years. It can be found by following the line of the tail of Scorpius, and is the fourth star from Antares, heading south. It lies at the point where the tail takes a sharp turn east. Actually, there are two stars there, Zeta 1 and Zeta 2. Zeta 1, the distant star, is a blue-white B1 type, while Zeta 2 is an orange K type, and at a distance of only 151 light years is 17 times closer.
Arching across the sky from the Southern Cross (low in the south-south-west) to Cygnus (low in the north) is a faint band of light called the Milky Way. Once thought to be a river of glowing gas, Galileo looked at it through his first telescope in 1609 and reported, "It is nothing but innumerable stars!" We now know that it is our own galaxy seen edge-on, for we are inside it. As well as billions of stars, it does contain millions of glowing gas clouds and dark clouds too.
About three-and-a-half
handspans west of the zenith is the brightest patch in the Milky Way, which is
the centre of our Galaxy seen behind the foreground stars of the constellation
of Sagittarius the Archer. Sagittarius teems
with stars, glowing nebulae and dust clouds. This month Sagittarius is host to
the bright planets Jupiter and Saturn and the very faint planet Pluto, which are all found together in its eastern
end. As Jupiter travels faster, on December 22, 2020, it will have caught up to Saturn, and they will be only 6.5 arcminutes apart - Saturn will appear almost as close to
Jupiter's disc as Jupiter's satellite Callisto.
By that time both planets will have moved into the faint constellation
Capricornus, the Sea-Goat (near the zenith) which they will traverse during 2021. Adjoining Sagittarius to the south, there is a beautiful curve of faint stars.
This is Corona Australis, the Southern Crown, and it is very elegant and delicate. The brightest star in this constellation has a magnitude of only 4.1.
Zeta 1 Scorpii is the upper star in the bright group of three at centre right. Zeta 2 is below it.
Between Scorpius and Centaurus
is an interesting constellation composed of mainly third magnitude stars, Lupus, the Wolf. Midway between Triangulum Australe and Scorpius is an asterism like a
small, elongated triangle. This is Ara, the Altar. High in the north-west, between Scorpius and Hercules, are two large but faint constellations,
Serpens, the Snake, and Ophiuchus, the Serpent Bearer. They have no stars brighter than magnitude 2. Ophiuchus is over two handspans
across in all directions, and may be found at 7 pm at mid-month about two handspans north-west of the zenith.
The constellations surrounding the Southern Cross The body of Scorpius is at right, with the two stars in the Sting to the left, at centre right of picture. The red supergiant star Antares appears close
to the top right corner. The stars in the left half of the picture are in Sagittarius. The well-known 'Teapot' shape may be seen. Near the lower left margin is a graceful
curve of fourth magnitude stars, Corona Australis, the Southern Crown. To see this view, stand facing south and look directly overhead at 7.00 pm at the beginning of the
month. Later in the month, the stars will be further to the west at the same time.
Antares, a red supergiant star
The star which we call Antares is a binary system. It is dominated by the great red supergiant Antares A which, if it swapped places with our Sun, would enclose all the planets out to Jupiter inside itself. Antares A is accompanied by the much smaller Antares B at a distance of between 224 and 529 AU - the estimates vary. (One AU or Astronomical Unit is the distance of the Earth from the Sun, or about 150 million kilometres or 8.3 light minutes.) Antares B is a bluish-white companion, which, although it is dwarfed by its huge primary, is actually a main sequence star of type B2.5V, itself substantially larger and hotter than our Sun. Antares B is difficult to observe as it is less than three arcseconds from Antares A and is swamped in the glare of its brilliant neighbour. It can be seen in the picture above, at position angle 277 degrees (almost due west or to the left) of Antares A. Seeing at the time was about IV on the Antoniadi Scale, or in other words below fair. Image acquired at Starfield Observatory in Nambour on July 1, 2017.
The Trifid Nebula, M20, in Sagittarius, is composed of a reflection nebula (blue), an emission nebula (pink), and dark lanes of dust.
The centre of our galaxy is teeming with stars, and would be bright enough to turn night into day, were it not for intervening dust and molecular clouds. This dark cloud is known as 'The Snake'. A satellite passed through the field of view at right.
Culminating high in the north, between Capricornus and Albireo, is the constellation of
Aquila, the Eagle (see picture). The
centre of this constellation is marked by a short line of three stars, of which the central star is the brightest. These stars, from north to south, are
Tarazed,
Altair and Alshain, and they indicate the Eagle's body. A handspan to the east of bright, first magnitude Altair is a faint but easily recognised
diamond-shaped group of stars, Delphinus the Dolphin. The line of the ecliptic along which the Sun, Moon and planets travel, passes through the following constellations this month: Libra, Scorpius, Sagittarius,
Capricornus, Aquarius and Pisces. The aborigines had a large constellation which is visible tonight, the
Emu. The Coalsack forms its head, with the faint sixth magnitude star in
the Coalsack, its eye. The Emu's neck is a dark lane of dust running east through the two Pointers, to Scorpius. The whole constellation Scorpius forms the Emu's
body. The Emu is sitting, waiting for its eggs to hatch. The eggs are the large star clouds of Sagittarius. More photographs of the amazing sights visible in our sky this month are found in our
Gallery. If you would like to become familiar with the constellations, we suggest that you access one of the world's best collections of constellation pictures
by clicking
Mira, the Wonderful
The amazing thing about the star Mira or Omicron Ceti is that it
varies dramatically in brightness, rising to magnitude 2 (brighter than any other star in Cetus), and then dropping to magnitude 10 (requiring a telescope
to detect it), over a period of 332 days. This drop of eight magnitudes means that its brightness diminishes over a period of five and a half months to one six-hundredth of what it had been,
and then over the next five and a half months it regains its original brightness. In the mid-17th century, the astronomer Johannes Hevelius watched the star fade away
during the year until it disappeared, and then it slowly reappeared again. Its not surprising that he named it Mira, meaning 'The Wonderful' or 'The Miraculous One'. We
now know that many stars vary in brightness, even our Sun doing so to a small degree, with a period of 11 years. One type of star varies, not because it is
actually becoming less bright in itself, but because another, fainter star moves around it in an orbit roughly in line with the Earth, and obscures it on each
pass. This type of star is called an eclipsing variable and they are very common. The
star Mira though, varies its light output because of processes in its interior. It is what is known as a pulsating variable. Stars of the Mira type are
giant pulsating red stars that vary between 2.5 and 11 magnitudes in brightness. They have long, regular periods of pulsation which lie in the range from 80 to
1000 days.
In 2019, Mira reached a maximum brightness of magnitude 3.4 on October 24 and then faded slowly,
dropping well below naked-eye visibility (magnitude 6) by mid-year. It is now
rapidly brightening again and will reach its next maximum on September 20.
Mira near minimum, 26 September 2008 Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope, the Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind
Mira as the star streaks through space. Galaxy Evolution Explorer ("GALEX" for short) scanned the well-known star during its ongoing survey of the entire
sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13
light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star.
More,
including pictures
Double and multiple stars
Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when a condensing Bok globule or protostar splits into two or more parts.
Binary stars may have similar components (Alpha Centauri A and
B are both stars like our Sun), or they may be completely dissimilar, as with
Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).
The binary stars Rigil Kentaurus (Foot of the Centaur, or Alpha Centauri) at left, and Beta Cygni (Albireo), at right.
Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky. Rigel A is a large white supergiant which is 500 times brighter than its small companion, Rigel B, Yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.) In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.
Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis. It is a close binary, circled by a third dwarf companion.
Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily
seen with a small telescope. The components are both solar-type main sequence stars, one of type G and the other, slightly cooler and fainter, of type K. Through a
telescope this star system looks like a pair of distant but bright car headlights. Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third
component, the 11th magnitude red dwarf Proxima Centauri, takes about 1 million years to orbit the other two. It is about one tenth of a light year from the bright
pair and a little closer to us, hence its name. This makes it our nearest interstellar neighbour, with a distance of 4.3 light years. Red dwarfs are by far the most
common type of star, but, being so small and faint, none is visible to the unaided eye. Because they use up so little of their
energy, they are also the longest-lived of stars. The bigger a star is, the shorter its life.
Close-up of the star field around Proxima Centauri
Knowing the orbital period of the two brightest stars A and B, we can apply
Kepler’s Third Law to find the distance they are apart. This tells us that Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours.
This makes them a little less than the distance apart of the Sun and Uranus (the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.)
Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separate them (Acrux, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.
Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
The Milky Way
A glowing band of
light crossing the sky is especially noticeable during September. This
glow is the light of millions of faint stars combined with that coming from
glowing gas clouds called emission nebulae. It is concentrated along the plane of our
galaxy, and this month it is seen crossing the sky, starting from the
south-south-west and passing through Crux to Centaurus, Scorpius, Sagittarius and Aquila to Cygnus in the north-east. The plane of our galaxy from Scutum (at left) through Sagittarius and Scorpius (centre) to Centaurus
and Crux (right). The Eta Carinae nebula is at the right margin, below centre. The Coalsack is clearly visible, and the dark dust lanes can be seen. Taken with an
ultra-wide-angle lens.
The centre of our galaxy. The constellations partly visible here are Sagittarius (left), Ophiuchus (above centre) and Scorpius (at right). The planet Jupiter is the bright object below centre left. This is a normal unaided-eye view.
The Season of the Scorpion
The spectacular constellation of Scorpius is about a handspan west of the zenith in mid-September. Three bright stars in a gentle curve mark his head, and another three mark his body. Of this second group of three, the centre one is a bright, red supergiant, Antares. It marks the red heart of the scorpion. This star is so large that, if it swapped places with our Sun, it would engulf the Earth and extend to the orbit of Mars. It is 604 light years away and shines at magnitude 1.06. Antares, an M type star, has a faint companion which can be seen in a good amateur telescope if the seeing is excellent.
The rest of the stars run around the scorpion's tail, ending with two blue-white B type stars, Shaula (the brighter of the two) and Lesath,
at the tip of the scorpion's sting. These two stars are at the eastern end of the constellation, and are near the bottom of the picture below. West of
Lesath in the body of the scorpion is an optical double star, which can be seen as two with the unaided eye.
Scorpius, with its head at top left and tail (with sting) at lower right.
Probably the two constellations most easily recognisable (apart from Crux, the Southern Cross) are Orion the Hunter and Scorpius the Scorpion. Both are
large constellations containing numerous bright stars, and are very obvious 'pictures in the sky'. Both also contain a very bright red supergiant
star, Betelgeuse in Orion and Antares in Scorpius.
The Lagoon Nebula, M8, in Sagittarius, adjacent to Scorpius
The centre of the Lagoon Nebula
Why are some constellations bright, while others are faint ?
The Milky Way is a barred spiral galaxy some 100000 – 120000 light-years in diameter which contains 100 – 400 billion stars. It may contain at least as many planets as well. Our galaxy is shaped like a flattened disc with a central bulge. The Solar System is located within the disc, about 27000 light-years from the Galactic Centre, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. When we look along the plane of the galaxy, either in towards the centre or out towards the edge, we are looking along the disc through the teeming hordes of stars, clusters, dust clouds and nebulae. In the sky, the galactic plane gives the appearance which we call the Milky Way, a brighter band of light crossing the sky. This part of the sky is very interesting to observe with binoculars or telescope. The brightest and most spectacular constellations, such as Crux, Canis Major, Orion and Scorpius are located close to the Milky Way.
If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is out of sight this month.
The southern window is in the constellation Sculptor, just south of the star Fomalhaut. This window is low in the south-east in the early evening, but later in the night it will rise high enough for distant galaxies to be observed. Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look across the millions of light years of space to thousands of distant galaxies.
Finding the South Celestial Pole:
The South Celestial Pole is that point in the southern sky around which the stars rotate in a clockwise direction. The Earth's axis is aimed exactly at this point. For an equatorially-mounted telescope, the polar axis of the mounting also needs to be aligned exactly to this point in the sky for accurate tracking to take place.
To find this point, first locate the Southern Cross. Project a line from the top of the Cross down through its base and continue straight on towards the south for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.
Another way to locate the South Celestial Pole is to draw an imaginary straight line joining Beta Centauri high in the south-west to Achernar low in the south-east. Both stars will be at about the same elevation above the horizon at 8 pm in the middle of September. Find the midpoint of this line to locate the pole.
Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open the shutter
for thirty minutes or more. The stars will move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different
colours, like the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
Star Clusters
The two clusters in Taurus, the Pleiades and the Hyades,
are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as
tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e.
within the disc of our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some
are relatively young. The photograph below shows a typical open cluster,
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters,
containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular
cluster gives the impression of almost a solid mass of faint stars. Most other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is
NGC 5139**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, but this month it is low and only observable before 8 pm. Shining at fourth magnitude, it is faintly visible to the unaided eye, but is easily seen with binoculars, like a light in a fog. A telescope of 20 cm aperture or better will reveal its true nature, with millions of faint stars giving the impression of diamond dust on a black satin background. It lies at a distance of 5 kiloparsecs, or 16 300 light years. Omega Centauri is not a typical globular cluster, being much larger than any other in our Galaxy, with millions of stars. It is also slightly oval in shape.
The globular cluster Omega Centauri
The central core of Omega Centauri
There is another remarkable globular, second only to Omega Centauri. Close to the SMC (see below), binoculars can detect a fuzzy star. A
telescope will reveal this faint glow as a magnificent globular cluster, lying at a distance of 5.8 kiloparsecs. Its light has taken almost 19 000 years to reach us. This is
NGC 104, commonly known as 47 Tucanae. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars
twinkling in its core are very beautiful. This month, neither Omega Centauri nor 47 Tucanae
is at its best position for viewing, but both are observable.
The globular cluster 47 Tucanae.
Observers aiming their telescopes towards the
SMC generally also look at the nearby 47 Tucanae, but there is another globular cluster nearby which is also worth a visit. This is
NGC 362, which is
less than half as bright as the other globular, but this is because it is more than twice as far away. Its distance is 12.6 kiloparsecs or 41 000 light years,
so it is about one-fifth of the way from our galaxy to the SMC. Both NGC 104 and NGC 362 are always above the horizon for all parts of Australia south of the
Tropic of Capricorn.
* M7: This number means that Ptolemy's Cluster in Scorpius is No. 7 in a list of 103 astronomical objects compiled and published in 1784 by Charles Messier. Charles was interested in the discovery of new comets, and his aim was to provide a list for observers of fuzzy nebulae and clusters which could easily be reported as comets by mistake. Messier's search for comets is now just a footnote to history, but his list of 103 objects is well known to all astronomers today, and has even been extended to 110 objects. As Messier had quite a small telescope, his list is an excellent guide to the brightest and most spectacular objects in the sky, although he did not include any of our excellent southern objects such as 47 Tucanae, Omega Centauri and Eta Carinae, as they were never visible from his home in Paris.
** NGC 5139: This number means that Omega Centauri is No. 5139 in the
New General Catalogue of
Non-stellar Astronomical Objects. This catalogue was first published in 1888 by J. L. E. Dreyer under the auspices of the Royal Astronomical Society, as his
New General Catalogue of Nebulae and Clusters of Stars. It contained only
objects that needed a telescope to be seen. It was an updated version of the
previous General Catalogue
The recent explosion of discovery in astronomy has meant that more and more catalogues are being produced, but they tend to specialise in particular types of objects, rather than being all-encompassing, as the NGC / IC try to be. Some examples are the H
I Parkes All-Sky Survey (HIPASS: HI = clouds of neutral hydrogen in space), Planetary Nebulae Catalogue (PK) which lists 1455 nebulae, the Washington Catalogue of Double Stars (WDS) which lists 12 000 binaries, the General Catalogue of Variable Stars (GCVS) which lists 28 000 variables, and the Principal Galaxies Catalogue (PGC) which lists 73 000 galaxies. The largest modern catalogue is the Hubble Guide Star Catalogue (GSC) which was assembled to support the Hubble Space Telescope's need for guide stars when photographing sky objects. The GSC contains nearly 19 million stars brighter than magnitude 15.
Two close galaxies
Very low in the south, two faint smudges of light may be seen. These are the two
Clouds of Magellan, known to astronomers as the LMC (Large
Magellanic Cloud) and the SMC (Small Magellanic Cloud). The LMC is above the SMC, and is noticeably larger.
They lie at distances of 190 000 light years for the LMC, and 200 000 light years for the SMC. They are about 60 000 light
years apart. These dwarf galaxies circle our own much larger galaxy, the Milky Way. The LMC is slightly closer, but this does
not account for its larger appearance. It really is larger than the SMC, and has developed as an under-sized barred spiral galaxy.
Astronomers have recently reported the largest star yet found, claimed to have 300 times the mass of the Sun, located in a cluster of stars embedded in the Tarantula Nebula (above). Such a huge star would be close to the Eddington Limit, and would have a short lifespan measured in only a couple of million years.
These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and
Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies, and so can be observed in much
clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our
nearest inter-galactic neighbours.