October
2020
Updated: 27 October 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 Virgo, the Virgin. It leaves Virgo and passes into Libra, the Scales on October 31.
Penumbral Lunar Eclipse, November 30, 2020:
Moon Phases:
Last Quarter:
New Moon:
October 31:
Moon at 8 days after New, as on October 24.
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 a crater in the Moon's rugged south
polar region, Moretus.
From its crisp and undamaged appearance, we can assume that Moretus is the
youngest large crater in the photograph above. Its rim is complete with quite
sharp edges, and the walls have collapsed down to the floor, forming huge
terraces around the circumference. The crater floor is flat, having been covered
with lava, upwelling through fissures created by the original impact. Projecting
from the centre of the floor is a large, monolithic mountain, at 2.7 kilometres
one of the highest central mountains on the Moon. We only see the summit,
the mountain's base being under the lava floor. There is a 1000 metre craterlet
just east of its summit, which may be volcanic.
North of this mountain and halfway across the floor to the north rim is a 4
kilometre craterlet with a 1000 metre craterlet on its eastern flank. Between
this pair and the eastern wall is a very fine fracture in Moretus' floor, barely
visible. The western and southern parts of the floor of Moretus are quite
rugged, with numerous craterlets and hills.
On the far side of Moretus is the 50 kilometre crater Short, which has a
9 kilometre craterlet in the centre of its bowl-shaped floor. Behind Short to
its south-west is 80 kilometre diameter Newton, damaged by later impacts
and only partly shown.
Theodorus Moretus (1602-1667) was a Flemish Jesuit priest who was also a
mathematician, geometer, theologian and philosopher. He published in 1664 a
treatise providing a mathematical analysis of the harmony of sounds.
Moretus is
the large, well-defined crater a little left of centre in the above picture. It
is only 580 kilometres from the Moon's South Pole and so is greatly flattened by
perspective. Each month libration takes it closer to the Moon's southern limb,
but on the date that the above image was acquired, libration had brought it to a
more favoured position. The whole of the Moon's southern hemisphere up to a
distance of 7000 kilometres from the Pole itself is very ancient, and so is
totally covered with impact craters overlapping each other. The sizes of these
craters vary from the giant crater Bailly (303 kilometres across, the
largest on the near side of the Moon) to the tiniest craterlets, some of which
can be seen above. The oldest craters are deformed by more recent impacts, and
their features are smoothed out by rock melt blasted over the moonscape as a
consequence of those impacts. The more clearly defined a crater is, then the
younger is its age.
Moretus
The crater Moretus is 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.
The Planets for this month:
M
ercury: During the first three weeks of October, the innermost planet is in the western twilight sky, setting soon after the Sun. It will be noticeable low in the west after sunset early in the month. 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 in the second week of November, below Venus. The waning crescent Moon will be just to the left of Mercury on the morning of November 14.
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 October 1 it may be spotted high in the north-east before dawn breaks, in the constellation of Leo. During October it rises at about 3:40 am. Venus reached its greatest elongation west (43º 17') on August 14, when it was at its greatest angular distance from the Sun. Shining at around magnitude -4.3, it is brighter than anything else in the night sky, except for the Moon and the occasional bolide. Venus is bright enough to cast faint shadows. Venus will cross into Virgo on October 23, and will remain a 'morning star' until early next year. At 4:20 am on the morning of October 3, Venus will be within 15 arcminutes of the first magnitude star Regulus (Alpha Leonis) and by 9 am they will be only 5 arcminutes apart. The waning crescent Moon will be close to Venus on the morning of October 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. 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.
Mars:
The red planet is at its best for viewing this month, as the Earth will catch up
to and overtake Mars on October 14. Mars is
slowly increasing in angular size as we approach, and by October 1 it reached 22 arcseconds in diameter,
with a brightness of -2.5 (brighter even than Jupiter). Mars spends this 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 October 1 Mars will rise almost due-east at about 7 pm, and
on October 31 before sunset. Mars is currently performing 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
Full Moon will be in the vicinity of Mars on October 3 and October 29 and 30
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.
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.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
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.
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 until midnight. On October 1 it will be near the zenith as soon as night falls, in the constellation of Sagittarius. It will reach eastern quadrature on October 11. The waxing crescent Moon will be 4.5º to the left and below Jupiter as darkness falls on October 22, with Saturn 6º above Jupiter.
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 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.
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.
Saturn:
Left: Saturn showing the Rings when edge-on. Right: Over-exposed Saturn surrounded by its satellites Rhea, Enceladus, Dione, Tethys and Titan - February 23/24, 2009.
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. This month, it reaches
eastern quadrature on October 15 (near the zenith at sunset) and so will be best observed in the early evening hours. 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 October 1, Jupiter and Pluto will be only 4.6º apart, but this separation will decrease as the days go by, so that on October 31 their
separation will be only 1.8º. Their closest approach (41 arcminutes apart) 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
the nights of October 22 and 23, the First Quarter Moon will pass by these planets, 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 has spent 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 October 1 Mars was 90º (five handspans) east of Saturn, and had become six times brighter than the ringed planet.
Mars came to a halt against the background stars of Pisces on September 10, and reversed direction. 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 halted again on September 29 and is now heading east once more. It will cross back into Capricornus on December 16,
followed by Jupiter 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 Betelgeuse. Associated with Comet Halley
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.
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 close
to or below the southern horizon for most of the night.
The Stars and Constellations for this
month:
These descriptions of the night sky are for 9 pm on October 1 and 7 pm on October
31. Broadly speaking, the following description starts low in the north-west and
follows the horizon to the right, heading round to the east, then south, then
west, then overhead and back to the north-west.
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 is the brightest star in the constellation. It appears close to the north-north-western horizon tonight. This star is
the first magnitude star Deneb, or Alpha Cygni. Deneb is a white giant star, and is the nineteenth brightest in the sky. Its name is Arabic for 'tail'. It will be
due north at about 7.00 pm at mid-month. High in the sky and approaching culmination is the Great Square of Pegasus. It will be standing directly above the northern horizon at 9.30 pm at
mid-month. It is very large, each side being around 15 degrees long. It is about as large as a fist held at arm's length, and is a similar distance above the horizon. 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 top-left one and moving in a clockwise direction around the Square) 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. Andromeda trails down from Alpheratz below the north-eastern horizon. To its right is the zodiacal constellation of
Aries, now risen in the
north-east. The brightest star in Aries is a second magnitude orange star called
Hamal. Above it is the white star Sheratan, slightly fainter.
The planet
Uranus is about 11 degrees to the south of
the star Hamal this month, close to the boundary with the constellation of Cetus. Just below the horizon is Taurus, soon to rise. In the east, a mv 2.2 star is about halfway up the sky. 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 being also known as Alpha Ceti should be
brighter, but Menkar is actually more than half a magnitude fainter than Diphda. Menkar may be seen rising above the east-north-eastern horizon. 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 ancient peoples noticed. The astronomer Hevelius named it Mira, the Wonderful (see below). Between Cetus and Pegasus is the zodiacal constellation of
Pisces, the Fishes. Pisces is found just above Aries, and contains a faint ring of stars, known as the 'Circlet'. This month, Pisces is host to the red planet Mars, which reached opposition on October 14. Mars will be at its closest to the Earth this month, and
is brighter than Jupiter. Despite this closeness, it is a small planet and through a telescope will look only half the diameter of Jupiter. Above Diphda is Fomalhaut, a bright, white first magnitude star in the faint constellation
Piscis Austrinus, the Southern Fish. Above
Fomalhaut and to the right is a large, flattened triangle of stars, Grus, the Crane. Fomalhaut and Grus are both almost directly overhead at this time. Very high in the south-east is Achernar, which is the ninth brightest star.
It is the main star in the constellation Eridanus the River,
which winds its way from Achernar towards the eastern horizon below 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 10.00 pm.
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. Between Achernar and the south-eastern horizon can be seen the brilliant supergiant
Canopus rising in the south-east. Canopus is the second-brightest star in the night sky, being outshone only by Sirius, which is smaller but much closer. To the left of Achernar, the faint constellation of Phoenix may be seen. Its brightest star is Ankaa, a mv 2.39 star which is
halfway between Diphda and Achernar, but slightly above. A little to the east of due south, the Large Magellanic Cloud (LMC) is gaining altitude. 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 our latitude both Magellanic Clouds are circumpolar. This means that they are closer to the
South Celestial Pole than that Pole's
altitude above the horizon, so they never dip below the horizon. They never rise nor set, but are always in our sky. Of course, they are not visible in daylight, but they
are there, all the same. The Southern Cross is almost out of sight below the southern horizon, and
Alpha and Beta Centauri are setting nearby. Near the
west-south-western horizon, we see the bright S-shaped constellation of Scorpius, the Scorpion, with the red-supergiant star
Antares marking the Scorpion's
heart. The planets Jupiter and Saturn are in the adjoining constellation of Sagittarius, the Archer, which is about 45º above the south-western horizon, just
above Scorpius. The star clouds in the centre of our Milky Way galaxy lie behind the stars of Sagittarius.
Jupiter and Saturn will spend all of October slowly moving eastward
through Sagittarius.
Antares, a red supergiant
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 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 was discovered by Edward Emerson Barnard in 1905 and is known as B72, 'The Snake'. A satellite passed through the field of view at right.
East of Sagittarius and a little to the west of the zenith is Capricornus, the Sea-Goat. Between Capricornus and Pisces is a rather faint constellation, Aquarius, the Water Bearer. Aquarius is almost directly overhead at this time, and this year contains the faint planet Neptune, which lies about two-thirds of a handspan south-east of the asterism known as the 'Water Jar'.
The constellations Sagittarius (top) and Scorpius (bottom), with the elegant curve of Corona Australis to the left of Sagittarius. They are high in the west at 8.00 pm this month.
High in the north-west, between Capricornus and Albireo, is the constellation of Aquila, the Eagle. The centre of this constellation is marked by a short line of three stars, of which the centre star is the brightest. These stars, from left to right, are Tarazed, Altair and Alshain, and they indicate the Eagle's body. A handspan east of the bright, first magnitude Altair is a faint but easily recognised diamond-shaped group of stars, Delphinus the Dolphin.
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.
The zodiacal constellations visible tonight, starting from the west-south-western horizon and heading north-east, are Scorpius, Sagittarius, Capricornus, Aquarius, Pisces and Aries.
More photographs of the amazing sights visible in our sky this month are found in our Picture 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
here. To see some of the best astrophotographs taken with the giant Anglo-Australian telescope, click here.
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.
The seventeenth century Polish astronomer Johannes Hevelius 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. In the last two months it rapidly brightened again and reached its 2020 maximum on September 20. It is now slowly fading again, and will reach its 2021 minimum in June next year.
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
Estimate
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 Kent (Alpha Centauri) at left, and Beta Cygni (Albireo), at right.
The binary star Rigel (Beta Orionis, left) 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. 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 the winter months,
and to a lesser extent in the spring, when it appears more to the west. This glow is the light of millions of faint stars combined with that coming from
glowing gas clouds called nebulae. It is concentrated along the plane of our galaxy, and this month it is seen crossing the western half of the sky, starting from the
south-south-west and passing through Crux to Norma, Ara, Scorpius, Sagittarius, Scutum and Aquila to Cygnus in the north-north-west.
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.
It is rewarding to scan along this band with a pair of binoculars, looking for star clusters and emission nebulae. Dust lanes along the plane of the Milky Way appear to split it in two in some parts of the sky. One of these lanes can be easily seen, starting near Alpha Centauri and heading towards Antares.
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 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 top 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.
The spectacular constellation of Scorpius is about a handspan above the western horizon at about 8.00 pm in mid-October. 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.
Scorpius, with its head at lower right, and the tail (with sting) at upper left.
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.
A very distant star that is easily seen with the unaided eye is Zeta Scorpii, in the tail of the Scorpion. Actually, there are two stars there, Zeta 1 and Zeta 2. Zeta 2 is an orange K-type giant star which is only 150 light years away. Zeta 1, though, is a blue-white B1 hypergiant, and at a distance of 2600 light years is over 17 times further away than Zeta 2. The light from Zeta 1 left it around 600 BC, when the Greek philosophers Socrates and Plato were alive. 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.
Zeta 1 Scorpii is the upper star in the bright group of three at centre right. Zeta 2 is below it.
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, not far from the star Fomalhaut. This window is 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 in the south-west to Achernar in the south-east. Both stars will be at about the same elevation above the horizon at 7.00 pm at the beginning of October. 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.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right
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 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,
M7*. It lies in the constellation Scorpius, just below the scorpion's sting, in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years. M7 is visible tonight.
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 a solid mass of faint stars. Many 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 is difficult to observe in October. 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.
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, 47 Tucanae will be observable all night.
The globular cluster 47 Tucanae.
The globular cluster NGC 6752 in the constellation Pavo.
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
* M42: This number means that the Great Nebula in Orion is No. 42 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.
** 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 could be seen through a telescope. Soon after it appeared, the new technique of astrophotography became available, revealing thousands more faint objects in space, and also dark, obscuring nebulae and dust clouds. This meant that the NGC had to be supplemented with the addition of two Index Catalogues (IC). Many non-stellar objects in the sky have therefore NGC numbers or IC numbers. For example, the famous Horsehead Nebula in Orion is catalogued as IC 434. The NGC was revised in 1973, and lists 7840 objects.
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 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 Galaxy 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
The Large Magellanic Cloud - the
bright knot of gas to left of centre is the famous Tarantula Nebula.
Low in the south-south-east, 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
directly below 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,
and are linked to it by the Magellanic Stream. 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.
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