August
2017
Updated: 19 August 2017
Welcome to the night skies of Winter, featuring Crux, Centaurus, Scorpius, Sagittarius, Aquila, Cygnus, Lyra 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 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 developed for our site with our equipment over the past year.
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.
A handspan at arm's length covers an angle of approximately
18 - 20 degrees.
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 = 100).
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. (The meridian is an imaginary semicircular line running from the due south point on the horizon and arching overhead through the zenith, and coming down to the horizon again at its due north point.) 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 constellation of Cancer the Crab. It leaves Cancer and passes into Leo, the Lion on August 10.
Partial Lunar Eclipse, August 8:
This eclipse of the Moon is partial - only about one fifth of the Full Moon will be immersed in the umbra (the darkest part of the Earth's shadow). The eclipse will begin at 1:49 am when the Moon enters the faint penumbra. This part of the eclipse is only noticeable to careful observers, as the Moon's light is only slightly dimmed. The Moon will begin to enter the umbra at 3:22 am - this is easily seen as a 'bite' out of the Full Moon. The maximum phase of the eclipse will occur at 4:23 am, and the Moon will leave the umbra at 5:18 am Moonset will occur at 6:26 am, and the penumbral phase of the eclipse will end at 6:54 am. This event will be visible across the western Pacific including Australia and New Zealand, Asia, Europe and Africa. It is perfectly safe to watch lunar eclipses as they occur at night. Solar eclipses are the dangerous ones, for looking at the Sun without special protection can ruin your eyesight.
Total Solar Eclipse, August 21 (USA time):
This eclipse of the Sun can be seen over the whole of the United States, so is being touted by them as the "American Eclipse". The path of totality begins in the North Pacific Ocean between Hawaii and the Aleutian Islands of Alaska. It proceeds in an east-south-easterly direction, reaching the continental USA just south of Portland, Oregon. The Moon's shadow continues over Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina and then passes over the North Atlantic, the eclipse ending in mid-ocean.
The eclipse will be perfectly timed for observers, lasting a little over two and a half hours in mid-morning in Oregon, and a little over three hours inmid-afternoon in South Carolina. 12 million people live within the path of totality, and 25 million within a day's drive of it, so NASA has sent out traffic warnings. The totality of this eclipse is unfortunately fairly short, averaging between two and two and a half minutes. The centre of the eclipse track is near the town of Hopkinsville in Kentucky, which will enjoy 2 minutes 40 seconds of darkness at 1:24 pm. Four minutes later, Nashville in Tennessee will also experience totality. The further east in the United States you are, the more likely it is that clouds may interfere with the eclipse.
This event will be seen as a partial eclipse from Hawaii, Alaska, Canada, Central America, Equador, the Caribbean Islands, countries in South America that border the Caribbean, and north Brazil. Ireland, the United Kingdom, France, Portugal and Spain may catch a glimpse just before sunset. No part of the eclipse will be observable from Africa, most of Europe, Asia or Australia.
Full Moon:
August 08 04:12 hrs
diameter = 30.3' Partial lunar eclipse
Last Quarter: August 15 11:16 hrs diameter =
32.2'
New Moon:
August 22 04:31 hrs diameter = 32.1'
Total solar eclipse (USA only)
First Quarter:
August 29 18:14 hrs diameter = 29.6'
Full Moon:
September
06 17:03 hrs
diameter = 31.1'
Last Quarter:
September
13 16:26 hrs diameter =
32.3'
New Moon:
September
20 15:30 hrs diameter =
31.2'
First Quarter:
September
28 12:54 hrs diameter = 29.6'
Lunar Orbital Elements:
August
03:
Moon at apogee (405 053 km) at 03:46 hrs, diameter = 29.5'
August
08: Moon at descending node at
20:54 hrs, diameter = 30.4'
August
18: Moon at perigee (366
120
km) at 22:56 hrs, diameter = 32.6'
August
21: Moon at ascending node at
20:35 hrs, diameter = 32.2'
August
30:
Moon at apogee (404 313 km) at 21:51 hrs, diameter = 29.6'
September
05: Moon at descending node at
04:39 hrs, diameter = 30.6'
September
14: Moon at perigee (369
859
km) at 01:17 hrs, diameter = 32.3'
September
18:
Moon at ascending node at
04:28 hrs, diameter = 31.9'
September 27:
Moon at apogee (404 350 km) at 16:42 hrs, diameter = 29.6'
Moon at 8 days after New, as on August 1 and 30.
The photograph above shows the
Moon when approximately eight days after New, just after First Quarter. A detailed map of the Moon's near side is available here. 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.
Click
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. Last month we described the great walled plain Plato. This month we will discuss some of the interesting features in the vicinity of Plato.
This area was photographed from Starfield Observatory, Nambour on October 10, 2017. East (where the Sun is rising) is to the right, north is to the top. The area is dominated by the large walled plain Plato at upper left, and the impact crater Cassini at lower right. Between the two is a rugged mountainous area called the Alps, to the west of which is a large basin filled with solidified lava, called Mare Imbrium (the Sea of Rains). Some of the more notable features are named.
The rectangle shows the location of the top image on the Moon. The dotted circle shows the full extent of the Mare Imbrium.
The Vallis
Alpes (Alpine Valley) is 134 km long. Along its length runs a delicate rille
which varies between 500 and 700 metres across, and averages 78 metres deep. Although it looks
like a water course, there is no running water on the Moon. It is probably
volcanic in origin. Both images above were photographed at Starfield Observatory
through the Alluna 20 inch Ritchey Chrétien
telescope on August 2, 2017.
This area includes parts of two lava plains, which as they are basically cold
basalt appear darker in colour than some other areas. In ancient times the Moon was thought to be a perfect, silver mirror, reflecting the seas and continents of
the Earth. This idea persisted until the 16th century. By 1600, some people including Leonardo da Vinci in Italy and William Gilbert in England believed
that the darker areas were reflections of our continents and the lighter areas were reflection of the Earthly seas and oceans, but most thinkers thought that
the opposite was true, that the darker areas were seas ("mare" singular and "maria" plural) and the sole larger one an ocean ("oceanus").
The lighter areas were lands ("terrae"). This second view was the one held by the first makers of Moon maps, Langrenus, Hevelius, Riccioli and Grimaldi.
As Riccioli and Grimaldi have given us most of the names of places on the Moon that they could see with their primitive telescope, the darker areas in the
pictures above are the Mare Imbrium (Sea of Rains) and the Mare Frigoris (Sea of Cold). The Mare Imbrium is a great circular plain on the Moon, caused by a massive
asteroid impact - see "The Imbrium Event" below. All of the seas and the solitary ocean were formed by such impacts and are also dry, cold plains of
solidified lava. The terrae around them are much more ancient and are totally covered with fairly large craters, most overlapping older craters, but the Seas are more
recent, and show fewer large craters, although they also are peppered with thousands of craterlets.
The impact of asteroids, meteoroids, rocks, and micrometeoroids with the Moon’s
surface over the eons has left it covered with a thick layer of dust, soil and shattered rocks. This layer averages 4 to 5 metres deep over the maria, but can
be up to 30 metres deep in the valleys of the ancient highlands or terrae. This layer is called the regolith. The dusty upper surface is fine and
powdery, like talcum powder, and holds astronaut’s bootprints very well. The Apollo 16 astronauts landed near the crater Descartes, at a small recent crater with rays, called North Ray Crater. The regolith near this crater was
found to be only a few centimetres thick. Many samples of the regolith have been brought to Earth for analysis. Those from sites on maria are generally rock fragments o
f basalt, with pyroxene and plagioclase. Those from highland sites are mainly broken plagioclase rocks and crystals.
Beneath the regolith of the maria there is a layer of basalt, averaging 20 kilometres deep,
formed when the initial asteroid fractured the surface down to a depth of 60
kilometres, to the Moon's mantle. These fractures released pressure on the
superheated rocks in the top layers of the mantle. They immediately liquefied
into molten basaltic magma which forced its way to the surface and spread out
over the landscape as lava, swamping or partially swamping existing craters and
mountains, and spreading out over thousands of square kilometres. The lava
cooled and hardened to form the "seas" that are such a notable feature
of our side of the Moon.
In the pictures above, the lava has covered the bases of the Teneriffe
Mountains, Mount Pico and Mount Piton to a great depth,
although their lofty peaks protrude above the plain of Mare Imbrium. The crater
Plato was caused not long after the Imbrium Event, but just south of Plato can
be seen the faint outline of a similar but slightly larger crater, that was
completely submerged under the lava except for some of the ramparts of its wall
which were high enough not to be swamped - these ramparts are now called the
Teneriffe Mountains (1.4 kilometres high) and Mount Pico (2.4 kilometres high).
Three impact craters on Mare Imbrium of more recent origin are Piazzi Smyth (22
km), Kirch (12 km) and spectacular Cassini (60 km).
Between Mare Imbrium and Mare Frigoris (Sea of Cold) is a rough chain of lofty mountains,
trending from north-west to south-east. Johannes Hevelius called this
mountainous area the "Montes Alpes", and Riccioli named it the "Terra Grandinis
(High Land"). This was one case where the name given by Hevelius was preferred
over Riccioli's alternative, and the range is usually known simply as the Alps.
Its highest mountain is Mont Blanc (White Mountain) and is 3.6 kilometres high.
The Alps are divided in two by a straight valley called the Vallis Alpes (Alpine
Valley). It is 134 kilometres long and is a linear fault or graben that was
filled with lava. Along its length runs a delicate rille that is shown in the
picture above and on our
Gallery webpage.
The Imbrium Event The Moon and planets coalesced out of debris in the solar
proto-nebula about 4.6 billion years ago. Most of the craters on the Moon
were formed early in its history, during the first 500 million years. At that time, there was a lot of
solid material in orbit around the Sun, and the planets attracted much of
it to themselves by their own gravity. There were major collisions of asteroid-sized rocks with all the rocky planets and moons, and they still show the scars. Earth,
despite the effects of weather and erosion, still has some large impact craters or astroblemes, two in Australia being Wolfe Creek Crater and Gosse’s Bluff. After the main bombardment had ended, most of the Moon was covered in craters overlapping each other. The south polar
region of the Moon and the far side still have this appearance. Some large
asteroids later struck our side of the Moon, the huge impact craters being filled with lava
which produced the large, dark-coloured level area which give the Moon its
patchy appearance to the naked eye. Early astronomers thought that the dark
areas were water, and in the 17th century the Jesuit astronomers Grimaldi and
Riccioli, working with primitive telescopes, named them, giving us 3 marshes, 17
lakes, 11 bays, 20 seas and one ocean. Of course, there is not one drop of
liquid water on the Moon - all of the dark areas are plains of cooled lava. The final large asteroid collided
with the Moon about 3.85 billion years ago. We don’t know its size, but it was probably half as big as Tasmania.
It struck the northern hemisphere, blasting out a crater over 800 kilometres across. Magma immediately rose from the Moon’s
mantle through fractures in the crust and flooded out across the surface, swamping the destroyed area with great lava flows which filled the floor of the
great new crater. This cataclysm is known as the 'Imbrium Event'. The circular area resulting from this
gigantic blast is known to us today as the Mare Imbrium, the Sea of Rains. The area is circled
by great mountain ranges, the Alps, the Caucasus, the Apennines and the
Carpathian Mountains, which are in fact the original ramparts of the impact crater. Because of its location halfway between the Moon's equator and
its north pole, it appears foreshortened to us, looking oval in shape. Protruding from the lava floor of Mare Imbrium are isolated mountain peaks, such as Pico
and Piton. Their bases are kilometres below the Mare surface. As different flows of lava occurred, ripples in the surface were caused, which are visible today as
wrinkle ridges. Some craters were formed after the initial lava flows had solidified, and then partially or completely flooded by later flows.
These are called ghost craters. When the initial Imbrium Event occurred, large blocks of rock the size of mountains were sent crashing for hundreds of kilometres
across the lunar surface, particularly heading south-south-east. These caused lines of smaller craters and great longitudinal scars to be formed as the blocks bounced
along, all of this damage radiating from the centre of the Imbrium Event impact site. Most
of these scars are seen in the area north,
north-west and north-east of the central crater Ptolemæus.
Since those times, new craters have been formed on what was once the level, smooth Mare Imbrium. We can see some in the pictures above. One large later collision occurred
at the northern edge of the Mare, creating the crater Plato. This was severe enough to fracture the crust and allow lava to well up and almost fill the crater, so
that instead of being bowl-shaped it has a flat floor at about the same level as the nearby Mare. Being made of
the same basaltic lava, the floor is the
same dark grey colour as the Mare, while the surrounding mountains are much lighter in tone.
Geocentric Events:
August 2: Mercury at
aphelion at 22:48 hrs (diameter = 8.2") September 2: Moon 3º north of Pluto at 05:44 hrs
August 3: Uranus at
western stationary point at 12:40 hrs (diameter = 3.6")
August 3: Moon 3.7º
north
of
Saturn at 16:20 hrs
August 5: Moon 1.9º
north of
the star Pi Sagittarii (mv=
3.00) at 18:31 hrs
August 5: Moon 2.5º
north of
Pluto at 21:38 hrs
August 10: Moon occults Neptune
between
9:43 and 10:18 hrs
August 13: Mercury at eastern
stationary point at 11:09 hrs (diameter = 9.8")
August 13: Moon 3.9º
south of Uranus at
16:52 hrs
August 16: Limb of Moon 21 arcminutes north of
the star Aldebaran (Alpha Tauri, mv= 0.87) at 17:22 hrs
August 19: Moon 1.9º
south
of Venus at
16:02 hrs
August 21: Moon 1.1º
south
of Mars at 15:38
hrs
August 22: Moon 6º
north
of Mercury at
19:58 hrs
August 25: Saturn at eastern
stationary point at 20:30 hrs (diameter = 17.1")
August 26: Moon 3.6º
north
of Jupiter at
01:39 hrs
August 27: Mercury at inferior
conjunction at 06:35 hrs (diameter = 10.8")
August 31: Moon 3.9º
north
of Saturn at 02:05 hrs
September 2: Saturn at eastern quadrature at 13:13 hrs (diameter = 16.6")
September 5: Mercury 3º south of Mars at 08:52 hrs
September 5: Neptune at opposition at 15:09 hrs (diameter = 2.3")
September 5: Mercury at western stationary point at 21:13 hrs (diameter = 8.9")
September 5: Mars 42 arcminutes north of the star Regulus (Alpha Leonis, mv= 1.36) at 21:02 hrs
September 5: Uranus 1º north of the star Omicron Piscium (mv= 4.25) at 21:33 hrs
September 6: Limb of Moon 8 arcminutes south of Neptune at 13:57 hrs
September 9: Moon 4º south of Uranus at 20:31 hrs
September 10: Mercury 35 arcminutes south-east of the star Regulus (Alpha Leonis, mv= 1.36) at 22:33 hrs
September 12: Mercury at Greatest Elongation West (17º 56') at 19:15 hrs (diameter = 7.2")
September 12: Limb of Moon 21 arcminutes north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 21:03 hrs
September 15: Mercury at perihelion at 22:25 hrs (diameter = 6.6")
September 17: Mercury 3.4 arcminutes north of Mars at 04:33 hrs
September 18: Moon occults Venus between 10:50 and 12:21 hrs
September 18: Moon occults the star Regulus (Alpha Leonis, mv= 1.36) between 16:30 and 16:49 hrs
September 19: Limb of Moon 32 arcminutes north of Mars at 4:36 hrs
September 19: Limb of Moon 22 arcminutes north of Mercury at 09:02 hrs
September 20: Venus 28 arcminutes north of the star Regulus (Alpha Leonis, mv= 1.36) at 12:14 hrs
September 22: Moon 3.7º north of Jupiter at 21:28 hrs
September 23: Spring equinox at 05:57 hrs
September 26: Jupiter 39 arcminutes south of the star 82 Virginis (mv= 5.03) at 16:28 hrs
September 27: Moon 4º north of Saturn at 08:56 hrs
September 29: Pluto at eastern stationary point at 01:31 hrs (diameter = 0.1")
September 29: Moon 2.1º north of the star Pi Sagittarii (mv= 3.00) at 10:38 hrs
September 29: Moon 2.9º north of Pluto at 11:05 hrs
Mercury:
On August 1, Mercury will be in the west-north-western twilight sky, about 6.6 degrees or a third of a handspan above the first magnitude star Regulus, the brightest star in the constellation Leo. It will be near its maximum angular distance from the Sun, 27 degrees, greatest elongation east having been reached on July 30. The first three weeks of August will be the best time to observe Mercury, as its angular size will increase from 8 arcseconds on August 1 to 11 arcseconds on August 21, while its phase will fall in the same period from 43% to 6%. During the next days it will become increasingly difficult to find due to the brightness of the twilight. Mercury will overtake the Earth, passing between us and the Sun on August 27, and will then move to the pre-dawn eastern sky.
Venus:
This, the brightest planet, is still dominating the pre-dawn sky as a 'morning star', although its angular distance from the Sun is diminishing day by day. For example, on August 1 it will be 38 degrees from the Sun, and by August 31 its elongation will have fallen to 32 degrees. This process will continue as speeding Venus curves around towards the far side of the Sun. Venus will pass behind the Sun (superior conjunction) on January 9 next year. After that, Venus will return to the western sky as an 'evening star'. In mid-August, Venus will appear in a small telescope as a tiny 'gibbous Moon' with a magnitude of -4.0 and an angular size of 13 arcseconds. Its phase will be 79%.On the morning of August 19, the waning crescent Moon will be just above Venus in the sky.
(The coloured fringes to the first and third images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus. 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).
April 2017 June 2017 December 2017
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 the first two months of 2017, Venus appeared as an 'Evening Star', but on March 25 it moved to the pre-dawn sky and became a 'Morning Star'. Each of these appearances lasts about eight to nine months. Venus will pass on the far side of the Sun (superior conjunction) on January 9, 2018, when it will return to the evening sky and become an 'Evening Star' once again.
Because Venus was 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:
Having passed through conjunction with the Sun on July 27, the red planet is extremely difficult to observe this month. By August 31 it will still be within 12 degrees of the Sun, and risky to observe due to the solar glare. However, from now on it will leave the environs of the Sun and will grow in size each day, as our Earth, travelling faster, begins to catch it up. We will overtake Mars on July 27 next year. This will be a very favourable opposition, as Mars will appear bigger (24.2 arcseconds in diameter) and brighter (magnitude -2.8) than it has for many years. It will be particularly favourable for us in the southern hemisphere, as during the month of opposition it will be in the constellation of Capricornus, almost directly overhead each night from the Sunshine Coast.
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.
Jupiter:
This gas giant planet is still a spectacular evening object as it passed through eastern quadrature (due north and crossing the meridian at sunset) on July 6. In mid-August it may be easily seen high in the sky, being about halfway between the zenith and the west-north-western horizon as darkness falls. It is the brightest object in the evening sky apart from the Moon, and is in the constellation Virgo, north-west of the first magnitude star Spica. The crescent Moon will be close to Jupiter as night falls on August 25.
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 as
it appeared at 7:29 pm on July 2, 2017. The Great Red Spot is 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 has 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 appears to be disappearing, and a
darker streak along the northern edge of the South Tropical Belt is moving
south. 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: The ringed planet is visible for most of the night this month, as it reached opposition on June 15. At mid-month it will be visible about a handspan east of the zenith as soon as darkness falls, underneath the huge S-shaped curve of Scorpius, the Scorpion. The almost-full Moon will be just underneath Saturn on August 3 and August 30.
Saturn with its Rings wide open on May 25, 2017. The shadow of its globe can be
seen on the far side of the Ring system. There are three main concentric rings:
Ring A is the outermost, and is separated from the brighter Ring B by a dark gap
known as the Cassini Division, which is 4800 kilometres wide, enough to drop
Australia through.
Uranus: This ice giant planet is an early morning object in August, as it reached western quadrature (rising at midnight) on July 21. This month Uranus rises above the theoretical horizon at 11:12 pm on August 1 and 9:13 pm on August 31. Uranus shines at about magnitude 5.8, so a pair of binoculars or a small telescope is required to observe it. It is currently in the constellation Pisces, near the south-west corner of Aries. The waning gibbous Moon will be in the vicinity of Uranus on August 13.
Neptune: The icy blue planet is a late evening object this month. It reached western quadrature on June 5, which means that on that date it rose at midnight. In mid-August it rises at about 7 pm. Neptune is located in the constellation of Aquarius, between the magnitude 3 star Skat (Delta Aquarii) and the four-star asterism known as the Water-Jar. As it shines at about magnitude 8, a small telescope is required to observe Neptune. The almost Full Moon will rise just above Neptune on the evening of August 9, and will occult Neptune on the following day. Neptune will reach opposition on September 5.
Neptune, photographed from Nambour on October 31, 2008
Pluto:
The erstwhile ninth and most distant planet can be observed almost all night this month, as it reached opposition on July 10, when it rose at sunset. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune. Located just east of the 'Teaspoon' which is north-east of the Sagittarius 'Teapot', it is a faint 14.1 magnitude object near the centre of Sagittarius. A telescope with an aperture of 25 cm or more is necessary to observe 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.
Meteor Showers:
N Delta Aquarids
August 13
Waning gibbous Moon, 77%
sunlit ZHR =
20
Radiant: Near the
Sadalsuud
Perseids
August 11 to 13
Waning gibbous Moon, 74%
sunlit ZHR =
usually 95
Radiant:
Just above the northern horizon at 5 am. Composed of debris associated with the
tail of Comet Swift-Tuttle.
This shower usually produces a number of very bright, explosive meteors called
'fireballs' or 'bolides'.
Alpha
Aurigids
September 1
Waxing gibbous
Moon, 74% sunlit ZHR =
10
Radiant: Near the bright star Capella
Use
this Fluxtimator to calculate the number of meteors predicted per hour for any meteor swarm on any date, for any place in the world.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'. Oan 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.
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.
Comets:
Comet Lulin
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.
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._____________________________________
Similar real-time charts can also be generated from another source, by following this second link:
Click here for a different real-time sky chart.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
View horizon at this observing site
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.
The Eta Carinae Nebula is nearly a handspan below the Southern Cross at 6:30 pm in mid-August.
The Stars and Constellations for this month:
These descriptions of the night sky are for 9 pm on August 1 and 7 pm on August 31. They start at the western horizon.
Tonight, Spica is at an altitude of 25 degrees above the west-south-western horizon. It is roughly halfway between the star Arcturus and the Southern Cross. For most of this year, Virgo has been dominated by the presence of the brilliant planet Jupiter - it is much brighter than any of Virgo's stars. It is at 9 pm tonight only a handspan above the western horizon. Jupiter will reach conjunction with the Sun on October 27, and will cross into Libra on November 14.
A handspan to the left of Jupiter is the constellation of Corvus the Crow. Corvus is a lopsided quadrilateral of four third magnitude stars. It is close to the west-south-western horizon. About 45 degrees above the western horizon is the faint constellation of Libra, the Scales, the brightest stars of which are two of magnitude 2.7 with exotic names, Zuben Elgenubi and Zuben Eschamali.
Low in the north-west, about a handspan above the horizon, we can find the fourth brightest star in the
night sky, Arcturus. It is outshone only by Sirius, Canopus and
Alpha Centauri. Arcturus differs from
those stars just named, for it is an obvious orange colour, a K2 star of zero magnitude. It is a particularly beautiful star,
and is the brightest in the constellation of Boőtes, the Herdsman. Boötes (pronounced 'Bo-oh-tees') will have set by 10.00 pm at mid-month.
Arcturus
Above Boötes and about a handspan above the north-western horizon is a fainter
circle of fourth magnitude stars, Corona Borealis, the Northern Crown. The brightest star in the Crown is
named Alphecca, and it shines at magnitude 2.3.
East of the Northern Crown is Hercules, a large constellation above the north-north-western horizon. Below Hercules is the head of the northern constellation Draco, the Dragon, containing the star Eltanin, famous as London's Zenith Star. To its right, about a handspan above the horizon a little east of north, is a bright white A0 star, Vega, which is the fifth brightest star, after Arcturus. Vega is the main star in the small constellation of Lyra the Lyre, which contains the famous Ring Nebula, M 57.
The Ring Nebula was ejected from the central star in a great explosion
Just climbing above the north-east horizon is another bright star,
Deneb. Deneb is the
nineteenth brightest star, and belongs to the constellation Cygnus, the Swan. Cygnus is known in the northern hemisphere as the 'Northern Cross', and it
appears upside-down to us in Australia. Tonight it is tilted, with the base star of the Cross, the binary (double star)
Albireo or Beta Cygni (see below)
about a handspan above Deneb and to the left. Albireo is a beautiful pairing of a bright golden star with a smaller electric-blue companion, and lies about
half-a-handspan to the right and slightly above Vega.
Until the early nineteenth century, people thought that all stars had much the same brightness. This led to the assumption that bright stars were close, and faint ones were far away. Deneb proved this to be wrong, as although it is a bright first magnitude star (visual magnitude 1.25), it is one of the most distant stars visible to the unaided eye, being 2600 light years away. In fact, most of the solar system's closest neighbours are faint red dwarf stars, much too dim for us to see without a telescope.
High in the north-east (nearly two handspans up) is the great main-sequence star Altair. This A7 white star is the eleventh brightest in the heavens. Altair is also known as Alpha Aquilae, as it is the brightest star in the constellation of Aquila, the Eagle. It marks the heart of the eagle, and is flanked by two lesser stars marking each wing, Gamma Aquilae and Beta Aquilae. This threesome, making a short horizontal line, is easy to find.
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.
Directly overhead is Sagittarius the Archer, through which the Milky Way passes. The centre of our galaxy is at the zenith at this time. Sagittarius teems
with stars, glowing nebulae and dust clouds, as it is in line with the centre of our galaxy. 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.
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.
To the west of Sagittarius, and also near the zenith, is the spectacular constellation of
Scorpius,
the Scorpion (see below), also very rich in objects to find with a small telescope or binoculars. This famous zodiacal
constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion.
The brightest star in Scorpius is Antares, a red type M supergiant of magnitude 0.9. Antares is the fifteenth brightest
star.
The body of Scorpius is at top, with the two stars in the Sting underneath, just above the centre of the picture. The red supergiant star Antares appears close to the top left corner. The stars in the lower half of the picture are in Sagittarius. Near the lower right margin is a graceful curve of fourth magnitude stars, Corona Australis, the Southern Crown.
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.
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.
Zeta 1 Scorpii is the upper star in the bright group of three at centre right. Zeta 2 is below it. High in the north, between Scorpius and Hercules, are two large but faint constellations, Serpens, the Snake, and Ophiuchus, the Serpent Bearer.
Between Scorpius and Hercules is a large constellation with
no stars brighter than magnitude 2, Ophiuchus, the Serpent Bearer.
Ophiuchus is over two handspans across in all directions, and may be found a
little north of the zenith at 7:20 pm at mid-month. At 9 pm on August 1,
Ophiuchus is about one handspan north-west of the zenith.
Though Ophiuchus is not as spectacular as Scorpius, this year his right foot is favoured by
the presence of the ringed planet Saturn, which is brighter than any of
the stars in that part of the sky.
Saturn will cross into Sagittarius on November 19, and in the week between
Christmas and New Year it will skirt the Trifid Nebula (M20) and will pass
across outlying stars of the open cluster, M21.
Adjoining Sagittarius on its eastern side is another large zodiacal constellation,
Capricornus, the Sea-Goat. It lacks any bright stars,
but it does have a pair of third-magnitude stars, Nashira and
Deneb Algedi. It also boasts the presence of
quite a bright globular cluster, M30.
Underneath Capricornus, the large zodiacal constellation of
Aquarius, the Water Bearer has just cleared the horizon. Aquarius has a
grouping of four stars, the 'Water Jar', and the planet
Neptune (too faint to be seen with the unaided eye) is also present
in this constellation. Neptune can be detected with 7x50 binoculars or a small telescope.
To the right of Aquarius is the first magnitude star Fomalhaut. This white star is the eighteenth brightest in the sky, and is the main star in the faint and inconspicuous constellation of Piscis Austrinus, the Southern Fish. Above Fomalhaut and slightly to the south is a large, flattened triangle of stars, Grus, the Crane.
Low in the south-south-east, brilliant Achernar may be seen climbing up from the horizon. A clear and low horizon will be needed to glimpse it. From areas 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 ninth brightest.
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 mv15.
High in the south-west, Crux (Southern Cross) has rotated round to a nearly horizontal position, at an altitude of about 27 degrees or one and a
half handspans. The two Pointers, Alpha and Beta Centauri lie above and to its left. The two pointers are 8 degrees apart. Alpha is the one further
away from Crux. Whereas Alpha Centauri 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. Alternative names for these two Pointers are
Rigel Kentaurus and
Hadar.
Beta Crucis (left) and the Jewel Box cluster
Herschel's Jewel Box
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.
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.
To the left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle.
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.
The constellations surrounding the Southern Cross.
Between Crux and the south-south-western horizon is an area of sky filled with interesting objects. This was once the constellation
Argo, named
for Jason’s famous ship used by the Argonauts in their quest for the Golden Fleece. The constellation Argo was found to be too large, so modern star atlases
divide it into three sections - Carina (the Keel), Vela (the Sails) and
Puppis (the Stern).
Dipping below the south-south-western horizon is the False Cross, larger and more lopsided than the Southern Cross. Both of these Crosses are actually more like kites in shape, for, unlike Cygnus (the Northern Cross) they have no star at the intersection of the two cross arms.
Between the Southern Cross and the False Cross may be seen a glowing patch of light. This is the famous Eta Carinae nebula, which is a remarkable sight through binoculars or a small telescope working at low magnification. Photographs of this emission nebula appear below.
The central part of the Eta Carinae nebula, showing dark lanes, molecular clouds, and glowing clouds of fluorescing hydrogen
The Keyhole, a dark cloud obscuring part of the Eta Carinae Nebula
The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae.
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 of 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 and Scutum.
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,
The Season of the Scorpion
The spectacular constellation of Scorpius is directly overhead at about 7.00 pm in mid-August. 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.
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 ?
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 low in the south-east, but later in the evening 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.
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 < 9 (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. To the ancients, they saw the familiar star fade away during the year until it disappeared, and then it slowly reappeared again. Its not surprising that they 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.
This year, Mira reached a maximum brightness of magnitude 3.4 on February 23 and has now dropped well below naked-eye visibility (magnitude 6) again. It will reach its minimum brightness of magnitude 9.3 on September 22. The next maximum will occur on January 19, 2018.
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
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.
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).
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 the
smallest 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 small
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
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. Albireo rises in the north-east at about 8 pm on July 1.
Binary stars may be widely spaced, as the two examples just mentioned, or so close that a telescope is struggling to separated
them (Acrux, Castor, 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.
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.)
A glowing band of light crossing the sky is especially noticeable during the winter months. 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 sky, starting from the south-west and passing through Carina to Crux, Centaurus, Scorpius and Sagittarius 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.
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. At 8.00 pm in mid-August, the Milky Way crosses the zenith, almost dividing the sky in two. It runs from south-west to north-east, and the very centre of our galaxy passes directly overhead.
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.
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 10.00 pm in the middle of August. Find the midpoint of this line to locate the pole. To find due south on the ground, first find the South Celestial Pole as described above, and from that point drop vertically to the horizon.
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 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 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
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, both Omega Centauri and 47 Tucanae are not at their best positions for viewing, but 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.
* 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. As larger telescopes built early in the 20th century discovered fainter objects in space, and also dark, obscuring nebulae and dust clouds, the NGC was supplemented with the addition of the Index Catalogue (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
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.
The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula.
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.
This month, the SMC can be observed from a dark site from about 8 pm, but the
LMC will reach a similar position in the sky five hours later. Both of these
Clouds are about two handspans above the southern horizon at 4:30 am.
Astronomy Picture of the Day
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