August
2020
Updated: 1 August 2020
Welcome to the night skies of Winter, featuring Virgo, Crux, Centaurus, Scorpius, Sagittarius, Aquila, Hercules, Lyra, 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 Cancer, the Crab. It leaves Cancer and passes into Leo, the Lion on August 10.
M
Last Quarter:
New Moon:
Moon at 8 days after New, as on August 27.
The photograph above shows the Moon when approximately eight days after New, just after First Quarter. A rotatable view of the Moon, with ability to zoom in close to the surface (including the far side), and giving detailed information on each feature, may be downloaded
here. A professional version of this freeware with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the Moon's surface) and many other useful features is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage.Click
here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool ! All these downloads are freeware, although the authors do accept donations if the user feels inclined to support their work.
Lunar Feature for this Month
Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at the Montes Riphaeus (Riphaeus Mountains).
To the west of the centre of the range is an uplifted, dome-shaped pyroclastic area, quite rugged and containing a cluster of over a dozen volcanic hills, some with small
summit craters. At the western extremity of this raised area is a 12-kilometre impact crater called Euclides (Euclid). In the image above, this crater is half-way
down the western (left-hand) margin. The whole area contained in this image exhibits linear damage appearing as striations running from the top right to bottom left. This
damage, called 'Imbrium Sculpture', was caused when a large asteroid about 250 kilometres across collided with the Moon's northern hemisphere 3.8 billion years ago.
The impactor was shattered in the collision, and large masses of broken rock and other debris was blasted out in all directions, leading to damage to the surrounding
landscape in the form of grooves and trenches that are radial to the point of impact, which was north-north-east of the Montes Riphaeus. The huge crater formed in the impact
was filled with upwelling magma which erupted up into the newly-formed basin, levelling out the surface to form the Mare Imbrium. The second person to produce a detailed map of the Moon (after Michael van Langren's effort in 1645, see item
Giovanni Riccioli and Francesco Grimaldi (see item
Montes Riphaeus
The Montes Riphaeus (Riphaeus Mountains) are named after a mountain range mentioned by authors in ancient times, but it is uncertain to which range the name referred. The Roman historian Pliny the Elder identified the Montes Riphaeus with the Ural Mountains, a 2500 kilometre-long range running north-south in Russia, marking the boundary between Europe and Asia. The Montes Riphaeus on the Moon also run north-south, so the identification appears feasible, and is probably correct.
The Montes Riphaeus are located inside the rectangle above.
Click
Geocentric Events:
It should be remembered that close approaches of Moon, planets and stars are only perspective effects as seen from the Earth - that is why they are called
'geocentric or Earth-centred phenomena'. The Moon, planets and stars do not really approach and dance around each other as it appears to us from the vantage
point of our speeding planet.
August 1: Moon 2.1º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 09:41 hrs
September 3: Moon 3.3º south of Neptune at 10:32 hrs
September 6: Limb of Moon 3 arcminutes north of Mars at 14:31 hrs
September 7: Moon 2.9º south of Uranus at 15:52 hrs
September 12: Moon 2.3º north of the star Tejat Posterior (Mu Geminorum, mv= 2.87) at 03:58 hrs
September 12: Neptune at opposition at 06:04 hrs (diameter = 2.3")
September 12: Limb of Moon 16 arcminutes south of the star Mebsuta (Epsilon Geminorum (mv= 3.06) at 15:04 hrs
September 13: Jupiter at eastern stationary point at 09:22 hrs (diameter = 42.8")
September 14: Moon 4.4º north of Venus at 18:01 hrs
September 16: Mars at perihelion at 21:42 hrs (diameter = 18.1")
September 19: Moon 6.3º north of Mercury at 12:03 hrs
September 19: Mercury at aphelion at 13:15 hrs (diameter - 5.7>)
September 22: Moon 1.4º north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 09:54 hrs
September 22: Mercury 16 arcminutes north of the star Spica (Alpha Virginis, mv= 0.98) at 21:20 hrs
September 24: Moon 1.2º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 22:32 hrs
September 25: Moon 2.5º north of the star Nunki (Sigma Sagittarii, mv=2.02) at 07:37 hrs
September 26: Limb of Moon 26 arcminutes south of Pluto at 03:24 hrs
September 26: Moon 1.6º south of Saturn at 07:23 hrs
September 28: Moon 1.4º south of the star Deneb Algedi (Delta Capricorni, mv= 2.85) at 12:14 hrs
September 30: Moon 3.7º south of Neptune at 13:59 hrs
The Planets for this month:
M
ercury: During the first half of August, the innermost planet is in the eastern pre-dawn sky between Venus and the east-north-eastern horizon. After the first few days it will become lost in the solar glare, as it approaches superior conjunction on August 18. On that date it will pass on the far side of the Sun and then move into the evening twilight sky, setting after the Sun. It will become visible low in the west after sunset in late August, but not really noticeable until early September. Mercury will pass very close to the first magnitude star Spica on the evening of September 22. Mercury will reach its greatest angular distance from the Sun (Greatest Elongation East, 25º 49') on the evening of October 1, when it will set 1 hour 57 minutes after the Sun. On that date its phase will be 60.7%. Mercury will pass between the Earth and the Sun (inferior conjunction) on October 26, and will then return to the eastern pre-dawn sky, becoming visible before sunrise.
Venus: This, the brightest planet, was an 'evening star' in the first five months of 2020. It passed through inferior conjunction (when it overtook the Earth by passing between us and the Sun) on June 4, after which it moved to the eastern pre-dawn sky. It is now a 'morning star', and on August 1 it may be spotted high in the north-east before dawn breaks, in the constellation Gemini. During August it rises at about 3:30 am. Venus will reach its greatest elongation west (43º 17') on August 14. This is when it is at its greatest angular distance from the Sun, and at magnitude -4.3 it will be very bright, bright enough to cast faint shadows. Venus will move through the upraised club of Orion between August 5 and 13, after which it will re-enter Gemini through which it will travel until it crosses into Cancer on September 4. Venus will remain as a 'morning star' until early next year. The waning crescent Moon will be close to Venus on the mornings of August 15 and 16.
(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 itself. The planet was closer to the horizon when these images were taken than it was for the second photograph, which was taken when Venus was at its greatest elongation from the Sun).
March 2020 mid-May 2020 early July 2020 mid-August 2020 December 2020
Click here for a photographic animation showing the Venusian phases. Venus is always far brighter than anything else in the sky except for the Sun and Moon. For all of 2019 up to July, Venus appeared as an 'Morning Star' in the eastern pre-dawn sky, but in August last year it moved to the evening sky to be an 'Evening Star'. It is now quite easy to find in the west as twilight begins to fade.
Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
Mars:
The red planet is becoming a better object for viewing this month, as the Earth is catching up to it. Mars is slowly increasing in angular size, and by August 1 it reached
15 arcseconds in diameter, with a brightness of -1.5 (brighter than the brightest star, Sirius). It passed through western quadrature (rising at midnight) on June 7. Mars
begins the month in the constellation of Pisces, between the Great Square of
Pegasus and the faint constellation of Cetus, the Whale. It is by far the brightest night object in that part of the sky, except for the Moon. On August 1 Mars will rise almost
due-east at about 10:30 pm, and on August 31 at 8:30 pm. Mars will perform its retrograde S-bend in Pisces between September 10 and November 14. During this period it will
be close to Earth and quite large and bright. At the midpoint between these two dates, Mars will be at its closest distance to Earth, an event called "opposition", as it
will be directly opposite the Sun in the sky, rising in the east as the Sun sets in the west. The actual date of opposition will be October 14, when the angular diameter of
Mars will be 22.3 arcseconds. The waning gibbous Moon will be in the vicinity of Mars on August 8 and 9. Mars will finally leave Pisces and cross into Aries on January 5,
2021.
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
Mars photographed from Starfield Observatory, Nambour on June 29 and July 9, 2016, showing two different sides of the planet. The north polar cap is prominent.
Brilliant Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374, and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure 5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905. He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were not there, the galactic centre would be so bright that it would turn night into day.
Mars near opposition, July 24, 2018
Mars, called the red planet but usually coloured orange, in mid-2018 took on a yellowish tint and brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena were caused by a great dust storm which completely encircled the planet, obscuring the surface features so that they were only seen faintly through the thick curtain of dust. Although planetary photographers were mostly disappointed, many observers were interested to see that the yellow colour and increased brightness meant that a weather event on a distant planet could actually be detected with the unaided eye - a very unusual thing in itself.
The three pictures above were taken on the evening of July 24, at 9:05, 9:51 and 11:34 pm. Although the fine details that are usually seen on Mars
were hidden by the dust storm,
some of the larger features can be discerned, revealing how much Mars rotates in two and a half hours. Mars' sidereal rotation period (the time taken for one complete
rotation or 'Martian day') is 24 hours 37 minutes 22 seconds - a little longer than an Earth day. The dust storm began in the Hellas Desert on May 31, and after two months
it still enshrouded the planet. In September it began to clear, but by then
the close approach had passed.
Central meridian: 295º.
The two pictures immediately above were taken on the evening of September 7, at 6:25 and 8:06 pm. The dust storm was finally abating, and some of the surface features were becoming visible once again. This pair of images also demonstrates the rotation of Mars in 1 hour 41 minutes (equal to 24.6 degrees of longitude), but this time the view is of the opposite side of the planet to the set of three above. As we were now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet had fallen to 20 arcseconds). Well past opposition, Mars on September 7 exhibited a phase effect of 92.65 %.
Central meridian: 180º.
Jupiter: This gas giant planet reached opposition on July 14, and now is suitable for viewing for most of the night. On August 1 it will be well up in the east as soon as night falls, in the constellation of Sagittarius. On that date it will be almost directly overhead at 10:33 pm. The waxing gibbous Moon will be between Jupiter and Saturn on August 2 and 29.
Jupiter as photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent, sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. All of these are features in the cloud tops of Jupiter's atmosphere.
Jupiter at opposition, May 9, 2018
Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was
taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc.
Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25
pm, with the Great Red Spot very close to Jupiter's central meridian. The third photograph was taken at 10:20 pm, when Europa was approaching
Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth
photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres.
It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º,
55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from
Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.
Jupiter at 11:34 pm on May 18, nine days later. Changes in the rotating cloud patterns are apparent, as some cloud bands rotate faster than others and
interact. Compare with the first photograph in the line of four taken on May 9. The Great Red Spot is ploughing a furrow through the clouds of the South
Tropical Belt, and is pushing up a turbulent bow wave.
Jupiter at opposition, June 11, 2019
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.
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:
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. Rising just before Sunset,
it will be visible in the constellation Sagittarius all night. The best time to search for Pluto this month will be in the hours around midnight. 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 August 1, Jupiter and Pluto will be only 3.3º apart, but this separation will increase to 5.1º by the end of the month. 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.
Planetary
Alignments
For the rest of this year, there will be a fine conjunction of the planets Jupiter and Saturn in the night sky. On August 2 and 29, the Full Moon
will pass through the grouping, 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 spends the rest of the year approaching Saturn, and follows it into Capricornus on December 19. They will be within 7 arcminutes of each other on December 22. During this period Mars left Saturn, travelling east through Capricornus. It passed into Aquarius on May 9 and into Pisces on June 25. On August 1 Mars was 80º (four handspans) east of Saturn, and had become five times brighter than the ringed planet. Mars will enter Aries on January 5, 2021. Meanwhile in Capricornus, Jupiter came to a halt against the stars of Sagittarius on May 14 and began its retrograde loop towards its point of opposition, which it reached on July 14. Similarly, Saturn came to a stop against the stars of Capricornus on May 11, and reversed direction, crossing back into Sagittarius on July 3. After it reached opposition on July 21, Saturn will halt again on September 29 and then head east once more, crossing back into Capricornus on December 16. A week later, Jupiter will pass by Saturn, 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: Perseids
August 11 to 13
Waning crescent Moon, 43%
sunlit ZHR =
usually 95 Alpha
Aurigids
September 1
Almost Full Moon, 98% sunlit ZHR =
10
N Delta Aquarids
August 13
Waning crescent Moon, 45%
sunlit ZHR =
20
Radiant: Near the
magnitude 2.9 yellow star, Sadalsuud
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'.
Radiant: Near the bright star Capella
Use this
Although most meteors are found in swarms associated with debris from comets, there are numerous 'loners', meteors travelling on solitary
paths through space. When these enter our atmosphere, unannounced and at any time, they are known as 'sporadics'. On an average clear and dark evening, an
observer can expect to see about ten meteors per hour. They burn up to ash in their passage through our atmosphere. The ash slowly settles to the ground as
meteoric dust. The Earth gains about 80 tonnes of such dust every day, so a percentage of the soil we walk on is actually interplanetary in origin. If a
meteor survives its passage through the air and reaches the ground, it is called a 'meteorite'. In the past, large meteorites (possibly comet nuclei or small asteroids) collided with the Earth and produced huge craters which still exist
today. These craters are called 'astroblemes'. Two famous ones in Australia are Wolfe Creek Crater and Gosse's Bluff. The Moon and Mercury are covered with such
astroblemes, and craters are also found on Venus, Mars, planetary satellites, minor planets, asteroids and even comets.
Comets:
Note: NEOWISE refers to the
Near-Earth Object Wide-field Infrared Survey Explorer satellite which
discovered this comet. This satellite was placed in orbit in December 2009
as the WISE (W
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.
Comet
Comet
In
December 2018, Comet 46P/Wirtanen swept past Earth, making one of the ten closest approaches of a comet to our planet since 1960. It was faintly visible to the naked eye for two weeks. Although Wirtanen's nucleus is only 1.2 kilometres across, its green atmosphere became larger than the Full Moon, and was an easy target for binoculars and small telescopes. It reached its closest to the Sun (perihelion) on December 12, and then headed in our direction. It passed the Earth at a distance of 11.5 million kilometres (30 times as far away as the Moon) on December 16. In the week preceding it was at its brightest at magnitude 4, but this was a cloudy week at Nambour. It passed between the Pleiades and Hyades star clusters on the night of December 19-20, but the light of the almost Full Moon made it difficult to see. It then headed towards the star Capella in Auriga, which it passed on December 24-25. It is currently 470 million kilometres away in the constellation Virgo, between Spica and Arcturus, and close to the star Heze (Zeta Virginis), but at 18th magnitude it is too faint for most amateur telescopes. Click here for more information and chart
This comet, (C/2007 N3), discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.
Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.
The
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click here to generate these charts.
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Similar real-time charts can also be generated from another source, by following this second link:
The first, circular chart will show the full hemisphere of sky overhead. The zenith is at the centre of the circle, and the cardinal points are shown around the circumference, which marks the horizon. The chart also shows the positions of the Moon and planets at that time. As the chart is rather cluttered, click on a part of it to show that section of the sky in greater detail. Also, click on Update to make the screen concurrent with the ever-moving sky.
The stars and constellations around the horizon to an elevation of about 40o can be examined by clicking on
The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.
The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.
This month, the Eta Carinae Nebula is not well-placed for viewing, as it is between the Southern Cross and the horizon as darkness falls 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. A handspan to the left of Virgo 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 by powerful stellar winds. It is called a 'planetary nebula'
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.
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.
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.
This month Sagittarius is host to the bright planets Jupiter and Saturn and the very faint planet Pluto, which are all found together in its eastern
end. As Jupiter travels faster, on December 22, 2020, it will have caught up to Saturn, and they will be only 6.5 arcminutes apart - Saturn will appear almost as close to
Jupiter's disc as Jupiter's satellite Callisto.
Both planets will remain in Capricornus during 2021. Adjoining Sagittarius to the south, there is a beautiful curve of faint stars.
This is Corona Australis, the Southern Crown, and it is very elegant and delicate. The brightest star in this constellation has a magnitude of only 4.1.
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.
Adjoining Sagittarius on its eastern side is another large zodiacal constellation,
Capricornus, the Sea-Goat. This constellation is lacking in any bright stars, and is fairly unremarkable,
but in December both Jupiter and Saturn will cross into Capricornus from
Sagittarius.
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.
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.
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.
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.
High in the north, between Scorpius and Hercules, are two large but faint constellations, Serpens, the Snake, and Ophiuchus, the Serpent Bearer. They have
no stars brighter than magnitude 2. Ophiuchus is over two handspans across in
all directions, and may be found a little north of the zenith at 7:20 pm at
mid-month.
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.
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.
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, we suggest that you access one of the world's best collections of constellation pictures by clicking
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.
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 < 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 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.
Last year, Mira reached a maximum brightness of magnitude 3.4 on
October 24 and then faded slowly,
dropping well below naked-eye visibility (magnitude 6) by mid-year. It is now beginning to
rapidly brighten again and will reach its next maximum on September 20.
Mira near minimum, 26 September 2008
Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope, the Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind
Mira as the star streaks through space.
Galaxy Evolution Explorer ("GALEX" for short) scanned the well-known star during its ongoing survey of the entire
sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13
light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star.
More,
including pictures
Double and multiple stars
Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when a condensing Bok globule or protostar splits into two or more parts.
Binary stars may have similar components (Alpha Centauri A and B are both stars like our Sun), or they may be completely dissimilar, as with Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).
The binary stars Rigil Kentaurus (Alpha Centauri) at left, and Albireo (Beta Cygni) at right.
Rigel (Beta Orionis, left) is a binary star which is the seventh brightest star in the night sky. Rigel A is a large white supergiant which is 500 times brighter than its small companion, Rigel B, Yet Rigel B is itself composed or a very close pair of Sun-type stars that orbit each other in less than 10 days. Each of the two stars comprising Rigel B is brighter in absolute terms than Sirius (see above). The Rigel B pair orbit Rigel A at the immense distance of 2200 Astronomical Units, equal to 12 light-days. (An Astronomical Unit or AU is the distance from the Earth to the Sun.) In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.
Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis. It is a close binary, circled by a third dwarf companion.
Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily seen with 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 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 or NGC 104.
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.
The globular cluster NGC 6752 in the
constellation Pavo.
* 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.
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