March
2019
Updated: 1 March 2019
Welcome to the night skies of Autumn, featuring Gemini, Orion, Canis Major, Carina and Crux
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 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.
The zenith is the point in the sky directly overhead
from the observer. 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.
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. 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 Aquarius, the Water-Bearer. It leaves Aquarius and passes into Pisces, the Fishes on March 12.
Moon Phases:
First Quarter:
Full Moon:
Last Quarter:
April 29:
Moon at apogee (404 583 km) at 04:40 hrs, diameter 29.5'
Moon at 8 days after New, as on March 15.
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 an interesting crater-plain in the south-eastern quadrant that has been flooded by lava, Fracastorius.
This picture is dominated by the large and ancient crater-plain
Fracastorius (128 km diameter). To its north is a large, flat lava plain
called the Mare Nectaris or Sea of Nectar. This Mare dates from about 3.9
billion years ago, when the Moon was only about 750 million years old. Yet
Fracastorius must be considerably older, as lava from the impact that created
the Mare Nectaris has flooded over its northern wall and pooled in the interior.
In the north-east corner of the image (top right) is a much more recent impact
crater, Rosse, which is 12 km in diameter. Rosse may be only about 1
billion years old. This image was taken at 5:40 pm on 18 July 2018. North is to the top, east to the right.
This large crater-plain appears like a huge bay on the southern edge of Mare Nectaris. Roughly circular in shape, its position in the Moon's southern hemisphere at latitude 21º and longitude 33º East makes it appear elliptical due to east-west foreshortening. Once a typical bowl-shaped crater with a low floor and central mountain peaks, all of the interior was swamped by lava from the Nectaris impact to the north. This lava swept over the north wall and filled the floor until it was level with the rest of Mare Nectaris. The violence of this inundation dislodged a huge part of the north wall about 10 km across, and carried it south about 15 km into the interior of Fracastorius. It can be easily seen in the image above, south of the demolished section of the north wall, and looks a little like a map of Australia. The rest of the north wall can only be seen under a low Sun as a number of low mounds.
Shrinkage of the lava as it cooled and solidified opened up a huge fracture or rille in the surface. This 200 km long rille crosses the floor jaggedly from west to east just below centre, then, as it approaches the eastern wall, it turns north and follows the base of the wall until it reaches the point where lava burst through the north wall. The lava has swamped the rest of the rille. A secondary rille runs south from near the centre of the main rille and peters out at a landslip from the southern wall.
Near the centre of Fracastorius are some low hills, which are the peaks of the original central cluster of mountains, the bases of which are buried deep below the lava's surface. The walls of Fracastorius are damaged by later impacts, the largest being on the western side. The eastern wall is quite well preserved. The south-eastern wall shows slumping, where sections of the wall have fallen down to the floor, forming a terrace and encroaching on the floor. There are numerous craterlets on the interior, the largest being the 5 km diameter Fracastorius L. Another, 4 km Fracastorius M, is 10 km west of the junction between the main rille and the secondary rille, and actually impinges on the main rille.
Fracastorius
Girolamo Fracastoro (Fracastorius, 1478-1553) was a Venetian physician who was the first to realise how epidemics were spread. He also was the first to declare that fossils were the petrified remains of living creatures. He was also an astronomer, and in his Homocentrica of 1538 he tried to replace Ptolemy’s geocentric system of deferents and epicycles by restoring the homocentric spheres of Eudoxus and Callippus, but without taking the revolutionary step of replacing the Earth at the centre of the spheres with the Sun, as Copernicus did. He advocated an inflexible system of 77 homocentric spheres as follows: 1 carrying the fixed stars, 7 carrying the planets, 6 for the daily rotation and precession, 10 for Saturn, 11 for Jupiter, 9 for Mars, 4 for the Sun, 11 for Venus, 11 for Mercury, 6 for the Moon, and 1 for a sublunary sphere. He added that it would “do the Sun a great deal of good to get two more spheres, making a total of 79.” He applied names to all of these spheres, some examples being circumducens, circitor, contravectus, anticircitor and ultimus contravectus. Where possible, they operated in sets of five. J. L. E. Dreyer has written that this kind of thinking by Fracastorius (and others like him) was like “trying to breathe life into a mummy”. Tycho Brahe would be scathing about Fracastorius’ attempts to turn the clock back 19 centuries.
Fracastorius said that the sublunary sphere was not his invention, but had originally been discussed by philosophers such as Seneca (3 BCE-AD 65) in the seventh book of his Naturales quaestiones. As its name implied, the sublunary sphere was above the zones of air and fire, and below the Moon’s sphere. He claimed it was a sphere of æther with dense and less dense parts. It provided a place for comets, which were thought to be clouds of very dense æther moving through it. These explained the variations in brightness of the planets, their light dimming as they passed behind a denser part. Others argued that comets were flaming emissions high in the zone of fire. Although very few astronomers followed Fracastorius’ lead, and a few others tried to account for small errors in predictions made by the Ptolemaic system by adding epicyclets onto epicycles, by far the great majority, including all of those who produced famous almanacs, adhered to the original system of Ptolemy as translated by Gerard of Cremona and Regiomontanus in their versions of the Almagest.
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.
March 1: Moon occults the star Pi Sagittarii (mv= 2.88) between 23:52 and 00:39 hrs
April 1: Limb of Moon 18 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 13:25 hrs
April 2: Moon 1.9º south of Venus at 17:41 hrs
April 3: Mercury 23 arcminutes north of Neptune at 06:57 hrs
April 3: Moon 2.6º south of Neptune at 12:31 hrs
April 3: Moon 2.9º south of Mercury at 12:41 hrs
April 7: Moon 4.2º south of Uranus at 01:43 hrs
April 8: Neptune 1.8º north north of the star Chi Aquarii (mv= 4.93) at 09:21 hrs
April 9: Moon 3.9º south of Mars at 19:38 hrs
April 10: Neptune 5 arcminutes north of the star Phi Aquarii (mv= 4.22) at 09:01 hrs
April 10: Venus 17 arcminutes south of Neptune at 16:13 hrs
April 10: Mercury at aphelion at 17:38 hrs (diameter = 7.9")
April 10: Saturn at western quadrature at 18:52 hrs (diameter = 16.5")
April 11: Jupiter at western stationary point at 02:51 hrs (diameter = 41.0")
April 11: Moon occults the star Zeta Tauri (mv= 2.97) between 04:14 and 4:54 hrs
April 11: Moon occults the star Propus (Eta Geminorum (mv= 3.31) between 21:08 and 21:59 hrs
April 12: Limb of Moon 3 arcminutes south of the star Mu Geminorum (mv= 2.87) at 00:46 hrs
April 12: Mercury at Greatest Elongation West (27 39') at 01:21 hrs (diameter = 7.8")
April 13: Pluto at western quadrature at 18:04 hrs (diameter = 0.1")
April 18: Venus at aphelion at 12:32 hrs (diameter = 12.1")
April 23: Uranus in conjunction with the Sun at 09:09 hrs (diameter = 3.4")
April 23: Moon 2.2º north of Jupiter at 20:16 hrs
April 24: Pluto at western stationary point at 22:44 hrs (diameter = 0.1")
April 25: Limb of Moon 7 arcminutes south of the star Pi Sagittarii (mv= 2.88) at 16:32 hrs
April 25: Moon occults Saturn between 22:23 and 23:20 hrs
April 26: Moon occults Pluto between at 05:34 and 06:45 hrs
April 28: Limb of Moon 10 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 19:11 hrs
April 30: Saturn at western stationary point at 09:51 hrs (diameter = 17.1")
April 30: Moon 2.7º south of Neptune at 20:12 hrs
Mercury:
Mercury passed through superior conjunction on January 30 and is now in the western twilight sky. It reached its greatest elongation east (18º 05') on February 27, so is now preparing to pass between the Earth and the Sun (inferior conjunction). This will occur on March 15, after which Mercury will move to the eastern pre-dawn sky. The innermost planet shines a little brighter than Alpha Centauri or Rigel, but is only visible when it is at a large angular distance from the glare of the Sun. As Mercury lies well inside the Earth's orbit and close to the Sun, it can never move more than 27.8º from the Sun. On March 1 it will be only 2º above the western horizon at 6:50 pm (half an hour after sunset). At this low altitude, an observer will need a clear horizon without hills, trees or buildings. Mercury will be very close to Neptune on the morning of April 3.
Venus:
This, the brightest planet, passed through inferior conjunction (between
the Earth and the Sun) on October 27, and disappeared from the western
twilight sky. It has since reappeared in the eastern pre-dawn sky where it is very
prominent in the early mornings as a so-called 'morning star'. Currently in the
constellation of
Sagittarius, it will cross into Capricornus on March 2, and into Aquarius on
March 25. The waning crescent Moon will be
close by Venus on the mornings of March 3 and April 2 and 3. Venus will remain a pre-dawn object until next August.
(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).
December 2018 January 2019 August 2019 March 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 most of 2018, Venus appeared as an 'Evening Star' in the western twilight sky, where it stayed for about nine months. Venus passed between us and the Sun (inferior conjunction) on October 27 last, and is now in the morning sky as a 'Morning Star'. It will return to the evening sky to be an 'Evening Star' once again on August 14 next, although it won't be away from the Sun's glare to be easily visible until next October.
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 now poorly positioned for viewing, as the Earth has left it far behind. Mars is much reduced in size, being only
5 arcseconds in
diameter during March. By mid-April its diameter will have fallen to 4 arcseconds.
During March its phase will increase slightly from
91.43% to 93.66%.
All through March at the onset of darkness (7 pm), Mars will be about
20º or a little more than a handspan
above the west-north-western horizon. It will begin the month in the constellation of
Aries, but will cross into Taurus on March 24.
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, has now taken on a yellowish tint and has brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena have been caused by a great dust storm which has completely encircled the planet, obscuring the surface features so that they are 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 are 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 August it began to clear. Unfortunately, dust swamped the solar panels on the Mars Rover Opportunity, and the
batteries went flat on June 10 last. As the skies cleared, the batteries failed to recharge and by this month over 1000 attempts have been
made to restore communication with the rover. The final attempt was made on Tuesday, February 12, 2019, but was not successful. NASA has therefore
reported that the rover is defunct. Designed to last for 90 days and travel 1 kilometre, it exceeded its life expectancy by a factor of 58 and travelled
45 kilometres. The rover Curiosity is nuclear powered and does not have solar panels. It was therefore unaffected by the dust storm and is
working normally. The lander InSight touched down on Mars on November 26, 2018 after the dust storm had abated. Like Opportunity, it
generates electricity from solar panels, but unlike the rovers is not mobile.
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 is finally abating, and some of the surface features are 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 are now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet has 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 passed through conjunction with the Sun (on the far side of its orbit) on November 26 and is now in the early morning eastern sky. On March 1 it will be in the constellation Ophiuchus, and rising a few minutes before midnight. By the first light of dawn it will be about three-and-a-half handspans above the eastern horizon. As the month progresses, Jupiter will become higher in the sky as dawn approaches, and on March 31 it will be almost at the zenith at 5 am (the beginning of dawn). Jupiter will remain in Ophiuchus until November 16, when it will pass into Sagittarius. On March 27 and 28 the waning gibbous Moon will be close to Jupiter's left.
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.
Jupiter at opposition, May 9, 2018
>
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.
Saturn:
Uranus:
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.
Neptune, photographed from Nambour on October 31, 2008
Pluto: The erstwhile ninth and most distant planet reached conjunction on January 11. It is now observable
as a pre-dawn object, being difficult to observe this month as it is still close to
the Sun. 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.No meteor showers in March.
Lyrids
April 23
Full Moon, 100% sunlit ZHR =
15
Radiant: Near the star
Vega.
Associated with Comet Thatcher.
Pi Puppids
April 24
Almost Full Moon, 99% sunlit ZHR = 10
Radiant: Between the False Cross and the tail of Canis Major
Use this
Although most meteors are found in swarms associated with debris from comets, there are numerous 'loners', meteors travelling on solitary
paths through space. When these enter our atmosphere, unannounced and at any time, they are known as 'sporadics'. On an average clear and dark evening, an
observer can expect to see about ten meteors per hour. They burn up to ash in their passage through our atmosphere. The ash slowly settles to the ground as
meteoric dust. The Earth gains about 80 tonnes of such dust every day, so a percentage of the soil we walk on is actually interplanetary in origin. If a
meteor survives its passage through the air and reaches the ground, it is called a 'meteorite'. In the past, large meteorites (possibly comet nuclei or small asteroids) collided with the Earth and produced huge craters which still exist
today. These craters are called 'astroblemes'. Two famous ones in Australia are Wolfe Creek Crater and Gosse's Bluff. The Moon and Mercury are covered with such
astroblemes, and craters are also found on Venus, Mars, planetary satellites, minor planets, asteroids and even comets.
Comets:
ZHR = zenithal hourly rate (number of meteors expected to be observed at the zenith in one hour). The
maximum phase of meteor showers usually occurs between 3 am and sunrise. The reason most meteors are observed in the pre-dawn hours is because at that time
we are on the front of the Earth as it rushes through space at 107 000 km per hour (30 km per second). We are meeting the meteors head-on, and the speed at
which they enter our atmosphere is the sum of their own speed plus ours. In the evenings, we are on the rear side of the Earth, and many meteors we see at that
time are actually having to catch us up. This means that the speed at which they enter our atmosphere is less than in the morning hours, and they burn up less
brilliantly.
Green comets in the news
Comet C/2018 V1 Machholz-Fujikawa-Iwamoto
In modern astronomy, most comets are found by large mountaintop telescopes photographing the skies under computer control. The photographs are scanned automatically to look for any new object that is not on the databases, such as an asteroid or comet. These on-going robotic surveys discover most new comets before they are bright enough for amateur astronomers to catch them. Surprisingly, three amateur astronomers (one in Arizona, two in Japan) have just discovered a bright new comet in the constellation Virgo that somehow escaped the notice of the automated surveys. This morning it was near the magnitude 2.9 star Porrima (Gamma Virginis), and heading east through the background stars. It will be near the magnitude 3.38 star Zeta Virginis on November 18. Named Comet Machholz-Fujikawa-Iwamoto after the three discoverers, it is plunging toward the Sun and could brighten to naked-eye visibility later this month. It will be at its closest approach to Earth on November 27 and closest approach to the Sun on December 4. The best time to observe it from November 13 to November 18 will be from 4 am to the first light of dawn, close to the due east horizon, and a little over half a handspan to the left of Venus. As the days go by and it becomes closer to the Sun, it will become lost in the solar glare. Visit the November 12 and subsequent editions of Spaceweather for the full story.
Comet 46P/Wirtanen
Last
December, 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 in the far northern constellation or Ursa Major and slowly coming south, but at 11th magnitude is only a telescopic object. Visit Spaceweather and here for more information and chart
This month it will shine at magnitude 7 so it will be easy to see in small telescopes and binoculars, but not with the unaided eye. It will only be observable in the hour
or so before dawn begins to light the sky, low to the north-east horizon. On September 10 it will be gliding through the stars of the constellation Auriga about 58 million
kilometres from our planet. In the week ahead, it will cross into Gemini and on September 15 it will pass right across the
rich star cluster M35, providing a spectacular photo-opportunity for amateur astronomers. Visit the September 9 edition of
Spaceweather
for details and observing tips.
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.
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.
In
March the Eta Carinae Nebula is directly above the Southern Cross soon after
sunset, and
is at its highest above the horizon at 11 pm.
It can be observed for most of the night in March.
The Stars and Constellations for this
month:
This description of the night sky is for 9 pm on March 1 and 7 pm on March 31. They start at Orion,
which is very high in the north-west.
This month, Orion (see below) is high in the north-west. By mid-month, Orion will have set by midnight. Canis Major (the Large Dog) is just west of the zenith at this time, with the brilliant white star Sirius (Alpha Canis Majoris) showing the Dog's heart. Sirius, also known as the Dog Star, is the brightest star in the night sky and is about a handspan from the zenith. The brilliant white star Rigel (Beta Orionis) is about two handspans west of the zenith. Nearly overhead are the constellations Puppis, the Stern (of the ship, Argo) and Columba, the Dove. Columba culminates at 8 pm on March 1.
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The constellation Taurus with the clusters Pleiades and Hyades is between Orion and the north-western horizon. The brightest star in Taurus is a star dominating
(but not actually a member of) the Hyades cluster. This is Aldebaran, a K5 orange star with a visual magnitude of 0.87. It is only half as far away as the Hyades. The Pleiades is
a small group like a question mark, and is often called the Seven Sisters, although excellent eyes are needed to detect the seventh star without optical aid. All the stars in this cluster
are hot and blue. They are also the same age, as they formed as a group out of a gas cloud or nebula. There are actually more than 250 stars in the Pleiades.
The Pleiades is the small cluster at centre left, while the Hyades is the much larger grouping at centre right.
Wisps of a nebula through which the Pleiades are passing can be seen around the brighter stars in the cluster.
Between Orion’s head and the north-western horizon is a large constellation shaped roughly like a pentagon. It is north of Taurus. This is Auriga the Charioteer, its
brightest star being Capella, at the bottom of the tilted pentagon. Capella is the sixth brightest star in the sky, after Sirius, Canopus, Alpha Centauri, Beta
Centauri and Vega. To the left of Capella is a small triangle of stars known as 'The Kids’. The lowest star in this triangle is Epsilon Aurigae, one of the
largest stars known. It is also very distant.
To the east of Auriga, Gemini is quite high, the two twin stars at its eastern end, Pollux and Castor being due north. At 9 pm at the beginning of the month they are straddling the meridian (the line that runs from due south to due north and passing through the zenith - directly overhead). When a sky object crosses the meridian, it is said to be culminating. At that point, it ceases rising and begins setting. The Twins will have set by 1.30 am.
Tonight, at a little over two handspans above the northern horizon, and directly above Pollux and Castor, is the first magnitude star Procyon, which is the brightest star in the constellation Canis Minor (the Small Dog).
High in the north-east is another zodiacal constellation, Leo, the Lion. The bright star Regulus (Alpha Leonis) marks the Lion’s heart, and Denebola, the star marking the tip of the lion's tail, is low in the east-north-east. From the southern hemisphere, we always see the Lion upside-down. His head and mane are marked by a curved line of stars shaped like an upside-down question mark. This line is also known as the 'Reaping Hook' or 'Sickle', the star Regulus marking the end of the Sickle's handle.
Between Gemini and Leo is the faint constellation of Cancer the Crab. Though a fairly unremarkable constellation in other ways, Cancer does contain a large star cluster called Praesepe or the Beehive, which presents well in binoculars. Also known as M44* , Praesepe is a little more than halfway along a line between Pollux and Regulus.
Rising above the eastern horizon is the next zodiacal constellation after Leo, Virgo, the Virgin. The brightest star in Virgo is Spica, an ellipsoidal variable star whose brightness averages magnitude 1. This makes it the sixteenth brightest star, and its colour is blue-white. Spica is about 10 degrees above the theoretical eastern horizon at this time. Between Denebola and Spica is a fainter star, Porrima, which has a magnitude of 2.74.
High in the east above Spica is the constellation Corvus the Crow, shaped like a quadrilateral of magnitude 3 stars. A large but faint constellation, Hydra, the Water Snake, winds its way from near Procyon around the north-eastern part of the sky at an altitude of about 60 degrees above the horizon. It passes over the top of Corvus and Virgo to end near Libra, which will not rise until 10 pm (at the beginning of the month).
Hydra has one bright star, Alphard, mv=2.2, an orange star that was known by Arabs in ancient times as ‘The Solitary One’, as it lies in an area of sky with no bright stars nearby.
Well up in the south-south-east, Crux (Southern Cross) is almost horizontal. The two Pointers Alpha and Beta Centauri lie below Crux. Crux will have rotated clockwise to a vertical position by 1 am at mid-month. Surrounding Crux on three sides is the large constellation Centaurus, and between Crux and the southern horizon are two brilliant stars, Alpha and Beta Centauri. Beta is the one nearer to Crux. These two stars are also known as the Guardians of the Cross.
To the right of Crux is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. Below and to the right of Alpha Centauri and underneath Musca is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle. It is about half a handspan above the south-south-eastern horizon.
Between Crux and Sirius is a very large area of sky filled with interesting objects. This was once the constellation Argo Navis, 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).
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
One and a half handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). On the border of Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. The False Cross is a little more than a handspan above Crux and to the right, and is also lying on its side at this time of year. It is high in the south, and will soon culminate. 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.
A handspan above the south-south-western horizon is Achernar, Alpha Eridani. It is the brightest star in Eridanus the River, which winds its way with faint stars from Achernar in a northerly direction to Cursa, a mv= 2.9 star close to brilliant Rigel in Orion. At magnitude 0.49, Achernar is the ninth brightest star. It swings down towards the south-south-westerly horizon during the evening, and sets soon after midnight.
High in the south, about 43 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. It is a little less than a handspan below (south of) Canopus. About a handspan below the LMC is the Small Magellanic Cloud (SMC), a smaller glowing patch. The LMC and SMC are described below.
The zodiacal constellations visible tonight, starting at the western horizon and heading east (passing about two handspans north of the zenith, are Aries, Taurus, Gemini, Cancer, Leo and Virgo.
The season of the Hunter and his Dogs
Two of the most spectacular constellations in the sky may be seen near the zenith as soon as darkness falls. These are
Orion the Hunter, and his
large dog, Canis Major. Orion straddles the celestial equator, midway between the south celestial pole and its northern equivalent. This means that the centre of the constellation,
the three stars known as Orion's Belt, rise due east and set due west.
Orion:
This is one of the most easily recognised constellations, as it really does give a very good impression of a human figure. From the northern hemisphere he appears to stand upright when he is high in the sky, but from our location ‘down under’ he appears lying down when rising and setting, and upside down when high in the sky. You can, though, make him appear upright when high in the sky (near the meridian), by observing him from a reclining chair, with your feet pointing to the south and your head tilted back
Orion has two bright stars marking his shoulders, the red supergiant Betelgeuse and Bellatrix. A little north of a line joining these stars is a tiny triangle of stars marking Orion’s head. The three stars forming his Belt are, from west to east, Mintaka, Alnilam and Alnitak. These three stars are related, and all lie at a distance of 1300 light years. They are members of a group of hot blue-white stars called the Orion Association.
The red supergiant star, Betelgeuse
To the south of the Belt, at a distance of about one Belt-length, we see another faint group of stars in a line, fainter and closer together than those in the Belt. This is Orion’s Sword. Orion’s two feet are marked by brilliant Rigel and fainter Saiph. Both of these stars are also members of the Orion Association.
The Saucepan, with Belt at right, M42 at upper left.
Orion is quite a symmetrical constellation, with the Belt at its centre and the two shoulder stars off to the north and the two
knee stars to the south. It is quite a large star group, the Hunter being over twenty degrees (a little more than a
handspan) tall.
The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. Tonight this asterism appears right-side up, as in the photographs above. The faint, fuzzy star in the centre of the Sword, or the Saucepan's handle, is a great gas cloud or nebula where stars are being created. It is called the ‘Great Nebula in Orion’ or ‘M42’ (number 42 in Messier’s list of nebulae). A photograph of it appears below:
The Sword of Orion, with the Great Nebula, M42, at centre
The central section of the Great Nebula in Orion. At the brightest spot is a famous multiple star system, the Trapezium, illustrated below.
New stars are forming in the nebula. At the brightest spot is a famous multiple star system, the Trapezium, illustrated below.
Canis MajorAbove Orion as twilight ends (facing west), a brilliant white star will be seen about one handspan away. This is Sirius, or Alpha Canis Majoris, and it is the brightest star in the night sky with a visual magnitude of -1.43. It marks the heart of the hunter's dog, and has been known for centuries as the Dog Star. As we see him tonight, the dog is on his feet with his tail at upper left. A front leg stretches down from Sirius to Mirzam. It is also known as Beta Canis Majoris, which tells us that it is the second-brightest star in the constellation. Mirzam is about one-third of a handspan below Sirius.
The hindquarters of the Dog are indicated by a large right-angled triangle of stars located above and to the left of Sirius. The end of his tail is the top-left corner of the triangle, about one handspan south (above and to the left) of Sirius. It is marked by a blue-white star, Aludra.
Both Sirius and Rigel are bright white stars and each has a tiny, faint white dwarf companion. Whereas a small telescope can reveal the companion to Rigel quite easily, the companion to Sirius the Dog Star, (called ‘the Pup’), can only be observed by using a powerful telescope with excellent optics during rare periods of a completely still atmosphere, as it is very close to brilliant Sirius and is usually lost in the glare.
Canis Major as it appears almost overhead at 9 pm at mid-month (observer facing west).
Canis Minor
By 8.00 pm at mid-month, this small constellation is about one and a half handspans due north of the zenith. It contains only two main stars, the brighter of which is Procyon (Alpha Canis Minoris). This yellow-white star of mv= 0.5 forms one corner of a large equilateral triangle, the other two corners being the red Betelgeuse and white Sirius. Beta Canis Minoris is also known as Gomeisa, a blue-white star of mv= 3.1.
Some fainter constellations
Between the two Dogs is the constellation Monoceros the Unicorn, undistinguished except for the presence of the remarkable Rosette Nebula. South of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Eridanus the River winds its way from near Orion west of the zenith to Achernar, high in the south-west. Between Achernar and the western horizon is the star Fomalhaut, a white star of first magnitude in the small constellation of Piscis Austrinus (the Southern Fish). To the left of Fomalhaut is the triangular constellation of Grus, the Crane. Between the zenith and the south-western horizon are a number of small, faint constellations, Horologium, Pictor, Caelum, Mensa, Tucana, Phoenix, Hydrus and Reticulum. The LMC lies in the constellation Dorado, and the South Celestial Pole is in the very faint constellation Octans.
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.
Project a line from the top of the Cross (the star Gacrux) down through its base (the star Acrux) 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 (a handspan above the south-south-eastern horizon) to Achernar (a handspan above the south-western horizon. At 8 pm at mid-month, both stars will be at similar altitudes and the line will be horizontal. Bisect this line to find the pole with an accuracy of two degrees.
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, as the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
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 - B is even said to have an Earth-sized planet), 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.
Alpha Centauri, with Proxima
Knowing the orbital period of the two brightest stars A and B, we can apply Kepler’s Third Law to find the distance they are apart. This tells us that
Alpha Centauri A and B are about 2700 million kilometres apart or about 2.5 light hours. This makes them a little less than the distance apart of the Sun and Uranus
(the orbital period of Uranus is 84 years, that of Alpha Centauri A and B is 80 years.) Albireo (Beta Cygni) is sometimes described poetically as a large topaz with a small blue sapphire. It is one of the sky’s most beautiful objects. The
stars are of classes G and B, making a wonderful colour contrast. It lies at a distance of 410 light years, 95 times further away than Alpha Centauri. Binary stars may be widely spaced,
as the two examples just mentioned, or so close that a telescope is struggling to 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. Star Clusters
The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some are relatively young.
The photograph below shows a typical open cluster, M7
* . It is found in the constellation Scorpius, just below the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years.
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters, containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is NGC 5139
**, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, but is also observable in autumn. Shining at fourth magnitude, it is faintly visible to the unaided eye, but is easily seen with binoculars, like a light in a fog. A telescope of 20 cm aperture or better will reveal its true nature, with hundreds of faint stars giving the impression of diamond dust on a black satin background. It lies at a distance of 5 kiloparsecs, or 16 300 light years.
The globular cluster Omega Centauri
The central core of Omega Centauri
There is another remarkable globular, second only to Omega Centauri. About two degrees below the SMC (see below), binoculars can detect a fuzzy star. A telescope will reveal this faint glow as a magnificent globular cluster, lying at a distance of 5.8 kiloparsecs. Its light has taken almost 19 000 years to reach us. This is NGC 104, commonly known as 47 Tucanae. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars twinkling in its core are very beautiful. This month, 47 Tucanae is low in the south-south-west, and not clearly visible. By 10 pm Omega Centauri is high enough for detailed viewing.
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 appears to lie above 47 Tucanae as we see it in mid-evening this month. It 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.
Globular Cluster NGC104 in Tucana
* 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
High in the south, to the left of Achernar, two large 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 to the left and above the SMC, and is noticeably larger. They lie at a distance of 160 000 light years, and are about 60 000 light years apart. They are dwarf galaxies, and they 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.
From our latitude both Magellanic Clouds are circumpolar. This means that they are closer to the South Celestial Pole than that Pole's altitude above the horizon, so they never dip below the horizon. They never rise nor set, but are always in our sky. Of course, they are not visible in daylight, but they are there, all the same.
The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula (below)
These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies referred to above, and so can be observed in much clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our nearest inter-galactic neighbours.
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 southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window is too low in the south-west in the early evenings this month for useful viewing. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It begins to rise in the east-north-east at 8 pm at mid-month, and is well placed for viewing at midnight.
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 out across the millions of light years of space to thousands of distant
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