April
2020
Updated: 1 April 2020
Welcome to the night skies of Autumn, featuring Venus, 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 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 Pisces, the Fishes. It leaves Pisces and passes into Aries, the Ram on April 18.
Full Moon:
April 28:
Moon at ascending node at 03:53 hrs, diameter = 30.5'
Moon at 8 days after New, as on April 02.
The photograph above shows the Moon when approximately eight days after New, just after First Quarter. A rotatable view of the Moon, with ability to zoom in close to the surface (including the far side), and giving detailed information on each feature, may be downloaded
here. A professional version of this freeware with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the Moon's surface) and many other useful features is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage.Click
here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool ! All these downloads are freeware, although the authors do accept donations if the user feels inclined to support their work.
Lunar Feature for this Month
Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at a large 'ghost' crater in the Moon's southern hemisphere, Kies .
Kies was once a bowl-shaped impact crater like most of the others in its neighbourhood, but is much older than most of them. The huge flat area that occupies most
of the image above is the Mare Nubium (Sea of Clouds). It is a lava plain caused by magma released when an asteroid-sized impactor struck the Moon early in its
history. Kies was already a crater in the area, and the lava flooded and levelled its surroundings and breached its western wall, filling up the original bowl-shaped
interior until it was level with the outside plain. Only the summits of Kies' rim remain above the new lava surface. These summits are highest in the south-western rim,
reaching altitudes of about 750 metres. The floor of Kies is dotted with numerous craterlets, ranging in size from the two
largest at 7 kiloimetres diameter down to the limit of visibility. 100 kilometres to the north-east (upper right) of Kies is a similar ghost crater
which is only 27 kilometres across. In the case of this example, the walls of the crater have virtually been swamped by lave entirely, making it a better
example of a 'ghost crater' than Kies. Two 49 kilometre craters, almost twins, can be found south-west of Kies, near the lower left corner of the image. These
are named, from left to right, Campanus and Mercator. The floor of the former is completely in shadow, while some of Mercator's floor is catching the rising
Sun.
Numerous 'wrinkle ridges' (solidified ripples in the lava flows) can be seen in the image, some over 100 kilometres long. Just off the northern margin of this image is the large
63 kilometre impact crater called Bullialdus (see
Lunar Features of the Month Archive
There is a volcano in this image, probably extinct. It is called Kies Pi and can be seen 21 kilometres
to the west-south-west of Kies' wall. It is just to the left of the Greek symbol for Pi, shown
as
Kies
Johann Kies (1713-1781) was a German mathematician and astronomer. He collaborated in astronomical and mathematical research with the Frenchmen Lalande and de Lacaille, and with his compatriot Leonhard Euler.
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.
April 1: Mars 54 arcminutes south of Saturn at 05:16 hrs
May 1: Mercury 18 arcminutes south of Uranus at 13:48 hrs
May 2: Mars 55 arcminutes north of the star Nashira (Gamma Capricorni, mv= 3.69) at
01:43 hrs
May 4: Mars 55 arcminutes north of the star Deneb Algiedi (Alpha Capricorni, mv= 2.85) at 13:45 hrs
May 5: Mercury at superior conjunction at 07:28 hrs
May 8: Moon 1.6º north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 22:10 hrs
May 10: Mercury at perihelion at 14:19 hrs
May 11: Moon 1.8º north of the star Kaus Borealis (Lambda Sagittarii, mv= 2.82) at 08:34 hrs
May 11: Saturn at western stationary point at 11:45 hrs
May 12: Mercury 2.9º south of the star Alcyone (mv= 2.85) at 11:30 hrs (Alcyone is the brightest star in
the Pleiades cluster, and is located in the cluster's centre)
May 12: Limb of Moon 22 arcminutes south of Pluto at 16:03 hrs
May 13: Moon 2.5º south of Saturn at 05:43 hrs
May 13: Venus at eastern stationary point at 17:18 hrs
May 14: Limb of Moon 30 arcminutes south of the star Nashira (Gamma Capricorni, mv= 3.69)
at 18:57 hrs
May 14: Moon 1.3º south of the star Deneb Algiedi (Alpha Capricorni, mv= 2.85) at 21:38 hrs
May 14: Jupiter at western stationary point at 23:06 hrs
May 15: Moon 1.9º south of Mars at 14:59 hrs
May 17: Moon 4º south of Neptune at 02:46 hrs
May 21: Moon 3.6º south of Uranus at 02:46 hrs
May 22: Venus 53 arcminutes north of Mercury at 19:02 hrs (Venus will have a phase of only 5.3%, but
its diameter will be extremely large at 54 arcseconds)
May 24: Moon 2.9º south of Venus at 13:25 hrs
May 24: Moon 2.5º south of Mercury at 22:30 hrs
May 25: Moon 1.9º north of the star Alheka (Zeta Tauri, mv= 2.97) at 00:10 hrs
May 25: Moon 1.9º north of the star Propus (Eta Geminorum, mv= 3.31) at 17:18 hrs
May 25: Moon 1.7º north of the star Tejat Posterior (Mu Geminorum, mv= 2.87) at 21:02 hrs
May 26: Moon 1º south of the star Mebsuta (Epsilon Geminorum,
mv= 3.06) at 03:45 hrs
The Planets for this month:
Mercury:
During all of April, the innermost planet will be in the morning sky. It will begin the month in the constellation of Aquarius, but each following night it will appear at a given time further in angular distance from the Sun, and heading east through the starry background. Mercury will cross into Pisces on April 10, and then cut across a corner of the non-zodiacal constellation of Cetus, the Whale on April 15, re-entering Pisces on April 18. On that day, Mercury will appear as a tiny gibbous Moon with a diameter of 5 arcseconds and a phase of 85%. The best time to observe Mercury will be in the first half of April. The crescent Moon will be 3 degrees to the right of Mercury on the morning of April 22. Mercury will pass through superior conjunction (on the far side of the Sun) on May 5, and will then return to the western twilight sky, becoming visible later that month.
Venus: This, the brightest planet, was a 'morning star' in the first half of 2019, It passed through superior conjunction (when it went behind the Sun on the far side of its orbit) on August 14 last, whereupon it moved to the western twilight sky, and for the past eight months it has appeared as a so-called 'evening star'. It is currently the brightest object in the evening sky apart from the Moon. Venus will begin the month in the constellation of Taurus, and each night the point on the horizon where it sets moves a little further north. At the beginning of April it will set almost exactly due west.
Venus will then move into the morning sky, rising at dawn just before the Sun. It will continue its south-western passage through Taurus, and will pass just north of the Hyades star cluster between June 12 and 22, but the Sun will be only a lttle more than a handspan away.
During April the angular diameter of Venus will increase from 25 arcseconds to 39 arcseconds, but its phase will decrease from 47% to 25%. This combination of increasing size but decreasing phase means that the brightness of Venus throughout March will increase only slightly, from magnitude -4.4 to -4.5. The waxing crescent Moon will appear to the left of Venus on April 26.
We will lose Venus from our evening sky at the end of May, as it will pass through inferior conjunction (overtaking the Earth and passing between us and the Sun) on June 4. It will then return to the eastern pre-dawn sky and become a 'morning star' again.
(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).
August 2019 March 2020 mid-May 2020 early July 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 remains a difficult object, as it is on the far side of its orbit as seen from the Earth. Mars is still quite small, being only 7 arcseconds in diameter. It passed through superior conjunction at 8:55 pm on September 2 last year. Mars begins the month in the constellation of Capricornus, and on the morning of April 1 it appears close to Saturn.
Mars will continue to head east through the background stars, and will enter Aquarius on May 9. Mars will remain very small and distant for another four months. The next close approach of Mars to the Earth will occur in six months' time on October 14, when its diameter will rapidly increase to 22.3 arcseconds. The waning crescent Moon will be 5 degrees above Mars on the morning of April 16.
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 passed through conjunction with the Sun on December 28, and has now moved to the pre-dawn eastern sky. It can now be observed, rising in the east-south-east soon after midnight in the constellation of Sagittarius. The waning crescent Moon will be to the right of Jupiter on the morning of April 15.
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 was in a similar position near Jupiter's eastern limb (edge) as in the fifth picture in the series above.
It will be seen that in the past two months the position of the Spot had drifted
when compared with the festoons in the Equatorial Belt, so must rotate around the planet at a slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the
planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less. This high rate of rotation has made the planet quite oblate. The prominent 'bay'
around the Red Spot in the five earlier images appeared to be disappearing, and a darker streak along the northern edge of the South Tropical Belt
was moving
south. In June this year the Spot began to shrink in size, losing about 20% of its
diameter. Two new white spots have developed in the South Temperate Belt, west of the Red Spot. The five upper images were taken near opposition, when the Sun was
directly behind the Earth and illuminating all of Jupiter's disc evenly. The July 2 image was taken just four days before Eastern Quadrature, when the angle
from the Sun to Jupiter and back to the Earth was at its maximum size. This angle means that we see a tiny amount of Jupiter's dark side, the shadow being
visible around the limb of the planet on the left-hand side, whereas the right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to
the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter, sitting in front of the North Tropical Belt, just to the left of its centre.
Jupiter at opposition, May 9, 2018
Jupiter reached opposition on May 9, 2018 at 10:21 hrs, and the above photographs were taken that evening, some ten to twelve hours later. The first image above was taken at 9:03 pm, when the Great Red Spot was approaching Jupiter's central meridian and the satellite Europa was preparing to transit Jupiter's disc. Europa's transit began at 9:22 pm, one minute after its shadow had touched Jupiter's cloud tops. The second photograph was taken three minutes later at 9:25 pm, with the Great Red Spot very close to Jupiter's central meridian.
The third photograph was taken at 10:20 pm, when Europa was approaching Jupiter's central meridian. Its dark shadow is behind it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:34 pm shows Europa and its shadow well past the central meridian. Europa is the smallest of the Galilean satellites, and has a diameter of 3120 kilometres. It is ice-covered, which accounts for its brightness and whitish colour. Jupiter's elevation above the horizon for the four photographs in order was 50º, 55º, 66º and 71º. As the evening progressed, the air temperature dropped a little and the planet gained altitude. The 'seeing' improved slightly, from Antoniadi IV to Antoniadi III. At the time of the photographs, Europa's angular diameter was 1.57 arcseconds. Part of the final photograph is enlarged below.
Jupiter at 11:34 pm on May 18, nine days later. Changes in the rotating cloud patterns are apparent, as some cloud bands rotate faster than others and
interact. Compare with the first photograph in the line of four taken on May 9. The Great Red Spot is ploughing a furrow through the clouds of the South
Tropical Belt, and is pushing up a turbulent bow wave.
Jupiter at opposition, June 11, 2019
The third photograph was taken at 10:41 pm, when Europa was about a third of its way across Jupiter. Its dark shadow is trailing it, slightly below, on the clouds of the North Temperate Belt. The shadow is partially eclipsed by Europa itself. The fourth photograph at 10:54 pm shows Europa and its shadow about a quarter of the way across. This image is enlarged below. The fifth photograph shows Europa on Jupiter's central meridian at 11:24 pm, with the Great Red Spot on Jupiter's limb. The sixth photograph taken at 11:45 pm shows Europa about two-thirds of the way through its transit, and the Great Red Spot almost out of sight. In this image, the satellite Callisto may be seen to the lower right of its parent planet. Jupiter's elevation above the horizon for the six photographs in order was 66º, 70º, 75º, 78º, 84º and 86º. As the evening progressed, the 'seeing' proved quite variable.
There have been numerous alterations to Jupiter's belts and spots over the thirteen months since the 2018 opposition. In particular, there have been major disturbances affecting the Great Red Spot, which appears to be slowly changing in size or "unravelling".
It was very fortuitous that, during the evenings of the days when the 2018 and
2019 oppositions occurred, there was a transit of one of the satellites as well as the appearance of the Great Red Spot. It was also interesting in that the
same satellite, Europa, was involved both times. This run of good fortune will continue for the 2020 opposition, except that the satellite in
transit will be Io.
This year, opposition will occur at 5:33 pm on July 14. On that date, the Sun will set at 5:13 pm, just eight minutes after Jupiter rises. The satellite Io
will commence its transit across the face of Jupiter at 5:31 pm, with its shadow almost exactly behind it. The Great Red Spot will be well on its way across
Jupiter's disc. Io will move off the face of Jupiter at 7:49 pm, when Jupiter will be at an elevation of 34.6 degrees. In 2021 the Great Red Spot will be
present, but there will be no transit of any satellite.
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 conjunction with the Sun on January 13. This
means that Pluto has left the evening sky and moved to the pre-dawn eastern sky. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of Neptune. Located
about 11 degrees east of the star Nunki in the handle of the Sagittarius
'Teapot' in the eastern third of Sagittarius, it is a faint 14.1 magnitude
object. On April 6, Jupiter and Pluto will be only 44 arcminutes apart. 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.
This month, there will be a fine grouping of the planets Mars, Jupiter and Saturn in the eastern pre-dawn sky. On
April 15 and 16, the crescent 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 will have reached Saturn, and the two planets will be
less than a degree apart, with Jupiter 6.3
Meteor Showers:
Radiant: Near the star
Vega.
Associated with Comet Thatcher.
Pi Puppids
April 24
New Moon, 0% 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.
It is behaving in a similar manner to Comet 2006 P1 (McNaught), which in January 2007 provided the finest display since the 1910 apparition of Comet
Halley. Comet McNaught was so bright that it became visible in daylight, and at night its tail arced across the sky in a most spectacular manner - see
our
Comet 46P/Wirtanen
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 500 million kilometres away in the constellation Virgo, between Spica and Arcturus, and close to the star Heze (Zeta Virginis), but at 19th magnitude it is too faint for most amateur telescopes. Click here for more information and chart
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.
This month,
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 all of the night in April.
The Stars and Constellations for this
month:
These descriptions of the night sky are for 9 pm on April 1 and 7 pm on April
30. They start at Orion, which is low in the west.
The constellation Taurus is straddling the west-north-westerly horizon, but its main features, the Pleiades and Hyades clusters, will have set by
9:10 pm. 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. On the evenings of April 3 and 4, the planet Venus will pass through outlying members of the
Pleiades. The Pleiades will have disappeared by 7.40 pm early in the month, and the rest of Taurus follows them below the horizon soon after.
Wisps of the nebula which formed the Pleiades can be seen around the brighter stars in the cluster.
Above the north-western horizon lies the constellation of Gemini, the
Twins. The two twin stars at its north-eastern end, Pollux and Castor,
are very distinctive, Pollux being vertically above Castor. These two stars are
about one handspan above the horizon, but will have set by 10.30 pm at
mid-month.
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-north-east is another zodiacal constellation, Leo, the Lion. The bright star Regulus (Alpha Leonis) marks the Lion’s heart, and is presently due north. Denebola, the star marking the lion's tail, is high in the north-east. We see the lion upside-down from the Southern Hemisphere. Regulus is the highest star in a pattern called 'The Sickle' (or reaping-hook). It marks the top of the Sickle's handle, with the other end of the handle, the star Eta Leonis, directly underneath. The blade of the Sickle curves around clockwise from Eta Leonis. The Sickle looks like an upside-down question mark, and forms the mane and head of the lion.
About four degrees to the right and below Eta Leonis is a beautiful double star,
Algieba or Gamma Leonis. With a total magnitude of 2.61, the two
stars are only 4.3 arcseconds apart, and may be distinguished with a small
telescope. Both are orange in colour.
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 is a good sight in binoculars.
Rising in the north-east is a particularly beautiful orange star with a fine name: Arcturus. This is the fourth brightest star (after Sirius, Canopus and Alpha Centauri). It is a K2 star of magnitude -0.06, and lies at a distance of 36 light years. It is the brightest star in the constellation Boötes the Herdsman (only partially risen) and therefore has the alternative name of Alpha Boötis.
High above the eastern horizon (about 45 degrees up) 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.
Below Spica and to its right is the faint constellation of Libra (the Scales). It has only two main stars, both about magnitude 2.7, but they have wonderful names: Zubenelgenubi and Zubeneschamali. Libra is low in the east, and as the night progresses it will be followed above the horizon by one of the brightest and most easily-recognisable constellations, Scorpius the Scorpion, which will be described next month.
Above Spica and approaching the zenith is the constellation
Corvus the Crow, shaped like a quadrilateral of third magnitude stars. A
large but faint constellation, Hydra (the Water-snake), winds its way
from near Procyon through the zenith and over the top of Corvus and Virgo to
Libra, which has just risen above the eastern horizon. Hydra has one bright
star, Alphard, mv=2.2. Alphard is 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. Tonight it is about 20 degrees north of the
zenith, near culmination. About a handspan to the south-east of Alphard is a
bright planetary nebula, the 'Ghost of Jupiter' NGC 3242. It is the
remnant left when the central star exploded (below).
Well up in the south-south-east, Crux (Southern
Cross) is at an angle of about thirty degrees. Just below the second
brightest star in the Cross (Beta Crucis) is a brilliant small star
cluster known as Herschel's Jewel Box. In the centre of the cluster is a
red supergiant star, which is just passing through.
Between Hydra and Leo is a faint constellation, Sextans, (The Sextant),
named by Johannes Hevelius to commemorate his favourite observing instrument
which was destroyed when his observatory was burned by arsonists. Between
Sextans and Corvus is another faint constellation,
Crater, (The Cup). Appearing above the east-south-eastern
horizon is the head of Scorpius, the Scorpion. This
famous zodiacal constellation will be fully risen by 10.00 pm at the beginning
of the month.
The two Pointers - Alpha
and Beta Centauri - lie below Crux, Alpha being below Beta. Crux will
have rotated clockwise to a vertical position by 11.00 pm at mid-month.
Surrounding Crux on three sides is the large constellation
Centaurus, and the Pointers are its two brightest stars. They are also
known as the Guardians of the Cross.
At top right - the Eta Carinae Nebula. Centre - Crux (Southern Cross) tilted on its side. Beneath Crux - the Coalsack. Bottom - the two Pointers, Alpha and Beta Centauri.
To the right and below Crux is a small, fainter quadrilateral of stars,
Musca, the Fly. Out of all the 88 constellations, it is the only insect.
Below Alpha Centauri is a (roughly) equilateral triangle of 3rd magnitude stars.
This is the constellation
Triangulum Australe, the Southern Triangle. It is well 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 huge 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 in the eighteenth century Nicholas Lacaille divided it into three sections - Carina (the Keel), Vela (the Sails) and Puppis (the Stern).
One and a half handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). Although appearing almost as bright as Sirius but a little more yellow, the two stars are entirely dissimilar. Sirius is a normal-sized star that is bright because it is close to us - only 8.6 light years away. Canopus, on the other hand, is a F0 type supergiant, over 100 times brighter than Sirius, but 36 times further away (312 light years).
On the border of Carina and Vela is the False Cross, larger and more lopsided than the Southern Cross. The False Cross is two handspans above Crux and to the right, and is also tilted to the left at this time of year. It has passed culmination, and is beginning to head for the south-south-western horizon. 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. It is a turbulent area of dark
dust lanes and fluorescing gas. There is a peculiar unstable star in its centre.
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
Close to 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 will have set by
9.00 pm at mid-month.
High in the south, about 30 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. It is about a handspan below (south of) Canopus. About a handspan below and to the left of the LMC is the Small Magellanic Cloud (SMC), a smaller glowing patch. The LMC and SMC are the closest other galaxies to our own Milky Way Galaxy, and are described below.
The zodiacal constellations visible tonight, starting at the west-north-western horizon and heading east (passing about two handspans north of the zenith, are Taurus, Gemini, Cancer, Leo, Virgo and Libra.
Two of the most spectacular constellations in the sky may be seen in the western half of the sky 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.
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 brilliant white star Rigel (Beta Orionis), once twice as bright as Betelgeuse, is
presently three times brighter. Some have speculated that the dimming may not have been intrinsic to Betelgeuse, but may have been caused by a small molecular cloud
passing across the supergiant in our line of sight, thereby obscuring some of its light. Future monitoring of Betelgeuse may shed light on this phenomenon. 
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 foot stars to the south. It is 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 deep sky objects). 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.
Above 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 nose 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, which 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.
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, 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).
By 8 pm at mid-month, this small constellation is a little more than two handspans above the north-western horizon. 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. To the left of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Underneath the star Canopus are the faint constellations Pictor (The Painter's Easel), Caelum (The Chisel), Reticulum (The Net) and Horologium (The Pendulum Clock). These were all invented by Nicolas de Lacaille and named after items in his workshop. Eridanus the River winds its way from near Orion west of the zenith to Achernar, very low in the south-south-west. The LMC lies in the constellation Dorado, with the faint constellations Mensa (Table Mountain) and Hydrus (The Water Snake) to its left. The South Celestial Pole is in the very faint constellation, Octans (Hadley's Octant).
Below the False Cross is the faint constellation of Piscis Volans (The Flying Fish), called Volans for short. Underneath and to the left of Volans is Chamaeleon, (The Chamaeleon Lizard). The Lizard has his tongue extended to catch the adjoining Fly, Musca.
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 (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 7 pm on April 1, both stars will be at similar altitudes and the line will be horizontal. Bisect this line to find the pole.
Interesting photographs of this area can
be taken by using a camera on time exposure. Set the camera on a tripod pointing
due south, and open the shutter for thirty minutes or more. The stars will move
during the exposure, being recorded on the film as short arcs of a circle. The
arcs will be different colours, like the stars are. All the arcs will have a
common centre of curvature, which is the south celestial pole.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.
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
The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some are relatively young.
The photograph below shows a typical open cluster,
M7*. It lies in the constellation Scorpius, just below the scorpion's sting. 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 close to the horizon in summer. 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 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
Globular Cluster NGC 104 in Tucana
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
* 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-south-west, below and t
o the left of Canopus, 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 right 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, 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 below the horizon in the early evenings this month. 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 sunset 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 galaxies.
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