February
2020
Updated: 19 February 2020
Welcome to the night skies of Summer, featuring Venus, Auriga, Taurus, Gemini, Orion, Canis Major and Carina
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 Capricornus, the Sea-Goat. It leaves Capricornus and passes into Aquarius, the Water-bearer on February 17.
Full Moon:
Moon at 8 days after New, as on February 03.
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 small crater with a long cleft passing through it, Hyginus .
Hyginus was not created by the impact of a meteor striking the lunar
surface, as all the other circular features in the above image were. Impact
craters of such a small size generally have a bowl-shaped floor and all of them
have raised rims. Hyginus, though, has no rim at all, and gives the appearance
that it is a portion of the lunar surface that has collapsed down into a void
beneath, like a 'sinkhole' on Earth. The obvious conclusion is that Hyginus and
its cleft were both formed by vulcanism in the area. This is supported by close
examination of the flat-floored cleft. Hyginus sits squarely on it, and further
along the cleft can be counted at least 20 similar but smaller craterlets, all
rimless, where the floor of the cleft has collapsed into voids underneath it.
Beneath the cleft there must be a long fissure where molten magma has been able
to reach the surface, and flood out in all directions, creating the huge flat
lava plain which forms about 60% of the above image. The extrusion of this magma
has left linear voids below the surface, which have collapsed down to create the
cleft and Hyginus itself. A void alongside Hyginus has also collapsed, deforming
the crater's northern wall.
The Rima Hyginus runs east and branches in two (near the right-hand
margin). The northern branch connects with the Rima Ariadaeus, also 226
kilometres long, which at its eastern end is swamped by the lavas of the Mare Tranquillitatis (Sea
of Tranquility) but reappears again as the Rima Hypatia (184 kilometres long)
south of the Apollo 11 landing site (see the
South of Hyginus there is a complex system of clefts, partially seen above. This
network is in the environs of the 27 kilometre crater Triesnecker, and can be
examined in full in
Hyginus
For information about Gaius Julius Hyginus (64 BCE - AD 17), see the
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.
February 1: Jupiter 1.6º south of the star Xi-2 Sagittarii (mv= 3.52) at 14:31 hrs
March 4: Moon 1.3º north of the star Alheka (Zeta Tauri, mv= 2.97) at 05:37 hrs
March 4: Moon 1.3º north of the star Propus (Eta Geminorum (mv= 3.31) at 22:21 hrs
March 5: Limb of Moon 55 arcminutes north of the star Tejat Posterior (Mu Geminorum, (mv= 2.87) at 02:04 hrs
March 5: Moon 1.6º south of the star Mebsuta (Epsilon Geminorum (mv= 3.06) at 08:38 hrs
March 6: Mars 2.9º south of the star Nunki (Sigma Sagittarii, mv= 2.04) at 05:14 hrs
March 8: Neptune in conjunction with the Sun at 22:38 hrs (diameter = 2.2")
March 9: Venus 2.2º north of Uranus at 04:16 hrs
March 10: Mercury at western stationary point at 23:34 hrs (diameter = 9.5")
March 11: Mars 2º south of the star Pi Sagittarii (mv= 2.88) at 21:23 hrs
March 15: Moon 1.9º north of the star Graffias (Beta-1 Scorpii, mv= 2.56) at 02:33 hrs
March 18: Moon occults Mars between 18:22 and 19:05 hrs
(not visible from eastern Australia)
March 18: Limb of Moon 35 arcminutes south of Jupiter at 20:20 hrs
March 19: Limb of Moon 11 arcminutes south of Pluto at 11:18 hrs
March 19: Moon 1.8º south of Saturn at 12:11 hrs
March 20: Venus at perihelion at 13:45 hrs (diameter = 22.4")
March 20: Autumn Equinox at 13:58 hrs
March 20: Mars 43 arcminutes south of Jupiter at 23:32 hrs
March 21: Limb of Moon 45 arcminutes south of the star Deneb Algiedi (Alpha Capricorni, mv= 2.85) at 10:32 hrs
March 22: Moon 3.3º south of Mercury at 05:34 hrs
March 23: Moon 3.3º south of Neptune at 13:10 hrs
March 23: Mars 40 arcseconds south of Pluto at 15:20 hrs
March 24: Mercury at greatest elongation west (27º 45') at 08:53 hrs (diameter = 7.4")
March 24: Venus at greatest elongation east (46º 00') at 17:42 hrs (diameter = 23.4")
March 27: Moon 3.7º south of Uranus at 07:42 hrs
March 27: Mercury at aphelion at 14:43 hrs (diameter = 7.1")
March 29: Moon 6.2º south of Venus at 01:06 hrs
March 31: Moon 1.8º north of the star Alheka (Zeta Tauri, mv= 2.97) at 10:47 hrs
The Planets for this month:
Mercury:
During most of February, Mercury will be in the evening sky, between brilliant Venus and the western horizon. It will be at its maximum angular distance from the Sun on February 10, so the first two weeks of the month will be the best time to look for the innermost planet. Mercury will overtake the Earth and pass between us and the Sun on February 26 (inferior conjunction).
Venus: This, the brightest planet, passed through superior conjunction (when it went behind the Sun on the far side of its orbit) on August 14 last, whereupon it returned to the western twilight sky, as a so-called 'evening star'. It is currently the brightest object in the evening sky apart from the Moon. As February progresses, Venus will appear each night a little further to the north, and at the end of the month it will set almost exactly due west. The waxing crescent Moon will appear to the left of Venus on February 27.
(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).
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 all of 2019 up to July, Venus appeared as an 'Morning Star' in the eastern pre-dawn sky, but in August last year it moved to the evening sky to be an 'Evening Star'. It is now quite easy to find in the west as twilight begins to fade.
Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
Mars: The red planet is still 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 4 arcseconds in diameter. It passed through superior conjunction at 8:55 pm on September 2. At the beginning of February Mars is in the constellation of Ophiuchus, but will join Jupiter in Sagittarius on February 11. The waning crescent Moon will be just below Mars after 2 am on the morning of February 19. Mars will remain very small and distant for another six months. The next close approach of Mars to the Earth will occur on October 14, 2020, when its diameter will rapidly increase to 22.3 arcseconds. See
Planetary Aligments below.
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 before the Sun in the constellation of Sagittarius. The waning crescent Moon will be just to the right of Jupiter on the morning of February 20.
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 observing Pluto will be impossible due to the glare of the Sun until late February. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth that of
Neptune. Located about 10.8 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 February 1, Pluto will be 1.7 degrees south-west of Saturn. On March 23, Mars and Pluto will be only 40 arcseconds apart, but this close
approach will occur at 3:15 pm during the afternoon. That morning, before dawn breaks, Mars and Pluto will be 17 arcminutes apart, with Jupiter close by at a
distance of 1.5 degrees. 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.For the next six months, there will be a fine grouping of the planets Mars, Jupiter and Saturn in the eastern pre-dawn sky. On occasion, the crescent Moon
will pass through the grouping, adding additional interest.
On February 1 at 4:30 am, Jupiter will be found at the eastern end of the constellation
Sagittarius. It is the brightest object in this part of the sky. Below Jupiter and close to the horizon will be Saturn (and also the very faint Pluto). The
separation between Jupiter and Saturn will be 12º, a little over half-a-handspan. Mars can be found 24º above Jupiter (a little more than a hand-span, in the constellation
of Ophiuchus (the Serpent Bearer). Mars and Saturn are two handspans apart. On February 18, the waning crescent Moon is above Mars in the sky, and the following night it
is between Mars and Jupiter. On February 18, Mars will be between the two bright nebulae M8 (the "Lagoon Nebula") and M20 (the "Trifid Nebula"). On
February 20, the crescent Moon is a little to the right of Jupiter, and on February 21, the Moon will be below Saturn. On February 29, Mars will be only 2 arcminutes from
the centre of the globular cluster M22. During February, Mars will move eastwards into Sagittarius, and it will close up the distance between itself and the two gas giants
Jupiter and Saturn.
Planetary Alignments
Meteor Showers:
Alpha Centaurids February
8
Waxing almost Full Moon,
Moon, 98% sunlit ZHR =
10
Radiant: Near the star
Alpha Centauri.
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:
In
Use this
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.
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 February, the Eta Carinae Nebula can be viewed low in the
south-south-east, in the hours after 9 pm, above the Southern Cross.
The Stars and Constellations for this month:
This description of the night sky is for 9 pm on February 1 and 7 pm on February 29. They start at Orion, which is very high, a handspan north of the zenith.
This month, the constellation of Orion (see below) is as high as he can ever appear from our latitude. He is about 25 degrees north of the zenith, and has just crossed 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. Orion will have set by 3.00 am.
The bright orange-red star Betelgeuse (Alpha Orionis) will culminate at 9 pm on February 2. See its image below and Astro News. Since late last year, Betelgeuse has been fading dramatically, and at present is only 64% as bright as it was previously. This drop of a full magnitude in brightness is easily noticeable to the naked eye. Whereas Betelgeuse was the 10th brightest star, and much brighter than its close neighbours Aldebaran and Bellatrix, it is now the 22nd brightest, much fainter than Aldebaran and about the same brightness as Bellatrix and the stars in Orion's Belt. This phenomenon has generated great interest among astronomers, and it has been accompanied by a distortion of the star's shape, as it now appears in photographs as 'lopsided'. Whether Betelgeuse will recover or become the Milky Way's first supernova in 400 years is unknown so far. The brilliant white star Rigel (Beta Orionis), once twice as bright as Betelgeuse, but now five times brighter, is at this time half a handspan past the meridian. About a hand-span to the south-east of Rigel is the brightest star in the night sky, Sirius, also known as Alpha Canis Majoris. Sirius is almost directly overhead at 9.00 pm at mid-month. At the zenith itself is a faint constellation, Columba, the Dove.
The constellation Taurus, the Bull, with the clusters the 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. 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. The group is also known as ‘Santa’s Sleigh’, as it appears around Christmas time. 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 the nebula which surrounds the Pleiades can be seen around the brighter stars in the cluster.
Setting in the west are the constellations Cetus, Pisces and Aries, none of which is spectacular. Low in the north-west, the three main stars of Aries (from the left, Gamma, Beta and Alpha Arietis, otherwise known as Mesarthim, Sheratan and Hamal), form a short bent line parallel with the horizon. One reasonably bright star, Diphda (Alpha Ceti) is low in the west, and another, Fomalhaut, is near the south-western horizon. By midnight the Pleiades will have disappeared, and the rest of Taurus follows them below the horizon soon after.
Between Orion’s head and the northern horizon is a large constellation shaped roughly like a pentagon. This is Auriga the Charioteer, its brightest star being Capella, at the left side of the base of the pentagon. Capella is the sixth brightest star in the sky, after Sirius, Canopus, Alpha Centauri, Beta Centauri and Vega. Above Capella and slightly to the left is a small triangle of stars known as 'The Kids'. The lower star in this triangle is Epsilon Aurigae, one of the largest stars known. It is also very distant. West of Auriga, the constellation Perseus is straddling the north-north-western horizon.
The top star of Auriga's pentagon is actually in the constellation Taurus. It is El Nath, also known as Beta Tauri. It marks the tip of one of the Bull's horns.
To the east of Auriga, Gemini is quite high, the two twin stars at its eastern end, Pollux and Castor being a little more than a handspan above the north-north-east horizon. East of Gemini is a faint zodiacal constellation, Cancer, the Crab. Though it has no bright stars, Cancer does contain a rich open cluster of stars, known as the Praesepe or the Beehive Cluster. Praesepe was known in antiquity, and is a wonderful sight in binoculars or a small telescope.
A handspan due east of Betelgeuse in Orion (see below) is Procyon, the brightest star in the small constellation of Canis Minor, the Lesser Dog. Procyon is midway between the bright stars Rigel and Regulus.
Rising in the north-east is another zodiacal constellation, Leo, the Lion. The bright star Regulus (Alpha Leonis) marks the Lion’s heart. Leo is fully risen by 10.00 pm, the star marking the tip of the lion's tail, Denebola, being the last star in Leo to rise.
Just beginning to appear above the east-south-eastern horizon is the constellation Corvus the Crow, shaped like a quadrilateral of magnitude 3 stars. A large but faint constellation, Hydra, winds its way from near Procyon around the eastern horizon and over the top of Corvus to Libra, which will not rise until 11:30 pm at mid-month. Hydra has one bright star, Alphard, mv=2.2, which tonight is about one-and-a-half handspans above the eastern horizon. 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.
Well up in the south-south-east, Crux (Southern Cross) is sloping, but will be almost horizontal by 10 pm at mid-month. Crux will have rotated clockwise to a vertical position by 3.15 am at mid-month. Surrounding Crux on three sides is the large constellation Centaurus, and below Crux and to the right are two brilliant stars, Rigil Kentaurus and Hadar. They are also known as Alpha and Beta Centauri. Beta is the one nearer to Crux. These two stars are also known as the Pointers or the Guardians of the Cross.
Above and 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. To the right of Alpha Centauri and below Musca is a (roughly) equilateral
triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle.
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).
About midway between Crux and the False Cross is a faintly glowing patch, easily seen with the naked-eye and a splendid view in binoculars or a small telescope. This is the famous Eta Carinae Nebula, a vast panorama of hydrogen gas being made to fluoresce or glow because of the intense radiation being emitted by the eruptive variable star at its centre, Eta Carinae itself.
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
Two beautiful star clusters are in the same part of the sky, one on either side of Eta Carinae. The most southerly in the cluster IC 2602, also called the 'Southern Pleiades'. The other is NGC 3532, the 'Wishing Well Cluster', a more distant but very rich cluster. All three objects can be seen together when viewed through binoculars.
The cluster IC 2602, known as the 'Southern Pleiades'.
The 'Wishing Well Cluster', NGC 3532.
Two handspans south-west of Canopus is Achernar, Alpha Eridani. It is the brightest star in
Eridanus the River and marks its mouth. Eridanus has its source at
High in the south, about 50 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. To its right and below is the Small Magellanic Cloud (SMC), a smaller glowing patch. The LMC and SMC are described below.
The zodiacal constellations visible tonight, starting from the western horizon and heading to the right between one and two handspans above the horizon until the due east point is reached, are Pisces, Aries, Taurus, Gemini, Cancer and Leo.
The amazing thing about the star Mira
or Omicron Ceti is that it varies dramatically in brightness, rising to magnitude 2 (brighter than any other star in
Cetus), and then dropping to magnitude 10 (requiring a telescope to detect it), over a period of 332 days. This drop of eight magnitudes means that its brightness
diminishes over a period of five and a half months to one six-hundredth of what it had been, and then over the next five and a
half months it regains its original brightness. To the ancients, they saw the familiar star fade away during the year until it
disappeared, and then it slowly reappeared again. Its not surprising that it became known as Mira, meaning 'The Wonderful' or
'The Miraculous One'. We now know that many stars vary in brightness, even our Sun
doing so to a small degree, with a period of 11 years. One type of star varies, not because it is actually becoming less bright
in itself, but because another, fainter star moves around it in an orbit roughly in line with the Earth, and obscures it on each
pass. This type of star is called an eclipsing variable and they are very common. The star Mira though, varies its light output because of
processes in its interior. It is what is known as a pulsating variable. Stars of the Mira type are giant pulsating red
stars that vary between 2.5 and 11 magnitudes in brightness. They have long, regular periods of pulsation which lie in the range
from 80 to 1000 days.
Mira near minimum, 26 September 2008 Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope, the
Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind
Mira as the star streaks through space.
Galaxy Evolution Explorer ("GALEX"
for short) scanned the well-known star during its ongoing survey of the entire
sky in ultraviolet light. Astronomers then noticed what looked like a comet with
a gargantuan tail. In fact, material blowing off Mira is forming a wake 13
light-years long, or about 20,000 times the average distance of Pluto from the
sun. Nothing like this has ever been seen before around a star.
The season of the Hunter and his
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 the Sword of Orion. 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. This asterism appears upside-down tonight, 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 clusters and nebulae). A photograph of it appears below:
The Sword of Orion, with the Great Nebula, M42, at centre
The central section of M42, the Great Nebula in Orion.
New stars are forming in the nebula. At the brightest spot is a famous multiple star system, the Trapezium, illustrated below.
Canis Major:
To the right of Orion as twilight ends (facing east), 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 neck of the hunter's dog, and has been known for centuries as the Dog Star. As he rises, the dog is on his back with his front foot in the air. The star at the end of this foot is called 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 above Sirius.
The hindquarters of the Dog are indicated by a large right-angled triangle of stars located to the right of Sirius. The end of his tail is the lower-right corner of the triangle, about one handspan south (to the right) of Sirius.
Both Sirius and Rigel are bright white stars and each has a tiny, faint 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 (see above).
Canis Major as it appears almost overhead at 9 pm at mid-month (observer facing west).
Canis Minor:
By 8 pm at mid-month, this small constellation is about 50 degrees above the north-eastern 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.
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) 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 (low in the south-south-east) to Achernar (low in the south-west). Both stars will be about a handspan above the horizon at 9:50 pm at mid-month, and a line joining them 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, 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 many 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.
The binary stars Rigil Kentaurus (Alpha Centauri) at left, and Beta Cygni (Albireo), 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) is a binary easily seen with the smallest telescope. The components are both solar-type main sequence stars, one of type G and the other, slightly cooler and fainter, of type K. Through a small telescope this star system looks like a pair of distant but bright car headlights.
Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third component, the 11th magnitude red dwarf Proxima Centauri, takes about 1 million years to orbit the other two. It is about one tenth of a light year from the bright pair and a little closer to us, hence its name. This makes it our nearest interstellar neighbour, with a distance of 4.3 light years. Red dwarfs are by far the most common type of star, but, being so small and faint, none is visible to the unaided eye. Because they use up so little of their energy, they are also the longest-lived of stars. The bigger a star is, the shorter its life.
Close-up of the star field around Proxima Centauri.
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 small telescope is struggling to separate them (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
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 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.
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 ?
Our galaxy is shaped like a flattened disc containing about 100 million stars. Our own star, the Sun, with its Solar System is located about two-thirds of the distance out from the centre. When we look along the plane of the galaxy, either in towards the centre or out towards the edge, we are looking along the disc through the teeming hordes of stars, clusters, dust clouds and nebulae. In the sky, the galactic plane gives the appearance which we call the Milky Way, a brighter band of light crossing the sky. This part of the sky is very interesting to observe with binoculars or telescope. The brightest and most spectacular constellations, such as Crux, Canis Major, Orion and Scorpius are located close to the Milky Way.
If we look at ninety degrees to the plane, either straight up and out of the galaxy or straight down, we are looking through comparatively few stars and gas clouds and so can see out into deep space. These are the directions of the north and south galactic poles, and because we have a clear view in these directions to distant galaxies, these parts of the sky are called the intergalactic windows. The southern window is in the constellation Sculptor, not far from the star Fomalhaut. This window is low in the south-west in the early evenings this month, but sets by 11 pm. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is below the horizon in the early evenings this month, but rises in the east-north-east at about 10 pm.
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