February
2019
Updated: 14 February 2019
Welcome to the night skies of Summer, featuring 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 developed for our site with our equipment over the past year.
Explanatory Notes:
Rise and set times are given for the theoretical horizon, which is a flat horizon all the
way round the compass, with no mountains, hills, trees or buildings to obscure the view. Observers will have to make allowance for their own actual horizon.
Transient phenomena are provided for the current month and the next. Geocentric phenomena are calculated as if the Earth were fixed in space
as the ancient Greeks believed. This viewpoint is useful, as otherwise rising and setting times would be meaningless. In the list of geocentric events, the nearer object is
given first.
When a planet is referred to as ‘stationary’, it means that its movement across the stellar background appears to have ceased, not that
the planet itself has stopped. With inferior planets (those inside the Earth’s orbit, Mercury and Venus), this is caused by the planet heading either directly
towards or directly away from the Earth. With superior planets (Mars out to Pluto), this phenomenon is caused by the planet either beginning or ending its
retrograde loop due to the Earth’s overtaking it.
Apogee and perigee: Maximum and minimum distances of the Moon or artificial satellite from the Earth.
Aphelion and perihelion: Maximum and minimum distances of a planet, asteroid or comet from the Sun.
The zenith is the point in the sky directly overhead
from the observer. The
meridian is a semicircle starting from a point on the horizon that is exactly
due north from the observer, and arching up into the sky to the zenith and
continuing down to a point on the horizon that is exactly due south. On the way
down it passes through the South Celestial Pole which is 26.6 degrees above the
horizon at Nambour. The elevation of the South Celestial Pole is exactly the
same as the observer's latitude, e.g. from Cairns it is 16.9 degrees above the
horizon, and from Melbourne it is 37.8 degrees. The Earth's axis points to this
point in the sky in the southern hemisphere, and to an equivalent point in the
northern hemisphere, near the star Polaris, which from Australia is always below
the northern horizon.
All astronomical objects rise until they reach the meridian, then they begin to
set. The act of crossing or 'transitting' the meridian is called 'culmination'.
Objects closer to the South Celestial Pole than its altitude above the southern
horizon do not rise or set, but are always above the horizon, constantly
circling once each sidereal day. They are called 'circumpolar'. A handspan at arm's length with fingers spread covers an angle of approximately 18 - 20 degrees.
mv = visual magnitude or brightness. Magnitude 1 stars are very bright, magnitude 2 less so, and
magnitude 6 stars are so faint that the unaided eye can only just detect them under good, dark conditions. Binoculars will allow us to see down to magnitude
8, and the Observatory telescope can reach visual magnitude 17 or 22 photographically. The world's biggest telescopes have detected stars and galaxies as faint as
magnitude 30. The sixteen very brightest stars are assigned magnitudes of 0 or even -1. The brightest star, Sirius, has a magnitude of -1.44. Jupiter can reach -2.4, and
Venus can be more than 6 times brighter at magnitude -4.7, bright enough to cast shadows. The Full Moon can reach magnitude -12 and the
overhead Sun is magnitude -26.5. Each
magnitude step is 2.51 times brighter or fainter than the next one, i.e. a magnitude 3.0 star is 2.51 times brighter than a magnitude 4.0. Magnitude 1.0
stars are exactly 100 times brighter than magnitude 6.0 (5 steps each of 2.51 times, 2.51x2.51x2.51x2.51x2.51 = 2.515 = 100).
The Four Minute Rule:
How long does it take the Earth to complete one rotation? No, it's not 24 hours - that is the time taken for the Sun to cross the meridian on successive days. This 24 hours is a little longer than one complete rotation, as the curve in the Earth's orbit means that it needs to turn a fraction more (~1 degree of angle) in order for the Sun to cross the meridian again. It is called a 'solar day'. The stars, clusters, nebulae and galaxies are so distant that most appear to have fixed positions in the night sky on a human time-scale, and for a star to return to the same point in the sky relative to a fixed observer takes 23 hours 56 minutes 4.0916 seconds. This is the time taken for the Earth to complete exactly one rotation, and is called a 'sidereal day'.
As our clocks and lives are organised to run on solar days of 24 hours, and the stars circulate in 23 hours 56 minutes approximately, there is a four minute difference between the movement of the Sun and the movement of the stars. This causes the following phenomena:
1. The Sun slowly moves in the sky relative to the stars by four minutes of time or one degree of angle per day. Over the course of a year it moves ~4 minutes X 365 days = 24 hours, and ~1 degree X 365 = 360 degrees or a complete circle. Together, both these facts mean that after the course of a year the Sun returns to exactly the same position relative to the stars, ready for the whole process to begin again.
2. For a given clock time, say 8:00 pm, the stars on consecutive evenings are ~4 minutes or ~1 degree further on than they were the previous night. This means that the stars, as well as their nightly movement caused by the Earth's rotation, also drift further west for a given time as the weeks pass. The stars of autumn, such as Orion are lost below the western horizon by mid-June, and new constellations, such as Sagittarius, have appeared in the east. The stars change with the seasons, and after a year, they are all back where they started, thanks to the Earth's having completed a revolution of the Sun and returned to its theoretical starting point.
We can therefore say that the star patterns we see in the sky at 11:00 pm tonight will be identical to those we see at 10:32 pm this day next week (4 minutes X 7 = 28 minutes earlier), and will be identical to those of 9:00 pm this date next month or 7:00 pm the month after. All the above also includes the Moon and planets, but their movements are made more complicated, for as well as the Four Minute Drift with the stars, they also drift at different rates against the starry background, the closest ones drifting the fastest (such as the Moon or Venus), and the most distant ones (such as Saturn or Neptune) moving the slowest.
Solar System
Sun:
The Sun begins the month in the zodiacal constellation of Capricornus, the Sea-Goat. It leaves Capricornus and passes into Aquarius, the Water-Bearer on February 16.
Moon Phases:
First Quarter:
Full Moon:
Last Quarter:
Moon at 8 days after New, as on February 14.
The photograph above shows the
Moon when approximately eight days after New, just after First Quarter.
A rotatable view of the Moon, with ability to zoom in close to the surface
(including the far side), and giving detailed
information on each feature, may be downloaded here. A professional version of this freeware
with excellent pictures from the Lunar Reconnaissance Orbiter and the Chang orbiter (giving a resolution of 50 metres on the
Moon's surface) and many other useful features
is available on a DVD from the same website for 20 Euros (about AU $ 33) plus postage.
Click
here for a photographic animation showing the lunar phases. It also shows the Moon's wobble or libration, and how its apparent size changes as it moves from perigee to apogee each month. It takes a little while to load, but once running is very cool ! All these downloads are freeware, although the authors do accept donations if the user feels inclined to support their work.
Lunar Feature for this Month
:Each month we describe a lunar crater, cluster of craters, valley, mountain range or other object, chosen at random, but one with interesting attributes. A recent photograph from our Alluna RC20 telescope will illustrate the object. As all large lunar objects are named, the origin of the name will be given if it is important. This month we will look at an interesting crater-plain near the centre of the Moon's disc, Albategnius.
This picture is dominated by the large crater-plain Albategnius (139 km diameter). Its south-western quadrant has been impacted by a later asteroid, creating the
46 km crater Klein. This image was taken at 8:33 pm on 18 September 2018. North is to the top, east to the right.
This large crater-plain lies adjacent to Ptolemaeus to the west and Hipparchus to the north. Both of those crater-plains are larger than Albategnius. Like most other impact craters, it is surrounded by melted rock that splashed out from the original impact. The terraces in the walls have to a large extent been obliterated by later impacts by smaller bodies. AS is typical, the impact that formed Albategnius rebounded off the deep bedrock and fractured the floor of the new crater and threw up massive blocks of rock as mountains. Molten magma then welled up through the fissures and covered the floor to a great depth, swamping the bases of the mountains so that only their summits remained visible. This flooding of Albategnius' floor by lava occurred in two stages. In the first, a lava plain was formed, which cooled and later was struck by further impactors, producing craters on the new, flat floor. A second upwelling of lave spread out on the floor, and swept over the new craters, covering them. These swamped craters can be seen as shallow, circular depressions dotted over the present floor like faint ghost craters, averaging 5 or 6 kilometres across. There are at least a dozen visible above. These should not be confused with the more recent craterlets, which are much smaller (2 kilometres across or less) and are quite sharply defined.
The central mountain peak is all that remains uncovered of the central mountain cluster. At its summit is a crater, not as clearly defined as the recent impact craters on the floor which are roughly the same size (2 kilometres). This latter fact, combined with its position exactly at the summit, indicates that this crater is probably an extinct volcanic crater, not caused by an impact.
There are two recent impacts that have damaged the floor of Albategnius. The largest is Klein, 46 km across. It also exhibits a typical central mountain. It has deformed the south-west wall of Albategnius. At the northern end of Albategnius' floor iand impinging slightly on the northern wall is a 20 km crater, Albategnius B. This crater itself has been struck by an impactor, leaving a 4 km crater a little to the east of centre.
On note in this part of the Moon are numerous linear features like grooves deforming the surface. These grooves run from north-north-west to south-south-east. At least seven can be seen in the picture above. They were caused by vast quantities of rock, many pieces as large as flying mountains, skittering, crashing and bounding across the surface for hundreds, even thousands of kilometres, radiating away from a major impact on the Moon by a large asteroid about 3.8 billion years ago. This impact blasted out a huge hole in the Moon 1287 kilometres across, which immediately filled with molten magma to create the huge lava-plain known as the Mare Imbrium (Sea of Rains). This titanic blast is called the 'Imbrium Event', and the linear damage radiating across the near side of the Moon (partially shown in the image above) is called 'Imbrium Sculpture'. All of the grooves shown in the image, if traced back to their sources, radiate from the centre of Mare Imbrium.
Albategnius
Abu Abdullah Al-Battani
(Albategnius, AD 858-929), is regarded as the greatest Arab astronomer who ever lived. Working at an observatory at Raqqa, he continued to push forward the boundaries of understanding, using the most accurate astronomical instruments of his day. Albategnius introduced trigonometry into astronomy and, with it, he re-determined the length of the year, timed the occurrence of the spring equinox to a precision of a few hours, improved the value for precession, and accurately measured the tilt of the Earth’s axis relative to the Sun. He studied the Moon, and computed tables of lunar, solar and planetary positions that were more accurate than those in the Almagest, and were published as a ‘zij’, which was of such superior quality that it was translated into Latin in Spain in the middle of the 12th century.Written in a new and refreshing style, al-Battani’s ‘zij’ did not slavishly repeat everything that had been set down in earlier works. Instead, it concentrated on recent developments and observations, such as his new figure for the obliquity of the ecliptic (and thus the tilt of the Earth’s axis) of 23⁰ 35’, and a figure for the precession of the equinoxes that was much more accurate than Ptolemy’s. Al-Battani also made improvements to the astrolabe. His work was praised by later astronomers such as Richard of Wallingford, Peurbach, Regiomontanus and Copernicus.
Albategnius also studied the changing apparent diameter of the Sun’s disc throughout the year, which we now know is caused by the eccentricity of the Earth’s orbit. He found that the point where the Sun’s diameter is smallest (and thus the Earth-Sun distance greatest, now called the aphelion), had moved from its position as recorded by Ptolemy. Modern astronomy has confirmed that the Earth’s aphelion point does in fact move around the orbit. Albategnius published his astronomical observations in a book On Stellar Motion which proved to be of great value to astronomers for centuries to follow. Copernicus mentioned his indebtedness to al-Battani and quoted him extensively in De Revolutionibus, the book that initiated the Copernican revolution.
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 2: Moon 1.4º north of Saturn at 18:08 hrs
March 1: Moon occults the star Pi Sagittarii (mv= 2.88) between
23:52 and 00:39 hrs
March 2: Limb
of Moon 23 arcminutes north of Saturn at 02:38 hrs
March 2: Limb
of Moon 45 arcminutes north of Pluto at 15:16 hrs
March 3: Limb
of Moon 52 arcminutes south of Venus at 07:05 hrs
March 5: Limb
of Moon 10 arcminutes north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 03:48 hrs
March 6: Mercury at eastern stationary point at 04:08 hrs (diameter = 8.9")
March 7: Moon 2.6º south of Neptune at 01:53 hrs
March 7: Neptune in conjunction with the Sun at 11:14 hrs (diameter = 2.2")
March 8: Jupiter 1.6º north of the star 44 Ophiuchi (mv= 4.16) at 01:56 hrs
March 8: Moon 7.7º south of Mercury at 04:09 hrs
March 10: Moon 3.9º south of Uranus at 18:34 hrs
March 12: Moon 5.1º south of Mars at 01:54 hrs
March 13: Moon 2.5º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 21:16 hrs
March 14: Jupiter at western quadrature at 11:29 hrs (diameter = 37.6")
March 15: Moon occults the star Zeta Tauri (mv= 2.97) between 00:12 and 01:02 hrs
March 15: Mercury in inferior conjunction at 11:39 hrs
March 15: Moon occults the star Mu Geminorum (mv= 2.87) between 16:29 and 17:33 hrs
March 21: Autumn Equinox at 07:46 hrs
March 22: Jupiter 1.3º north of the star 51 Ophiuchi (mv= 4.78) at 03:34 hrs
March 22: Venus 2.2º north of the star Deneb Algedi (Alpha Capricorni, mv= 2.85) at 00:35 hrs
March 27: Moon 2.4º north of Jupiter at 13:57 hrs
March 28: Mercury at western stationary point at 23:55 hrs (diameter = 9.9")
March 29: Limb of Moon 21 arcminutes south of the star Pi Sagittarii (mv= 2.88) at 09:48 hrs
March 29: Limb of Moon 19 arcminutes north of Saturn at 15:54 hrs
March 29: Limb of Moon 40 arcminutes north of Pluto at 20:39 hrs
Mercury:
Mercury passed through superior conjunction on January 30 and has now returned to the western twilight sky. It shines brighter than Rigel, but is only visible when it is at a large angular distance from the glare of the Sun. As Mercury lies well inside the Earth's orbit and close to the Sun, it can never move more than 27.8º from the Sun. On February 1 it will be too close to the Sun to observed safely, but it will become a feature of the early evening sky by mid-month. Look close to the western horizon about 7 pm (half an hour after sunset). Mercury will be very low, so a clear horizon without hills, trees or buildings will be necessary. The innermost planet will be at greatest elongation east (18º 05') on February 27, so that week will be the best time to observe it. Even though its distance from the Sun will be over 18º along the ecliptic on that day, at Sunset its altitude will be only 10º, or about half-a-handspan.
Venus:
This, the brightest planet, passed through inferior conjunction (between
the Earth and the Sun) on October 27, and has now disappeared from the western
twilight sky. It has reappeared in the eastern pre-dawn sky where it is very
prominent in the early mornings as a so-called 'morning star'. Currently in the
constellation of
Sagittarius, it will cross into Capricornus on March 2. The waning crescent Moon will be
close by Venus on the mornings of February 1 and March 3. Venus will remain a pre-dawn object until next August.
(The coloured fringes to the first and third images below are due to refractive effects in our own atmosphere, and are not intrinsic to Venus. The planet was closer to the
horizon when these images were taken than it was for the second photograph, which was taken when Venus was at its greatest elongation from the Sun).
December 2018 January 2019 August 2019 March 2020
Click here for a photographic animation showing the Venusian phases. Venus is always far brighter than anything else in the sky except for the Sun and Moon. For most of 2018, Venus appeared as an 'Evening Star' in the western twilight sky, where it stayed for about nine months. Venus passed between us and the Sun (inferior conjunction) on October 27 last, and is now in the morning sky as a 'Morning Star'. It will return to the evening sky to be an 'Evening Star' once again on August 14 next, although it won't be away from the Sun's glare to be easily visible until next October.
Because Venus is visible as the 'Evening Star' and as the 'Morning Star', astronomers of ancient times believed that it was two different objects. They called it Hesperus when it appeared in the evening sky and Phosphorus when it was seen before dawn. They also realised that these objects moved with respect to the so-called 'fixed stars' and so were not really stars themselves, but planets (from the Greek word for 'wanderers'). When it was finally realised that the two objects were one and the same, the two names were dropped and the Greeks applied a new name Aphrodite (Goddess of Love) to the planet, to counter Ares (God of War). We use the Roman versions of these names, Venus and Mars, for these two planets.
Venus at 6.55 pm on September 7, 2018. The phase is 36 % and the angular diameter is 32 arcseconds.
Mars: The red planet is now poorly positioned for viewing, as the Earth has left it far behind. Mars is much reduced in size, being only 6 arcseconds in diameter on February 1. By February 28 its diameter will have fallen to 5 arcseconds, and during the month its phase will increase slightly from 89.3% to 91.35%. All through February at the onset of darkness (7:30 pm), Mars will be about 30º or one-and-a-half handspans above the west-north-western horizon, in the constellation of Pisces. By February 28 at end of twilight, Mars will be about 30º or one-and-a-half handspans above the west-north-west horizon. For the first half of February it will be in the constellation Pisces, and on February 13 Mars will cross into Aries. On March 24 it will enter Taurus. By then the speeding Earth will have left it far behind, and its angular diameter will have fallen from the value in July last year of 24 arcseconds to only 4 arcseconds in April. On February 10 and 11, the waxing crescent Moon will be close to Mars in the early evening sky. On February 13, Mars and Uranus will be only 1º apart.
In this image, the south polar cap of Mars is easily seen. Above it is a dark triangular area known as Syrtis Major. Dark Sinus Sabaeus runs off to the left, just south of the equator. Between the south polar cap and the equator is a large desert called Hellas. The desert to upper left is known as Aeria, and that to the north-east of Syrtis Major is called Isidis Regio. Photograph taken in 1971.
Mars
photographed from Starfield Observatory, Nambour on June 29 and July 9, 2016, showing two different sides of the planet. The north polar cap is prominent.
Brilliant Mars at left, shining at magnitude 0.9, passes in front of the dark molecular clouds in Sagittarius on October 15, 2014. At the top margin is the white fourth magnitude star 44 Ophiuchi. Its type is A3 IV:m. Below it and to the left is another star, less bright and orange in colour. This is the sixth magnitude star SAO 185374, and its type is K0 III. To the right (north) of this star is a dark molecular cloud named B74. A line of more dark clouds wends its way down through the image to a small, extremely dense cloud, B68, just right of centre at the bottom margin. In the lower right-hand corner is a long dark cloud shaped like a figure 5. This is the Snake Nebula, B72. Above the Snake is a larger cloud, B77. These dark clouds were discovered by Edward Emerson Barnard at Mount Wilson in 1905. He catalogued 370 of them, hence the initial 'B'. The bright centre of our Galaxy is behind these dark clouds, and is hidden from view. If the clouds were not there, the galactic centre would be so bright that it would turn night into day.
Mars near opposition, July 24, 2018
Mars, called the red planet but usually coloured orange, has now taken on a yellowish tint and has brightened by 0.4 magnitude, making it twice as bright as previous predictions for the July 27 opposition. These phenomena have been caused by a great dust storm which has completely encircled the planet, obscuring the surface features so that they are only seen faintly through the thick curtain of dust. Although planetary photographers were mostly disappointed, many observers were interested to see that the yellow colour and increased brightness meant that a weather event on a distant planet could actually be detected with the unaided eye - a very unusual thing in itself.
The three pictures above were taken on the evening of July 24, at 9:05, 9:51 and 11:34 pm. Although the fine details that are usually seen on Mars are hidden by the dust storm,
some of the larger features can be discerned, revealing how much Mars rotates in two and a half hours. Mars' sidereal rotation period (the time taken for one complete
rotation or 'Martian day') is 24 hours 37 minutes 22 seconds - a little longer than an Earth day. The dust storm began in the Hellas Desert on May 31, and after two months
it still enshrouded the planet. In August it began to clear. Unfortunately, dust swamped the solar panels on the Mars Rover Opportunity, and the
batteries went flat on June 10 last. As the skies cleared, the batteries failed to recharge and by this month over 1000 attempts have been
made to restore communication with the rover. The final attempt was made on Tuesday, February 12, 2019, but was not successful. NASA has therefore
reported that the rover is defunct. Designed to last for 90 days and travel 1 kilometre, it exceeded its life expectancy by a factor of 58 and travelled
45 kilometres. The rover Curiosity is nuclear powered and does not have solar panels. It was therefore unaffected by the dust storm and is
working normally. The lander InSight touched down on Mars on November 26, 2018 after the dust storm had abated. Like Opportunity, it
generates electricity from solar panels, but unlike the rovers is not mobile.
Central meridian: 295º.
The two pictures immediately above were taken on the evening of September 7, at 6:25 and 8:06 pm. The dust storm is finally abating, and some of the surface features are becoming visible once again. This pair of images also demonstrates the rotation of Mars in 1 hour 41 minutes (equal to 24.6 degrees of longitude), but this time the view is of the opposite side of the planet to the set of three above. As we are now leaving Mars behind, the images are appreciably smaller (the angular diameter of the red planet has fallen to 20 arcseconds). Well past opposition, Mars on September 7 exhibited a phase effect of 92.65 %.
Central meridian: 180º.
Jupiter: This gas giant planet passed through conjunction with the Sun (on the far side of its orbit) on November 26 and is now in the pre-dawn eastern sky. On February 1 it will be in the constellation Ophiuchus, and rising at 1:20 am. By the first light of dawn it will be about two handspans above the eastern horizon. As the month progresses, Jupiter will become higher in the sky as dawn approaches, and on February 28 it will be over three handspans above the horizon at 4:40 am (the beginning of dawn). On that date the waning crescent Moon will be close to Jupiter's left.
Jupiter as photographed from Nambour on the evening of April 25, 2017. The images were
taken, from left to right, at 9:10, 9:23, 9:49, 10:06 and 10:37 pm. The rapid rotation of this giant planet in a little under 10 hours is clearly seen. In the
southern hemisphere, the Great Red Spot (bigger than the Earth) is prominent, sitting within a 'bay' in the South Tropical Belt. South of it is one of the
numerous White Spots. All of these are features in the cloud tops of Jupiter's atmosphere
Jupiter as it appeared at 7:29 pm on July 2, 2017. The Great Red Spot is in a similar position near Jupiter's eastern limb (edge) as in the fifth picture in the series above.
It will be seen that in the past two months the position of the Spot has drifted
when compared with the festoons in the Equatorial Belt, so must rotate around the planet at a slower rate. In fact, the Belt enclosing the Great Red Spot rotates around the
planet in 9 hours 55 minutes, and the Equatorial Belt takes five minutes less. This high rate of rotation has made the planet quite oblate. The prominent 'bay'
around the Red Spot in the five earlier images appears to be disappearing, and a darker streak along the northern edge of the South Tropical Belt is moving
south. Two new white spots have developed in the South Temperate Belt, west of the Red Spot. The five upper images were taken near opposition, when the Sun was
directly behind the Earth and illuminating all of Jupiter's disc evenly. The July 2 image was taken just four days before Eastern Quadrature, when the angle
from the Sun to Jupiter and back to the Earth was at its maximum size. This angle means that we see a tiny amount of Jupiter's dark side, the shadow being
visible around the limb of the planet on the left-hand side, whereas the right-hand limb is clear and sharp. Three of Jupiter's Galilean satellites are visible, Ganymede to
the left and Europa to the right. The satellite Io can be detected in a transit of Jupiter, sitting in front of the North Tropical Belt, just to the left of its centre.
Jupiter at opposition, May 9, 2018
Jupiter 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.
Saturn:
Uranus:
Neptune: The icy blue planet will pass through
conjunction on March 7, so it will be a difficult object to observe this month.
At the beginning of February, Neptune is only 22º above the western horizon at
Sunset, and sets at 8:25 pm, only 40 minutes after the end of twilight. The thin, waxing crescent Moon will be
above and to the left of Neptune on February 7.
The photograph above was taken when Saturn was close to opposition, with the Earth between Saturn and the Sun. At that time, the shadow of Saturn's globe upon the
Ring system was directly behind the planet and hardly visible. The photograph below was taken at 7:14 pm on September 09, 2018, when Saturn was near eastern quadrature.
At such a time, the angle from the Sun to Saturn and back to the Earth is near its maximum, making the shadow fall at an angle across the Rings as seen from
Earth. It may be seen falling across the far side of the Ring to the left side of the globe.
Neptune, photographed from Nambour on October 31, 2008
Pluto: The erstwhile ninth and most distant planet reached
conjunction on January 11. It is
impossible to observe
this month as it is too close to the Sun. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth
that of Neptune. Located just east of the 'Teaspoon' which is north-east of the Sagittarius 'Teapot', it is a faint 14.1 magnitude object near the centre of Sagittarius. A
telescope with an aperture of 25 cm or more is necessary to observe Pluto.
The movement of the dwarf planet Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field: 200 arcseconds
This is a stack of four images, showing the movement of Pluto over the period October 22 to 25, 2014. Pluto's image for each date appears as a star-like point at the upper right corner of the numerals. The four are equidistant points on an almost-straight line. Four eleventh magnitude field stars are identified. A is GSC 6292:20, mv = 11.6. B is GSC 6288:1587, mv = 11.9. C is GSC 6292:171, mv = 11.2.
D is GSC 6292:36, mv = 11.5. (GSC = Guide Star Catalogue). The position of Pluto on October 24 (centre of image) was at Right Ascension = 18 hours 48 minutes 13 seconds, Declination = -20º 39' 11". The planet moved 2' 51" with respect to the stellar background during the three days between the first and last images, or 57 arcseconds per day, or 1 arcsecond every 25¼ minutes.
Alpha Centaurids February
8
Waxing crescent Moon,
Moon, 9% sunlit ZHR =
10
Radiant: Near the star
Alpha Centauri.
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.
Comet C/2018 V1 Machholz-Fujikawa-Iwamoto
Green comets in the news
In modern astronomy, most comets are found by large mountaintop telescopes photographing the skies under computer control. The photographs are scanned automatically to look for any new object that is not on the databases, such as an asteroid or comet. These on-going robotic surveys discover most new comets before they are bright enough for amateur astronomers to catch them. Surprisingly, three amateur astronomers (one in Arizona, two in Japan) have just discovered a bright new comet in the constellation Virgo that somehow escaped the notice of the automated surveys. This morning it was near the magnitude 2.9 star Porrima (Gamma Virginis), and heading east through the background stars. It will be near the magnitude 3.38 star Zeta Virginis on November 18. Named Comet Machholz-Fujikawa-Iwamoto after the three discoverers, it is plunging toward the Sun and could brighten to naked-eye visibility later this month. It will be at its closest approach to Earth on November 27 and closest approach to the Sun on December 4. The best time to observe it from November 13 to November 18 will be from 4 am to the first light of dawn, close to the due east horizon, and a little over half a handspan to the left of Venus. As the days go by and it becomes closer to the Sun, it will become lost in the solar glare. Visit the November 12 and subsequent editions of Spaceweather for the full story.
Comet 46P/Wirtanen
Last
December, Comet 46P/Wirtanen swept past Earth, making one of the ten closest approaches of a comet to our planet since 1960. It was faintly visible to the naked eye for two weeks. Although Wirtanen's nucleus is only 1.2 kilometres across, its green atmosphere became larger than the Full Moon, and was an easy target for binoculars and small telescopes. It reached its closest to the Sun (perihelion) on December 12, and then headed in our direction. It passed the Earth at a distance of 11.5 million kilometres (30 times as far away as the Moon) on December 16. In the week preceding it was at its brightest at magnitude 4, but this was a cloudy week at Nambour. It passed between the Pleiades and Hyades star clusters on the night of December 19-20, but the light of the almost Full Moon made it difficult to see. It then headed towards the star Capella in Auriga, which it passed on December 24-25. Visit Spaceweather and here for more information and chart
This month it will shine at magnitude 7 so it will be easy to see in small telescopes and binoculars, but not with the unaided eye. It will only be observable in the hour
or so before dawn begins to light the sky, low to the north-east horizon. On September 10 it will be gliding through the stars of the constellation Auriga about 58 million
kilometres from our planet. In the week ahead, it will cross into Gemini and on September 15 it will pass right across the
rich star cluster M35, providing a spectacular photo-opportunity for amateur astronomers. Visit the September 9 edition of
Spaceweather
for details and observing tips.
Comet Lulin
This comet, (C/2007 N3), discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now in the outer Solar System, and has faded below magnitude 15.
Comet Lulin at 11:25 pm on February 28, 2009, in Leo. The brightest star is Nu Leonis, magnitude 5.26.
The
LINEARNearly all of these programs are based in the northern hemisphere, leaving gaps in the coverage of the southern sky. These gaps are the areas of sky where amateur astronomers look for comets from their backyard observatories.
To find out more about current comets, including finder charts showing exact positions and magnitudes, click
here. To see pictures of these comets, click here.
The 3.9 metre Anglo-Australian Telescope (AAT) at the Australian Astronomical Observatory near Coonabarabran, NSW.
Deep Space
Sky Charts and Maps available on-line:
There are some useful representations of the sky available here. The sky charts linked
below show the sky as it appears to the unaided eye. Stars rise four minutes earlier each night, so at the end of a week the stars have gained about half an
hour. After a month they have gained two hours. In other words, the stars that were positioned in the sky at 8 pm at the beginning of a month will have the
same positions at 6 pm by the end of that month. After 12 months the stars have gained 12 x 2 hours = 24 hours = 1 day, so after a year the stars have returned
to their original positions for the chosen time. This accounts for the slow changing of the starry sky as the seasons progress.
The following interactive sky charts are courtesy of Sky and Telescope magazine. They can simulate a view of the sky from any location on Earth at any time of day or night between the years 1600 and 2400. You can also print an all-sky map. A Java-enabled web browser is required. You will need to specify the location, date and time before the charts are generated. The accuracy of the charts will depend on your computer’s clock being set to the correct time and date.
To produce a real-time sky chart (i.e. a chart showing the sky at the instant the chart is generated), enter the name of your nearest city and the country. You will also need to enter the approximate latitude and longitude of your observing site. For the Sunshine Coast, these are:
latitude: 26.6o South longitude: 153o East
Then enter your time, by scrolling down through the list of cities to "Brisbane: UT + 10 hours". Enter this one if you are located near this city, as Nambour is. The code means that Brisbane is ten hours ahead of Universal Time (UT), which is related to Greenwich Mean Time (GMT), the time observed at longitude 0o, which passes through London, England. Click here to generate these charts.
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Similar real-time charts can also be generated from another source, by following this second link:
The first, circular chart will show the full hemisphere of sky overhead. The zenith is at the centre of the circle, and the cardinal points are shown around the circumference, which marks the horizon. The chart also shows the positions of the Moon and planets at that time. As the chart is rather cluttered, click on a part of it to show that section of the sky in greater detail. Also, click on Update to make the screen concurrent with the ever-moving sky.
The stars and constellations around the horizon to an elevation of about 40o can be examined by clicking on
The view can be panned around the horizon, 45 degrees at a time. Scrolling down the screen will reveal tables showing setup and customising options, and an Ephemeris showing the positions of the Sun, Moon and planets, and whether they are visible at the time or not. These charts and data are from YourSky, produced by John Walker.
The charts above and the descriptions below assume that the observer has a good observing site with a low, flat horizon that is not too much obscured by buildings or trees. Detection of fainter sky objects is greatly assisted if the observer can avoid bright lights, or, ideally, travel to a dark sky site. On the Sunshine Coast, one merely has to travel a few kilometres west of the coastal strip to enjoy magnificent sky views. On the Blackall Range, simply avoid streetlights. Allow your eyes about 15 minutes to become dark-adapted, a little longer if you have been watching television. Small binoculars can provide some amazing views, and with a small telescope, the sky’s the limit.
In mid-February at 9 pm, the Eta Carinae Nebula can be found about 45 degrees (two handspans) above the south-eastern horizon, above the tilted 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 28. 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. The brilliant white star Rigel (Beta Orionis), twice as bright as Betelgeuse, 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 'Firefly 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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