September
2017
Updated: 19 September 2017
Welcome to the night skies of Spring, featuring Saturn, Scorpius, Sagittarius, Aquila, Lyra and Cygnus
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 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.
A handspan at arm's length 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. (The meridian is an imaginary semicircular line running from the due south point on the horizon and arching overhead through the zenith, and coming down to the horizon again at its due north point.) 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 constellation of Leo the Lion. It leaves Leo and passes into Virgo, the Virgin on September 16.
Moon Phases: Lunations (Brown series): #1172, 1173
Full Moon:
September
06 17:03 hrs
diameter = 31.1'
Last Quarter:
September
13 16:26 hrs diameter =
32.3'
New Moon:
September
20 15:30 hrs diameter =
31.2'
First Quarter:
September
28 12:54 hrs diameter = 29.6'
Full Moon:
October 06
04:40 hrs diameter
= 32.0'
Last Quarter:
October
12 22:26 hrs diameter =
32.2'
New Moon:
October
20
05:12 hrs diameter = 30.4'
First Quarter:
October
28
08:22 hrs diameter = 29.9'
Lunar Orbital Elements:
September
05: Moon at descending node at
04:39 hrs, diameter = 30.6'
September
14: Moon at perigee (369
859
km) at 01:17 hrs, diameter = 32.3'
September
18:
Moon at ascending node at
04:28 hrs, diameter = 31.9'
September 27:
Moon at apogee (404 350 km) at 16:42 hrs, diameter = 29.6'
October
02: Moon at descending node at
12:03 hrs, diameter = 30.6'
October
09: Moon at perigee (366
855
km) at 15:37 hrs, diameter = 32.6'
October
15: Moon at ascending node at
08:13 hrs, diameter = 31.7'
October
25:
Moon at apogee (405 164 km) at 12:38 hrs, diameter = 29.5'
October
29: Moon at descending node at
16:38 hrs, diameter = 30.3'
Moon at 8 days after New, as on September 29.
The photograph above shows the
Moon when approximately eight days after New, just after First Quarter. A detailed map of the Moon's near side is available here. 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.
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 unusual feature in the Moon's southern hemisphere, the Rupes Recta or "Straight Wall".
The "Straight Wall" and its surroundings on the Moon, photographed from Nambour on August 2, 2017.
Rupes Recta, or the Straight Wall, is a long escarpment in the Moon's Mare Nubium (Sea of Clouds). 100 km long and 1 to 1.5 kilometres wide, it is a fault
in the lunar crust. Despite
its appearance under a low Sun as above, it is neither particularly high nor steep. The eastern or right-hand side is about 250 metres higher than the
western, near the wall's mid-section. The slope is actually quite gentle, some sources quoting
it as being as little as seven degrees. It can be detected with quite a small
telescope. At the time near First Quarter as above, the wall is in shadow and
appears as a dark line. Two weeks later, around Last Quarter when the Sun is
coming from the west, it appears brighter than its surroundings - a bright line.
Also in the image are some craters. To the west (left) of the Wall, is the 17 km crater Birt, the eastern rim of which is deformed by the later impact that created 6.8 km Birt A. Starting in a craterlet just to the west of Birt is a curved valley or rille called Rima Birt, which is 51 km long and turns to the north-west before fading out. It has a gap in its middle and a slight S-bend.
The large crater to the right of the image is 60 km Thebit. It has a rille on its floor which is unusual in that it is composed of four straight sections joined at abrupt angles, almost like half of a hexagon. Most rilles are sinuous. Thebit A (20 km) has deformed the western rim of the larger crater, and is itself deformed by Thebit L (12 km). In Thebit A quantities of soil and rocks have slumped down the walls and covered much of the floor.
Birt
William Radcliffe Birt (1804-1881) was an English amateur astronomer who worked extensively with Sir John Herschel in the middle of the 19th century. He was connected with the Kew Observatory in London. Mainly interested in meteorology and atmospheric phenomena, he was also very interested in mapping the Moon. The crater Birt was named after him by Schmidt in 1878.
Thebit
The crater Thebit is named after Thabit ibn Qurra (AD 826 to AD 901). Thabit was born in Assyria (Turkey) but moved to Baghdad which was at the time the greatest seat of Arab learning. It was there that a House of Wisdom had been established in AD 800, for the translation into Arabic of the works of the Greek and Alexandrian philosophers. Thabit was a physician, mathematician, astronomer and translator, and made important discoveries in algebra, geometry and astronomy. He translated Ptolemy's Almagest from its original Greek into Arabic, and made corrections to Ptolemy's work where he thought they were needed. For example, Ptolemy had described Hipparchus' discovery of Precession of the Equinoxes, but Thabit and others saw that there was an irregularity in the precession, a slow wobble which varied back and forth over an average, causing planetary tables to need constant correction. Thabit described this wobble in his De motu octavae sphaerae (On the motion of the eighth sphere), the eighth sphere being the sphere of 'fixed' stars.
To account for this irregularity, which was later called Trepidation(unease), later astronomers sometimes added an extra crystalline sphere between the sphere of precession and the Primum Mobile. They named this new sphere ‘Trepidationis’. Not until 1728 was it found that this irregularity is caused by a small 18.6-year swaying of the Earth’s axis called nutation, which is superimposed upon the large 26 000-year swing of 46.87º diameter that causes precession of the equinoxes. In comparison with precession, the wobble in nutation is very tiny, with maximum variations of ±17 arcseconds in longitude and ± 9 arcseconds in obliquity. Although many astronomers did not think that this minuscule movement warranted the inclusion of a Trepidationis Sphere, the map-maker Andreas Cellarius believed that it did in his star atlas Harmonia Macrocosmica of 1660.
Copies of Thabit's version of the Almagest were carried to Spain by the Moors, and one was used at the Toledo School of Translators by Gerard of Cremona in the 12th century when he was given the task of translating the Arabic Almagest into Latin for dissemination into Europe. The translation of such ancient Greek texts (at first by means of the Arabic versions) provided the stimulus for the ending of the Dark Ages in Europe. By the 14th century, when the original Greek versions of the ancient texts were rediscovered at Constantinople, their translation into Latin helped bring about the Renaissance.
The rectangle shows the location of the Rupes Recta ("Straight Wall").
It is also visible in the large picture above of the Moon at 8 days after New.
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.
September 2: Moon 3º north of Pluto at 05:44 hrs
October 3: Venus at perihelion at 08:28 hrs (diameter = 11.1")
September 2: Saturn at eastern quadrature at 13:13 hrs (diameter = 16.6")
September 5: Mercury 3º south of Mars at 08:52 hrs
September 5: Neptune at opposition at 15:09 hrs (diameter = 2.3")
September 5: Mercury at western stationary point at 21:13 hrs (diameter = 8.9")
September 5: Mars 42 arcminutes north of the star Regulus (Alpha Leonis, mv= 1.36) at 21:02 hrs
September 5: Uranus 1º north of the star Omicron Piscium (mv= 4.25) at 21:33 hrs
September 6: Limb of Moon 8 arcminutes south of Neptune at 13:57 hrs
September 9: Moon 4º south of Uranus at 20:31 hrs
September 10: Mercury 35 arcminutes south-east of the star Regulus (Alpha Leonis, mv= 1.36) at 22:33 hrs
September 12: Mercury at Greatest Elongation West (17º 56') at 19:15 hrs (diameter = 7.2")
September 12: Limb of Moon 21 arcminutes north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 21:03 hrs
September 15: Mercury at perihelion at 22:25 hrs (diameter = 6.6")
September 17: Mercury 3.4 arcminutes north of Mars at 04:33 hrs
September 18: Moon occults Venus between 10:50 and 12:21 hrs
September 18: Moon occults the star Regulus (Alpha Leonis, mv= 1.36) between 16:30 and 16:49 hrs
September 19: Limb of Moon 32 arcminutes north of Mars at 4:36 hrs
September 19: Limb of Moon 22 arcminutes north of Mercury at 09:02 hrs
September 20: Venus 28 arcminutes north of the star Regulus (Alpha Leonis, mv= 1.36) at 12:14 hrs
September 22: Moon 3.7º north of Jupiter at 21:28 hrs
September 23: Spring equinox at 05:57 hrs
September 26: Jupiter 39 arcminutes south of the star 82 Virginis (mv= 5.03) at 16:28 hrs
September 27: Moon 4º north of Saturn at 08:56 hrs
September 29: Pluto at eastern stationary point at 01:31 hrs (diameter = 0.1")
September 29: Moon 2.1º north of the star Pi Sagittarii (mv= 3.00) at 10:38 hrs
September 29: Moon 2.9º north of Pluto at 11:05 hrs
October 3: Limb of Moon 6 arcminutes south of Neptune at 22:55 hrs
October 6: Venus is 12 arcminutes north of Mars at 02:36hrs
October 7: Moon 3.2º south of Uranus at 05:28 hrs
October 8: Mars at aphelion at 09:46 hrs (diameter = 3.7")
October 9: Mercury in superior conjunction at 06:37 hrs (diameter = 4.8")
October 10: Moon 1.3º north of the star Aldebaran (Alpha Tauri, mv= 0.87) at 04:45 hrs
October 10: Pluto at eastern quadrature at 09:58 hrs (diameter = 0.1")
October 14: Mercury 2.6º north of the star Spica (Alpha Leonis, mv= 0.98) at 03:55 hrs
October 15: Limb of Moon 14 arcminutes north of the star Regulus (Alpha Leonis, mv= 1.36) at 21:11 hrs
October 17: Moon 2º north of Mars at 21:33 hrs
October 18: Moon 2.1º north of Venus at 12:54 hrs
October 18: Mercury 57 arcminutes south of Jupiter at 20:20 hrs
October 20: Uranus at opposition at 03:19 hrs (diameter = 3.7")
October 20: Moon 3.8º north of Jupiter at 16:46 hrs
October 20: Neptune 33 arcminutes south of the star Hydor (Lambda Aquarii, mv= 3.73) at 19:40 hrs
October 20: Moon
October 21: Saturn 1.7º north of the star 51 Ophiuchi (mv= 4.78) at 17:10 hrs
October 24: Moon 3.7º north of Saturn at 23:20 hrs
October 26: Moon 1.7º north of the star Pi Sagittarii (mv= 3.00) at 21:16 hrs
October 26: Moon 2.6º north of Pluto at 22:32 hrs
October 27: Jupiter in conjunction with the Sun at 04:14 hrs (diameter = 30.6")
October 27: Mercury 1.2º south of the star Zuben Elgenubi (Alpha Librae, mv= 2.75) at 11:32 hrs
October 29: Mercury at aphelion at 22:04 hrs (diameter = 4.9")
October 31: Limb of Moon 2 arcminutes south of Neptune at 07:23 hrs
Mercury: On September 1, Mercury will be in the eastern pre-dawn sky, close to the planet Mars. Unfortunately, both will be too close to the glare of the Sun to be easily observed. This situation will continue all month, Mercury and Mars both remaining little more than a handspan from the Sun. Both of them are at the opposite sides of their orbits from us. On the morning of September 19, the waning crescent Moon will be adjacent to the two planets.
Venus:
This, the brightest planet, is still to be found in the pre-dawn sky as a 'morning star', although its angular distance from the Sun continues to diminish day by day. For example, on September 1 Venus will be 32 degrees from the Sun, and by September 30 its elongation will have fallen to 25 degrees, taking it close to the solar glare. This process will continue as speeding Venus curves around towards the far side of the Sun. Venus will pass behind the Sun (superior conjunction) on January 9 next year. After that, Venus will return to the western sky as an 'evening star'. In mid-September, Venus will appear in a small telescope as a tiny 'nearly Full Moon' with a magnitude of -3.9 and an angular size of 12 arcseconds. Its phase will be 87%.On the morning of September 19, the waning crescent Moon will be just above Venus in the sky. Later that morning, between 10:50 am and 12:21 pm, the Moon will occult Venus.
(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).
April 2017 June 2017 December 2017
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 the first two months of 2017, Venus appeared as an 'Evening Star', but on March 25 it moved to the pre-dawn sky and became a 'Morning Star'. Each of these appearances lasts about eight to nine months. Venus will pass on the far side of the Sun (superior conjunction) on January 9, 2018, when it will return to the evening sky and become an 'Evening Star' once again.
Because Venus was 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.
Mars:
Having passed through conjunction with the Sun on July 27, the red planet is extremely difficult to observe this month. On September 1 it will be less than 12 degrees from the Sun, and risky to observe due to the solar glare. By the end of September Mars will still be only 21 degrees from the Sun, a little more than a handspan. However, as the weeks pass it will leave the environs of the Sun and will grow in size each day, as our Earth, travelling faster, begins to catch it up. We will overtake Mars on July 27 next year. This will be a very favourable opposition, as Mars will appear bigger (24.2 arcseconds in diameter) and brighter (magnitude -2.8) than it has for many years. It will be particularly favourable for us in the southern hemisphere, as during the month of opposition it will be in the constellation of Capricornus, almost directly overhead each night from the Sunshine Coast.The waning crescent Moon will appear close to Mars in the hour before dawn on September 19. Mars and Mercury will be within 15 degrees of each other for all of September. Venus will be closing in on Mars, the two being 20 degrees apart at the beginning of the month and less than 4 degrees apart on September 30. Their closest approach will be on October 6, when they will be only 6 arcminutes (one tenth of a degree) 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.
Jupiter:
We will be losing this gas giant planet as a spectacular evening object soon, as it passed through eastern quadrature (due north and crossing the meridian at sunset) on July 6. In mid-September it will be only a handspan above the western horizon as darkness falls. It is in the constellation Virgo, north-west of the first magnitude star Spica. The crescent Moon will be close to Jupiter as night falls on September 22. Jupiter will be involved in the twilight glow towards the end of September, and will be in conjunction with the Sun on October 27.
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.
Saturn: The ringed planet is visible for most of the night this month, as it reached opposition on June 15. On September 1 it will be almost directly overhead as soon as darkness falls, 6:30 pm. The nearly First Quarter Moon will be close by Saturn on September 27. Saturn will reach eastern quadrature on September 2, and will be in conjunction with the Sun on December 22.
Saturn with its Rings wide open on July 2, 2017. The shadow of its globe can
just be
seen on the far side of the Ring system. There are three main concentric rings:
Ring A is the outermost, and is separated from the brighter Ring B by a dark gap
known as the Cassini Division, which is 4800 kilometres wide, enough to drop
Australia through.
T
he 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 on September 18, 2017, 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: This ice giant planet is a middle of the night object in September, as it reached western quadrature (rising at midnight) on July 21. This month Uranus rises above the theoretical horizon at 9:09 pm on September 1 and 7:11 pm on September 30. Uranus shines at about magnitude 5.8, so a pair of binoculars or a small telescope is required to observe it. It is currently in the constellation Pisces, near the south-west corner of Aries. The waning gibbous Moon will be in the vicinity of Uranus on September 9. Uranus will reach opposition (in the opposite direction to the Sun, rising in the east as the Sun sets in the west) on October 20.
Neptune: The icy blue planet is a late evening object this month. It reached western quadrature on June 5, which means that on that date it rose at midnight. Neptune will reach opposition on September 5, when it will be visible all night. Neptune is located in the constellation of Aquarius, between the magnitude 3 star Skat (Delta Aquarii) and the four-star asterism known as the Water-Jar. As it shines at about magnitude 8, a small telescope is required to observe Neptune. The Full Moon will be close to Neptune on the evening of September 6.
Neptune, photographed from Nambour on October 31, 2008
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.
By the 19th, the Moon will have moved down to Mercury and Mars, and will be sitting between them. Meanwhile, Venus will be moving towards Regulus, and will pass close by that star on September 20, when the Moon will be too close to the Sun to observe. Mercury will also be heading towards the Sun, and in the last week of September it will be lost to view. Venus and Mars will be coming together, and they will be only 12 arcminutes apart on October 6. Mercury will pass on the far side of the Sun on October 9 (superior conjunction), and will then move to the western twilight sky, becoming visible there in mid-October, near Jupiter. It will become easily seen with the unaided eye during November, becoming close to Saturn and the waxing crescent Moon on November 21.
Alpha
Aurigids
September 1
Waxing gibbous
Moon, 74% sunlit ZHR =
10
Radiant: Near the bright star Capella
Use
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'. Oan 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.
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.
Comets:
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.
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._____________________________________
Similar real-time charts can also be generated from another source, by following this second link:
Click here for a different real-time sky chart.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
View horizon at this observing site
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.
September is not a good month to observe the Eta Carinae Nebula, as it is only seven degrees above the south-south-western horizon as darkness falls and remains below the horizon at Nambour for most of the night. It will become visible as an evening object again, early in 2018.
The Stars and Constellations for this
month:
These descriptions of the night sky are for 9 pm on September 1 and 7 pm on September
30. Broadly speaking, the following description starts low in the north-west and
follows the horizon to the right, heading round to the east, then south, then
west, then overhead.
The Ring Nebula was ejected from the central Sun-sized star towards the end of its life, as powerful stellar winds blew away the outer layers of its atmosphere to form a ring.
Approaching due north is the constellation Cygnus, the Swan. Cygnus is also known as the
Northern
Cross, but to us in the southern hemisphere it appears this month upside-down and tilted to the left. The star at the bottom of the Cross, or the
highest one above the horizon as we see it tonight, is a beautiful double star or binary, called Albireo (see below).
Whereas most binaries are a pair of similar stars, there are many in which the two stars are very different, such as brilliant Sirius the Dog Star with its tiny white dwarf companion known as 'The Pup'. Albireo's two components have a marked colour contrast, the brighter star being a golden yellow, and the fainter companion being a vivid electric blue. It is a wonderful object to view with a small telescope.
At the top of the Northern Cross (which is the star closest to the horizon as we see it) is the bright first magnitude star
Deneb, or Alpha Cygni.
Deneb is a white star, and is the nineteenth brightest in the sky. It will be due north at about 9.00pm at mid-month. Its name means 'The Tail', as it marks the tail
of the Swan. Deneb, although bright, is one of the most distant individual stars we can see with the unaided eye. It lies at a distance of about 2600 light years. This means
that the light entering your eye tonight actually left Deneb in the 6th century BC, about the time when the world's first 'scientist', Thales of Miletus, was studying the
night sky from ancient Ionia, part of today's Turkey. Everything you see in the night sky is a view into the past, due to the finite speed of light.
Vega is the brightest star at centre left, with the stars of Lyra to its right. Deneb is the brightest star near the bottom edge. Cygnus, or the 'Northern Cross', stretches up vertically from Deneb to Albireo, above centre. We see the Northern Cross upside-down. The three bright stars of Aquila form a line at upper right, with Altair, the brightest, being the middle one. The small diamond-shaped constellation of Delphinus, the Dolphin, is above centre-right.
In the north-east, the Great Square of Pegasus has cleared the horizon, and is standing on one corner. It is very large, each side being around
15 degrees long. It is about as large as a fist held at arm's length. The Great Square is remarkable for having few naked-eye stars within it.
The names of the four stars marking the corners of the Square (starting at the uppermost one and moving in a clockwise direction) are Markab, Algenib, Alpheratz and Scheat. Although these four stars are known as the Great Square of Pegasus, only three are actually in the constellation of Pegasus, the Winged Horse. In point of fact, Alpheratz is the brightest star of the constellation Andromeda, the Chained Maiden.
In the east, a mv 2.2 star is about a handspan above the horizon. This is Beta Ceti, the brightest ordinary star in the constellation Cetus, the Whale. Its common name is Diphda, and it has a yellowish-orange colour. By rights, the star Menkar or Alpha Ceti should be brighter, but Menkar is actually more than half a magnitude fainter than Diphda. Menkar does not rise at the beginning of September until about 11 pm.
Cetus is a large constellation, running around the eastern horizon tonight, and to the unaided eye it appears unremarkable. But it does contain a most interesting star, which even men of old noticed, naming it Mira, the Wonderful (see below). Between Cetus and Pegasus is the faint zodiacal constellation of Pisces, the Fishes, which contains a faint ring of stars known as the 'Circlet'. Near this asterism, this year, Pisces is distinguished by the presence of the planet Uranus, about a handspan east of the Circlet.
Above Diphda is Fomalhaut, a bright, white first magnitude star in the faint constellation Piscis Austrinus, the Southern Fish. The first planet to be photographed circling another star is embedded in the dust ring surrounding Fomalhaut. Above Fomalhaut and to the right is a large, flattened triangle of stars, Grus, the Crane.
A little more than a handspan above the south-eastern horizon is
Achernar, which is the ninth brightest star. From locations south of
Newcastle, Achernar is circumpolar, i.e. it never dips below the horizon but is always in the sky. A hot blue-white star, Achernar is the main star in the
constellation Eridanus the River, which winds its way from Achernar towards the south-eastern horizon and then turns along the horizon to the east towards
Cetus. It then continues below the horizon all the way to Orion, which this month will not rise above the eastern horizon until a little after midnight.
Achernar's visual magnitude ( mv) is 0.45, and it is a hot blue-white star of B3 spectral type. The width of the field is 24 arcminutes and the faintest stars are mv 15.
To the left and higher than Achernar, the faint constellation of Phoenix may be seen.
Its brightest star is Ankaa or Alpha Phoenicis, a mv 2.39 star which is halfway between Diphda and Achernar, but slightly above.
One-and-a-half degrees to the right of Ankaa is a fourth magnitude star,
Kappa Phoenicis.
Due south, the Large Magellanic Cloud (LMC) is a faint, diffuse glowing patch low to the horizon, only eight degrees up from the latitude of Nambour. The Small Magellanic Cloud (SMC) is about a handspan above it, and to the right of Achernar. Both of these Clouds appear as faint smudges of light, but in reality they are dwarf galaxies containing millions of stars.
From Nambour's latitude, these two clouds never set. Each day they circle the South Celestial Pole, which is a point in our sky 26.6 degrees above the horizon's due south point. Objects in the sky that never set are called 'circumpolar'. The LMC and SMC are described below.
The Southern Cross (Crux) is setting low in the south-west, with the two Pointers Alpha and Beta Centauri vertically above it. Alternative names for these two Pointers are Rigel Kentaurus and Hadar. The two pointers are eight degrees apart. Alpha is the one further away from Crux. Whereas Alpha Centauri (see below) is the nearest star system to our Sun, only 4.2 light years distant, Beta is eighty times further away. Beta Centauri must have an absolute magnitude much greater than Alpha, in order to appear nearly as bright.
At top - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross) with the
dark cloud of dust known as the Coalsack above Alpha Crucis. Bottom - star clusters in the Milky Way with the Eta Carinae nebula near the lower edge.
Just above the second brightest star in the Cross (Beta Crucis) is a brilliant small
star cluster known as 'Herschel's Jewel Box'. In the centre of the cluster is a red supergiant star, which is just passing through.
Beta Crucis (left) and the Jewel Box cluster
If the night is dark and the skies are clear, a black dust cloud known as the Coalsack can be seen just above
Acrux, the left-most and
brightest star of the Cross. Surrounding Crux on three sides is the large constellation
Centaurus, its two brightest stars being the brilliant Alpha and
Beta Centauri. The rest of the constellation of Centaurus arches over Crux from above it, to its right-hand side, and then underneath it, where it adjoins
Carina and Vela.
Adjoining Crux on its left-hand side is a small, fainter quadrilateral of stars, Musca, the Fly. Out of all the 88 constellations, it is the only insect. To the upper left of Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle.
Six of the zodiacal constellations are overhead tonight. The faint constellation of Libra is low in the west. Starting from Libra and looking above it, we see the bright constellation of Scorpius, the Scorpion, with the red-supergiant star Antares marking the Scorpion's heart. This famous zodiacal constellation is like a large letter 'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion. Antares is the brightest star in Scorpius, and is a red type M supergiant of magnitude 0.9. Antares is the fifteenth brightest star.
Another very distant star that is easily seen with the unaided eye is
Zeta Scorpii, which, like Deneb, also lies at a distance of 2600 light years. It
can be found by following the line of the tail of Scorpius, and is the fourth star from Antares, heading south. It lies at the point where the tail takes a
sharp turn east. Actually, there are two stars there, Zeta 1 and Zeta 2. Zeta 1, the distant star, is a blue-white B1 type, while Zeta 2 is an orange K type, and
at a distance of only 151 light years is 17 times closer.
Zeta 1 Scorpii is the upper star in the bright group of three at centre right. Zeta 2 is below it.
Adjoining Scorpius to its north is Ophiuchus, the serpent bearer.
Though Ophiuchus is not as spectacular as Scorpius, this year it is favoured by
the presence of the ringed planet Saturn, which is brighter than any of
the stars in that part of the sky.
Saturn will cross into Sagittarius on November 19, and in the week between
Christmas and New Year it will skirt the Trifid Nebula (M20) and will pass
across outlying stars of the open cluster, M21.
Between Scorpius and Centaurus is an interesting constellation composed of mainly third
magnitude stars, Lupus, the Wolf. Midway between Triangulum Australe and Scorpius is an asterism like a small, elongated triangle. This is
Ara, the Altar.
The constellations surrounding the Southern Cross
High in the north-west, between Scorpius and Hercules, are two large but faint constellations,
Serpens, the Snake, and Ophiuchus, the Serpent Bearer.
Though Ophiuchus is not as spectacular as Scorpius, this year it is favoured by
the presence of the ringed planet Saturn, which is brighter than any of
the stars in that part of the sky.
Saturn will cross into Sagittarius on November 19, and in the week between
Christmas and New Year it will skirt the Trifid Nebula (M20) and will pass
across outlying stars of the open cluster, M21.
East of Scorpius is Sagittarius the Archer, which is almost directly overhead, having just passed the meridian. The 'Teapot' asterism in this constellation is illustrated below.
The Milky Way passes through Sagittarius, and the centre of our galaxy is close to the zenith at
this time. Sagittarius teems with stars, glowing nebulae and dust clouds, as it is in line with the centre of our galaxy.
Adjoining Sagittarius to the south, there is a beautiful curve of faint stars This is
Corona Australis, the Southern
Crown, and it is very elegant and delicate. The brightest star in this constellation has a magnitude of only 4.1.
The body of Scorpius is at right, with the two stars in the Sting to the left, at centre right of picture. The red supergiant star Antares appears close to the top right corner. The stars in the left half of the picture are in Sagittarius. The well-known 'Teapot' shape may be seen. Near the lower left margin is a graceful curve of fourth magnitude stars, Corona Australis, the Southern Crown. To see this view, stand facing south and look directly overhead at 7.00 pm at the beginning of the month. Later in the month, the stars will be further to the west at the same time.
Antares, a red supergiant star
The star which we call Antares is a binary system. It is dominated by the great red supergiant Antares A which, if it swapped places with our Sun, would enclose all the planets out to Jupiter inside itself. Antares A is accompanied by the much smaller Antares B at a distance of between 224 and 529 AU - the estimates vary. (One AU or Astronomical Unit is the distance of the Earth from the Sun, or about 150 million kilometres or 8.3 light minutes.) Antares B is a bluish-white companion, which, although it is dwarfed by its huge primary, is actually a main sequence star of type B2.5V, itself substantially larger and hotter than our Sun. Antares B is difficult to observe as it is less than three arcseconds from Antares A and is swamped in the glare of its brilliant neighbour. It can be seen in the picture above, at position angle 277 degrees (almost due west or to the left) of Antares A. Seeing at the time was about IV on the Antoniadi Scale, or in other words below fair. Image acquired at Starfield Observatory in Nambour on July 1, 2017.
The Trifid Nebula, M20, in Sagittarius, is composed of a reflection nebula (blue), an emission nebula (pink), and dark lanes of dust.
The centre of our galaxy is teeming with stars, and would be bright enough to turn night into day, were it not for intervening dust and molecular clouds. This dark cloud is known as 'The Snake'. A satellite passed through the field of view at right.
East of Sagittarius and approaching the zenith is Capricornus, the Sea-Goat. This constellation is lacking in any bright stars, and is fairly
unremarkable.
Between Capricornus and Pisces is another rather faint constellation, Aquarius, the Water Bearer. There are no stars brighter than third magnitude in this constellation, but it does contain many interesting objects, including a group of four stars known as the 'Water Jar'. Also, this year faint Neptune may be found in its centre, about two-thirds of a handspan east of the star Deneb Algedi.
Culminating high in the north, between Capricornus and Albireo, is the constellation of Aquila, the Eagle (see picture). The centre of this constellation is marked by a short line of three stars, of which the central star is the brightest. These stars, from north to south, are Tarazed, Altair and Alshain, and they indicate the Eagle's body. A handspan to the east of bright, first magnitude Altair is a faint but easily recognised diamond-shaped group of stars, Delphinus the Dolphin.
The line of the ecliptic along which the Sun, Moon and planets travel, passes through the following constellations this month: Libra, Scorpius, Sagittarius, Capricornus, Aquarius and Pisces.
The aborigines had a large constellation which is visible tonight, the Emu. The Coalsack forms its head, with the faint sixth magnitude star in the Coalsack, its eye. The Emu's neck is a dark lane of dust running east through the two Pointers, to Scorpius. The whole constellation Scorpius forms the Emu's body. The Emu is sitting, waiting for its eggs to hatch. The eggs are the large star clouds of Sagittarius.
More photographs of the amazing sights visible in our sky this month are found in our Gallery.
If you would like to become familiar with the constellations, we suggest that you access one of the world's best collections of constellation pictures by clicking
here. To see some of the best astrophotographs taken with the giant Anglo-Australian telescope, click here.
Mira, the Wonderful
The amazing thing about the star
Mira or Omicron Ceti is that it
varies dramatically in brightness, rising to magnitude 2 (brighter than any other star in Cetus), and then dropping to magnitude 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. In the mid-17th century, the astronomer Johannes Hevelius watched the star fade away
during the year until it disappeared, and then it slowly reappeared again. Its not surprising that he named it Mira, meaning 'The Wonderful' or 'The Miraculous One'. We
now know that many stars vary in brightness, even our Sun doing so to a small
degree, with a period of 11 years. One type of star varies, not because it is
actually becoming less bright in itself, but because another, fainter star moves
around it in an orbit roughly in line with the Earth, and obscures it on each
pass. This type of star is called an eclipsing variable and they are very
common. The
star Mira though, varies its light output because of processes in its interior.
It is what is known as a pulsating variable. Stars of the Mira type are
giant pulsating red stars that vary between 2.5 and 11 magnitudes in brightness.
They have long, regular periods of pulsation which lie in the range from 80 to
1000 days.
Mira near minimum, 26 September 2008 Mira near maximum, 22 December 2008
Astronomers using a NASA space telescope, the Galaxy Evolution Explorer, have spotted an amazingly long comet-like tail behind
Mira as the star streaks through space. Galaxy Evolution Explorer ("GALEX" for short) scanned the well-known star during its ongoing survey of the entire
sky in ultraviolet light. Astronomers then noticed what looked like a comet with a gargantuan tail. In fact, material blowing off Mira is forming a wake 13
light-years long, or about 20,000 times the average distance of Pluto from the sun. Nothing like this has ever been seen before around a star.
More,
including pictures
Double and multiple stars
Estimates vary that between 15% and 50% of stars are single bodies like our Sun, although the latest view is that less than 25% of stars are solitary. At least 30% of stars and possibly as much as 60% of stars are in double systems, where the two stars are gravitationally linked and orbit their mutual centre of gravity. Such double stars are called binaries. The remaining 20%+ of stars are in multiple systems of three stars or more. Binaries and multiple stars are formed when a condensing Bok globule or protostar splits into two or more parts.
Binary stars may have similar components (Alpha Centauri A and
B are both stars like our Sun), or they may be completely dissimilar, as with
Albireo (Beta Cygni, where a bright golden giant star is paired with a smaller bluish main sequence star).
The binary stars Rigil Kentaurus (Foot of the Centaur, or 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, Rigil Kent or Toliman) is a binary easily
seen with a small 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
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 telescope is struggling to separate them (Acrux, 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.
The Milky Way
A glowing band of
light crossing the sky is especially noticeable during September. This
glow is the light of millions of faint stars combined with that coming from
glowing gas clouds called emission nebulae. It is concentrated along the plane of our
galaxy, and this month it is seen crossing the sky, starting from the
south-south-west and passing through Crux to Centaurus, Scorpius, Sagittarius and Aquila to Cygnus in the north-east. The plane of our galaxy from Scutum (at left) through Sagittarius and Scorpius (centre) to Centaurus
and Crux (right). The Eta Carinae nebula is at the right margin, below centre. The Coalsack is clearly visible, and the dark dust lanes can be seen. Taken with an
ultra-wide-angle lens.
The centre of our galaxy. The constellations partly visible here are Sagittarius (left), Ophiuchus (above centre) and Scorpius (at right). The planet Jupiter is the bright object below centre left. This is a normal unaided-eye view.
The Season of the Scorpion
The spectacular constellation of Scorpius is about a handspan west of the zenith in mid-September. Three bright stars in a gentle curve mark his head, and another three mark his body. Of this second group of three, the centre one is a bright, red supergiant, Antares. It marks the red heart of the scorpion. This star is so large that, if it swapped places with our Sun, it would engulf the Earth and extend to the orbit of Mars. It is 604 light years away and shines at magnitude 1.06. Antares, an M type star, has a faint companion which can be seen in a good amateur telescope if the seeing is excellent.
The rest of the stars run around the scorpion's tail, ending with two blue-white B type stars, Shaula (the brighter of the two) and Lesath,
at the tip of the scorpion's sting. These two stars are at the eastern end of the constellation, and are near the bottom of the picture below. West of
Lesath in the body of the scorpion is an optical double star, which can be seen as two with the unaided eye.
Scorpius, with its head at top left and tail (with sting) at lower right.
Probably the two constellations most easily recognisable (apart from Crux, the Southern Cross) are Orion the Hunter and Scorpius the Scorpion. Both are
large constellations containing numerous bright stars, and are very obvious 'pictures in the sky'. Both also contain a very bright red supergiant
star, Betelgeuse in Orion and Antares in Scorpius.
The Lagoon Nebula, M8, in Sagittarius, adjacent to Scorpius
The centre of the Lagoon Nebula
Why are some constellations bright, while others are faint ?
The Milky Way is a barred spiral galaxy some 100000 – 120000 light-years in diameter which contains 100 – 400 billion stars. It may contain at least as many planets as well. Our galaxy is shaped like a flattened disc with a central bulge. The Solar System is located within the disc, about 27000 light-years from the Galactic Centre, on the inner edge of one of the spiral-shaped concentrations of gas and dust called the Orion Arm. 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 northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is out of sight this month.
The southern window is in the constellation Sculptor, just south of the star Fomalhaut. This window is low in the south-east in the early evening, but later in the night it will rise high enough for distant galaxies to be observed. Some of the fainter and apparently insignificant constellations are found around these windows, and their lack of bright stars, clusters and gas clouds presents us with the opportunity to look across the millions of light years of space to thousands of distant galaxies.
Finding the South Celestial Pole:
The South Celestial Pole is that point in the southern sky around which the stars rotate in a clockwise direction. The Earth's axis is aimed exactly at this point. For an equatorially-mounted telescope, the polar axis of the mounting also needs to be aligned exactly to this point in the sky for accurate tracking to take place.
To find this point, first locate the Southern Cross. Project a line from the top of the Cross down through its base and continue straight on towards the south for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.
Another way to locate the South Celestial Pole is to draw an imaginary straight line joining Beta Centauri high in the south-west to Achernar low in the south-east. Both stars will be at about the same elevation above the horizon at 8 pm in the middle of September. Find the midpoint of this line to locate the pole.
Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open the shutter
for thirty minutes or more. The stars will move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different
colours, like the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
Star trails between the South Celestial Pole and the southern horizon. All stars that do not pass below the horizon are circumpolar.
Star Clusters
The 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 the disc of 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,
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 almost a solid mass of faint stars. Most 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, and this month it is observable before midnight. 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 millions 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. Omega Centauri is not a typical globular cluster, being much larger than any other in our Galaxy, with millions of stars. It is also slightly oval in shape.
The globular cluster Omega Centauri
The central core of Omega Centauri
There is another remarkable globular, second only to Omega Centauri. Close to the SMC (see below), binoculars can detect a fuzzy star. A
telescope will reveal this faint glow as a magnificent globular cluster, lying at a distance of 5.8 kiloparsecs. Its light has taken almost 19 000 years to reach us. This is
NGC 104, commonly known as 47 Tucanae. Some regard this cluster as being more spectacular than Omega Centauri, as it is more compact, and the faint stars
twinkling in its core are very beautiful. This month, neither Omega Centauri nor 47 Tucanae
is at its best position for viewing, but both are observable.
The globular cluster 47 Tucanae.
Observers aiming their telescopes towards the
SMC generally also look at the nearby 47 Tucanae, but there is another globular cluster nearby which is also worth a visit. This is
NGC 362, which is
less than half as bright as the other globular, but this is because it is more than twice as far away. Its distance is 12.6 kiloparsecs or 41 000 light years,
so it is about one-fifth of the way from our galaxy to the SMC. Both NGC 104 and NGC 362 are always above the horizon for all parts of Australia south of the
Tropic of Capricorn.
