February
2012
Updated: 1 February 2012
Welcome to the night skies of Summer, featuring Taurus,
Orion, Canis Major and Jupiter
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. 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 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 magnitude 15. 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 Sun 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).
Solar System
Sun:
The Sun begins the month in the constellation of Capricornus, the Sea-Goat, and passes into Aquarius, the Water-bearer, on February 17.
Moon Phases: Lunations this month: #1101, 1102
Full Moon:
February
8 07:55 hrs
diameter = 31.8'
Last Quarter:
February 15 03:05 hrs
diameter = 32.2'
New Moon:
February 22
08:36 hrs
diameter = 30.5'
First Quarter:
March 1 11:22 hrs
diameter = 29.8'
Full Moon:
March 8 19:40 hrs
diameter = 32.7'
Last Quarter: March 15
11:26 hrs
diameter = 31.9'
New Moon:
March 23 00:38 hrs
diameter = 29.8'
First Quarter: March 31
05:41 hrs
diameter = 30.3'
Lunar Orbital Elements:
February
3:
Moon at descending node at 05.57 hrs, diameter = 30.0'
February
12: Moon at
perigee (367 928 km) at 04:11 hrs, diameter = 32.5'
February
16:
Moon at ascending node at 06:17 hrs, diameter = 32.0'
February
27:
Moon at apogee (404 842 km) at 23:44 hrs, diameter = 29.5'
March 1: Moon at descending node at
08:32 hrs, diameter = 29.8'
March
10: Moon at
perigee (362 409 km) at 20:01 hrs, diameter = 33.0'
March
14: Moon at
ascending node at 06:43 hrs, diameter = 32.3'
March 26: Moon at apogee
(405 775 km) at 03:46 hrs, diameter = 29.4'
March 28: Moon
at descending node at 10:15 hrs, diameter = 29.6'
Moon at 8 days after New, as on February 1
Moon at 9 days after New, as on February 2
The two photographs above show the Mare Imbrium area in the Moon's northern hemisphere. They were taken a day apart, just after First Quarter. Mare Imbrium (the Sea of Rains) is a large lava flow caused by the Imbrium Event - a cataclysmic collision of an asteroid with the Moon many millions of years ago. A comparison of the two photographs will show how the appearance of lunar features changes with the angle of the Sun.
In the first photograph, Mare Imbrium (left) is separated from Mare Serenitatis (right) by two ranges of mountains, the Alps to the north and the Apennines to the south. Two large craters at upper right are Aristoteles and Eudoxus. The straight Alpine Valley may be seen cutting through the Alps. Mt Piton (height 2000 metres) is visible as a bright spot with a shadow, due south of the southern end of the Alpine Valley. Archimedes is the large crater at left. It is a walled plain 80 kilometres in diameter with a flat floor. To its right are two bowl-shaped craters, Aristillus and Autolycus. These craters are all formed by impact with large meteors. Apollo 15 landed close by the Apennines, in a small enclosed area to the right and below Archimedes, on the picture's central vertical axis.
In the second photograph, the sunrise line (called the 'terminator') has moved to the left, revealing a large walled plain in the Alps, known as Plato. South of Plato, an isolated mountain protruding through the lava flow is called Mt Pico. Ripples in the lava, called 'wrinkle ridges', are visible. The crater at lower left is Timocharis, 42 kilometres in diameter.
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, 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 !<
Geocentric Events:
February 4: Moon
1.1º
north of the
star Zeta Tauri (mv= 2.97) at 07:23 hrs
February 4: Limb of Moon 9
arcminutes south of the
star Propus (Eta Geminorum, mv=
3.31) at 23:38 hrs
February 5: Limb of Moon 41
arcminutes south of the
star Mu Geminorum (mv=
2.87) at 03:39 hrs
February 7: Mercury in superior
conjunction at 18:47 hrs (diameter = 4.8")
February 7: Saturn at western
stationary point at 22:37 hrs (diameter = 17.7")
February 10: Venus 19 arcminutes north of Uranus at 14:45
hrs
February 11: Mercury 38 arcminutes
north of the star Deneb
Algedi
(Delta Capricorni, mv=
2.85) at 00:55 hrs
February 12: Limb of Moon 42 arcminutes
south of the star Spica
(Alpha Virginis, mv=
0.98) at 21:09 hrs
February 13: Moon 5.9º
south of Saturn at 11:45
hrs
February 14: Mercury 1.2º
south of Neptune at
17:23 hrs
February 15: Moon
1.8º
north of the star Delta Scorpii
(mv=
2.29)
at 15:59 hrs
February 15: Moon 1.0º
south of the star Graffias
(Beta
1 Scorpii, mv=
2.56)
at 16:53 hrs
February 16: Mars at aphelion at 10:02 hrs
(diameter = 13.1")
March 2:
Mercury at perihelion at 15:21 hrs (diameter = 6.7")
March 2:
Moon
1.3º
north of the star Zeta Tauri
(mv= 2.97)
at 15:29 hrs
March 3:
Moon
1.1º
south of the star
The Planets for this month:
Mercury: The innermost planet will pass behind the Sun (superior conjunction) on February 7. By the end of the month, Mercury will appear in the twilight sky, between Venus and the western horizon.
Venus:
The brilliant planet Venus is now dominating the twilight sky. It is high above the western horizon soon after sunset and sets at about 8:45 pm at the beginning of the month. As February progresses, the angular distance between Venus and the Sun will increase from 40 degrees to 44 (almost its maximum). The phase will decrease from 74% to 64%, but as it approaches the Earth, its angular size will increase from 15 arcseconds to 18. This will result in the brightness of the planet remaining constant, at magnitude -4.2. Venus is the brightest object in the night sky except for the Moon, and can even cast shadows on a moonless night.Venus will be a feature of the western twilight sky for the next four months. On the evenings of February 25 and 26, the waxing crescent Moon will appear just to the right of Venus. Venus is heading towards Jupiter, and the two brightest planets will be together in the sky (only 3 degrees apart) on March 14 next. The crescent Moon will move between them on the evening of March 26.
(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).
October 2011 to January 2012 March to April 2012 May 2012
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. Up until last August, Venus appeared as a 'Morning Star', but since then it has been an 'Evening Star'. Each of these appearances lasts about eight to nine months. Venus will appear as an 'Evening Star' until the end of May 2012.
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: The red planet is now significantly brighter as it approaches opposition, rising at about 9 pm on February 1, and is located in front of the back legs of the constellation Leo, the Lion. It is the brightest object in that part of the sky, and its orange colour is very obvious. Look above the east-north-eastern horizon at about 10 pm at the beginning of the month, or 8 pm at the end of February. Mars will appear a little larger and brighter each night, as the Earth catches it up. On March 4, we will overtake Mars, the red planet at that time being at its closest distance to us. Even so, it will be only one-third the diameter of Jupiter, and less than half as bright.
Sometimes, when the Earth passes by Mars, the two planets are quite close, less than 45 million kilometres apart. The last such "favourable opposition" took place in 2003. The oppositions since then have been less favourable. The variations in the distances between the planets at opposition are due to the oval shape of Mars's orbit.
The disc of Mars on February 1 will be 12 arcseconds across. By opposition on March 4, its angular size will have increased to 14 arcseconds. On the night of February 10, the rising Full Moon will 9 degrees to the right of Mars, just after 9 pm. On the night of March 16 - 17, Mars will pass less than 9 arcminutes north of the spiral galaxy M96, and will pass through the grouping of galaxies M95, M96 and M105.
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.
Jupiter:
At the beginning of February, the giant planet may be found 40 degrees (two handspans) above the north-western horizon at 7 pm. It lies near the border between the faint constellations of Pisces and Aries, and is the brightest object in that part of the night sky. Jupiter is about 80 degrees from the Sun, this angular distance decreasing slowly night by night. Jupiter will be in conjunction with the Sun on May 13. The Moon will be nearby on the evening of February 27. A pair of binoculars is all that is required to detect the four brightest moons of Jupiter, that Galileo discovered in 1609 with a small telescope he had constructed. Even the cheapest telescope available today will reveal them.Saturn: The ringed planet can be found in the constellation Virgo, becoming visible above the eastern horizon a little before 11 pm at the beginning of February. It is about 7 degrees below the first magnitude blue-white star Spica, and is slightly brighter than that star. Through a small telescope, Saturn’s rings are a magnificent sight, and are now opening wider. The waning gibbous Moon will make a fine grouping with Saturn and Spica at about 11 pm on February 13.
Uranus: This ice giant planet is currently in the constellation Pisces, near a faint asterism called 'The Circlet'. Uranus has a magnitude of 5.9, at the limit of the unaided eye. Its apparent diameter is 3 arcseconds. Venus and Uranus will be together in the sky on February 10, and Uranus will pass behind the Sun (conjunction) on March 25.
Neptune:
The icy blue planet is in the constellation of Aquarius at present, and is unobservable as it is lost in the solar glare. It passes through conjunction on February 20.
Neptune, photographed from Nambour on October 31, 2008
Pluto:
The erstwhile ninth and most distant planet is a faint 14.1 magnitude object in the constellation Sagittarius, near the boundaries with Scutum (The Shield) and Serpens Cauda (The Snake's Tail). It is presently passing through a rich star field and at mid-month will be about one degree east of the galactic cluster M25. This month it is very difficult to observe, as it is only 47 degrees from the Sun at mid-month, rising at about 2 am. Pluto's angular diameter is 0.13 arcseconds, less than one twentieth of the size of Neptune.
The movement of Pluto in two days, between 13 and 15 September, 2008. Pluto is the one object that has moved.
Width of field: 200 arcseconds
Meteor Showers:
Alpha Centaurids
February 8
Full Moon, 100% sunlit ZHR =
10
Radiant: Near the star
Alpha Centauri.
Use this Fluxtimator to calculate the number of meteors predicted per hour for any meteor swarm on any date, for any place in the world.
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.
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'. One caused great alarm in Canada recently, being recorded on a
camera mounted on the dashboard of a police cruiser. 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:
Comet Lovejoy
This comet was discovered on November 27, 2011 by Terry Lovejoy, a Brisbane amateur astronomer. It is the third comet to bear his name. The following photograph of Comet Lovejoy was taken from Starfield Observatory at Nambour at 3:38 am on Tuesday, December 27, 2011. The comet's nucleus was at R.A. 16 hours 58 minutes, Dec. -46 degrees 53 minutes, near the boundary between the constellations Scorpius and Ara. The tail appears on the image to be at least 26 degrees long. As it was superimposed on the molecular clouds that make a dark lane extending from Alpha Centauri to the galactic centre and beyond, it was quite easy to see with the unaided eye. Some stars of the Southern Cross (Alpha and Beta Crucis) appear near the top right-hand corner of the image. The comet is still moving away from the Sun, heading towards the South Celestial Pole. It is now very faint.
Comet Lulin
This comet, (C/2007 N3), discovered in 2007 at Lulin Observatory by a collaborative team of Taiwanese and Chinese astronomers, is now heading towards the outer Solar System, and has faded below magnitude 15.
The
Nearly 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._____________________________________
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 onView 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.
The Eta Carinae Nebula, above the Southern Cross tonight
The 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, just north of
the zenith.
The brilliant white star Rigel (Beta Orionis) is approaching the zenith at this time. 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. Directly overhead is a faint constellation, Columba, the Dove.
The constellation Taurus with the clusters
Pleiades and Hyades is between Orion and
the north-western horizon. The brightest star in Taurus is a star dominating (but not actually a member of) the Hyades cluster.
This is Aldebaran, a K5 orange star with a visual magnitude of 0.87. 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.
Above then is brilliant Jupiter, brighter than any star. One reasonably bright star,
Diphda (Alpha Ceti) is low in the west, and another,
Fomalhaut, is on 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. At present the red planet
Mars is crossing through Leo, and outshines all of that constellation's
stars. 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
midnight. 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 underneath Musca is a (roughly) equilateral
triangle of 4th magnitude stars. This is the constellation Triangulum Australe, the Southern Triangle. It is very low on
the horizon, just east of south. Between Crux and Sirius is a very large area of sky filled with interesting objects. This was once the
constellation Argo, 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). A handspan south of the zenith and two handspans south of Sirius is the second brightest star in the night
sky, Canopus (Alpha Carinae). On the border of Carina and Vela is the False Cross, larger and more lopsided
than the Southern Cross. The False Cross is two handspans above Crux, and is also lying on its side at this time of year. It is
high in the south-south-east. Both of these Crosses are actually more like kites in shape, for, unlike
Cygnus (the
Northern Cross) they have no star at the intersection of the two cross arms. Two handspans south-west of Canopus is Achernar, Alpha Eridani. It is the brightest star in
Eridanus the River, which winds its way with faint stars from Achernar in a northerly direction to
Cursa, a
mv= 2.9 star close to brilliant Rigel in Orion. Achernar curves down towards the south-south-westerly horizon
during the evening, and has set by 2 am. 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 season of the Hunter and his Dogs:
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
Two of the most spectacular constellations in the sky may be seen high in the east as soon as darkness falls. These are Orion the Hunter,
and his large dog, Canis Major. Orion straddles the celestial equator, midway between the south celestial pole and its northern equivalent. This means that the centre of
the constellation, the three stars known as Orion's Belt, rise due east and set due west.
Orion:
To the south of the Belt, at a distance of about one Belt-length, we see another faint group of stars in a line, fainter and closer together than those in the Belt. This is Orion’s Sword. Orion’s two feet are marked by brilliant Rigel and fainter Saiph. Both of these stars are also members of the Orion Association.
The Saucepan, with Belt at right, M42 at upper left.
Orion is quite a symmetrical constellation, with the Belt at its centre and the two shoulder stars off to the north and the two knee stars to the south. It is quite a large star group, the Hunter being over twenty degrees (a little more than a handspan) tall.
The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. 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 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 heart 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 white dwarf companion. Whereas a small telescope can reveal the companion to Rigel quite easily, the companion to Sirius the Dog Star, (called ‘the Pup’), can only be observed by using a powerful telescope with excellent optics, as it is very close to brilliant Sirius and is usually lost in the glare..
Canis Major as it appears almost overhead at 9 pm at mid-month (observer facing west).
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.
Some fainter constellations: Between the two Dogs is the constellation Monoceros the Unicorn, undistinguished except for the presence of the remarkable Rosette Nebula. South of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Eridanus the River winds its way from near Orion west of the zenith to Achernar, high in the south-west. Between Achernar and the western horizon is the star Fomalhaut, a white star of first magnitude in the small constellation of Piscis Austrinus (the Southern Fish). To the left of Fomalhaut is the triangular constellation of Grus, the Crane. Between the zenith and the south-western horizon are a number of small, faint constellations, Horologium, Pictor, Caelum, Mensa, Tucana, Phoenix, Hydrus and Reticulum. The LMC lies in the constellation Dorado, and the South Celestial Pole is in the very faint constellation of Octans.
Finding the South Celestial Pole:
The South Celestial Pole is that point in the southern sky around which the stars rotate in a clockwise direction. The Earth's axis is aimed exactly at this point. For an equatorially-mounted telescope, the polar axis of the mounting also needs to be aligned exactly to this point in the sky for accurate tracking to take place.
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 10 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.
The binary star Rigel (Beta Orionis, left) 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. In the centre of the Great Nebula in Orion (M42) is a multiple star known as the Trapezium (right). This star system has four bright white stars, two of which are binary stars with fainter red companions, giving a total of six. The hazy background is caused by the cloud of fluorescing hydrogen comprising the nebula.
Acrux, the brightest star in the Southern Cross, is also known as Alpha Crucis. It is a close binary, circled by a third dwarf companion.
Alpha Centauri (also known as Rigil Kentaurus, Rigil Kent or Toliman) is a binary easily seen with the smallest telescope. The components are both
solar-type main sequence stars, one of type G and the other, slightly cooler and fainter, of type K. Through a small telescope this star system looks like a pair
of distant but bright car headlights.
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 separated them (Acrux, Castor, Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.
Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
Star clusters:
The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the
fact that the stars in the cluster are grouped together, but not as tightly as in globular clusters (see below). The stars
appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within our
galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a
nebula into stars, and some are relatively young. The photograph below shows a typical open cluster, M7
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.
Some fainter constellations Between the two Dogs is the constellation Monoceros the Unicorn,
undistinguished except for the presence of the remarkable Rosette Nebula. South of Orion is a small constellation,
Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove.
Between the zenith and
the south-western horizon are a number of small, faint constellations, Phoenix, Hydrus, Reticulum,
Indus and Pavo. Clustered around the South Celestial Pole are Dorado, Octans, Apus,
Chamaeleon, Mensa and Volans.
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