May
2013
Updated: 11 May 2013
Welcome to the night skies of Autumn,
featuring Carina, Southern Cross, Centaurus, Leo, Virgo and Saturn
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 visual magnitude 15 or 19 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 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 Aries, the Ram, and passes into Taurus, the Bull on May 14.
Eclipse:
Partial solar eclipse early on May 10
On the morning of May 10, there will be a partial eclipse of the Sun. It will begin at 7:41 am, and will reach its maximum phase at 8:58 am, when about half of the Sun will be obscured by the Moon. The eclipse will be over by 10:28 am.
The eclipse will be at a maximum along a track beginning in the middle of Western Australia. It will cross the Northern Territory and then Cape York Peninsula north of Cooktown. As the Moon will be near apogee, its furthest distance from Earth in its elliptical orbit, it will not appear large enough to fully cover the Sun as with last November's total eclipse at Cairns. Instead, the Moon will cover the centre of the Sun, with a bright, narrow ring of Sun surrounding it. Such an eclipse is called an 'annular' eclipse, and is only seen as such by observers on the eclipse track. At the Sunshine Coast, we will see about half of the Sun covered at the maximum phase.
Warning:
Observing the Sun either with the naked eye or through binoculars or telescope is extremely hazardous at any time, and should not be attempted unless the observer has the proper safety equipment. A retinal burn is a possibility, and even blindness can result from looking at the Sun.
Moon Phases:
Lunations this month: #1116, 1117
Last Quarter:
May 2
21:16 hrs
diameter = 31.9'
New Moon:
May 10
10:30 hrs
diameter = 29.8' Annular Solar Eclipse
First Quarter: May 18
14:36 hrs
diameter = 30.3'
Full Moon:
May
25 14:26 hrs
diameter = 33.3'
Last Quarter:
June 1 05:00 hrs
diameter = 31.5'
New Moon:
June 9
01:58 hrs
diameter = 29.4'
First Quarter: June
17
03:25 hrs diameter =
30.9'
Full Moon:
June 23
21:33 hrs diameter =
33.5'
Last Quarter: June 30
14:55 hrs diameter =
30.9'
Lunar Orbital Elements:
May
10: Moon
at descending node at 05:13 hrs, diameter = 29.8'
May
13: Moon at
apogee (405 854 km) at 23:51 hrs, diameter = 29.4'
May
24: Moon at ascending node at
10:35 hrs, diameter = 33.0'
May
26: Moon at perigee
(358 387 km) at 11:54 hrs, diameter = 33.3'
June
06:
Moon at descending node at 10:59 hrs, diameter = 29.8'
June
10: Moon at
apogee (404 486 km) at 08:34 hrs, diameter = 29.4'
June 20: Moon at
ascending node at
19:49 hrs, diameter = 32.7'
June 23:
Moon at perigee (357 014 km) at 20:59 hrs, diameter = 33.5'
Moon at 8 days after New, as on May 19
Moon at 9 days after New, as on May 20
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:
May 2:
Limb of Moon 40 arcminutes north of
the star Dabih (Beta 1 Capricorni,
mv=
3.05)
at 07:30 hrs
May 4: Moon
6.3º
north
of Neptune at 15:38 hrs
May 7: Moon
4º
north
of Uranus at 06:23 hrs
May 8: Mercury
24 arcminutes south of Mars at 11:14 hrs
May 9: Limb of
Moon 19 arcminutes north of Mars at 23:56 hrs
May 10:
Limb of Moon 11 arcminutes north
of Mercury at 03:29 hrs
May 11: Limb of Moon 38
arcminutes south of Venus at 09: 25 hrs
May 12: Mercury in
superior conjunction at 06:57 hrs (diameter = 5.1")
May 12: Moon
2.4º
south
of Jupiter at 23:48 hrs
May 13: Limb of Moon 26
arcminutes south of
the star
Zeta Tauri (mv=
2.97) at 07:54 hrs
May 16: Mercury at
perihelion at 11:39 hrs (diameter = 5.2")
May 22: Moon occults
the
star Spica
(Alpha Virginis,
mv= 0.98)
between 20:58 and 21:06 hrs
May 23: Moon
3º
south of Saturn at
16:59 hrs
May 24: Moon
occults the star Zuben Elgenubi (Alpha Librae,
mv= 2.75)
) between 08:33 and 09:13 hrs
May 25: Mercury
1.4º
north of Venus at
06:58 hrs
May 25: Moon
1.25º
north of the star Graffias (Beta 1
Scorpii, mv=
2.56) at 13:03
hrs
May 26: Neptune at western
quadrature at 20:10 hrs (diameter = 2.2")
May 27: Mercury
2.4º
north of Jupiter at
16:46 hrs
May 28: Limb of
Moon 51 arcminutes
north of Pluto
at 01:01 hrs
May 29: Venus
1º
north of Jupiter at
04:31 hrs
May 29:
Moon 1.2º
north of the star Dabih (Beta 1 Capricorni,
mv=
3.05)
at 14:45 hrs
May 30: Venus
3º
north of
the star
Zeta Tauri (mv=
2.97) at 07:36 hrs
May 31: Moon
5.9º
north
of Neptune at 20:04 hrs
June 3:
Moon
4.2º
north of
Uranus at 15:04 hrs
June 4: Mercury
2.9º
north of
the star Mu Geminorum
(mv= 2.87)
at 01:58 hrs
June 4: Jupiter 1.9º
north
of
the star
Zeta Tauri (mv=
2.97) at 01:12 hrs
June 7: Neptune at
western stationary point at 13:23 hrs (diameter = 2.3")
June 7: Mercury 13
arcminutes south of
the star Mebsuta
(Epsilon Geminorum, mv= 3.06)
at 17:13 hrs
June 7: Venus 1.9º
north
of the star Mu Geminorum
(mv= 2.87)
at 22:51 hrs
June 7: Moon
1.7º
south
of Mars at 23:22 hrs
June 9: Limb of Moon
22 arcminutes south of
the star Zeta Tauri (mv=
2.97) at 16:54 hrs
June 9: Moon
2.8º
south of
Jupiter at18:05 hrs
June 10: Moon
5.1º
south of Venus
at 19:35 hrs
June 11: Moon
5.4º
south of
Mercury at 06:51 hrs
June 11: Venus 52
arcminutes south of the star Mebsuta
(Epsilon Geminorum, mv= 3.06)
at 19:14 hrs
June 13: Mercury at Greatest
Elongation East (24.1º)
at 05:35 hrs (diameter = 8.2")
June 13: Venus at perihelion at
20:46 hrs (diameter = 10.5")
June 19: Limb of Moon 23
arcminutes north of the
star Spica
(Alpha Virginis,
mv= 0.98)
at 06:38 hrs
June 20: Moon 3.2º
south of Saturn at
01:45 hrs
June 20: Jupiter in conjunction
with the Sun at 02:11 hrs (diameter = 32.1")
June 20: Mercury 1.9º
south of Venus at
17:18 hrs
June 20: Moon occults the star Zuben Elgenubi (Alpha Librae,
mv= 2.75)
) between 16:47 and 18:05 hrs
June 22: Limb of Moon 24
arcminutes north of the
star Graffias (Beta 1
Scorpii, mv=
2.56) at 00:49
hrs
June 24: Moon 1.7º
north of Pluto at
10:26 hrs
June 25: Limb of Moon 18
arcminutes north of the star Dabih (Beta 1 Capricorni,
mv=
3.05)
at 23:10 hrs
June 26: Mercury at eastern
stationary point at 23:24 hrs (diameter = 10.8")
June 28: Moon 5.9º
north of Neptune at
03:38 hrs
June 29: Mercury at aphelion at
11:19 hrs (diameter = 11.2")
June 30: Moon 3.7º
north of Uranus at
22:27 hrs
The Planets for this month:
Mercury:
Venus:
This, the brightest planet, passed behind the Sun (superior conjunction) on March 29. It has now moved into the western twilight sky, but will be too close to the solar glare to be easily seen until mid-May. It will dominate our twilight sky as an 'evening star' from June until the end of this year. The New Moon will be just above Venus, but hard to find in the glow of the Sun, on May 11.(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 2013 October 2013 December 2013
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 June, Venus appeared as an 'Evening Star', but since then it has been a 'Morning Star'. Each of these appearances lasts about eight to nine months. Venus will become an 'Evening Star' again next May.
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 moving eastwards through the background stars, in the constellation of Taurus, the Bull. It passed through conjunction on April 18, and this month rises as dawn is breaking. It will become visible in the pre-dawn sky during June.
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:
This gas giant planet may be found low in the north-west soon after sunset this month, but as the weeks pass it will become lost in the twilight. It will reach conjunction on June 20, after which it will move into the eastern pre-dawn sky.
Saturn: The ringed planet passed through opposition on April 28. This is the time when it rises in the east simultaneously as the Sun sets in the west. Presently found in the constellation Libra, Saturn is located beneath the star Spica, which is nearly as bright. These are the brightest objects in that part of the sky. Through a small telescope, Saturn’s rings are a magnificent sight, and are now well open. Saturn is presently moving westwards along its retrograde loop. It will reach the end of the loop on July 8, whereupon it will turn and head eastwards once more, heading towards the spectacular constellation of Scorpius. Saturn will grace our evening skies until October. The Full Moon will be just to the right of Saturn on May 23.
Uranus: During May, this ice giant planet rises at 4 am, about an hour before the appearance of the first light of dawn. Currently in the constellation of Pisces, Uranus has a magnitude of 5.9, near the limit of the unaided eye. Its apparent diameter is 3 arcseconds. The waning crescent Moon will be four degrees to the left of Uranus on the morning of May 7.
Neptune: The icy blue planet is in the constellation of Aquarius at present, and passed behind the Sun (conjunction) on February 21. At the beginning of May it will rise at about 1:20 am. Neptune is a telescopic object - its magnitude is 7.9, and its angular diameter is 2 arcseconds.
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 boundary with Scutum (The Shield). It may be found just above the handle of Sagittarius's 'Teapot'. At the beginning of May it will rise a little south of east at about 9:15 pm. Its 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
Planetary alignments:
In the last week of May, Mercury, Venus and Jupiter will be grouped close together in the western twilight sky, soon after sunset. During that week, they will change their positions relative to each other from night to night, a true 'dance of the planets' which is easily visible to the naked eye. Binoculars will help to find them in the twilight glow - look just above a low west-north-western horizon before 5:45 pm. Jupiter will leave the other two planets in the first week of June, but the New Moon will join Mercury and Venus on June 10 and 11.
Meteor Showers:
Alpha Scorpids
May 4
Waning crescent Moon, 35%
sunlit ZHR = 10 Eta Aquarids
May 5
Waning crescent Moon, 23% sunlit
ZHR = 30+
Radiant: Near the star Antares
Radiant: Near the boundary between Aquarius and Pegasus.
Associated with Comet Halley.
Use this
ZHR = zenithal hourly rate (number of meteors expected to be observed at the zenith in one hour). The
maximum phase of meteor showers usually occurs between 3 am and sunrise. The reason most meteors are observed in the pre-dawn hours is because at that time
we are on the front of the Earth as it rushes through space at 107 000 km per hour (30 km per second). We are meeting the meteors head-on, and the speed at
which they enter our atmosphere is the sum of their own speed plus ours. In the evenings, we are on the rear side of the Earth, and many meteors we see at that
time are actually having to catch us up. This means that the speed at which they enter our atmosphere is less than in the morning hours, and they burn up less
brilliantly.
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:
Coming later this year: Comet Ison
Comet
LulinThis 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
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. A large and dangerous bushfire swept through the Observatory grounds on the afternoon of January 13, 2013. The outside air temperature as shown by recording thermometers reached 104 degrees Celsius (hotter than boiling water). Though ash and debris entered some of the buildings, the numerous telescopes on the site were undamaged. Unfortunately, the astronomers' living quarters, some outbuildings and the Visitors' Centre were burnt out. The AAT returned to normal operations on the evening of February 14.
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.
The Eta Carinae Nebula, to the right of the Southern Cross tonight.
The Stars and Constellations for this month:
These descriptions of the night sky are for 8 pm on May 1 and 6 pm on May 31. They start at Orion, which is
due west.
This month, Orion
(see below) will be setting on the western horizon. It will not be
visible in June. Canis Major (the Large Dog) is above him, with the
brilliant white star Sirius (Alpha Canis Majoris) showing the Dog's
heart. Sirius, the Dog Star, is the brightest star in the night sky and is about
a handspan above the western horizon.
Above the Dog is the constellation, Puppis, the Stern (of the ship, Argo).
Approaching the north-western horizon is the constellation of Gemini, the Twins. The two twin stars at its eastern end,
Pollux and Castor, are very
distinctive, Pollux being brighter than Castor. Both of these stars will have set by 9.30 pm on May 1.
A handspan above and to the left of Pollux and Castor, is the first magnitude star
Procyon, which is the brightest star in the constellation Canis Minor (the Small Dog). High in the north is another zodiacal constellation, Leo, the Lion. The bright star
Regulus (Alpha Leonis) marks the Lion’s heart, and is on the
left-hand side of the constellation, in the north-north-west. Denebola, the star marking the
root of the lion's tail, is approaching culmination. Between Gemini and Leo is the faint constellation of Cancer the Crab. Though a fairly unremarkable constellation in other ways, Cancer does
contain a large star cluster called Praesepe or the Beehive, which is a good sight in binoculars. Skimming the northern horizon is the constellation of Ursa Major, the Great Bear. Known in the northern hemisphere as the 'Big Dipper' or 'The Plough',
it always appears to us upside-down. We only see Ursa Major at this
time of year, and it is always very low in the north. It can never be seen from the southern states of Australia. The further north an observer goes, the higher Ursa Major
will appear above the northern horizon. If the observer travels to Europe or North America, the Great Bear will always be seen in the night sky,
circling the Pole Star, Polaris, as it is circumpolar from those latitudes.
High in the north-east is a particularly beautiful orange star with a fine name:
Arcturus, meaning 'the follower of the Bear'.
This is the third brightest star (after Sirius and Canopus). It is a K2 star of magnitude -0.06, and lies at a distance of 36 light years. It is
the brightest star in the constellation Boötes the Herdsman, and therefore has the alternative name of
Alpha Boötis.
Just appearing above the horizon, slightly to the east of Boötes, is a circle of stars called
Corona Borealis, the Northern Crown. The brightest
star in the crown is called Alphecca, and it shines at magnitude 2.3. Between Leo and Arcturus may be seen a large Y-shaped cluster of faint stars. This is
Coma Berenices, the Hair of Berenice. Its chief claim to fame is that it is near
the northern galactic window (see below), and a small telescope can detect dozens of galaxies in this area. Large telescopes equipped with sensitive
cameras can detect millions of galaxies in this part of the sky. About 70 degrees above the eastern horizon is the next zodiacal constellation after Leo,
Virgo, the Virgin. The brightest star in Virgo is Spica, an
ellipsoidal variable star whose brightness averages magnitude 1. This makes it
the sixteenth brightest star, and its colour is blue-white.
Above Spica and almost directly overhead is the constellation Corvus the Crow, shaped like a small quadrilateral of magnitude 3 stars. A large but faint constellation,
Hydra
the Water-snake, winds its way from near Procyon west of the zenith and around Corvus and Virgo to
Libra, which is now above the eastern horizon. Hydra
has one bright star, Alphard, mv=2.2. Alphard is an orange star that
was known by Arabs in ancient times as ‘The Solitary One’, as it lies in an area of sky with no bright stars nearby. Tonight it is about 25 degrees
north-west of the zenith. About a handspan to the south-east of Alphard is a bright planetary nebula, the 'Ghost of Jupiter' NGC 3242. It is the remnant left when
the central star exploded (below).
Above the east-south-eastern horizon is Scorpius, the Scorpion. This famous zodiacal constellation is like a large letter
'S', and, unlike most constellations, is easy to recognise as the shape of a scorpion. At this time of year, he has his tail down and claws raised. Between Scorpius and Virgo is the fainter zodiacal constellation of
Libra, the Scales.
The brightest object in Libra this year is the planet Saturn, presently
located about seven degrees north-west of the star Zuben Elgenubi.
On the eastern horizon, another
fainter constellation, Ophiuchus, the Serpent Bearer, is nearly completely risen. Both of these constellations are completely outshone by
brilliant Scorpius and, later in the night, Sagittarius, which is below the Scorpion. High in the south-south-east, Crux (Southern Cross) is at an angle of about sixty degrees.
Herschel's Jewel Box
The two Pointers
Alpha and Beta
Centauri lie below Crux and to the left. Crux will have rotated clockwise to a
vertical position by 9.00 pm at mid-month. Surrounding Crux on three sides is
the large constellation Centaurus, its two brightest stars being the
Pointers of the Southern Cross, brilliant Alpha and Beta Centauri. Beta is the one nearer to Crux.
At left - the two Pointers, Alpha and Beta Centauri. Centre - Crux (Southern Cross)
with the dark cloud of dust known as the Coalsack at its lower left. Right - star clusters in the Milky Way and the Eta Carinae nebula. Slightly to the right and below Crux is a small, fainter quadrilateral of stars,
Musca, the Fly. Out of
all the 88 constellations, it is the only insect. Below Alpha Centauri is a (roughly) equilateral triangle of 4th magnitude stars. This is the constellation
Triangulum Australe, the Southern Triangle. It is well above the south-south-eastern horizon. Between
Crux and Sirius is a very large area of sky filled with interesting objects. This was once the 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). Two
handspans south of Sirius is the second brightest star in the night sky, Canopus (Alpha Carinae). Although appearing almost as bright as Sirius but a little more
yellow, the two stars are entirely dissimilar. Sirius is a normal-sized star that is bright because it is close to us - only 8.6 light years away. Canopus,
on the other hand, is a F0 type supergiant, over 100 times brighter than Sirius, but 36 times further away (312 light years). On the border of Carina and Vela is the
False Cross, larger and more lopsided than the Southern Cross. The
False Cross is two handspans to the right of Crux, and is also lying tilted to the left at this time of year. It has passed culmination, and is beginning to head for
the south-south-western horizon. Both of these Crosses are actually more like kites in shape, for, unlike
Cygnus (the Northern Cross) they have no star
at the intersection of the two cross arms. Between
the Southern Cross and the False Cross may be seen a glowing patch of light. This is the famous
Eta Carinae Nebula, which is a remarkable sight
through binoculars or a small telescope working at low magnification.
It is a
turbulent area of dark dust lanes and fluorescing gas. There is a peculiar unstable star in its centre.
The Keyhole, a dark cloud
obscuring part of the Eta Carinae
Nebula
The Homunculus, a tiny planetary nebula ejected by the eruptive variable star, Eta Carinae
Extremely close to the south-south-western horizon and soon to set is Achernar, Alpha Eridani. It is the brightest star in Eridanus the River, which winds its way with faint stars from Achernar around the south-western horizon to Cursa, a mv= 2.9 star close to brilliant Rigel in Orion. At magnitude 0.49, Achernar is the ninth brightest star.High in the south-south-west, about 25 degrees above the horizon, the Large Magellanic Cloud (LMC) is faintly visible as a diffuse glowing patch. It is about a handspan south of Canopus. About a handspan below and to the left of the LMC is the Small Magellanic Cloud (SMC), a smaller glowing patch, not far above the horizon. From Nambour's latitude, these two clouds never set. Each day they circle the South Celestial Pole, which is a point in the 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 in actual fact nearby dwarf galaxies and are described below.
The line of the ecliptic along which the Sun, Moon and planets travel passes through the following constellations this month: Gemini, Cancer, Leo, Virgo, Libra, Scorpius and Sagittarius.
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 .
The Season of the Lion
We see Leo the Lion upside-down from the Southern Hemisphere. Its brightest star is Regulus, which means 'the King star'. Regulus is the highest star in a pattern called 'The Sickle' (or reaping-hook). It marks the top of the Sickle's handle, with the other end of the handle, the star Eta Leonis, directly underneath. The blade of the Sickle curves around clockwise from Eta Leonis. The Sickle forms the mane and head of the lion. The star Denebola, a handspan east of Regulus, marks the tip of the lion's tail. The planet Saturn is currently between the Lion's legs.
About four degrees to the right and below Eta Leonis is a beautiful double star, Algieba or Gamma Leonis. With a total magnitude of 2.61, the two stars are only 4.3 arcseconds apart, and may be distinguished with a small telescope. Both are orange in colour.
There are also numerous galaxies in this area of the sky. On one of Leo's back legs, the three galaxies M65, M66 and NGC 3628 can be viewed together in the same low-power telescopic field.
Between Leo and the northern horizon is a faint grouping of three fourth magnitude stars. This is the small and inconspicuous constellation of Leo Minor, the small lion. Leo Minor is halfway between Leo and Ursa Major.
The Hunter and his Dogs
Two of the most spectacular constellations in the sky may be seen low in the western sky as soon as darkness falls. These are Orion the Hunter, and his large dog, Canis Major.
Orion: 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.
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.
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, rises due east and sets due west.
The star Mintaka is actually less than 18 arcminutes south of the celestial equator.
At the beginning of May he is close to the horizon. The central part of Orion, popularly called 'The Saucepan', is very easy to recognise and is due west tonight.
The red supergiant star Betelgeuse
To the south of the Belt, at a distance of about one Belt-length, we see another faint group of stars in a line, fainter and closer together than those in the Belt. This is the Sword of Orion. Orion’s two feet are marked by the hot, blue-white stars, 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.
The stars forming the Belt and Sword are popularly known in Australia as ‘The Saucepan’, with the Sword forming the Saucepan’s handle. Tonight this asterism appears right-side up, as in the photographs above. The faint, fuzzy star in the centre of the Sword, or the Saucepan's handle, is a great gas cloud or nebula where stars are being created. It is called the ‘Great Nebula in Orion’ or ‘M42’ (number 42 in Messier’s list of nebulae). A photograph of it appears below:
The Sword of Orion, with the Great Nebula, M42, at centre
The central section of the Great Nebula in Orion.
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 upper left of Orion as twilight ends, 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 below Sirius.
The hindquarters of the Dog are indicated by a large right-angled triangle of stars located to the upper left of Sirius and tilted. The end of his tail is the upper left corner of the triangle, about one handspan south (to the upper left) of Sirius. It is marked by the star Aludra (Eta Canis Majoris).
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.
At the onset of darkness, this small constellation is about 45 degrees (about two handspans) above the northern horizon. It contains only two main stars, the brighter of which is Procyon (Alpha Canis Minoris). This yellow-white star of mv= 0.5 forms one corner of a large equilateral triangle, the other two corners being the red Betelgeuse and white Sirius. Beta Canis Minoris is also known as Gomeisa, a blue-white star of mv= 3.1.
Some fainter constellations
Between the two Dogs is the constellation Monoceros the Unicorn, undistinguished except for the presence of the remarkable Rosette Nebula. To the left of Orion is a small constellation, Lepus the Hare. Between Lepus and the star Canopus is the star group Columba the Dove. Between Corvus and the star Regulus are two faint constellations, Crater the Cup and Sextans the Sextant. Between the zenith and the south-western horizon are a number of small, faint constellations: above the Milky Way are Antlia and Pyxis, while Volans and Mensa are below it. The LMC lies in the constellation Dorado, and the South Celestial Pole is in the very faint constellation 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 red star Gacrux) down through its base (the star Acrux) and continue straight on towards the south for another four Cross lengths. This will locate the approximate spot. There is no bright star to mark the Pole, whereas in the northern hemisphere they have Polaris (the Pole Star) to mark fairly closely the North Celestial Pole.
Interesting photographs of this area can be taken by using a camera on time exposure. Set the camera on a tripod pointing due south, and open the shutter for thirty minutes or more. The stars will move during the exposure, being recorded on the film as short arcs of a circle. The arcs will be different colours, like the stars are. All the arcs will have a common centre of curvature, which is the south celestial pole.
A wide-angle view of trails around the South Celestial Pole, with Scorpius and Sagittarius at left, Crux and Centaurus at top, and Carina and False Cross at right.
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 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 Kent (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. Alpha Centauri A and B take 80 years to complete an orbit, but a tiny third component, the 11th magnitude red dwarf Proxima, 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 separated them (Antares, Sirius). Even closer double stars cannot be split by the telescope, but the spectroscope can disclose their true nature by revealing clues in the absorption lines in their spectra. These examples are called spectroscopic binaries. In a binary system, closer stars will have shorter periods for the stars to complete an orbit. Eta Cassiopeiae takes 480 years for the stars to circle each other. The binary with the shortest period is AM Canum Venaticorum, which takes only 17½ minutes.
Sometimes one star in a binary system will pass in front of the other one, partially blocking off its light. The total light output of the pair will be seen to vary, as regular as clockwork. These are called eclipsing binaries, and are a type of variable star, although the stars themselves usually do not vary.
Star Clusters
The two clusters in Taurus, the Pleiades and the Hyades, are known as Open Clusters or Galactic Clusters. The name 'open cluster' refers to the fact that the stars in the cluster are grouped together, but not as tightly as in globular clusters (see below). The stars appear to be loosely arranged, and this is partly due to the fact that the cluster is relatively close to us, i.e. within our galaxy, hence the alternate name, 'galactic cluster'. These clusters are generally formed from the condensation of gas in a nebula into stars, and some are relatively young.
The photograph below shows a typical open cluster, M7*. It lies in the constellation Scorpius, just below the scorpion's sting. It lies in the direction of our galaxy's centre. The cluster itself is the group of white stars in the centre of the field. Its distance is about 380 parsecs or 1240 light years.
Galactic Cluster M7 in Scorpius
Outside the plane of our galaxy, there is a halo of Globular Clusters. These are very old, dense clusters, containing perhaps several hundred thousand stars. These stars are closer to each other than is usual, and because of its great distance from us, a globular cluster gives the impression of a solid mass of faint stars. Many other galaxies also have a halo of globular clusters circling around them.
The largest and brightest globular cluster in the sky is NGC 5139, also known as Omega Centauri. It has a slightly oval shape. It is an outstanding winter object, and this month it is readily observable.
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 globular cluster Omega Centauri
The central core of Omega Centauri
There are about 145 globular cluster in a halo surrounding our galaxy. Many other galaxies are also surrounded by a large family of globular clusters.
* 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
A little more than a handspan above the south-south-western horizon, and below and to the left of Canopus, a large smudge of light may be seen. This is the Large Cloud of Magellan, and there is also a Small Cloud close to the horizon. They are known to astronomers as the LMC (Large Magellanic Cloud) and the SMC (Small Magellanic Cloud). The SMC is due south and quite low to the horizon (about ten degrees up), and the LMC is above it and to its right. The LMC is noticeably larger and brighter. They lie at a distance of 200 000 light years, and are about 60 000 light years apart. They are dwarf galaxies, and they circle our own much larger galaxy, the Milky Way. The LMC is slightly closer, but this does not account for its larger appearance. It really is larger than the SMC, and has developed as an under-sized barred spiral galaxy.
From our latitude both Magellanic Clouds are circumpolar. This means that they are closer to the South Celestial Pole than that Pole's altitude above the horizon, so they never dip below the horizon. They never rise nor set, but are always in our sky. Of course, they are not visible in daylight, but they are there, all the same.
The Large Magellanic Cloud - the bright knot of gas to left of centre is the famous Tarantula Nebula (below)
These two Clouds are the closest galaxies to our own, but lie too far south to be seen by the large telescopes in Hawaii, California and
Arizona. They are 15 times closer than the famous Andromeda and Triangulum galaxies, and so can be observed in much
clearer detail. Our great observatories in Australia, both radio and optical, have for many years been engaged in important research involving these, our
nearest inter-galactic neighbours.
Why are some constellations bright, while others are faint ?
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 not well placed for viewing this month, being below the horizon until well after midnight, but when it is higher many distant galaxies can be observed in this area of the sky. The northern window is between the constellations Virgo and Coma Berenices, roughly between the stars Denebola and Arcturus. It is ideally placed for viewing in May, culminating at 8.45 pm at mid-month. The winter months are best for observing galaxies in this window, particularly those in the Virgo cluster.
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.
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