©2009 Full Circle Magazine.
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September 2009 Vol6 No9
southern stars
by Case Rijsdijk
Should any readers have any questions,
please feel free to email me at:
The month starts with a waxing gibbous
Moon that becomes full on the 4th, with
New Moon following about two weeks later
on the 18th. The actual time of the new
Moon is 8.46pm, so the first crescent will
not become visible till the evening of the
19th, the sighting of which marks the start
of Eid.
On the 22nd is the Spring Equinox (Latin for
‘equal night’) – the time when the length of the
day is the same as the length of the night.
Because it is “‘equal night’ you might expect that
the length of day and night should be equally 12
hours. Instead, daylight lasts for an extra eight
minutes due to the size of the Sun and the
bending of light (refraction) … the actual 12-
hour day occurred on the 19th!
Some regard this as the beginning of Spring,
while others say that 1 September marks the
beginning of Spring. I won’t enter this debate,
but it is the time of the year when the days are
getting longer most rapidly in the Southern
Hemisphere. Careful observers will also notice
that the Sun rises to the south of east and sets
to the south of west from now on. However, this
Equinox does mark the end of the southern
winter-half of the year, lasting 186.4 days; and
the start of the northern winter-half of the year,
lasting 178.8 days. The colder half of the year
lasts about 7.5 days longer for us because the
Earth’s orbit is not circular but elliptical (slightly
egg-shaped) and the Earth travels more slowly
at this part of it orbit.
Mercury is visible just after sunset until the
20th when it sets with the Sun, after which it
becomes a morning object, rising early in the
dawn sky. Jupiter is well up in the evening sky
and sets as the Sun rises. The other planets are
morning objects: Venus unmistakably bright in
the east; Mars visible much earlier; and Saturn
rising a little before dawn.
The evening sky to the west shows the
Scorpion heading for the horizon, with the red
supergiant star Antares at its heart … it sets
around midnight. To the left (south) of
Scorpious, the Southern Cross and the Pointers
will be earning their name of Thutlwa, or the
‘Giraffe Stars’ as they ‘graze’ the tops of the
trees … this well-known group of stars is low
above the horizon. To the north, the bright star
Vega is unmistakable, low above the horizon. To
the right (east) of this is Deneb, the brightest
star in the constellation of Cygnus, the Swan.
On a par with Rigel, it is one the ‘great stars’: 25
times more massive than the Sun, 60 000 times
more luminous and about 1 500 light-years
away.
The morning sky is truly spectacular, and
early-risers can enjoy some beautiful sights.
Orion is now well up, with isiLimela and the
Hyades to the left. Sirius shines very brightly
further left and forms an equilateral triangle with
Betelgeuse (a red supergiant) and Procyon (the
Little Dog star). Castor and Pollux can be seen
low in the northeast. On the morning of the 10th
a waning gibbous Moon can be seen close to
isiLimela, and by the following morning it will
have passed by. For midnight owls there is a
minor feast! At 1am or later, Orion can be seen
rising, along with Aldebaran (the red giant star in
the small cluster of the Hyades to the left).
isiLimela is further left, and then, low in the
northeast, our twin galaxy, Andromeda, can be
seen. It is the remotest object visible to the
naked eye, 2.3 million light-years away, and a
pair of binoculars will reveal its shape clearly.
T Tauri
It is now fairly well established that stars form
when clouds of gas and dust condense due to
gravitational contraction, and there are many
regions that are classed as ‘stellar nurseries’, the
Great Nebula in Orion being one of them. T Tauri
stars, named after the first stars of their type to
be discovered in the constellation of Taurus, are
typical of an intermediary evolutionary process:
they are stars that are on the point of becoming
what might be termed ‘normal stable stars’.
But, before getting to that stage, they show
considerable variation in brightness over time, as
does the remaining cloud of gas and dust from
which they form. They are today recognized as
young, Sun-like stars a few million years old,
starting their life of about 10 billion years.
Infrared imaging has shown that T Tauri itself is
probably part of a multiple-stars system. The
image here shows the bright, orange-coloured T
Tauri in the centre, about 400 light-years away,
surrounded by a dusty yellow cloud. The
stunning image shown spans a region about four
light-years across: the distance from our Sun to
the nearest star.
September
STAR LORE
In Zulu a midwinter Moon is called
unTlangula, meaning ‘The Moon of the Bare
Trees’. A spring Moon is called unCwaba, or
‘new green Moon’ as Ncwaba means ‘to
bloom’.
