Crux, the Southern Cross, is visible from latitudes
south of 25 degrees North and completely invisible in latitudes above
35 degrees North. Crux was unknown to the ancient world, Ptolemy drew
it as part of Centaur in the original 48 constellations, The Romans
called it Thronis Caesaris in honor of emperor Augustus, although Crux
is invisible in Italy, it was visible from Alexandaria. Another interesting
sidelight is that Crux was last seen in Jerusalem (latitude 31 degrees,
46 minutes, and 45 seconds) about the time that Christ was crucified.
The invention of Crux is generally attributed to Royer in 1679, but
we know it was written about some two centuries earlier than that. Amerigo
Vespucci has references in his ships logs from 1507 to Crux. Crux was
an aid to early sailors, the southern hemisphere does not have a bright
star near the south pole, so many used Crux which is some 25 degrees
from true south, they would draw a line using the stars in the cross
to the south pole and could then make a determination of their position.
--- Crux, the Southern Cross, is the most familiar
constellation in the southern hemisphere. This tiny constellation (the
smallest in the entire sky) was once part of Centaurus, but the sight
of such a brilliant cross in the sky was so compelling that it became
a constellation of its own in the sixteenth century. Besides the cross
itself, the constellation contains a unique dark nebula, a famous star
cluster, and a remarkable binary. Apart from the four bright stars that
form the cross, the constellation's stars are generally fourth-magnitude.
Note that while gammaA and gammaB are labelled as binary components,
these stars only form an optical double. The two theta stars are also
not gravitationally bound to each other; on the other hand mu1 and mu2
do form a binary system (see below). --------------------------------------------------------------------------------
Thousands of years ago these four stars were an object of reverence
in the countries of the Near East. In the Biblical days, two thousand
years ago, they were just visible at the horizon. Some might find a
religious connotation, linking their disappearance with the Crucifixion
of Christ. Over the millennia precession has brought the cross far to
the south; it is no longer visible at latitudes north of 25 degrees.
It was the European explorers of the early sixteenth
century who "rediscovered" the Southern Cross. For these adventurers
the constellation was an important clock, for when it passed the meridian
it was (more or less) straight up and down. Thus, by studying the constellation's
inclination from the perpendicular, navigators could calculate their
present time. The principal star of note in the constellation is Acrux
(alpha Crucis), a splendid binary (see below). The combined visual magnitude
of both stars results in a magnitude of 0.72. The stars are 320 light
years away, and each is approximately one and a half to twice the size
of our Sun. Alpha Crucis has an apparent proper motion of 236º. (That
is, from our viewpoint, it seems to be moving very slowly in this direction.)
Others in this constellation with similar motions, and therefore part
of a moving star cluster, are beta, delta, zeta, lambda, and mu Crucis.
The group as a whole is quite large, forming what is called the "Scorpio-Centaurus
Association".
Beta Crucis (Mimosa) is the brightest star of the
group, a blue-white giant (nearly five times the Sun's size) with a
visual magnitude of 1.25. The star is an estimated 580 light years away,
and has a luminosity of nearly 8000. The star is a variable (see below)
Gamma Crucis (Gacrux) forms the top of the cross. The reported distance
may be erroneous; it's been calculated from the visual and absolute
magnitudes. The resulting parallax is so large that it should be measurable.
Although gammaA and gammaB have been so named because of a suspected
duplicity (that is, that they form a binary system) the facts are different.
The stars are moving in different directions (174 degrees, 129 degrees)
and are therefore not held together gravitationally. Delta Crucis is
the western arm, very similar in size and distance to alpha Crucis,
and part of the star cluster mentioned above. The star is a beta-CMa
type variable (see below). --------------------------------------------------------------------------------
Double stars in Crux: Alpha Crucis is by far the best of the group:
a splendid binary of equal blue-white stars: 1.58, 2.09; PA 115º, separation
4.4". Beta Crucis has a very faint (11m) companion: PA 322º, separation
44.3". Eta Crucis has a distance companion, rather faint: 3.6, 10; PA
299º, separation 44". Iota Crucis is an easy binary to resolve: 4.7,
7.5; PA 22º, 26.9". Mu1 and Mu2 Crucis form a fixed binary, also an
easy one for small telescopes: 4, 5.2; PA 17º and separation 35". --------------------------------------------------------------------------------
Variable stars in Crux: Crux has four beta CMa type variables (also
called beta Cephei stars). These are very hot giant stars which pulsate
for some inexplicable reason. Their variation is extremely small (from
less than 0.01 to 0.25 magnitudes). Below are listed the beta CMa stars
in Crux and their range. Beta Crucis: 1.23 to 1.31 every 5h40m34s. Delta
Crucis: 2.78 to 2.84 every 3h37m30s. Theta2 Crucis: 4.7 to 4.74 every
2h8m1s. Lambda Crucis: 4.62 to 4.64 every 9h28m57s. Mu2 is a gamma Cas
variable, with a range from 4.99 to 5.18. Finally, R Crucis isn't (as
one might think) a Mira-type long-period variable, but rather a cepheid,
ranging from 6.4 to 7.23 every 5d19h49m5s. --------------------------------------------------------------------------------
Deep Sky Objects in Crux: "Brilliant" is the word usually used to describe
The Jewel Box (NGC 4755)
. Also called the Kappa Crucis star cluster, this open
cluster is composed of over a hundred stars, about fifty of which are
a mixture of colourful supergiants: reds and blues intermingled with
yellows and whites in a profusion of sparkling light. The cluster is
just a baby, perhaps no older than ten million years. Many of the stars
have very high luminosities, approaching 100,000 Suns. The central star
is kappa Crucis, a blue sixth-magnitude supergiant. The cluster is considered
to be from 6800 to 7800 light years away. To locate The Jewel Box, find
beta Crucis and drop down to the southeast one and a half degrees.
The Coal Sack is a large dark nebula only 550 light
years away, just to the south of the Jewel Box,
visible to the naked eye. Dark nebulae are massive clouds of interstellar
gases and dust, dense enough to block out most of the light from stars
behind it. The Coal Sack and Horsehead Nebulae (in Orion) are
the two best known dark nebulae; of all dark nebulae, the Coal Sack
is the largest one visible to the unaided eye. --------------------------------------------------------------------------------
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