The
Southern Cross was first identified
as a separate constellation by
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
labeled 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 millenia
precession has brought the cross far to the
south; it is no longer visible at latitudes north
of 27 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 principle 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 236 degrees. (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.