Grus, Phoenix, Tucana, Pavo, Indus,
G
rus, the Crane, is much the most prominent of the
four Southern Birds; one way to identify it is to contin-
ue the line from ·and ‚Pegasi, in the Square, through
Fomalhaut. The line of stars running from Áthrough ‚
and on to Âand ̇really does give some impression of a
bird in flight. The little pairs making up ‰and Ìgive the
impression of being wide doubles, though both are due to
nothing more than line-of-sight effects.
Of the two leaders of the Crane, ·(Alnair) is a bluish-
white B-star, 150 light-years away and 100 times as lumi-
nous as the Sun. ‚(Al Dhanab) is an M-type giant, 228
light-years away and 750 Sun-power. The two are almost
equally bright, and the contrast between the steely hue of
Alnair and the warm orange of Al Dhanab is striking in
binoculars – or even with the naked eye. Grus contains a
number of faint galaxies, but there is not much of interest
here for the user of a small telescope.
Phoenixwas the mythological bird which periodically
burned itself to ashes, though this did not perturb it in the
least and it soon recovered. ·(Ankaa) is the only bright star;
it is of type K, decidedly orange, lying at a distance of 78
light-years. It is 75 times as luminous as the Sun. It makes
up a triangle with Achernar in Eridanus and Al Dhanab in
Grus, which is probably the best way to identify it.
The main object of interest is ̇Phoenicis, which is a
typical Algol eclipsing binary with a range from magni-
tude 3.6 to 4.4; the variations are easy to follow with the
naked eye, and there are suitable comparison stars in ‚
(3.31), ‰(3.95) and Ë(4.36). Both components are of type
B. This is actually the brightest of all stars of its kind apart
from Algol itself and ÏTauri.
The interesting variable SX Phoenicis lies less than
seven degrees west of Ankaa. It is a pulsating star of the
‰Scuti type, with the remarkably short period of only
79 minutes, during which time the magnitude ranges
between 7.1 and 7.5 – though the amplitude is not constant
from one cycle to another. The spectrum, too, is variable,sometimes being of type A and at others more like type F.
The distance is no more than 150 light-years, and the
luminosity is roughly twice that of the Sun. Stars of this
type are sometimes known as dwarf Cepheids. SX itself
forms a triangle with Ankaa and È(4.71), but the field is
not very easy to identify without a telescope equipped
with good setting circles.
Tucana, the Toucan. Though the dimmest of the Southern
Birds, Tucana is graced by the presence of the Small
Magellanic Cloud and two superb globular clusters. The
brightest star is ·, which is of type K and is decidedly
orange; ‚is a wide double in a fine binocular field. The
fainter component is a close binary.
The Small Cloud is very prominent with the naked
eye; it is further away than the Large Cloud, but the two
are connected by a ‘bridge’ of material, and are no more
than 80,000 light-years apart. The Small Cloud contains
objects of all kinds, including many short-period variables- in fact it was by studying these, in 1912, that Henrietta
Leavitt was able to establish the period–luminosity re-
lationship which has been so invaluable to astronomers.
It has been suggested that the Small Cloud may be of com-
plex form, and that we are seeing it almost ‘end-on’.
Almost silhouetted against the Cloud is NGC104
(47 Tucanae), the brightest of all globular clusters apart
from ˆCentauri. It has even been claimed that 47 Tucanae
is the more spectacular of the two, because it is small
enough to be fitted into the same moderate-power tele-
scopic field. It is surprisingly poor in variable stars, but
there are several of the ‘blue stragglers’ referred to earlier;
photographs taken with the Hubble Space Telescope
resolve the cluster right through to its centre. It is about
15,000 light-years away. Telescopically, or even with
binoculars, it is evident that its surface brightness is much
greater than that of the Small Cloud.
NGC362 (C104) is another globular cluster in the same
region. It is close to naked-eye visibility, and telescopically
ATLAS OF THE UNIVERSE
The region of the
‘Southern Birds’ is apt
to be somewhat confusing,
because only Grus is
distinctive, and the other
Birds are comparatively
ill formed – though in Tucana
we find the Small Cloud of
Magellan together with the
splendid globular cluster
47 Tucanae. However,
the fact that Achernar
lies nearby is a help in
identification. The other
constellations in this map –
Indus, Microscopium and
Sculptor – are very obscure.MagnitudesVariable starGalaxyPlanetary nebulaGaseous nebulaGlobular clusterMagellanic Cloud–1
0 1 2 3 4 5SCULPTOR
GRUS
PHOENIXINDUS
MICROSCOPIUM
TUCANA
PAVO
ERIDANUSAchernar362
·‚‰
Á·
Î ÂÈÌ Ï
Á ‚„ ‰
̇ Ë·
ÁÂ
‚ ̇ı
È
‚Â
̇·‰‰ ÌÏÁÁ
‚
·ı ̇·Ëı
̉  ‚ · Á ‚ ‰ ÂÏ
Î̇Í
ˉ1
2
SXSXS6752
HOROLOGIUM
HYDRUS
OCTANSTELESCOPIUM
Î ·̄Tucanae
SMC104 + 47
55
300
253
SR̄ÓÍGb Atl of Univ Phil'03stp 2/4/03 7:43 pm Page 258