neighbour. ̇or Ascella is a very close, difficult binary.
The components are almost equal, and the revolution
period is 21 years; the separation is only about 0.3 of a
second of arc, so that a telescope of at least 38-centimetre
(15-inch) size is needed to resolve the pair. Users of tele-
scopes of around this size will find it a useful test object.
There are comparatively few bright variables, but RY
Sagittarii, again in the southern part of the constellation, is
an R Coronae star; usually it is of magnitude 6, but falls to- Unfortunately, its southern declination makes it an
awkward target for British and North American observers.
Sagittarius abounds in clusters and nebulae. There are
three superb galactic nebulae. M20 (the Trifid) and M8
(the Lagoon) are not far from Ïand Ì; they can be seen as
whitish patches in binoculars. Photography brings out their
vivid clouds. Close by is the open cluster M21, easy to
resolve. M17, in the northern section, and known variously
as the Omega, Swan or Horseshoe Nebula, is magnificent.
Of the globular clusters, M22 in particular is very fine, and
was the first member of the class to be discovered (by
Abraham Ihle, as long ago as 1665).
There is a great deal to be seen in Sagittarius, and one
has to be systematic about it; for example, once Ìhas been
identified it is not hard to move on to M25, M17, M21,
M20 and M18, though care must be taken not to confuse
them. Incidentally, M24 is not a true nebular object at all,
but merely a star cloud in the Milky Way – though it does
contain an open cluster, NGC6603, which lies in its north-
ern part. When the star clouds in the area are high above
the horizon, sweeping along them with binoculars or a
low-power telescope will give breathtaking views.
Corona Australis, or Corona Austrinus, is one of
Ptolemy’s original constellations, but no legends appear to
be attached to it. It is small, with no stars above the fourth
magnitude, but the little semi-circle consisting of Á, ·, ‚, ‰
and ıis distinctive enough, close to the relatively obscure
·Sagittarii. Áis a close binary, and makes a useful test
object. NGC6541 is a globular cluster, just detectable with
binoculars; it lies some way from the main pattern,
between ıCoronae and ıScorpii. The variable nebula
NGC6729 surrounds the erratic variable R Coronae
Australis (do not confuse it with R Coronae Borealis). The
changes in the nebula mimic the fluctuations of the star,
but this is faint, below the range of small telescopes.STAR MAP 11
BRIGHTEST STARS
No. Star R.A. Dec. Mag. Spectrum Proper name
hm s ° ‘ “
2 · 16 29 24 26 25 55 0.96 M1 Antares
35 Ï 17 33 36 37 06 14 1.63 B2 Shaula
ı 17 37 19 42 59 52 1.87 F0 Sargas
26 Â 16 50 10 34 17 36 2.29 K2 Wei
7 ‰ 16 00 20 22 37 18 2.32v B0 Dschubba
Î 17 42 29 39 01 48 2.41 B2 Girtab
8 ‚ 16 05 26 19 48 19 2.64 B0+B2 Graffias
34 ̆ 17 30 46 37 17 45 2.69 B3 Lesath
23 Ù 16 35 53 28 12 58 2.82 B0
20 Û 16 21 11 25 35 34 2.85 B1 Alniyat
6 15 58 51 26 06 50 2.89 B1
È^117 47 35 40 07 37 3.03 F2
Ì^116 51 52 38 02 51 3.04 B1
G174951 37 02 36 3.21 K2
Ë 17 12 09 43 14 21 3.33 F2
Also above magnitude 4.3: Ì^2 (3.57), ̇^2 (3.62), Ú(3.88), ˆ^1 (3.96), Ó(4.00), Í(4.16), H (4.16),
N (4.23), Q (4.29).
VARIABLE
Star R.A. Dec. Range Type Period Spectrum
hm °’ (mags) (d)
RR 16 55.6 30 35 5.0–12.4 Mira 279 M
DOUBLES
Star R.A. Dec. P.A. Sep. Mags
hm ° ’ ° “
Í 16 04.4 11 22 051 7.6 4.8, 7.3 A is double
‚ 16 05.4 19 48 021 13.6 2.6, 4.9 A is double
Û 16 21.2 25 36 273 20.0 2.9, 8.5
· 16 29.4 26 26 273 2.7 1.2, 5.4 Binary, 878y,
red, green
CLUSTERS AND NEBULAE
M NGC R.A. Dec. Mag. Dimensions Type
hm ° ’ ’
4 6121 16 23.6 26 32 5.9 26.3 Globular cluster.
6 6405 17 40.1 32 13 4.2 50 Open cluster
(Butterfly)
7 6475 17 53.9 34 49 3.3 80 Open cluster
80 6093 16 17.0 22 59 7.2 8.9 Globular
cluster
6124 16 25.6 40 40 5.8 29 Open cluster
6242 16 55.6 39 30 6.4 9 Open clusterBRIGHTEST STARS
No. Star R.A. Dec. Mag. Spectrum Proper
h m s ° ‘ “ name
20 Â 18 24 10 34 23 05 1.85 B9 Kaus Australia
34 Û 18 55 16 26 17 48 2.02 B3 Nunki
38 ̇ 19 02 37 29 52 49 2.59 A2 Ascella
19 ‰ 18 20 59 29 49 42 2.70 K2 Kaus
Meridonalis
22 Ï 18 27 58 25 25 18 2.81 K2 Kaus Borealis
41 19 09 46 21 01 25 2.89 F2 Albaldah
10 Á 18 05 48 30 25 26 2.99 K0 Alnasr
Ë 18 17 37 36 45 42 3.11 M3
27 Ê 18 45 39 26 59 27 3.17 B8
40 Ù 19 05 56 27 40 13 3.32 K1
Also above magnitude 4.3: Í^2 (3.51), Ô(3.77), Ì(Polis) (3.86), Ú(3.93), ‚^1 (Arkab) (3.93),
·(Rukbat) (3.97), È(4.13), ‚^2 (4.29).SCORPIUSSAGITTARIUSVARIABLES
Star R.A. Dec. Range Type Period Spectrum
hm °’ (mags) (d)
X1747.6 27 50 4.2–4.8 Cepheid 7.01 F
W1805.0 29 35 4.3–5.1 Cepheid 7.59 FG
RS 18 17.6 34 06 6.0–6.9 Algol 2.41 BA
Y1821.4 18 52 5.4–6.1 Cepheid 5.77 F
RY 19 16.5 33 31 6.0–15 R Coronæ – Gp
RR 19 55.9 29 11 5.6–14.0 Mira 335 M
DOUBLES
Star R.A. Dec. P.A. Sep. Mags
hm °’ ° “
Ë 18 17.6 36 46 105 3.6 3.2,7.8
‚^119 22.6 44 28 077 28.3 3.9,8.0 Wide naked-
eye pair
with ‚^2
CLUSTERS AND NEBULAE
M NGC R.A. Dec. Mag. Dimensions Type
hm ° ’ ’
8 6523 18 03.8 24 23 6.0 90 x 40 Nebula
(Lagoon)
17 6618 18 20.8 16 11 7.0 46 x 37 Nebula
(Omega)
18 6613 18 19.0 17 08 6.9 9 Open cluster
20 6514 18 02.6 23 02 7.5 29 x 27 Nebula (Trifid)
21 6531 18 04.6 22 30 5.9 13.0 Open cluster
22 6656 18 36.4 23 54 5.1 24.0 Globular
cluster
23 6494 17 56.8 19 01 5.5 27 Open cluster
24 6603 18 16.9 18 29 4.5 90 Star cloud
25 IC4725 18 31.6 19 15 4.6 31.0 Open cluster
round
28 6626 18 24.5 24 52 6.9 11.0 Globular
cluster
54 6715 18 55.1 30 29 7.7 9.1 Globular
cluster
55 6809 19 40.0 30 58 6.9 19.0 Globular
cluster
69 6637 18 31.4 32 21 7.7 7.1 Globular
cluster
70 6681 18 43.2 32 18 8.1 7.8 Globular
cluster
75 6864 20 06.1 21 55 8.6 6.0 Globular
clusterThe brightest stars are ·(Meridiana) and ‚, each 4.11; the position of ·is R.A. 19h 09m 28s.2,
dec. 37° 54’ 16”. The other stars making up the little semi-circle are Á(4.21), ‰(4.59) and ̇(4.75).
DOUBLES
Star R.A. Dec. P.A. Sep. Mags
hm ° ’ ° “
Î 18 33.4 38 44 359 21.6 5.9, 5.9
Á 19 06.4 37 04 109 1.3 4.8, 5.1 Binary, 12y,
good test
CLUSTERS AND NEBULAE
M NGC R.A. Dec. Mag. Dimensions Type
hm ° ’ ’
6541 18 08.0 43 42 6.6 13.1 Globular
cluster
6729 19 01.9 36 57 var 1 (var) Variable
nebula
round R
Coronae
AustralisCORONA AUSTRALISGb Atl of Univ Phil'03stp 2/4/03 7:41 pm Page 239