medicine was legendary. He was able even to restore the
dead to life, and this angered the God of the Underworld,
whose realm was starting to become depopulated. Jupiter
reluctantly disposed of Aesculapius by striking him with a
thunderbolt, but then relented sufficiently to transport him
to the sky.
Serpens. Of the two halves of Serpens, the head (Caput)
is much the more conspicuous, and there is one fairly
bright star, the reddish ·or Unukalhai: magnitude 2.65,
distance 88 light-years, luminosity 90 times that of the
Sun. The actual head is formed by a little triangle of
stars; Î(magnitude 4.09), ‚(3.67) and Á(3.85). Directly
between ‚and Áis the Mira variable R Serpentis, which
can rise to naked-eye visibility when at its best but which
becomes very faint at minimum. Like almost all members
of its class, it is very red. Its period is only nine days
less than a year, so that when it peaks at times when it is
above the horizon only during the hours of daylight, the
maxima are virtually unobservable for several consecutive
years. The date of maximum in 1994 was 25 March.
M5, which lies some way from Unukalhai, is one of
the finest globular clusters in the entire sky; only ˆ
Centauri, 47 Tucanae, M13 in Hercules and M22 in
Sagittarius are brighter. It is very evident in binoculars,
and has been known ever since 1702, when Gotfried Kirch
discovered it. M5 is easy to resolve; the distance is 27,000
light-years, and, unlike M13, it is particularly rich in
variable stars.
The Serpent’s body (Cauda) is less prominent, and
the brightest star, Ëor Alava, is only of magnitude 3.26
(distance is 52 light-years, luminosity 17 times that of the
Sun). However, Cauda does contain two objects of note.
One is M16, the Eagle Nebula, in which is embedded the
cluster NGC6611. It is on the fringe of the constellation,
adjoining Scutum, and in fact the guide star to it is ÁScuti,
magnitude 4.7. The Eagle is a large, diffuse area of
nebulosity, while the cluster is reasonably well marked.
The two are not difficult to locate, but of course photo-
graphs are needed to bring out their full glory; there is a
mass of detail, and there are areas of dark nebulositySTAR MAP 10
BRIGHTEST STARS
No. Star R.A. Dec. Mag. Spectrum Proper name
hm s ° ‘ “
55 · 17 34 56 12 33 36 2.08 A5 Rasalhague
35 Ë 17 10 22 15 43 30 2.43 A2 Sabik
13 ̇ 16 37 09 10 34 02 2.56 O9.5 Han
1 ‰ 16 14 21 03 41 39 2.74 M1 Yed Prior
60 ‚ 17 43 28 04 34 02 2.77 K2 Cheleb
27 Î 16 57 40 09 22 30 3.20 K2
2 Â 16 18 19 04 41 33 3.24 G8 Yed Post
42 ı 17 22 00 24 59 58 3.27 B2
64 Ó 17 59 01 09 46 25 3.34 K0
Also above magnitude 4.3: 72 (3.73), Á(3.75), Ï(Marfik) (3.82), 67 (3.97), 70 (4.03), Ê(4.28), 45 (4.29).
VARIABLES
Star R.A. Dec. Range Type Period Spectrum
hm °’ (mags) (d)
̄ 16 27.0 18 27 4.2–5.0 Irregular – B
U1 7 16.5 01 13 5.9–6.6 Algol 1.68 BB
X1 8 38.3 08 50 5.9–9.2 Mira 334 MK
RS 17 50.2 06 43 5.3–12.3 Recurrent -– OM
nova
DOUBLES
Star R.A. Dec. P.A. Sep. Mags
hm °’ ° “
Ú 16 25.6 23 27 344 3.1 5.3, 6.0
Ë 17 10.4 15 43 247 0.5 3.0, 3.5 Binary, 64 years
Difficult test
CLUSTERS AND NEBULAE
M NGC R.A. Dec. Mag. Dimensions Type
hm °’ ’
9 6333 17 19.2 18 31 7.9 9.3 Globular cluster
10 6254 16 57.1 04 06 6.6 15.1 Globular cluster
12 6218 16 47.2 01 57 6.6 14.5 Globular cluster
14 6402 17 37.6 03 15 7.6 11.7 Globular cluster
19 6273 17 02.6 26 16 7.1 13.5 Globular cluster
62 6266 17 01.2 30 07 6.6 14.1 Globular cluster
107 6171 16 32.5 03 13 8.1 10.0 Globular cluster
6633 18 27.7 06 34 4.6 27 Open cluster
IC4665 17 46.3 05 43 4.2 41 Open clusterBRIGHTEST STARS
No. Star R.A. Dec. Mag. Spectrum Proper name
hm s ° ‘ “
(Caput)
24 · 15 44 16 06 25 32 2.65 K2 Unukalhai
(Cauda)
58 Ë 18 21 18 02 53 56 3.26 K0 Alava
63 ı 18 56 13 04 12 13 3.4 (combined) A5A5 Alya
Also above magnitude 4.3: Caput: Ì (3.54), ‚ (3.67), Â (3.71), ‰ (3.80), Á (3.85), Î (4.09); Cauda: Í (3.54), Ô (4.26).
VARIABLE (Caput)
Star R.A. Dec. Range Type Period Spectrum
hm °’ (mags) (d)
R1 5 50.7 15 08 5.1–14.4 Mira 356 M
DOUBLES
Star R.A. Dec. P.A. Sep. Mags
hm °’ ° ”
‰ 16 34.8 10 32 177 4.4 4.1, 5.2 Binary, 3168y
(Caput)
ı 18 56.2 04 12 104 22.3 4.5, 4.5 (Cauda)
CLUSTERS AND NEBULAE
M NGC R.A. Dec. Mag. Dimensions Type
hm °’ ’
5 5904 15 18.6 02 05 5.8 17.4 Globular cluster
(Caput)
16 6611 18 18.8 13 47 35 28 Nebula and cluster
(Cauda) Eagle
Nebula and
cluster NGC6611OPHIUCHUSSERPENStogether with small, circular ‘globules’, which will eventu-
ally condense into stars.
The other main feature of Cauda is ı(Alya), a particu-
larly wide and easy double. The components are identical
twins, each of magnitude 4.5 and of spectral type A5. With
the naked eye, ıappears as a single star of magnitude 3.4,
but good binoculars will show both components. Of course
they make up a genuine binary system, but they are a
long way apart – perhaps 900 times the distance between
the Earth and the Sun, so that the revolution period is
immensely long. The distance from us is just over 100
light-years. It may be said that ıSerpentis is one of the
best ‘demonstration doubles’ in the entire sky. To find it,
follow through the line of ı, Ëand ‰Aquilae (Star Map 8);
if this is prolonged for an equal distance beyond, it will
reach ıSerpentis.Ga Atl of Univ Phil'03stp 2/4/03 7:38 pm Page 237