not far from the Dragon’s head. It rose to magnitude 1.2,
so that it remains the brightest nova to have appeared in
the northern hemisphere of the sky since Nova Aquilae- During its decline it was strongly green for a while,
and was also unusual inasmuch as it remained a naked-
eye object for several months. It has now faded back to
around its pre-outburst magnitude of 15, and has been
found to be an eclipsing binary with the very short period
of 4 hours 39 minutes. Both components are dwarfs – one
white, one red.
M13, which lies rather more than halfway between
̇and Ë, is the brightest globular cluster north of the
celestial equator; its only superiors are ˆCentauri and
47 Tucanae. M13 is just visible with the naked eye on a
clear night, but it is far from obvious, and it is not sur-
prising that it was overlooked until Edmond Halley
chanced upon it in 1714 – describing it as ‘a little patch,
but it shows itself to the naked eye when the sky is serene
and the Moon absent’. William Herschel was more
enthusiastic about it: ‘A most beautiful cluster of stars
exceedingly condensed in the middle and very rich.
Contains about 14,000 stars.’ In fact this is a gross under-
estimate; half a million would be closer to the truth.
Like all globulars, M13 is a long way away. Its dis-
tance has been given as 22,500 light-years and its real
diameter perhaps 160 light-years, with a condensed central
region 100 light-years across. It lies well away from
the main plane of the Milky Way, so that it has not been
greatly disturbed by the concentration of mass in the
centre of the Galaxy, and is certainly very old indeed.
Binoculars give good views of it, and even a small tele-
scope will resolve the outer parts into stars.
The second globular, M92, lies directly between Ë
and È. It is on the fringe of naked-eye visibility – very
keen-eyed observers claim that they can glimpse it – and
telescopically it is not much inferior to M3. It is rather
further away, at a distance of 37,000 light-years, and in
most respects it seems to be similar to M13, though it
contains a larger number of variable stars.
STAR MAP 9
The area enclosed in
the quadrilateral formed
by imaginary lines joining
Arcturus, Vega, Altair and
Antares is occupied by three
large, dim constellations:
Hercules, Ophiuchus and
Serpens. The region is best
seen during evenings in the
northern summer (southern
winter), but there are no
really distinctive patterns.
Although Hercules is so
extensive it has no star
much brighter than the
third magnitude.BRIGHTEST STARS
No. Star R.A. Dec. Mag. Spectrum Proper name
hm s ° ‘ “
27 ‚ 16 30 13 21 29 22 2.77 G8 Kornephoros
40 ̇ 16 41 17 31 36 10 2.81 G0 Rutilicus
64 · 17 14 39 14 23 25 3.0 (max) M5 Rasalgethi
65 ‰ 17 15 02 24 50 21 3.14 A3 Sarin
67 17 15 03 36 48 33 3.16 K3
86 Ì 17 46 27 27 43 15 3.42 G5Also above magnitude 4.3: Ë(3.53), Í(3.70), Á(3.75), È(3.80), Ô(3.83), 109 (3.84), ı(3.86), Ù(3.89), Â(3.92),
110 (4.19), Û(4.20), 95 (4.27).
VARIABLES
Star R.A. Dec. Range Type Period Spectrum
hm °’ (mags) (d)
· 17 14.6 14 23 3–4 Semi-reg. 100? M
30 (g) 16 28.6 41 53 5.7–7.2 Semi-reg. 70 M
68 (u) 17 17.3 35 06 4.6–5.3 ‚ Lyrae 2.05 BB
DOUBLES
Star R.A. Dec. P.A. Sep. Mags
hm °’ ° “
· 17 14.6 14 23 107 4.7 var, 5.4 Binary, 3600y
red, green
̇ 16 41.3 31 36 089 1.6 2.9, 3.5 Binary, 34.5y
Ú 17 23.7 37 09 316 4.1 4.6, 5.6
CLUSTERS AND NEBULAE
M NGC R.A. Dec. Mag. Dimensions Type
hm °’ ’
13 6205 16 41.7 36 28 5.9 16.6 Globular cluster
92 6341 17 17.1 43 08 5.5 11.2 Globular clusterHERCULES
MagnitudesVariable starGalaxyPlanetary nebulaGaseous nebulaGlobular clusterOpen cluster–1
0 1 2 3 4 5DRACO
CORONA BOREALIS
SERPENS CAPUT
OPHIUCHUS
AQUILA
SERPENS CAUDA
HERCULES
LYRA
VegaAlphekkaM57
M13
Èı Ú ËÛÙ
ÊÂ ̇‰
Ì ÏÍÓ95
102‚
Á·10911111030 (g)68(u)M92
Á ‚Í
ÓËı‚‰Î
Á ‚̄·
‰ Ì
ÂÏÈ
΂ Û
ÁË
Ïı
‰̇Â
· Rasalgethi
Rasalhague‰ ̇ ÎÔı‚
 ‰ÁBOÖTES
Á ‚Ga Atl of Univ Phil'03stp 2/4/03 7:37 pm Page 235