Philips Atlas of the Universe

Scorpius, Sagittarius, Corona Australis

S

corpiusis often, incorrectly, referred to as Scorpio.

The leader is Antares, which is sufficiently close to the

celestial equator to reach a reasonable altitude over Britain

and the northern United States – though the extreme south-

ern part of the Scorpion does not rise in these latitudes; the

southernmost bright star, ıor Sargas, has a declination of

almost 
43 degrees. Antares is generally regarded as the

reddest of the first-magnitude stars, though its colour is

much the same as that of Betelgeux. It is interesting to

compare these two red supergiants. Antares is 600 light-

years away, and 12,000 times as luminous as the Sun, so

that it has only about half the power of Betelgeux; it is

slightly variable, but the fluctuations, unlike those of

Betelgeux, are too slight to be noticed with the naked eye.

Antares has a companion which looks slightly green-

ish by comparison. Both are enveloped in a huge cloud of

very rarefied material, detected at infra-red wavelengths.

The brightest part of the cloud associated with ÚOphiuchi

lies less than four degrees to the north-north-west.

The long chain of stars making Scorpius is striking; it

ends in the ‘sting’, where there are two bright stars close

together – Ï(Shaula) and ̆(Lesath). They give the

impression of being a wide double, but there is no true

association, because Lesath is 1570 light-years away,

Shaula only 275. Lesath is extremely luminous, and could

match 9000 Suns, so that it rivals Antares; Shaula – only

just below the first magnitude as seen from Earth – is a

mere 1300 Sun-power. Both Shaula and Lesath are hot and

bluish-white. Antares is flanked by Ùand Û, both above the

third magnitude. Ìand ̇, further south, look like naked-

eye doubles – again a line-of-sight effect. The separation

between the two stars of ̇is nearly 7 minutes of arc; the

fainter of the pair is 2500 light-years away, more remote

than its brighter, orange neighbour.

The scorpion’s head is made up of ‚(Graffias or

Akrab), Óand ˆ. ‚is a fine double, so wide virtually any

telescope splits it; the primary is a spectroscopic binary.

Scorpius is crossed by the Milky Way, and there are

many fine star fields. There are also four clusters in

Messier’s list. M6 and M7 are among the most spectacular

open clusters in the sky, though they are inconveniently

far south from Britain and the northern United States. Both

are easily seen with the naked eye, and can be resolved

with binoculars; M7, the brighter of the two, was

described by Ptolemy as ‘a nebulous cluster following the

sting of Scorpius’. Because of its large size, it is best seen

with a very low magnification. M6, the Butterfly, is also

very prominent, and is further away: 1300 light-years, as

against 800 light-years for M7. Another bright open clus-

ter is NGC6124, which forms a triangle with the Ìand ̇

pairs. It can be detected with binoculars without difficulty.

The other two Messier objects are globulars. M4 is

very easy to locate, because it is in the same binocular

field with Antares and less than two degrees to the west. It

is just visible with the naked eye, and binoculars show

it well; it is one of the closest of all globulars. No more

than 7500 light-years away, it is very rich in variable stars.

M80 is not so prominent, but can be located easily

between Antares and ‚. It is 36,000 light-years away.

More remote than M4, it looks much smaller; it is also

more compact, with a diameter of perhaps 50 light-years.

It is relatively poor in variable stars, but in 1860 a bright

nova was seen in it, rising to the seventh magnitude; in the

lists it is given as T Scorpii. It soon faded away, but in

case it is a recurrent nova, M80 is worth monitoring.

Sagittariusis exceptionally rich, and the glorious star

clouds hide our view of that mysterious region at the

centre of the Galaxy. Since Sagittarius is the southernmost

of the Zodiacal constellations, it is never well seen from

Britain or the northern United States; part of it never rises at

all. The brightest stars are Â(Kaus Australis) and Û(Nunki),

with ·and ‚only just above the fourth magnitude.

‚or Akrab, in the far south of the constellation, is

an easy double, and makes up a naked-eye pair with its

ATLAS OF THE UNIVERSE

 The two southernmost

constellationsof the Zodiac.

From the British Isles or the

northern United States, parts

of Scorpius and Sagittarius

never rise. Antares can be

seen, and is at its best during

summer evenings. From

southern countries Scorpius

passes overhead, and rivals

Orion for the title of the most

glorious constellation in the

sky, while the star clouds

in Sagittarius hide our view

of the centre of the Galaxy.

Scorpius adjoins Libra,

which was once known

as the Scorpion’s Claws,

and the star formerly known

as ÁScorpii has been given

a free transfer, so that it

is now called ÛLibrae.

Magnitudes

Variable star

Galaxy

Planetary nebula

Gaseous nebula

Globular cluster

Open cluster

–1

0 1 2 3 4 5

SAGITTARIUS

LUPUS

CORONA AUSTRALIS

SCUTUM

LIBRA

CAPRICORNUS

SERPENS CAUDA

M8

M20

M25

M7

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Ï

Ì

Ù

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G

SCORPIUS

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RY

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M75

M55

M17

M18

M24

M23

M21

M22

M54 M69

M70

6729

M6

6541

M80

M4

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