Sky and Telescope - July 2015 - 58

OBSERVING

Deep-Sky Wonders

M18 as viewed by Jeremy Perez through his 6-inch Newtonian
reﬂector at 48x magniﬁcation.

count 24 stars loosely scattered across approximately 7½′.
Two richer star clusters bracket M24. To the west we
ﬁnd Messier 23, which blossoms into a populous collection of 9th- to 12th-magnitude stars when seen through
my 130-mm scope at 37×. The cluster spans about ½°
and boasts about 90 stars strung in glittering chains
meandering through the group. Pushing the scope east
from M24 brings me to Messier 25, a lovely cluster of 70
mixed bright and faint stars prettily displayed in sprays
and gently curving lines. The four brightest stars shine
gold, and there's an adorable little D of seven faint stars

Select Objects from the Small Sagittarius Star Cloud
Object

Type

Mag(v)

Size/Sep

at the group's heart. The nice double star 3′ south of the
D is Burnham 966 CD (β966 or BU 966), with a yellow
primary whose companion rests 11″ to its south. Altogether, the stars form a vaguely rectangular gathering
that's about 40′ long, tipped a bit east of north.
The Small Sagittarius Star Cloud plumbs our galaxy
to considerable depth. Its stars spread from about 10,000
to 15,000 light-years away from us. This would include
stars from the Scutum-Centaurus Arm, the major spiral
arm between us and the galactic center. M24 may also
include stars within a minor spiral arm farther inward,
the Norma Arm. Thus, M24 is not a true physical object,
but merely a pile-up of stars along our line of sight.
At a distance of 12,000 light-years, NGC 6603 is actually within the bounds of the Star Cloud. It's diﬃcult
to tell where NGC 6567 ﬁts into the scene. As is often
the case with planetary nebula, even recent distance
estimates are widely varied - in this case from 5,500 to
12,000 light-years. The other objects are in the foreground, and we simply see them projected against M24.
Collinder 469 is about 4,800 light-years distant, while the
dark nebulae are roughly 700 light-years away from us.
Most Messier objects are also listed in the 1888 New
General Catalogue of Nebulae and Clusters of Stars (NGC),
but neither M24 nor M25 is among them. They were
added to the 1908 supplement to the NGC, known as the
Second Index Catalogue of Nebulae and Clusters of Stars
(IC). In the catalog, IC 4715 (M24) is described as a most
extremely large cloud of stars and nebulae and IC 4725
(M25) as a cluster, pretty compressed. We can just call
them beautiful. ✦

M25

Dec.

Messier 24

Star cloud

2.5

2° × 1°

18h 16.8m

-18° 33′

NGC 6567

Planetary Nebula

11.0

12″

18h 13.8m

-19° 05′

Messier 18

Open cluster

6.9

7.2′

18h 20.0m

-17° 06′

Messier 23

Open cluster

5.5

29′

17h 57.1m

-18° 59′

Messier 25

Open cluster

4.6

32′

18h 31.8m

-19° 07′

Angular sizes and separations are from recent catalogs. Visually, an object's size is often
smaller than the cataloged value and varies according to the aperture and magniﬁcation of the
viewing instrument. Right ascension and declination are for equinox 2000.0.

Right: The double star Burnham 966 CD (β966) lies 3′ south of the
D-shaped ﬁgure resting at the heart of open star cluster M25.

July 2015 sky & telescope

β966

POSS-I / STSCI / NATIONAL GEOGRAPHIC SOCIETY / CALTECH/ POSS-II / PALOMAR OBSERVATORY

M18

Sky and Telescope - July 2015

Table of Contents for the Digital Edition of Sky and Telescope - July 2015