SkyandTelescope.com August 2014 57Sue French
Summer
Christmas Treebqah+m1529SAGITTARIUSLagoon
NebulaTrifid
NebulaM236520M2165466568
65836595M25654465536624M286642
M22M54M20(^6559) M8
6589
IC 1284
6537
6567
SwSt 1
6629
6644
WNO 21
–20°
18 h 40 m 18 h 20 m 18 h 00 m
–25°
–30°
Star magnitudes
4
3
2
5
6
7
8
Sue French welcomes your comments at [email protected].
Just 1.4° north-northwest of the Lagoon Nebula, the
Tr ifi d Nebula (M20) is another site of recent and ongoing
star formation. I’ve seen it look wonderful in a dark sky
through a 92-mm refractor, but it can be disappointing
even through an 8-inch scope in a suburban sky. At home
my 105-mm refractor isn’t quite up to the task of show-
ing a lot of detail in the Trifi d, so I sketched it with my
130-mm refractor at 117×. Careful study shows that dark
nebulae carve the southern part of the Trifi d into a more
complex shape than its name implies. Although a narrow-
band fi lter may help you discern details here, it won’t off er
much aid when it comes to the nebulosity surrounding
the 7th-magnitude star to the north. This is primarily a
refl ection nebula that scatters light from the A-type super-
giant star at its heart, and such fi lters are designed for
nebulae that emit their own light.
The ancient stars of globular clusters can be seen
in NGC 6553 and NGC 6544, southeast of the Lagoon
Nebula. They share the fi eld of view through my 105-mm
refractor at 28×. NGC 6553 sits 27′ south-southeast of the
double star WNO 21, an unequal pair (magnitudes 6.8
and 8.8) whose companion star lies east-northeast of its
primary. NGC 6553 presents a granular hazy patch about
3 ′ across that grows softly brighter toward its center. An
11th-magnitude star pins its west-southwestern edge.
NGC 6544 lies 36′ northwest of WNO 21. It’s about the
same size as its neighbor, but it appears highly granular
in a fi eld richly speckled with faint stars. At 87× NGC
6544 presents a bright core and seems augmented by
the fi eld stars sprinkled around it. NGC 6553 looks a bit
larger, and a superimposed star gleams in the northwest.
Through my 10-inch scope at 213×, I count 10 stars
freckling the face of NGC 6544, but only a few extremely
faint glints favor NGC 6553. This isn’t surprising once
we realize that the former is 9,800 light-years distant,
whereas the latter is twice as far away.
These globular clusters are indeed timeworn. NGC
6544 is roughly 11 billion years old, while NGC 6553 may
need as many as 13 billion candles on its birthday cake.
Although not as elderly as globular clusters, the central
stars of planetary nebulae are near the ends of their active
lives. According to a 2010 paper in Astronomy & Astrophys-
ics by Walter J. Maciel and colleagues, the age distribution
of these stars peaks at 2 billion to 4 billion years.
One seldom-visited planetary nebula in Sagittarius is
Swings-Struve 1, also called PN G1.5-6.7 or PK 1-6.2. It’s
named for Pol Swings and Otto Struve, who revealed its
Facing page: The area
around M20, the Trifi d Neb-
ula, and M8, the Lagoon,
is awash in both bright and
dark nebulosity. The Trifi d’s
southern half glows reddish
from narrowband emis-
sions, which respond well
to fi lters. The blue north-
ern section is a refl ection
nebula, best viewed without
a fi lter. Right: The author
sketched M8 as seen in her
105-mm refractor at 87× and
M20 as seen in her 130-mm
refractor at 117×.