Astronomy

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although it’s likely the dense

clouds of Sagittarius hide a few

dozen more.

While the best and the

brightest globular clusters

appear more prominently

in the summer sky, our focus

will be on those often over-

looked specimens best viewed

during the rest of the year.

We will survey both Messier

and NGC objects, all discov-

ered by the 1800s and all

visible through backyard

telescopes.

Autumn

As the more sparsely populated

fall sky replaces the jeweled

treasures of summer, the num-

ber of globular clusters dimin-

ishes. The two premier exam-

ples are M2 in Aquarius and

M15 in Pegasus. Elsewhere, we

find a sprinkle of other Messier

globulars. Greater challenges

loom with a handful of NGC

globular clusters scattered over

this season’s sky.

Let’s start our tour with a

bonus object, in the eastern

corridor of Sagittarius. In an

overlooked corner of this con-

stellation, away from the bright

star clouds and nebulae associ-

ated with summer, is M75.

Within the reach of all but the

smallest telescopes, it’s fairly

tight and consequently not

resolvable into individual stars.

It more or less typifies the pro-

file of an off-season globular.

Not so for M2 and M15,

which are separated by about

13° nearly due north and south.

Through a 6-inch telescope,

both show as concentrated balls

of stars, although M15 is a bit

looser and consequently easier

to resolve into stars inside

its edges. M15, the second-

brightest cluster in our survey,

is astrophysically noteworthy

for two reasons. First, it is one

of only four members of its

class known to host a planetary

nebula: 14th-magnitude Pease

1, discovered spectroscopically

bular clusters

THE GLOBULARS OF AUTUMN

OBJECT CON. R.A. DEC. MAG. SIZE

M2 Aquarius 21h34m –0°49' 6.6 12.9'

M15 Pe g a s u s 21h3 0 m 12 °10 ' 6. 3 12. 3 '

M75 Sagittarius 20h06m –21°55' 8.5 6'

M30 Capricornus 21h40m –23°11' 7.3 11'

M56 Lyra 19h17m 30°11' 8. 3 7.1'

M71 Sagitta 19h54m 18°47' 8.0 7.2'

M72 Aquarius 20h54m –2°32' 9.3 5.9'

NGC 6934 Delphinus 20h34m 7°24' 8.7 5.9'

NGC 7006 Delphinus 21h02m 16°11' 10.5 2.8'

NGC 1466 Hydrus 3h45m –71°40' 11.6 3.5'

NGC 1841 Mensa 4h45m –83°59' 11.4 0.9'

NGC 7492 Aquarius 23h08m –15°37' 11.2 6.2'

NGC 1049 Fornax 2h40m –34°15' 12.6 0.4'

NGC 288 Sculptor 0h53m –26°35' 8.1 13.8'

Key: Con. = Constellation; R.A. = Right ascension (2000.0); Dec. = Declination (2000.0);

Mag. = Magnitude

in 1927. Lost among thousands

of stellar pinpoints, it is next to

impossible to identify visually.

If Pease 1 is not enough of an

oddity, M15 is one of only a few

globular clusters that emit

X-rays. The X-ray radiation

undoubtedly results from stel-

lar material streaming from

one star of a pair onto the

superdense surface of its com-

panion (be it a neutron star or a

black hole), which no optical

telescope will reveal.

M15

KEN SIARKIEWICZ/ADAM BLOCK/NOAO/AURA/NSF