OF THE ROGER N. CLARK
OF THE ROGER N. CLARK
OF THE ROGER N. CLARK
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--<br />
MI5 (NGC 7078), GLOBULAR CLUSTER<br />
IN PEGASUS<br />
R.A. 21h 30.0m, Dec. 12° 10' (2000.0)<br />
VISUAL ASTRONOMY <strong>OF</strong> <strong>THE</strong> DEEP SKY<br />
Technical. MI5 is an unusual globular cluster<br />
in that it is the only one known to contain<br />
a planetary nebula. The cluster was discovered<br />
by J. D. Maraldi in 1746, but it was not<br />
until 1927 that the small planetary (K 648)<br />
was found, by F.G. Pease on photographs<br />
made with the WO-inch Mount Wilson reflector.<br />
The nebula seems definitely to be a true<br />
member of MI5. The reason only one planetary<br />
nebula is known among the millions of<br />
stars in globular clusters is that planetary<br />
nebulae are extremely short-lived compared<br />
with the age of the ancient stars that populate<br />
the globulars.<br />
MI5 also contains an X-ray source. Some<br />
astronomers have suggested that this source<br />
at the core of the cluster, is a black hole. I<br />
mass is thought to be about 800 suns.<br />
The cluster is 30 000 to 40 000 light-years<br />
away, and its total luminosity is about<br />
200 000 times that of the Sun. It is about 130<br />
light-years in diameter.<br />
15 is one of the brightest globulars<br />
VtSua '<br />
orthern sky. With a total magnitude<br />
In . ilie n .<br />
5 it can be seen by the unaided eye<br />
of cl6 . ,<br />
un er e r<br />
b ' h<br />
ID arc-minutes yields a mean sunace rig t-<br />
f 20. 1 magnitudes per square arcness<br />
0 . .<br />
h b . h<br />
secon cl<br />
near the c.enter.<br />
. . .<br />
A VISUAL ATLAS <strong>OF</strong> DEEP-SKY OBJECTS<br />
xcellent conditions. Its size of about<br />
- though of course It IS muc rig ter<br />
MI5 is very rich, wJt ItS tars tightly<br />
k cl So it takes a medIUm-Size telescope<br />
p ac e .<br />
to e<br />
h 'n good skies the best view was at 188 x.<br />
WC I '<br />
At this power the cluster was resolved and<br />
b gin resolving the stars. Through the 8-<br />
the stars appeared like many pinpoints. In<br />
the drawing the brighter stars are positioned<br />
correctly; the many faint ones are placed only<br />
to show the overall impression. The center of<br />
the cluster is very bright, with the intensity<br />
decreasing rapidly outward. The small<br />
planetary nebula is magnitude 13.8 and only<br />
one arc-second across; I can find no reports of<br />
it ever having been seen visually.<br />
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