40 ASTRONOMY • JANUARY 2022
52 Cygnus X-1
A black hole on a list of the best deep-sky targets? That
sounds like an oxymoron! But for the observer who likes
to track down the unusual, this is an ideal target. In a
telescope, Cygnus X-1 looks like another faint star among
many — and to be honest, this is true. But astronomical
research shows us that this seemingly insignificant star
is so much more.
The first indication it was peculiar was noted in 1964,
when a pair of rocketborne Geiger counters discovered
an X-ray source. Astronomers traced the X-rays to the
9th-magnitude star HDE 226868 in Cygnus. Its X-1 desig-
nation refers to its identification as the first X-ray source
in that constellation.
We now know Cygnus X-1 contains a black hole and a
star in a binary system. The star is also classified as the
variable star V1357 Cygni, a hot supergiant (think Rigel)
with an O9.7 spectral classification. Its brightness ranges
from magnitude 8.72 to 8.93, though it may fall below
9th magnitude on occasion. Long-term investigations
show these brightness fluctuations occur as the star’s size
varies. When it grows too large, matter is pulled from the
outer layers into an accretion disk around the black hole.
The star has been fading slightly but steadily since 1999.
Cygnus X-1’s claim to fame is that it was the first candi-
date black hole. In December 1974, two famous theoretical
astrophysicists, Stephen Hawking and Kip Thorne, bet each
other magazine subscriptions over whether it was a black
hole. Thorne was pro, Hawking was con. Thorne ultimately
won his one-year subscription to Penthouse magazine.
Hawking would have won four years to Private Eye.
Cygnus X-1 is 7,200 light-years away. While a typical
black hole formed from a collapsing star ranges from
3 to 10 solar masses, this one is an estimated 21 solar
masses — making it the most massive stellar-mass black
hole known. If that isn’t cool enough, scientists recently
found that the black hole is rotating at close to the speed
of light! — A.G.53 The Bubble Nebula
William Herschel first observed NGC 7635 in 1787 as a glow
around the magnitude 8.7 star SAO 20575 (cataloged earlier as
BD+60°2522). In a telescope, the Bubble Nebula looks more like
a comma because the full extent of its spherical shell is so faint
that it was only discovered through photography. This emission
nebula, which is composed of ionized hydrogen energized by an
O-type star, is part of a larger complex of glowing gases.
Viewing the Bubble Nebula requires skies free of light pollu-
tion. A Hydrogen-II filter or UHC filter are helpful in bringing out
detail. If you observe under extremely dark skies, this nebula
can be seen in a 6-inch telescope. It is one of those objects that
benefits from increased aperture.
Finding NGC 7635 is easy — it lies about 0.5° from the
Cepheus border in Cassiopeia. If you can find the bright open
cluster M52, scan less than a degree southwest and you may
find the Bubble’s pale hydrogen glow. It is equidistant from the
HII region NGC 7538, located to the Bubble’s northwest. The
Bubble Nebula is 15' by 8' with a visual magnitude of about 10,
although that light is spread out, making it appear much fainter.
A 7th-magnitude star in the area can interfere with an observer’s
ability to get fully dark adapted.
The star SAO 20575 is the creator of the Bubble Nebula and
is estimated to weigh 45 solar masses. Its peculiar spectral clas-
sification of O6.5 indicates its surface temperature is a whopping
67,000 degrees Fahrenheit (37,200 degrees Celsius). Astronomers
also estimate this star is losing mass at a rate of 1 solar mass
every million years. The Bubble Nebula itself marks the edge of a
shock wave interacting with the hydrogen atoms in the interstellar
medium.
The Bubble Nebula falls within the Cassiopeia OB2 stellar
association — a group of young, hot stars that formed together
and live fast and die hard. The nebula and stellar association lie
some 7,000 to 8,000 light-years from us, in the Perseus Arm. — A.G.ANTHONY AYIOMAMITIS