Sky & Telescope - December 2019 - 7

size of the Airy disk by about 9%, so you
get a 9% improvement in resolution.
Also, it's well known that blue stars
can be resolved at closer separations
than red or yellow stars can, and γ 2
Andromedae's stars are intensely blue
in color.
J. B. Sidgwick also wrote that some
visual observers put central obstructions
on their refractors to get better resolving
power on some doubles. For example,
S. W. Burnham detected some equal or
nearly equal doubles separated by just
0.2″ with his 6-inch Alvan Clark refractor - a feat that was difﬁcult to equal
even with the 36-inch Lick refractor!
Scattered throughout the astronomical literature are many examples
of resolution below the Dawes limit.
Sometimes resolution and detection
are confused, but Ferguson has done an
excellent job of getting rid of any confusion between them.
Rodger W. Gordon
Nazareth, Pennsylvania

A Big Event (Horizon)
Your article on imaging M87's supermassive black hole (S&T: Sept. 2019,
p. 18) states that it would take light
1½ days to cross this black hole. That
means the distance from its center to its
event horizon is more than four times
the distance from the Sun to Neptune.
Is that right?!
James Folsom
Portola Valley, California

Camille Carlisle replies: That's
correct! Using the calculated mass
of 6.5 billion Suns, M87* would have a
Schwarzschild radius of 128 astronomical
units - compared to Neptune's semimajor
axis of 30 a.u. But this calculation assumes
it's a non-spinning black hole. If M87* spins,
it will drag spacetime closer around it and
effectively shrink the event horizon's size.

1969

1994

FOR THE RECORD

* Two labels in the graph of potentially resolvable supermassive black
hole silhouettes (S&T: Sept. 2019, p. 23)
were duplicated. The labels on the leftmost dotted vertical lines should read,
"0.8 mm (ground+GEO)" and "1.3 mm
(ground+MEO)".

* In "The Great Attractor" (S&T: Oct.
2019, p. 17), Sandra Faber (not Donald
Lynden-Bell) led "The Seven Samurai"
research team.
* In "A Full Deck of Kings" (S&T: Nov.
2019, p. 60), the correct Trumpler classification for King 10 is II1m.

SUBMISSIONS: We welcome your comments and questions! Write to Sky & Telescope, 90 Sherman St.,
Cambridge, MA 02140-3264, USA, or by email to: letters@skyandtelescope.com. Please limit your comments to
250 words; letters may be edited for brevity and clarity.

75, 50 & 25 YEARS AGO by Roger W. Sinnott
1944

Since all the supermassive black holes that
we've measured spins for generally have
rapid rotations, it's likely that the event
horizon for M87* is somewhat smaller than
128 a.u.

º December 1944
Galaxy Centers Resolved "One
of the observational problems that
most astronomers expected could
be solved only with the new 200inch telescope has been successfully tackled with the Mount Wilson
100-inch by Dr. [Walter] Baade.
Making long exposures on red-sensitive photographic plates, he has
[resolved] into stars the nebulous
nucleus of the Andromeda galaxy
and its two companions, Messier 32
and NGC 205. The Grifﬁth Observer
attributes this success to the Los
Angeles-Pasadena dimout, which
allowed longer exposure times than
were previously possible."
The term "dimout" refers to the
nighttime use of blackout curtains
in homes and restrictions on outdoor lighting during World War II.
º December 1969
Messier's Mistake "It can be
simply demonstrated that the lost
Messier object M91 is very prob-

ably the galaxy NGC 4548 . . . M91
was the last of a group of eight nebulae observed by Charles Messier
on the night of March 18, 1781. The
French astronomer described it as
a 'nebula without stars' and fainter
than M90. . . .
"My solution of the puzzle
assumes that Messier determined
the position of NGC 4548 by
measuring its right ascension and
declination relative to those of the
nearby galaxy M89 (since there is
no suitable reference star in the
vicinity) . . . It is further assumed
that, in calculating the coordinates
of the new object, by mistake he
applied the observed differences to
the coordinates of M58, a 9th-magnitude galaxy Messier had recorded
two years earlier. . . . [This switch]
reproduces the [erroneous] Messier
position to 0m.1 in right ascension
and 1′ in declination. . . .
"Since NGC 4548 is probably
the long-lost M91, as a Messier
Club member I would be pleased to
see this fine spiral galaxy returned
to the Messier catalogue."

This reasoning by amateur William C. Williams is now generally
accepted; M91 is missing no more.
º December 1994
Bigger, Biggest "What kind of
large-scale structure exists in
the universe? Are galaxies really
arrayed in great sheets wrapped
around giant voids? . . . In an
attempt to answer these challenging questions, a team of French
astronomers led by Georges Paturel
(Lyon Observatory) has mapped
more than 24,000 galaxies . . .
"When . . . each galaxy is plotted in three dimensions, a striking
pattern emerges. A giant shell
of galaxies stretches more than
650 million light-years [in extent],
roughly centered on the Local
Supercluster of which the Milky
Way is a part. . . . It is currently the
largest known structure in the universe, incorporating the 'Great Wall'
of galaxies discovered in 1989 . . ."
We now know about arrangements of remote quasars stretching
across several billion light-years.

sk yandtele scope.com * D EC E M B E R 2 019

http://www.skyandtelescope.com

Sky & Telescope - December 2019

Table of Contents for the Digital Edition of Sky & Telescope - December 2019