Sky & Telescope - April 2024 - 7

Stupefying Spin
As a retired engineer, I am blown away
by the scientific progress being made
in astronomy. But on a more mundane
level, I am baffled by the response to Dan
Crow's " Speeding Spiral " letter (S&T:
Jan. 2024, p. 6) about galaxy rotational
speed. What baffles me is that the rotational
speed is being measured in km/s
instead of rpm. What did I miss?
Jerry O'Shaughnessy
Vienna, Virginia
Monica Young replies: For a
solid object such as, say, a frisbee,
rotational speed is indeed typically given
in revolutions per minute (rpm). The actual
speed of a particle stuck to the frisbee will
be lower if the particle is near the center and
higher if it's sitting farther out. However, the
Milky Way is not a solid object. The bulge
near the center does experience rotation like
a solid body would, but beyond a certain radius,
the rotation curve flattens out, so that
particles farther out rotate around the center
"
at the same speed. Because the rotation
curve is flat over much of the star-forming
disk, 240 km/s applies to most of the disk.
Welcome Back, Stephen!
It is only fitting that I am " over the
Moon " at the return to S&T of Stephen
J. O'Meara, a great contributor to the
astronomy world. O'Meara's observations
have inspired many and have been
sadly missed in S&T. His Deep-Sky Companions
series is a must in any astronomer's
library. Welcome back, Stephen!
Robert Rukrigl
Severna Park, Maryland
Section Correction
On page 25 of the September 2023
issue, in " Vesto Slipher's Fast Stars and
Hotrod Galaxies, " Douglas MacDougal
writes that Slipher " chaired the Nebular
Section of the International Astronomical
Union. " The IAU indeed oscillated
between Committees and Commissions
(eventually settling for the latter, with
Comm. doing service for both), but it
never had Sections. From the get-go,
Comm. 28 was Nebulae, later Nebulae
and Star Clusters, and finally Galaxies,
but all were led by Presidents, as are current
IAU Commissions and Divisions,
never by chairs or chairmen.
Virginia Trimble
Past President
IAU Comm. 28, Galaxies, 1994-97
Irvine, California
FOR THE RECORD
* The plot on page 19 of the October 2023
issue incorrectly colored the two highredshift
galaxies below and below-right
of GN-z11 gray: These galaxies were not
Hubble candidates but JWST ones and
should have been orange and black.
SUBMISSIONS: Write to Sky & Telescope, 1374 Massachusetts Ave., 4th Floor, Cambridge, MA 02138, USA, or email:
letters@skyandtelescope.org. Please limit your comments to 250 words; letters may be edited for brevity and clarity.
75, 50 & 25 YEARS AGO by Roger W. Sinnott
1949
1974
º April 1949
Telescope Artist " The story is
told that one day when Mrs. Porter
came home she found that in her
absence Russell had cut a hole
in the living room wall so that
he could thrust the barrel of his
telescope through and sit inside
to study the stars out of reach of
mosquitoes. Russell himself refuted
this statement, denying that he ever
cut a hole through the living room
wall - it was through his den!
" In 1921, Porter wrote an article
1999
for Popular Astronomy describing
a simple telescope. Among those
who read it and became enthusiastic
was Albert G. Ingalls, of the Scientific
American magazine. [Porter]
later said, 'It was a fortunate thing
for our fraternity that brought us
together, and it was not long before
the first thin edition of Amateur
Telescope Making was published.' "
Russell W. Porter's obituary,
penned by close friends Leo and
Margaret Scanlon, goes on to
describe his role as both architect
and illustrator for the Hale 200-inch
telescope project.
º April 1974
Truly Dark? " Most people who
believe that nighttime is synonymous
with darkness are surprised
at how bright a clear, moonless
night can be, simply due to natural
sources of light from the night sky
[free of any light pollution or aurorae].
These principal sources are
nighttime airglow (nightglow), the
interplanetary [or zodiacal] light,
and integrated starlight. . . . For a
middle-latitude observatory, each
of these three sources provides on
the average about one-third of the
total light of the night sky.
" The contribution from extragalactic
nebulae (cosmic light) is
probably less than one percent of
the total. "
Robert G. Roosen added that
the richest parts of the Milky Way
are more than 10 times brighter than
the sky at the galactic poles.
º April 1999
Unseen Supernova " Nearly seven
centuries ago a celestial event took
place that ought to have astounded
observers at the time. An inconspicuous
star in the southern constellation
Vela should have brightened
millions of times, rivaling Venus as
the most brilliant point of light in
the evening sky. . . . So it comes as
a surprise to astronomers that no
historical records of this recently
discovered supernova have been
found. . . .
" Bernd Aschenbach (Max Planck
Institute for Extraterrestrial Physics)
uncovered evidence for this stellar
explosion while studying X-ray
images of a much older supernova:
the one that created the . . . Vela
supernova remnant. . . .
" Independent observations by
Anatoli F. Iyudin [and colleagues],
this time using NASA's Compton
Gamma Ray Observatory, [imply]
an age of 680 years and a distance
of 650 light-years. [So] why did the
apparition of RX J0852.0-4622
around A.D. 1320 go unnoticed? "
sk yand tele scope .o r g * APRIL 2024
7

http://skyandtelescope.org

Sky & Telescope - April 2024

Table of Contents for the Digital Edition of Sky & Telescope - April 2024