Sky & Telescope - July 2024 - 6

RICH TANSEY
FROM OUR READERS
Little Sun Scout
I enjoyed " Make a Solar Finder " by
Jerry Oltion (S&T: Mar. 2024, p. 72). I
thought he might be interested in the
method I used. I took a finder mount
from Orion that used to hold a penlight
laser. Similar to his technique, I
put red tape with a hole in front and
white tape with a spot for Sun alignment
on the back. The advantage is
that it fits on any of my telescopes with
a dovetail bracket.
Rich Tansey * Reno, Nevada
Labeyrie Lowdown
In response to Peter B. van der Wal's
letter " The Bottle of Labeyrie " (S&T:
Mar. 2024, p. 6): Yes, I have been aware
of their existence for many decades. I
believe these two telescopes were proofof-concept
instruments built as prototypes
for a more elaborate interferometer
project. In this 1993 paper, Antoine
Labeyrie and colleagues describe in detail
how the instruments work, including
tracking: https://is.gd/GI2TCalern. The
computer tracking system used for these
telescopes is extremely complex, much
more so than what I came up with in
1994 (S&T: Jan. 1996, p. 33). I wonder
if the French " Grand Interféromètre
à 2 télescopes " prototypes might have
benefited from my dual-axis, ball-scopetracking-platform
design had I published
it years earlier.
Pierre Lemay
Blainville, Quebec, Canada
Tracking T Coronae Borealis
I just read " Get Ready for a Nova's
Bright Return " by Bradley Schaefer
(S&T: Mar. 2024, p. 34) with extreme
interest. I've been an avid amateur
astronomer since my teens, and as a
professional astronomer I work for the
Italian Space Agency.
Back in 1992, I wrote a paper (in
Italian) in the July and August issue
of Astronomia, the magazine of the
Italian Amateur Astronomers Union.
It describes the results of an intensive
observing campaign of T Coronae
Borealis organized in Europe, resulting
6
JULY 2024 * SKY & TELESCOPE
in 884 visual estimates by 10 observers
over a period of 930 days.
The main result was clear evidence
of the orbital motion of the system
causing a modulation in luminosity
induced by the prolateness of the
main star. This effect was shown using
photoelectric photometry by H. C.
Lines et al. in results published in the
May 1988 Astronomical Journal, but
my analysis has shown that carefully
analyzed visual estimates could detect
a modulation of +/- 0.16 magnitudes
over a period of roughly 230 days.
Giuseppe Bianco
Matera, Italy
The Speed of Sound
As someone accustomed to dealing
with sound speeds ranging from more
than 330 m/sec (in freezing air) to
almost 6,000 m/sec (in steel), I was
amazed to learn in " 100,000-LightYear
Bow Shock in the Milky Way
Halo " by Monica Young (S&T: Apr.
2024, p. 11) that the speed of sound in
the Milky Way halo has a value of 165
km/sec. Could you please explain why
this value is so " astronomically " high?
Robert Janus
Hope, Rhode Island
"
Monica Young replies: Great question!
I'm glad you asked it, because
I learned something, too . I asked the
astronomers involved in the research your
question, and here is their reply:
That's a really great question! Robert Janus
has correctly identified that one can get
quite a bit of variety in the speed of sound in
different media, related to how easily compressible
a medium is as well as the density
of the medium. In the case of an ideal gas
like air or the circumgalactic medium surrounding
our Milky Way, the math works out
that the speed of sound is proportional to
the temperature of the gas (specifically, the
square root of the temperature).
When dealing with room temperature
(roughly 300 kelvin) like we experience
here on Earth, one calculates values for the
speed of sound on the order of a few hundred
meters per second. But the circumgalactic
medium is a super hostile and hot
environment, where the gas is flying around
at temperatures more like 1,000,000 kelvin.
As a result, the speed of sound can reach
incredibly high values like the one quoted
in our paper.
And because of the enormous gravitational
potential of the Milky Way, the Large
Magellanic Cloud has been accelerated to
a velocity that is almost two times faster
than that extremely high sound speed,
leading to the supersonic motion that we
predict in our simulation.
Dazzling Dark Energy
Elizabeth Fernandez's " Dark Energy: A
Brief History " (S&T: Feb. 2024, p. 26)
was the most mind-bending, mindexpanding,
thought-provoking article
I've read in a long time! Thank you for
showcasing such profound science.
Thomas Tella
Wolcott, Connecticut
The article on dark energy by Elizabeth
Fernandez in the February issue
of S&T was fascinating and set me to
thinking. The universe, including time
and space, was born in the Big Bang
and has been expanding since then,
and all galaxies are flying away from
each other at ever-increasing speeds.
But where was the universe born? Was
there nothing before the universe was
born? Or were space and time already
present and our universe got its birth
at some unknown coordinates in an
infinite and empty space?
Will such a line of reasoning also
explain that dark energy was already
there and is tearing us apart now that
https://www.is.gd/GI2TCalern
Sky & Telescope - July 2024

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