Sky & Telescope - December 2021 - 6

CYGNUS LOOP: KEES SCHERER / FLICKR; SKETCH: HOWARD BANICH
FROM OUR READERS
Admiring the Veil
Howard Banich's exquisite sketches
of deep-sky objects published in Sky
& Telescope through the years have
helped me immensely in teasing out
details during visual observing sessions.
Using an 18-inch (46-cm)
f/4.5 Dobsonian and his drawings
in " The Five Veils of the Cygnus
Loop " (S&T: Sept. 2021, p. 28), I spent
90 minutes completely blown away by
the detail that I saw while using his
drawings as a guide. Once I got a feel
for the extent and layout of the Veil
Nebula, I could freely (and slowly)
move the scope and tour the grandness
of this object. The best views were at
 The Veil Nebula is a large supernova
remnant in Cygnus. It's around 120 light-years in
diameter and stretches across 3° of the sky.
91× magnification with an OIII filter,
though I spent a satisfying first half of
my time observing without a filter.
Thank you, Sky & Telescope staff and
contributors, for all the information and
tools that enable me to see even more of
the beauty hidden in the clear night sky.
John Teleska
Rochester, New York
I'm an assiduous reader of S&T, and
of all the articles I have read to date,
" The Five Veils of the Cygnus Loop " is
one of my favorites. Howard Banich's
descriptions are exquisite. I had an
image of the Cygnus Loop on my iPad
while reading the article. The author's
descriptions guided me through the
many details of the Veil Nebula that
often go unnoticed. During my next
imaging session, the first thing I am
going to do is point my telescopes and
cameras towards that region to capture
and enjoy its beauty to the fullest.
Banich wrote the article in such a
way that it doesn't matter whether one
is a visual observer, sketcher, or imager,
we can all greatly benefit from the
detailed description he provides. I look
forward to reading more articles like
that in the future.
Wanda Conde
Caguas, Puerto Rico
The articles on Cygnus and the Veil
Nebula in the September 2021 issue are
outstanding. One of the most influential
images for me [though not appearing
in the issue] was the picture of the
Veil in The Universe by David Bergamini.
I was 10 or 12 years old when I saw the
photograph, and it set my course to
learn more about astronomy.
I have a copy of that book from
1971, from my grandfather's library,
that I hadn't looked at in many years.
After reading the September articles, I
opened it back up to that image. These
articles brought back great memories
from when I started my journey as an
amateur astronomer.
Ken Caine
Crafton, Pennsylvania
Pondering Plasma
In his article " The First 10 Seconds "
(S&T: Sept. 2021, p. 22), David Garrison
says, " Our oldest fossil is the
cosmic microwave background (CMB),
the radiation released when the early
universe cooled down enough for photons
to travel freely. " He also says that
it formed approximately 380,000 years
after the Big Bang. The accompanying
graphic shows that this is also when
neutral atoms formed, and the universe
became transparent.
Statements like these always puzzle
me because plasma is transparent to
light above a certain frequency, which
depends on the density of the electrons.
The current universe is full of plasma
6
DECEMBER 2021 * SKY & TELESCOPE
and yet transparent. By my calculations,
the density of electrons 380,000 years
after the Big Bang would have led to a
plasma frequency of about 2 megahertz,
well below that of visible light. So what
is the reason for the statement that the
universe became transparent when neutral
atoms formed?
R. C. Carlson
Folsom, California
"
David Garrison replies: There are
two things happening at the same
time here: the formation of neutral atoms
and the process of photon decoupling.
The first happened during an era known as
recombination, when particles' declining
energy made the binding of nuclei and
electrons possible. The second occurred
when photons became free to travel
without scattering. You are correct that
the formation of neutral atoms was not
necessary to enable the photons to travel
without scattering (photon decoupling).
The latter occurred once the plasma was
no longer dense enough to impede the
photons' progress. A plasma's density
determines how energetic a photon needs
to be in order to travel through it. Before
recombination, the density of particles was
so high that the energy threshold was much
higher than the energy of the photons
suffusing the plasma. But as time went
on, the universe expanded and cooled.
It's only around the time of recombination
that the density dropped enough for the
Sky & Telescope - December 2021

Table of Contents for the Digital Edition of Sky & Telescope - December 2021
