Sky & Telescope - September 2022 - 7

However, others have not had the
favorable experience with their DallKirkhams
that I've had. Alignment
between the secondary and primary
mirrors is critical, which may explain
some of this. The mirrors in my DallKirkham
are rigidly held in place in a
3
/16-inch-thick fiberglass tube that my
machinist friend made with layers of
cloth fiberglass.
Stanley Gorodenski
Dewey, Arizona
DIY Spectroscopy
I just finished reading Diana Hannikainen's
exciting article on " Decoding
Light " (S&T: May 2022, p. 57). I've been
capturing the spectra of stars with my
6-inch refractor and a Star Analyser and
RSpec for about 10 years. When I show
my astronomy friends how much science
I can do with a few hundred dollars'
worth of equipment, they're always surprised.
Hannikainen's article helps dispel
the myth that spectroscopy is expensive
or hard to do. She quotes respected
professional astronomer Steven Shore
regarding the care required to capture
research-quality spectra. My experience
is that after one or two sessions with my
telescope, I was able to capture spectra
that contained all sorts of useful scientific
data. I think it's important that we
not let perfection get in the way.
Spectroscopy isn't only for PhDs who
have access to professional telescopes.
Anyone who has captured astronomical
images can spend years capturing exciting
star spectra that they can submit to
the AAVSO spectroscopy database.
Rick Hiestand
Milton, Florida
The Galilean Moons
I enjoyed Benjamin Skuse's article
" Ocean Underworlds " (S&T: Apr. 2022,
p. 14) on the subterranean oceans on the
Galilean moons. Are the ages and origin
of the moons known?
Benjamin Skuse didn't address it in
the article, and when it comes to what
type of organisms might be discovered,
it's important.
Steve Bernstein
Jackson, New Jersey
"
Camille M. Carlisle replies:
Planetary scientists think that the
Galilean moons are primordial - that is,
that they formed with or soon after Jupiter
some 4½ billion years ago. There's a
good overview of recent work on this at
https://is.gd/galileanmoonsformed.
FOR THE RECORD
* In " A Star Where It Shouldn't Be " (S&T:
June 2022, p. 10), the peculiar properties
of HD 93521 were identified in the 1980s,
rather than in 1993.
SUBMISSIONS: Write to Sky & Telescope, One Alewife Center, Suite 300B, Cambridge, MA 02140, 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
1947
1972
º September 1947
Meteorite Ages " Assuming that
the helium contents of meteorites
are affected only by the radioactive
decay of uranium and thorium, the
relative abundances of these elements
have in the past been used
for estimating the ages of meteorites.
. . . Carl A. Bauer, of Harvard
College Observatory, questions the
infallibility of such estimates . . .
" Pointing out that cosmic rays
1997
produce nuclear disruptions in
which alpha particles (helium
nuclei) are among the disintegration
products, Mr. Bauer [notes
that] 'the largest observed helium
content can be produced in a
small meteoroid by cosmic radiation
in a time less than the present
assigned age.' "
Fortunately, other forms of
radioactive decay not involving
helium can also be used for dating
meteorites. Most meteorites have
the same age as the solar system,
4.5 billion years.
º September 1972
Aperture Fever " Last year at the
Hale Observatories in California,
the English astronomer R. F. Griffin
spent several nights measuring the
radial velocities of stars in star clusters.
. . . 'You would have to be a
very hard-boiled astronomer indeed
not to be impressed at the sight of
M13 in the [200-inch] telescope.
The dense central part is several
field diameters wide, and you can
see the colours of the stars down
to about the fifteenth magnitude.
In addition to the densely packed
mass of individually resolved stars,
in good seeing the whole cluster
shows a granular background - it's
rather like looking into a bowl of
sugar! . . . There must be perhaps
200 [red giant stars] . . . and you can
pick them out at sight, even in the
very middle of the cluster.' "
º September 1997
Hungry Galaxy " For several years
now, astronomers have used small
satellite galaxies - our Milky
Way's Magellanic Clouds being a
prime example - to 'weigh' the
much larger spirals they orbit. . . .
Claude Carignan (University of
Montreal) and his collaborators
have measured the line-of-sight
velocities of eight dwarf galaxies
that appear to orbit NGC 5084.
" [They] ascribe a total mass
of 6 to 10 trillion Suns to NGC
5084 - making it the 'heaviest'
disk galaxy known. In addition
they noted whether each satellite
was moving away from Earth at a
faster or slower speed than NGC
5084 itself. This showed an intriguing
trend - nearly all the dwarfs
follow retrograde orbits. (That is,
they appear to circle NGC 5084's
center in the opposite way than do
the galaxy's own stars and gas.)
" Computer models have shown
that dwarf galaxies in prograde
orbits are much more likely than
retrograde orbiters to be consumed
by tidal interactions with
the parent spiral. Thus, NGC 5084
presumably once had a larger
population of satellite galaxies but
has absorbed many of them. "
sk yand tele scope .o r g * SEPTEMBER 2022
7

https://www.eos.org/articles/jupiters-galilean-moons-may-have-formed-slowly http://skyandtelescope.org

Sky & Telescope - September 2022

Table of Contents for the Digital Edition of Sky & Telescope - September 2022