Sky & Telescope - September 2019 - 7

escape. When it does, it will travel straight
along the escape trajectory - barring any
more gravitational detours, that is.

Light a Standard Candle
"Lighting a Cosmic Fuse" by Shannon
Hall (S&T: June 2019, p. 14) paints an
interesting picture of an important
issue in cosmology. The need for "standard candles" is further highlighted
by Govert Schilling's article "Constant
Controversy" in the same issue (p. 22).
Hall's article concludes that astronomers now believe there are several
mechanisms behind Type Ia supernovae (single- versus double-degenerate
systems), and that they probably span a
range of absolute luminosities. She goes
on to explain how understanding these
mechanisms can help in improving
cosmological distance scales. Two observational studies are mentioned in this
regard that seem to point to vastly different percentages for single-degenerate
occurrences (25% versus less than 5%).

I wonder whether anyone has applied
the results of stellar evolution models
to predict what fraction of white dwarfs
are paired with ordinary stars and what
fraction with another white dwarf.
This could lend another independent
test to the distribution of single- versus
double-degenerate supernovae that have
been observed. On the other hand, if
observational studies can sort the distribution out, couldn't this help validate
the evolutionary models?
Dave Billesbach
Lincoln, Nebraska

Thank you for Govert Schilling's
articles over the last several years and
particularly his most recent article on
the Hubble-constant controversy. His
articles on black holes, gravitational
waves, multi-messenger astronomy, and
cosmology in general reﬂect scientiﬁc

1969

1994

Nick Anderson
Arlington, Virginia

Star Power
I greatly enjoyed the three-part indepth look at stars in the March, April,
and May 2019 issues. I'm a long-time
reader, and it's informative, wellwritten articles like those that make me
value your magazine so highly.
Nick Britt
Lewis Center, Ohio

FOR THE RECORD

* In Sky at a Glance (S&T: June 2019, p. 41)
on June 30th Aldebaran was 3° from the
waning, not the waxing, lunar crescent.

SUBMISSIONS: Write to Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, U.S.A. or email: letters@
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75, 50 & 25 YEARS AGO by Roger W. Sinnott
1944

journalism at its best - lucid and informative. He, Monica Young, and Camille
Carlisle constitute the best "troika" in
astronomical journalism, especially on
astrophysics and cosmology.

º September 1944
Pleiades Probed "Dr. J. A. Pearce
[at the Dominion Astrophysical Observatory has studied the
spectra] and physical characteristics of the 12 brightest stars in the
Pleiades. [He] collected some 214
observations of 10 of these stars
made during the past 40 years, and
combined these with 111 unpublished [radial] velocities of his own.
From these he finds that the Pleiades appear to be receding from
us at . . . five miles per second, a
figure in good agreement with that
predicted by observations of the
apparent motions of these stars
across the face of the sky. . . .
"The distance of the cluster,
obtained from Dr. Pearce's work, is
found to be about 240 light-years,
in close agreement with the results
from other methods."
Notoriously hard to measure,
the famed cluster's distance is now
pegged at 446 light-years (S&T:
Mar. 2019, p. 26).

º September 1969
Apollo 11 "One of astronaut [Neil]
Armstrong's tasks at Tranquillity
Base was to carry an 18-inchsquare aluminum block about 30
feet from the Eagle and leave it
there. Emplanted in this block are
100 retro-reflectors 1½ inches in
diameter. Each is a corner cut from
a perfect cube of fused silica. . . .
"The purpose of this retroreflector array is to allow a terrestrial telescope to send a laser light
beam to the moon and observe
its reflection. By using such a
compact lunar 'bench mark,' a very
great improvement in Earth-Moon
distance measurements is possible
[and] should ultimately reduce the
uncertainty in measured distance
to about six inches."
That retro-reﬂector is still being
targeted to help pin down the
Moon's orbit. Ranging uncertainty
is now only a millimeter.
º September 1994
Keck Performs "The night I witnessed Keck in operation [Steven]

Vogt and others measured the
radial velocities of 19th- and 20thmagnitude stars in the nearby dwarf
elliptical galaxy Leo II. Before Keck,
it was a three-hour struggle to
obtain a spectrum of one of these
stars with the Palomar 5-meter.
These astronomers . . . knock them
off one every 10 minutes. . . .
"They were searching for the
gravitational signature of dark matter. . . . Simply put, [large velocity
variations mean] some unseen
mass would be needed to keep
the galaxy from flying apart -the
luminous stars and gas can't do
it alone. Their preliminary results
suggest that the dispersion is
about 7 or 8 km per second. Thus,
as several earlier studies had suggested, dwarf ellipticals like Leo II
contain lots of dark matter, just like
large spiral galaxies."
Kevin Krisciunas, who is now
at Texas A & M University, visited
the ﬁrst 10-meter Keck telescope
on Mauna Kea two years after its
dedication.

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Sky & Telescope - September 2019

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