Sky & Telescope - February 2019 - 8

FROM OUR READERS
own lenses for the slide projectors
for his lecture. But, of course, he was
right! What I had thought of as the best
available lens quality from a renowned
German supplier fell totally short in
comparison, when we changed the light
bulbs to fresh ones and compared his
images to mine with his totally amazing projecting lenses. On the obligatory city tour the day after, I regained
a bit of pride seeing David's reaction to
our small cathedral of 1190. Ancient
architecture is apparently another of his
great interests.
Ole J. Knudsen
Aarhus, Denmark

Spectral Evidence
Some clariﬁcation is needed for the
discussion of the Hertzsprung-Russell
diagram (S&T: Dec. 2018, p. 27). The
statement "Spectral class indicates
the relative abundance of the different elements in stars and correlates
with temperature, so O and B stars are

hotter than K and M stars" is incorrect. Spectral classes are based on the
different appearance of absorption lines
in a star's spectrum. A B star has strong
lines of helium and a K star has prominent lines of calcium, but this does not
mean that a B star is mostly helium
while a K star is primarily calcium.
These spectral lines are produced when
the atoms in a star's surface absorb certain wavelengths of light, and an atom
can only absorb these wavelengths if its
electrons are in the right orbits, which
depends on surface temperature.
As ﬁrst demonstrated by Cecelia
Payne in 1925, knowing which orbits
the electrons are in can be used to calculate the abundances of the elements
in a star. In her doctoral thesis, Payne
found that all stars are composed of
mostly hydrogen and helium, with a
smattering of lighter elements.

1969

1994

º February 1944
Diminishing Returns "When F. C.
Brown and I first mounted a selenium cell at the focus of a 12-inch
refractor and pointed the telescope
at Jupiter there was no detectable
response whatever. Since then
the faintest object which [A. E.]
Whitford and I have measured with
a photocell is a star of magnitude
16.1 with the 100-inch reflector. As
the probable error of measurement
was about 10 per cent, the limit of
detection may fairly be called magnitude 18. From Jupiter at magnitude -2 to a star at +18 the change
is 20 magnitudes. This advance is
perhaps not so much a measure of
the excellence of the latest developments as of the crudeness of the
first attempts. . . .
"The limit of magnitude 16 was
reached six or seven years ago,
and the law of diminishing returns
is working now. We can predict
with confidence that the next 20
magnitudes will be harder to get."

FE B RUA RY 2 019 * SK Y & TELESCOPE

Bradley W. Carroll
Ogden, Utah

Diana Hannikainen replies: Yes,

we goofed in editing and used
"abundance" when we were thinking of
strength. Thanks for keeping us honest.

FOR THE RECORD

* In the sidebar "Moon Hides Star" (S&T:
Nov. 2018, p. 51), the first line should read
". . . occults Chi1 (Ȥ1) Orionis."
* In "New Year's Eve: Celestial Celebration"
(S&T: Dec. 2018, p. 22), in the table of bright
stars, Antares' constellation is Scorpius.

* In "Imbrium's Eyebrow" (S&T: Dec. 2018,
p. 52), the crater labeled Atlas is actually
Hercules. Atlas is the larger crater to the
right of Hercules in the image.

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75, 50 & 25 YEARS AGO by Roger W. Sinnott
1944

This must have slipped past your
usually knowledgeable editors.

Joel Stebbins (Washburn
Observatory) was a leading pioneer
in photoelectric photometry. True
to his prediction, and despite
CCDs, space telescopes, and
much greater apertures on the
ground, astronomers are still about
5 magnitudes shy of reliably reaching visual magnitude 36.
º February 1969
Earth's Tail "Two Polish astronomers, M. Jerzykiewicz and A.
Opolski of Wroclaw University
Observatory, report an unexpected
by-product of a large program of
photoelectric photometry carried
on at Lowell Observatory. . . .
[T]hey found that when a star
is within a few degrees of the
antisolar point, it appears about
0.01 magnitude fainter than when
observed at other times. . . .
"This dimming could be caused
by about one thousand dust
particles of 10-micron size in a
column of one square centimeter
cross section extending from star
to observer. . . . [The] astronomers

believe that their finding is evidence, like the gegenschein, for a
dust tail of the earth. But they could
not rule out the possibility of an
unknown instrumental effect. . . ."
º February 1994
Polaris's Pulses "Look on almost
any sky atlas and you'll see Polaris
shown by the symbol for a variable
star. Look in reference catalogs
and you'll find it listed as a classical
Cepheid with a 4-day period and
an amplitude of 0.1 magnitude. . . .
"Now Polaris has almost totally
ceased pulsing, according to a
report by J. Donald Fernie, Karl W.
Kamper, and Sara Seager (University of Toronto). . . . The variation
was down to a microscopic 0.010
± 0.002 magnitude as of mid-1992
when the astronomers made their
last measurements. They expect it
to become perfectly constant any
year now."
Fernie's team retracted its forecast in 1998, noting that Polaris's
variations had settled at about 0.03
magnitude.

Sky & Telescope - February 2019

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