Sky & Telescope - August 2021 - 7

sions, and other default indicators. Sure,
they'd usually report the oxygen content
and occasionally the surface gravity, but
those are relatively easy. Until I read this
piece, I didn't realize there were so many
other chemical clues to consider. Thanks
for opening my eyes to the fascinating
future of biosignature detection.
Dan Heim
New River, Arizona
The writing in S&T is always top-notch,
from the News Notes to the feature
articles. I'm sure this must have as much
to do with the editing as with the quality
of the authors' writing. But the article on
biosignatures is a cut above, what with
its full explanation of every point, anticipating
any questions a reader might
have, and the distinctive, fun phraseology
- not to mention treating data as
the plural word it is. I look forward to
more articles by Arwen Rimmer.
Joel Marks
Milford, Connecticut
Absorption Line Overload
" How Well Do We Know the Sun? "
by Colin Stuart (S&T: Apr. 2021, p.
12) states that 67 elements have been
identifi ed in the Sun so far. The photograph
on page 13 shows a beautiful and
extremely detailed solar spectrum. Its
caption notes that the dark absorption
lines result from the presence of specifi c
elements in the Sun's atmosphere. But
the image shows far more than 67 elements.
Why are there so many more
absorption lines than elements?
James Schwartz
Valrico, Florida
Monica Young replies: You're right,
there are a lot more absorption lines
than individual elements. Every element absorbs
light at multiple wavelengths due to
the many energy levels available to its electrons.
Even hydrogen has a number of pos "
sible
energy levels for its single electron and
therefore can absorb several wavelengths
of sunlight. Atoms with additional electrons
can produce even more absorption lines.
FOR THE RECORD
* The image of Pine Mountain Observatory
in " Working With What You Have " (S&T:
Jun. 2021, p. 54) was a 30-minute exposure
shot with a tripod-mounted Canon
80D DSLR set to ISO 200 and a Canon kit
lens zoomed to 20 mm and f/6.3.
* The " Selected Jupiter Mutual Satellite
Events " tables in the May, June, and July
2021 issues had incorrect values in the
" Mag. change " column. The correct ones
are May 1: 0.0; 11: 0.7; 20: 0.5; 27: 0.4;
June 1: 0.7; 10: 0.1; 18: 0.2; 21: 0.1; 28: 0.0;
July 5: 0.0. Events with a change in magnitude
of less than 0.3 are diffi cult to perceive,
and those with a value of 0.0 are unobservable
and shouldn't have been included.
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
1946
1971
 August 1946
UV Frontier " Films developed by
Eastman Kodak Company with
special fl uorescent coatings will be
used in spectrographs mounted in
the noses of [captured] V-2 rockets.
Ultraviolet sunlight, unable to
penetrate either our atmosphere or
ordinary photographic emulsions,
will be recorded when the rockets
reach altitudes of about 100 miles.
The fl uorescent fi lm coating glows
when ultraviolet light strikes it, and
the glow is recorded on the fi lm. "
These early rocket fl ights, the
1996
fi rst " baby steps " into ultraviolet
astronomy, successfully recorded
the spectral line of singly ionized
magnesium at 280 nanometers.
 August 1971
Shrinking Planet " [T]he mass of
the planet Pluto was determined to
be about 0.18 that of the earth, in a
study by [U. S. Naval Observatory
astronomers in 1968]. Their method
was to evaluate the size of the perturbations
that Pluto's gravitational
attraction produces in the orbital
motion of Neptune . . .
" This study has now been
repeated and refi ned . . . The
mass of Pluto is found to be 0.11
that of the earth . . . If the diameter
of Pluto is accepted as 6,400
kilometers, then the mean density
of the planet is about 0.88 the
earth's. Thus it appears that Pluto
may closely resemble Mars in size,
mass, and density. "
The fl aw in all these early determinations
was to assume, following
Percival Lowell, that Pluto perturbs
Neptune noticeably. It doesn't. We
now know Pluto has just 0.002 the
mass of Earth and about two-thirds
the diameter of our own Moon.
 August 1996
Stardust? " When a backyard
observer witnesses a meteor, he
or she sees the fl aming residue of
a piece of cosmic debris usually
the size of a pebble. Many
more particles are so small . . .
that they vaporize unseen in the
Earth's atmosphere. Nevertheless,
astronomers have used radar to
detect the ionized trails of such
invisible meteors. One such system
is the Advanced Meteor Orbit Radar
(AMOR) in New Zealand. Not only
can it 'see' the tracks of meteors
as faint as 13th magnitude, but it
allows astronomers to determine an
incoming particle's trajectory and,
hence, its orbit. . . .
" [As] Andrew D. Taylor (University
of Adelaide) and his colleagues
explain . . . about 14 percent of
the more than 350,000 meteors
AMOR detected traveled at more
than 73 kilometers per second -
fast enough to escape (or to have
entered) the solar system along
hyperbolic trajectories. . . .
" [With AMOR, they] hope to
identify specifi c stellar sources of
extrasolar dust. "
But astronomers have since
questioned the extrasolar origin
of these tiny particles. The much
larger bodies 1I/'Oumuamua (2017)
and 2I/Borisov (2019) are our only
confi rmed visitors from other stars.
sk yandtelescope.org * AUGUST 2021
7

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Sky & Telescope - August 2021

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