Sky and Telescope - July 2016 - 8

Letters
I truly enjoyed William Sheehan's March
Focal Point. It was marvelous and uplifting. He masters the gentle touch of imagery that blossoms into a world of wonders.
Melita Wade Thorpe
San Jose, California

Construction Zone Ahead
While the possibility of detecting an alien
Dyson sphere would be incredibly exciting
(S&T: Mar. 2016, p. 16), I have to take a step
back and look at the possibility rationally.
The light curve I've seen from the star
KIC 8462852 indicates some obstructions
blocking as much as 20% of the star's
light, but most of the time 0% is blocked.
Were some alien civilization to build such
a structure as a Dyson sphere, would it
really make sense to have sections many
thousands of miles wide put in place while
having most other sections not built at all?
I don't want to be a buzzkill, but it's

75, 50 & 25 Years Ago
July 1941
Nova Forecast "[At]
the 30th annual spring
meeting of the American
Association of Variable
Star Observers, at Vassar College, . . . Leon
Campbell, recorder of the
A.A.V.S.O., spoke on 'Do
Ordinary Novae Repeat?' This question was
answered in the possible aﬃrmative by Russian
astronomers in 1934. By plotting the range
of magnitude of various well-known types of
variables (particularly those which ﬂare up at
intervals, such as T Pyxidis, U Scorpii, and SS
Cygni-type stars) against the logarithm of the
period between outbursts, a curve is obtained
on which ordinary novae may have a place.
[Based on this relationship,] such novae as
those in Aquila and Perseus may recur after an
interval of 70,000 years."
Nova Persei 1901 peaked at magnitude 0.2
and Nova Aquilae 1918 at magnitude -1.4. The
speculation as to when they will erupt again is
untestable, but astronomers now have a much
better idea about what makes novae tick. Each
is a binary star in which a white dwarf accretes
hydrogen from its companion until a fusion reaction (as in a hydrogen bomb, only more gradual)
takes place on the white dwarf's surface.

July 2016 sky & telescope

hard to imagine these supposedly superior beings building such a structure in
that haphazard manner. The most logical
way would be to build a complete ring
around the star ﬁrst, and then make it
broader and broader until the sphere was
eventually complete. This would produce
a light curve for KIC 8462852 that would
be pretty much uniform (depending on
its orientation relative to our line of sight)
- not one exhibiting the stark ﬂuctuations that have been observed to date.
Brian Cobb
El Cajon, California

Undersampled Exposures
I would like to add a little more to the
advice on choosing a camera for astrophotography (S&T: Apr. 2016, p. 66). Richard
S. Wright, Jr., notes that undersampled
exposures can arise when the detector's
pixels are too large for the telescope used.

NASA came up with a process to improve
the resolution of undersampled exposures
from the Hubble Space Telescope called
"drizzle" that's very eﬀective and easy to
do. You should have six or more exposures,
taken with the telescope shifted very
slightly (dithered) between each. The exposures should then be resampled by a factor
of two - that is, resize a 500-by-500-pixel
frame to 1,000 by 1,000 pixels before aligning and stacking. This allows you to shift
the resized exposures by half the width of
the original pixels and results in improved
resolution. Note that this process only
works with undersampled exposures.
Ted Raﬀerty
Gaithersburg, Maryland

For the Record
✹ The paired images of a rising Moon and
Sun (S&T: May 2016, p. 74) were taken on
September 27, 2015.

Roger W. Sinnott
July 1966
Extragalactic X-rays "For
the ﬁrst time, X-radiation
coming from the direction of individual galaxies
has been measured. This
is only one of several
remarkable results
reported by E. T. Byram,
who with H. Friedman and T. A. Chubb has
analyzed data from an Aerobee rocket in April,
1965. The ﬂight was part of the U. S. Naval
Research Laboratory's continuing program of
X-ray astronomy. . . .
"To map the sky at wavelengths between 1
and about 15 angstroms, the Aerobee carried
two Geiger counters, each behind a honeycomb
baﬄe to limit its angular ﬁeld of view. . . . X-rays
were observed from the vicinities of two intense
radio galaxies, Messier 87 (Virgo A) and Cygnus
A. It appears that the X-ray emission from each
is 10 to 100 times greater than the combined
radio and optical emission. This result adds to
the diﬃculty that theoretical astrophysicists
have in explaining the enormous energy outﬂow from radio galaxies."
July 1991
Dinosaurs' Doom "In its heyday Chicxulub [on
the northern coast of the Yucatán peninsula]

thrived on the cultivation and export of sisal
ﬁber. . . .
Unbeknown to its
residents, Chicxulub (a
Maya phrase for 'horns
of the devil') has again
become a mecca of sorts
- but for geologists. A
growing number of them suspect that a huge
impact crater lies directly below it, buried far
from sight by a thick layer of limestone sediment. But this would be no ordinary crater. If
early indications prove correct, it is the Holy
Grail of impact geology - the long-sought site
of the blast that triggered the eradication of
most life on Earth 65 million years ago.
Geologists have been combing the Earth for
such a crater since 1980. That year a research
team at the University of California, Berkeley,
proposed that an asteroid's devastating arrival
created the abrupt [geochemical] boundary
found worldwide between the Cretaceous and
Tertiary geologic periods."
Senior Editor Kelly Beatty, who followed this
unfolding story closely, went on to tell how an
Arizona graduate student, a Texas reporter, Mexico's national oil company, and a three-decadeold gravity map all played a role in ﬁnding the
sunken, 180-km-wide crater.

Sky and Telescope - July 2016

Table of Contents for the Digital Edition of Sky and Telescope - July 2016