Sky & Telescope - May 2019 - 7

Making a Contribution

Fleeing the Scene

In Shawn Dilles's letter (S&T: Dec. 2018,
p. 6) he expressed interest in doing "some
observing that can really contribute to
science." He and other similarly interested readers should seek out The Sky
Is Your Laboratory: Advanced Astronomy
Projects for Amateurs by Robert K. Buchheim, which provides leads to dozens of
astronomical sub-disciplines in which
amateurs can make important contributions to science.
As to using video cameras, amateurs
can contribute by using their equipment
to measure asteroid and lunar occultations; you need only be able to make out
the so-called target star, not necessarily
the asteroid, and it doesn't require rural
dark skies. With only amateur equipment, you can help measure bodies that
would otherwise be too faint to see, and
with size precision exceeded only by
near-Earth radar methods.

I enjoyed your article on hypervelocity
stars (S&T: Dec. 2018, p. 30), but I'm
confused about the exact process that
allows these stars to break away from
their companions. Does the supernova
explosion provide the force needed to
eject the hypervelocity stars from their
orbits, or does the loss of centralized
mass in the exploding star reduce its
gravitational pull on its companion? Is
it a combination of these processes or a
different process altogether?

Ted Swift
Davis, California

SUBMISSIONS: Write to Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, U.S.A. or email: letters@
skyandtelescope.com. Please limit your comments to 250 words; letters may be edited for brevity and clarity.

Jerry Cheney
Ocklawaha, Florida

Ken Croswell replies: The article's

second paragraph explains the
escape mechanism; it's not the push from
the explosion but the vanishing act of the
exploding star, whose mass had gravitationally tethered the companion star. The

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

1969

1994

º May 1944
Cosmic Rays "Traveling with
speeds a million times greater than
that of a bullet from the highest
power rifle known, these particles
are now shooting right through your
bodies at the rate of about 10 per
second for each one of you. Each
of them is tearing apart about a million of your personal molecules as
it passes through you. . . . Do not
be alarmed, however. . . . We now
know that their penetrating power
is [such that] a measurable number
of these rays can pass through as
much as 75 feet of lead."
Thus W. F. G. Swann introduced
his four-part overview of cosmic
rays. These high-energy particles,
mostly atomic nuclei, still fascinate
cosmologists, who point to supernovae and active galactic nuclei
among likely sites of origin.
º May 1969
Umbral Flashes "Jacques M.
Beckers described a new solar

companion ﬂees fast because white dwarfs
are so tiny that the companion can orbit
near the white dwarf's center and thus
revolve rapidly, at speeds exceeding 1,000
kilometers per second. In contrast, were the
Sun to explode, the planets would sail off
more slowly, because their orbital velocities
are modest: 48 km/s for Mercury, 30 km/s
for Earth, and 4.7 km/s for Pluto.

FOR THE RECORD

* The last sentence of the caption for
the photo of M34 (S&T: Jan. 2019, p. 54)
should read: "M34 lies about halfway between the stars Beta (ȕ) Persei and Gamma
(Ȗ) Andromedae."

* The "Sunset for Dawn" News Note (S&T:
Feb. 2019, p. 12) states that Dawn was
NASA's first deep-space mission to use
ion propulsion. That distinction belongs to
NASA's Deep Space 1 mission, which was
launched in October 1998.

phenomenon that he and Paul E.
Tallant have discovered at Sacramento Peak Observatory in New
Mexico. Observing in the H and K
lines of ionized calcium . . . they
found brief flashes of light in the
dark umbras of sunspots.
"Lasting only about 50 seconds on the average, each flash
undergoes rapid enhancement to
maximum brightness, then declines
much more slowly. In a mediumsized umbra there are an average
of three to five flashes in progress
at any instant. . . . The diameters of
the flashing areas are very small,
from the telescope's resolution limit
up to about 3,000 kilometers. . . .
"At peak intensity the umbral
flashes were always blue-shifted,
corresponding to upward motions
(toward the observer) of about six
kilometers per second. . . . As each
flash faded, the narrow emission
line moved toward the red."
Umbral ﬂashes and "running
waves" are today studied as important clues to the magnetic ﬁeld in
sunspots.

º May 1994
Contact Binary "The power
of radar astronomy to probe
solar-system objects was vividly
demonstrated in Science for February 18th. There R. Scott Hudson
(Washington State University) and
Steven J. Ostro (Jet Propulsion
Laboratory) show off their painstaking reconstruction of the nearEarth asteroid 4769 Castalia. Ostro
and three colleagues had used
the powerful radio dish at Arecibo,
Puerto Rico. . . .
"The initial analysis showed
that Castalia . . . has a dumbbell
shape with two distinct lobes each
about ¾ kilometer across. . . .
Nearly four more years of work . . .
reveals a striking, narrow 'waist' at
its midsection [which implies] that
the two lobes were once separate
but came together in a relatively
gentle collision."
Approximately six dozen contact
binaries are now known, the most
famous of these being Ultima
Thule, thanks to close-up imaging
this past January.
s k y a n d t e l e s c o p e . c o m * M AY 2 0 1 9

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Sky & Telescope - May 2019

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