Sky & Telescope - October 2022 - 7

Catching Gravitational Waves
A friend sent me his copy of the June
issue, as I have a deep engagement in
the gravitational-wave field. Specifically,
I led the Advanced Laser Interferometer
Gravitational-Wave Observatory (LIGO)
Project that realized the detectors that
have been so successful, and I served as
the Spokesperson for the LIGO Scientific
Collaboration from 2017 to 2019.
I really liked the overall tenor and
depth of " What Gravitational Waves
Have Taught Us About Black Holes "
(S&T: June 2022, p. 12) by Camille M.
Carlisle. It felt like it could serve as both
a gentle introduction and a deep coverage
that would satisfy many astrophysicists
who do not work in the field (yet!).
David Shoemaker
Acton, Massachusetts
I really enjoyed your recent article on
gravitational waves and black holes.
I have a couple of questions. When
gravitational waves alter the lengths of
the detectors, for how long a period of
time will the length stay changed, or is
it just a split-second occurrence?
On page 13, it says " If you could
convert the energy of a binary black hole
merger into light, it would be brighter
than all the stars in the observable
universe. " Does it mean all the stars put
together? If so, that's incredible!
Mike Witkoski
Enola, Pennsylvania
Camille M. Carlisle replies: It
depends on the masses of the objects
involved, but ballpark, the detectors'
lengths oscillate for about a second in total.
Mergers involving more massive objects
shake the detectors for a shorter period of
time, because the waves' frequency goes
down as the mass goes up - the result of
which is, the signal falls in LIGO's sensitivity
range for a shorter period of time. There's a
"
nice comparison of four signals' lengths at
https://is.gd/LIGO2017.
As to your second question: Yes, that's
exactly what it means - it's crazy! LIGO
scientist Shane Larson (Northwestern University)
explains the calculation in his blog
https://is.gd/MyBrainIsMelting.
Thank you for the article by Camille M.
Carlisle on black hole mergers. It's an
excellent article. Kudos to Carlisle for her
thorough and engaging writing style.
James Edgar
Editor, RASC Observer's Handbook
Melville, Saskatchewan
FOR THE RECORD
* In the " Select Targets for Beginning
Sketchers " sidebar on page 59 of the June
issue, the Owl Cluster is NGC 457.
* The image at the top right of page 64 in
the August issue is M8.
SUBMISSIONS: Write to Sky & Telescope, 1374 Massachusetts Ave., 4th Floor, Cambridge, MA 02138, 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
1947
1972
º October 1947
Biggest Sunspot " Never before
has the sun's surface been so
closely watched as now. . . . This
interest was fathered by the discovery
that radio reception varies with
changes in solar activity. . . . Wartime
needs stimulated the study of
these solar effects, with the result
that many people who had never
heard of sunspots before became
quite conscious of them. . . .
" The largest single sunspot
1997
(penumbral area including umbrae)
ever recorded appeared in March,
1947. By April, this spot had
divided into several parts which
grew to form the largest group of
spots ever observed. . . .
When largest, it appeared to
cover more than one [percent]
of the apparent solar disk, its
area being 6,300,000,000 square
miles, 5,400 millions of the actual
solar hemisphere. "
The records mentioned by Seth
B. Nicholson still hold.
º October 1972
Telescope Meet " As is traditional
at Stellafane, awards were
tailored to the telescopes on hand.
. . . The 12-inch f/5.3 reflector of
Albert H. Nagler received first
prize for Newtonians. An engineer
from Spring Valley, New York, he
refigured another amateur's mirror
and made the unique perforated
diagonal [that] permits guiding on
an object while it is being photographed
. . . Most of the light
(equivalent to that gathered by
a 10-inch aperture) is reflected
toward the camera . . . but some
(from a six-inch annulus at the
center of the primary) enters the
perforation and goes through a
2x Barlow and diagonal to the
guiding eyepiece . . . "
Many optical entrepreneurs got
their start at this annual telescopemaking
convention in Springfield,
Vermont. In 1977, the same Al
Nagler would form Tele Vue Optics,
well-known for its line of exquisite
eyepieces and small refractors.
º October 1997
Moon's Birth " The idea that something
the size of Mars sideswiped
the Earth in its infancy to form the
Moon is not new; researchers William
K. Hartmann and Donald R.
Davis first offered the 'Big Splat'
scenario in the mid-1970s. . . . Building
[on] this earlier work, Shigeru Ida
(Tokyo Institute of Technology) and
Robin M. Canup and Glen R. Stewart
(University of Colorado) have
now simulated the collision's aftermath
with unprecedented detail.
" They find that the massive
cloud of vaporized rock ejected by
the impact flattens into a disk within
a few months. . . . Canup notes that
only impactors with 2½ to 3 times
the mass of Mars can create a disk
with a Moon's worth of matter outside
the Roche limit. The catch, she
says, is that the Earth-Moon system
is then left with about twice the
angular momentum it has today . . . "
Canup and others continue to
refine details of this impact, which
remains the leading theory of the
Moon's origin.
sk yand tele scope .o r g * OCTOBER 2022
7

https://www.is.gd/LIGO2017 https://www.is.gd/MyBrainIsMelting http://skyandtelescope.org

Sky & Telescope - October 2022

Table of Contents for the Digital Edition of Sky & Telescope - October 2022