Sky and Telescope - June 2017 - 7

Small Details Matter
I very much enjoyed retracing Alan
Whitman's tour of edge-on galaxies in
Ursa Major (S&T: March 2017, p. 57).
A small but very signiﬁcant detail in
the caption under the ﬁne picture of
NGC 4085/4088 caught my eye.
Many of us visual astronomers use
the work of astrophotographers like Jim
Thommes, who took that shot, in the
same way I imagine earlier observers
used the photographic plates to which
they had access. These are invaluable
to us because they serve to verify our
observational notes and often prompt
further observing of an object or area.
The signiﬁcant detail is your inclusion of the width of the image ﬁeld. This
is appreciated. It increases the illustration's value as a reference and guide
to visual observing. I hope that you'll
consider including this crucial detail in
all the astrophotos that you publish.
Dick Gentry
Dallas, Texas

How Do Astronomers Gauge
Intergalactic Divides?
Thanks for the timely publication of
the fascinating Perseus Galaxy Cluster
article (S&T: Jan. 2017, p. 57), since
Abell 426 dominates my observing at
the moment. How certain is author
Ted Forte that NGC 1293 and 1294 are
"more than a hundred light-years apart"
or that "about 129 million light-years"
separate NGC 1282 and 1283? If those
distances are determined by standard
candles, such as type Ia supernovae, no
problem. Redshifts, on the other hand,
are presumably rather less reliable in
such a densely packed cluster because of
gravitational interference. M31 actually
shows a negative redshift, but nobody (I
hope!) claims it's at a negative distance
from us.
Paul Dawson
Olancha, California

1967

1992

Ted Forte replies: Distance estimates made by different methods
often vary greatly for the same object -
and, as you've noted, redshift can be most
problematic to interpret. Yet for some galaxies only redshift surveys are available. So
some of the comparisons made in my article
are not at all certain. Also, it's important to
remind ourselves of the three-dimensional
nature of objects like the Perseus Galaxy
Cluster. Visual observing, limited to two
dimensions, is an imaginative exercise. My
intent was simply to offer a reminder of that
elusive third dimension.

FOR THE RECORD

* The large nighttime image (S&T: March
2017, p. 30) combines the summertime
Milky Way with a winter foreground scene.

* The two open clusters in Cancer are M44
and M67, not M26 (S&T: April 2017, p. 43).

LETTERS TO THE EDITOR: Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264 or

email: letters@skyandtelescope.com. Please limit comments to 250 words.

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

1942

º June 1942
Deep Sky "Behind the stars of
Virgo lies the richest cluster of
galaxies that can be seen through
amateur telescopes. This nest . . .
dominates northern Virgo and scatters across into southern Coma
Berenices. . . .
"In amateur telescopes these
marvels appear wee and ghostly.
"The Sombrero, one of the
brightest spirals, lies between [the
neighboring finder stars] Porrima and δ Corvi. . . . If you have a
telescope, be sure to look for this
handsome deep-sky wonder. . . ."
Columnist Leland S. Copeland,
always a colorful wordsmith, seems
to be the ﬁrst person ever to use
"deep sky" when speaking of the
universe beyond naked-eye stars
- and so a new expression was
born. In November that year, editor
Charles A. Federer, Jr., made DeepSky Wonders the title of Copeland's
department, and it has endured
under Copeland's successors, Walter Scott Houston and Sue French.

º June 1967
Evolving Galaxies "In general,
elliptical galaxies are many times
more massive than spirals, and
in some parts of the sky we see
a single very large elliptical in
association with one or more spiral
or irregular systems. The members of such groups seem to be
moving away from each other, as if
expanding from an explosion. This
possibility is discussed by Jose
Luis Sersic of Cordoba Observatory, Argentina . . .
"He and H. A. Abt of Kitt Peak
National Observatory have proposed that many galaxies may be
fragments of larger masses that
exploded some time ago. Statistical data indicate that it is the giant
ellipticals which produce the fragments seen now as smaller elliptical, spiral, and irregular systems."
Today most cosmologists would
reverse this process. Collisions
seem common among galaxy
groups and clusters, and their giant
ellipticals probably grew that way
by mergers with smaller galaxies.

º June 1992
Gravitational Waves "Groundbreaking for a revolutionary type of
astronomical telescope may occur
by the end of the year. In late February the National Science Foundation announced the selection of
sites for a pair of observatories
designed to detect gravitational
radiation. Hanford, Washington,
and Livingston, Louisiana, beat out
17 other prospective sites for the
Laser Interferometer GravitationalWave Observatory (LIGO). . . .
"Nevertheless, a third site will
still be needed to pinpoint the origin
of [a candidate] signal in the sky. A
LIGO clone is planned for Europe,
but financial concerns among the
countries involved have cast considerable doubt on its completion."
Called Virgo, the European sister facility was completed near Pisa,
Italy, in 2003, but was then turned
off in 2011 for a major upgrade. So
when the two LIGOs successfully
detected gravitational waves in
2015, scientists couldn't say what
direction they came from. Virgo
should rejoin the search soon.

s k y a n d t e l e s c o p e .c o m

* JUNE 2017

http://www.skyandtelescope.com

Sky and Telescope - June 2017

Table of Contents for the Digital Edition of Sky and Telescope - June 2017