Sky & Telescope - October 2019 - 15

had reason to complain: Gravity behaves in such a complex
way that, with the exception of the most trivial situations,
we cannot simply look at a galaxy now and predict precisely
what it looked like at a previous time - Humpty Dumpty can
never be put back together again.
But although we cannot yet reconstruct our galaxy's past
this way, it turns out that with a little ingenuity, we can sidestep the impossibilities and truly turn back the gravitational
clock in an attempt to map our galaxy's neighborhood.

Nature vs. Nurture

Milky Way

One of the most fundamental questions in cosmology is how
galaxies form and grow into the magnificent objects we see
today. Just as with children, the answer is not merely a matter of genes; it's also one of environment. Our home galaxy,
the Milky Way, lives very close to its sibling, the Andromeda
Galaxy, and a smattering of smaller dwarf galaxies. This
neighborhood of galaxies (which we call the Local Group) lies
off the edge of the Virgo Cluster, a metropolis of some 2,000
galaxies about 50 million light-years away. In the opposite
direction, a void abuts our location. Together with these and
other clusters and voids, we are part of a vast, watershed-like
conglomeration of galaxies that forms our cosmic nation.
To understand what made the Milky Way turn out the way
it ultimately did, with its large spiral disk and rich collection

30

Distance
(million light-years)

ALFAZOA J1952+1428

KK 246

The Local Void

M83

Milky Way

M81

Maffei 2
Maffei 1

p COSMIC FLOWS Each sphere in this wide-angle perspective represents a galaxy in the nearby universe. Arrows indicate the streamlines
the galaxies follow, and colors correspond to a region's density (red for
dense clusters, blue for voids).

Distance
(million light-years)

Centaurus A

Andromeda
IC 342
NGC 253

-20
Dist
a
(m nce
ligh illion
t-ye
ars)

LE AH TISCIONE / S&T, SOURCE: DA NIEL
POM ARÈDE (CE A UNIVERSITÉ PA RIS-SACL AY )

0

His school of thought produced some of the greatest lumi0
naries in modern astrophysics, a number of whom are still
active today, mostly spread across the Western world in a
-450
post-Soviet diaspora.
-450
0
20
SGX (km/s)
The voice berating our method came from Andrei Doroshp LOCAL SHEET Seen in supergalactic coordinates, the Milky Way and
kevich - a scion in the field who, now at more than 80 years
its largest neighbors (labeled) occupy the Local Sheet, which abuts the
of age, is one of the last of Zeldovich's living disciples. He
Local Void. Circles mark 6.5 million, 13 million, and 20 million light-years.
sk yandtele scope.com * OCTOBE R 2 019

15



http://www.skyandtelescope.com

Sky & Telescope - October 2019

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

Contents
Sky & Telescope - October 2019 - Cover1
Sky & Telescope - October 2019 - Cover2
Sky & Telescope - October 2019 - 1
Sky & Telescope - October 2019 - Contents
Sky & Telescope - October 2019 - 3
Sky & Telescope - October 2019 - 4
Sky & Telescope - October 2019 - 5
Sky & Telescope - October 2019 - 6
Sky & Telescope - October 2019 - 7
Sky & Telescope - October 2019 - 8
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