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in diameter. Half a million miles is not a meaningful
distance it is about twice the distance of the Moon
from the Earth. You can neither walk nor even drive
that far in any meaningful sense.
And yet there are galaxies beyond the 100,000
light-year disk of the Milky Way which are said to be
"nearby" us. The Large and Small Magellanic Clouds
are small irregular galaxies which appear to be orbiting
the Milky Way at distances of 169,000 and 205,000
light-years, respectively. M31 (the galaxy which Edwin
Hubble observed to confirm his hypothesis that some
nebulae actually lie outside the Milky Way galaxy and
are in fact galaxies themselves) appears to be about
2.2 million light-years away. In fact there are about
twenty galaxies of varying sizes and shapes within
about 3,000,000 light-years of the Milky Way. All
together these twenty galaxies are called the Local
Group.
Astronomers estimate that there are perhaps ten
billion galaxies in the observable universe. The Local
Group is near another group called the Virgo Cluster
(because it is in the constellation Virgo), which spans
a diameter of about 10 million light-years. The Virgo
Cluster of galaxies appears to be moving, along with
the Local Group, the Coma Cluster, and other clusters
of galaxies, toward another cluster of clusters - or
supercluster in Hercules, which is something like
400-600 million light-years across. In another
direction, another supercluster called Coma-A1367
seems to be traveling in the same direction as the
Hercules super-cluster. Something over beyond the
Hercules supercluster appears to be pulling everything
toward it. No one knows what it is, though some
presently call it "The Great Attractor" and speculate
that it is composed primarily of "dark matter," which
simply put means an enormous mass of undetected
subatomic particles. There is some evidence to show
that galaxies themselves are arranged in gigantic shells
or spirals, which might be roughly concentric out to
the edge of the universe, which is at best guess
nowadays between 10 and 20 billion light-years away
from the Earth.
An interesting detail of this kind of gigantism is
nothing. That is, we've been speaking of visible
objects - material reality - such as planets, stars and
galaxies, which actually exist in actual regions of
space-time. We take these material objects for granted,
as constants. But astronomers are beginning to show,
for example, that space-time is not uniformly dense
with material objects. The solar system, for example, is

pretty dense. The planets, far away as they seem, orbit
relatively compactly around the Sun, and clouds of
dust and gas and ionized molecules in the form of
"plasmas," not to mention comets and asteroids and
clouds of dirt and grit, also orbit the Sun. There is a lot
of material in our solar system, despite the enormous
voids in between.
The great interstellar voids in the galaxy, empty as
they are to us, are actually broken up by clouds of gas
and dust and nebulae where stars are forming. And
orbiting the outskirts of the galaxy's central bulge are
"globular clusters" of very old stars packed together as
densely as one star per cubic light-year. A galaxy like
M31, which apparently is very similar to the Milky
Way, is an opaque blur in the sky through binoculars,
revealing that, despite the immense distances between
stars, galaxies are in fact very dense. Clusters of
twenty or so galaxies, despite distances between them
of 2 million light-years and more, could also, on the
right scale, be shown to have considerable density.
And so could superclusters.
Yet despite these densities, there is all this empty
space between stars and galaxies and clusters. And to
complicate matters, there apparently are fantastic
bubbles of nothing surrounded by superclusters and
chains of galaxies arranged in filaments between one
supercluster and another. The superclusters, that is,
are not uniformly distributed through intergalactic
space. An area lying some 520 to 780 million
light-years from the Milky Way, and covering 30 x
1024 cubic light-years of space, seems to contain no
galaxies at all. Nothing.
The fact that we can describe these vacant distances
does not mean we comprehend them. In a sense they
prohibit comprehension. And if they prohibit
comprehension, then these distances surely prohibit
travel. When Carl Sagan and Frank Drake said, in a
Scientific American article, that "Electromagnetic
radiation is the fastest and also the cheapest method
of establishing contact [with other worlds] ...
Interstellar space vehicles cannot be excluded a priori,
but in all cases they would be a slower, more
expensive and more difficult means of transportation,"
they put it somewhat mildly. You can get there from
here by bouncing light signals into space, but you can't
get there from here in a spaceship.
What we're really hoping for, when we talk about
exploring the galaxy, is to cross the interstellar
vacancies and stand there on some reddish planet with
three suns perpetually dancing along the horizon -