50 2. LOOKING OUTWARD
2.6.3 SUPERCLUSTERS AND VOIDS
e Local Group, the Virgo Cluster, and a few other small clusters of galaxies are known to
be physically bound together because of the mutual gravity they feel for each other. Together
they make up what is known as the Local Supercluster, or the Virgo Supercluster. And so a
supercluster can be thought of as a cluster of clusters of galaxies.
Between the superclusters are large regions that are relatively free of galaxies. ese are
called voids, and they are roughly spherical in shape. Superclusters on the other hand are often
shaped like elongated filaments or flattened sheets. e typical size of both superclusters and
voids is roughly 100 Mpc; they are the largest structures in the universe [Ryden, 2017, pp. 10,
209].
2.7 CONSTELLATIONS AND THE VIEW FROM EARTH
e traditional patterns of stars visible to the naked eye are called constellations. We have no
depth perception when looking at the celestial sphere, so the constellations are essentially two-
dimensional patterns that tell us nothing per se about the three-dimensional arrangement of the
stars.
It turns out that the visual appearance of most constellations is wildly misleading. Many
of the brightest stars visible to the naked eye are quite distant, while most of the nearest stars
are too faint to see without a telescope. Some stars emit more light than others, by many powers
of ten—and so just because a star appears bright, it does not mean that it is relatively nearby.
And so most of the constellation patterns are chance arrangements of stars that simply
happen to be roughly in the same direction, but at vastly different distances. ere are a few
exceptions however. e seven bright stars that form the familiar pattern of the Big Dipper in
the Northern sky, for example, really are relatively close to each other in space. ey form what
is essentially a very sparse, nearby open star cluster. e same is true for many of the bright stars
in Orion, and the V-shaped head of Taurus the Bull.
If we include other astronomical objects besides stars, the appearance in the night-sky can
be even more misleading. Figure 2.19 is a time exposure, with an ordinary camera lens, of comet
Hale–Bopp, which graced the skies in 1997. e trees in the foreground are blurred because the
camera was tracked to counteract Earth’s rotation, and so track with the stars during the roughly
15-min exposure.
e comet was roughly 1 AU from Earth when this picture was taken; the stars, of course,
are in the background, many light years distant. e visible comet was nothing but sunlight
reflected off a large but insubstantial cloud of dust (the white part) and glowing gas (the blue
part) that stretched tens of millions of miles. But the solid part of the comet—its nucleus, hidden
in the depths of the dust cloud—was only some 60 km across.
Figure 2.19 also shows another object. e faint, somewhat fuzzy blob to the lower left
of the head of the comet is the Andromeda Galaxy, some 2.5 million light years distant.
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