User`s guide

Appearance of the Sky 51
you to see many galaxy images at the same
time, producing some spectacular views.
Visibility range: Use this slider to display
only those galaxies that lie within a certain
range. If you move the slider all the way to
the left, only those nearby galaxies that are
bright enough will be displayed, moving
the slider all the way to the right will
display all the galaxies in the Tully
Collection at the same time.
Colour saturation: Each different type of
galaxy is represented with its own colour
image. The dots used to represent a distant
galaxy are the same colour as the full-size
image. Moving this slider to the right
makes the colour of the dots closer to that
of the galaxy image, while moving the
slider to the left strips away all colour
distinctions until all galaxies are
represented by white dots. The advantage
of having the slider far to the right is that
you can quickly identify galaxy type, for
example, a cluster of blue dots indicates a
group of elliptical galaxies. The
disadvantage is that the sky becomes
gaudy and unrealistic-looking.
Brightness: When a galaxy is too far away
for its image to be visible, Starry Night
will represent its position in space with a
dot. This slider controls the brightness of
these dots. The ideal position of this slider
will depend on the brightness of your
monitor and the lighting conditions in your
computer room. Moving this slider to the
right makes the dots larger and brighter,
while moving it to the left makes the dots
smaller and dimmer.
Selecting Filaments and Groups in
the Tully Database
When exploring the Tully Collection of
galaxies, you can get more information by
pointing the cursor at an object (you
should see the cursor icon change to a
pointer) and then right-click or Ctrl-click
(Macintosh). This opens a contextual
menu with specific options for the object
you have clicked on. Some of the options
specific to the Tully Collection are
described below. But first lets look at some
terms you may not be familiar with.
Groups are gravitationally bound clumps
of galaxies. Membership can range from a
few to a few thousand galaxies. Large
groups are also known as clusters.
Filaments - or their kin walls, clouds, and
arcs - are formations of galaxies in
expanding space, which are not
necessarily gravitationally bound. These
formations can be string-like or sheet-like.
They can have curvature like a bow or like
a sail. Filaments can be grouped together
into even larger structures called filament
families.