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<title>StarPlot - 2. The Program Display</title>
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<h1>StarPlot Documentation</h1>
<h2><a href = "index.html">Contents</a> | <a href = "ch1.html">Previous</a> |
<a href = "ch3.html">Next</a></h2>
<h2>2. The Program Display</h2>
<p> The StarPlot user interface consists mainly of one graphical window
that displays a perspective view of interstellar space. Many of the
StarPlot features can be accessed through this window using only
the mouse, without any need to access the program menu or remember
keyboard shortcuts. This chapter will describe these features.
Some menu options will be mentioned, but they will be described
more systematically in <a href = "ch3.html">Chapter 3</a>.
</p>
<p> As shown in the diagram below, the main StarPlot window consists of
six parts: the menu bar at top; the chart display at center; the
chart status at upper left; the chart legend at upper right; the
button bar along the left edge; and the file status bar at bottom.
All of these but the menu bar will be described in this chapter.
</p>
<img src = "images/ui-parts.png" alt = "[Diagram of the StarPlot interface]" />
<a name = "sec21"></a>
<h3>2.1. The Chart Display</h3>
<p> The StarPlot chart display shows a perspective view of a spherical
volume of interstellar space, displayed as if the volume was being
viewed from some point outside it. Within this sphere are shown
a number of colored dots of different sizes representing stars.
Some (or all) of these are labeled in some way.
</p>
<h4>2.1.1. The chart grid</h4>
<p> Think of this sphere as a globe that you are viewing at an oblique
angle. Only stars within the interior of the globe (having a
distance from the globe center of less than the globe radius)
are shown. The equator of the globe is
represented by an ellipse, due to the angle. (Viewed from above one
of the poles, the equator would
look like a perfect circle; viewed from the side, edge-on, it
would be only a horizontal line.) The plane of the globe's equator is
parallel to the plane defined by the equator of the Earth and of
the celestial sphere. The axis
of the globe is parallel to the axis of the Earth and of the
celestial sphere. In the simplest case when the Earth is at the
center of the chart, the word "parallel" in the previous two sentences
should be replaced with the word "identical."
</p>
<p> As described later, the globe may be tilted by different amounts from
your point of view. If you are viewing it from a point above its
equator, the nearest point of its equator to you is towards the
bottom of the computer monitor. When viewed from below,
the nearest point of the globe's equator to you is towards the
top of the computer monitor. To distinguish between the two cases,
the equator is shown in gray in the first case, but in dark blue in
the second case. (As a reminder of this convention, the top half of
the circular outline of the globe is always shown in gray, and the
bottom half is always shown in dark blue. Perhaps these colors
will be user-settable in a future version of StarPlot.)
</p>
<p> On the equatorial plane of the chart are drawn two perpendicular,
horizontal axes intersecting at the center. If the chart is centered
on the Earth, these two axes represent the four directions of
0 hours, 6 hours, 12 hours, and 18 hours right ascension (at 0 degrees
declination). Otherwise they are parallel to the "real" such
directions as seen from the Earth. The four points where these
axes intersect the chart equator are labeled accordingly.
</p>
<p> Together, the equator, horizontal axes, and circular outline of the
globe are referred to as the chart grid. If you find them distracting
or confusing, you will be happy to know that they can be made
invisible (or visible again) through the menu option
Options->Toggle Grid.
</p>
<h4>2.1.2. The star symbols</h4>
<img src = "images/star-parts.png" alt = "[Star-related symbols]" />
<p> Above is shown a close-up of part of the StarPlot chart display.
This picture illustrates that every star shown in the display
is represented by a colored dot and may in addition have a label
and/or a vertical reference bar. (Part of the chart equator,
chart outline, and the direction parallel to 12 hours of right
ascension, all described in the previous section, are also shown.)
</p>
<p> The color of a star symbol is based on the star's spectral type.
The size is based either on the luminosity (absolute magnitude) of the
star, or upon its Morgan-Keenan luminosity class, depending upon
the selection in the Options->Star Diameters menu option.
The label may or may not be shown, or may be given only as a numerical
index, depending upon the selection in the
Options->Star Labels menu item. The label, if shown, is
always above and to the right of the star symbol.
</p>
<p> The vertical reference bar indicates how far a star is above or
below the equatorial plane of the chart. It is a line segment,
parallel to the axis of the globe, that extends from the star at
one end to the equatorial plane at the other end. (The intersection
with the equatorial plane is marked by a small ellipse.) The bar
is colored gray if a star is above the equatorial plane of the chart,
and dark blue if a star is below it.
</p>
<p> Without the vertical reference bars, it would be impossible to judge
above which point on the chart equatorial plane a star lay, or how
far above the plane it was. Nevertheless, you can make the reference
bars invisible (or visible again), for instance for aesthetic reasons,
via the Options->Toggle Bars menu item.
</p>
<h4>2.1.3. Interacting with the star symbols</h4>
<img src = "images/star-popup.png" alt = "[Pop-up window of star vital stats]"
style = "float: right;" />
<p> To find out information about a specific star shown on the chart,
you can right-click its symbol with the mouse. This will pop up a
small window giving the star's vital statistics. An example is
shown at right.
If you (or your distribution) have compiled StarPlot against a
reasonably current version of the GTK+ library, clicking on the
star's name, with the small arrowhead pointing at it, will reveal
all other names by which the star is known. (Click on the arrowhead
again to hide the other names.) Hit the "Close" button to get rid of
the small window.
</p>
<p> To re-center the chart about a different star, you can left-click
on the symbol for that star. This is an interesting way to explore
the space surrounding a given star. For instance, you can start
with a chart centered at the Sun. Left-clicking on the symbol for
Sirius will make the center of the chart leap to that star, and you
can continue to travel, leap by leap, as far away as you want
(as long as the file being used includes stars at that distance).
Note that the radius of the chart always remains the same, so the Sun
will quickly disappear off the edge.
</p>
<a name = "sec22"></a>
<h3>2.2. The Chart Status and Legend</h3>
<p> Though the chart status and chart legend provide useful information,
you may wish to turn them off for aesthetic reasons. This may be
done via the Options->Toggle Legend menu item.
</p>
<h4>2.2.1. The chart status information</h4>
<img src = "images/status.png" alt = "[The chart status]" style = "float: left;" />
<p> The chart status, at the upper left of the StarPlot main window,
provides information about the
current position, size, and limiting magnitudes of the chart.
The first bit of information is the "Location of Chart Origin".
If the chart is centered at the Sun, it will read "0 LY from Earth".
Otherwise, the spherical coordinates of the chart center will be
given: Right Ascension, Declination, and distance from Earth.
(You can change the center of the chart by left-clicking on a star
as described above, or through the Chart->Define Chart
dialog.)
Below this will always be given the radius of the chart globe,
in light-years. (This may be changed via the button bar or the
Chart->Define Chart dialog.)
</p>
<p> The last two bits of information are the brightest and dimmest
absolute magnitudes of stars to display on the chart. Only stars
whose absolute magnitudes are greater (dimmer) than the current bright
magnitude limit, and smaller (brighter) than the current dim magnitude
limit, are displayed.
</p>
<p> By default, the bright magnitude limit is set to a large negative
number, -25, which is much brighter than any existing star. Hence
this limit is effectively ignored. Indeed, in the most recent
StarPlot versions (0.95.5 or newer),
it will not be displayed if set to the default
value. You can set it to a meaningful non-default
value only via the Chart->Star Filter dialog.
</p>
<p> This is not the case for the dim magnitude limit. By default, the
dim magnitude limit is changed whenever you change the radius of
the chart! This is done so that the chart includes a reasonable
number of stars. Only the brightest stars within the chart volume
are shown if you are viewing a very large region of space; otherwise
the chart would look cluttered and unreadable. But all the stars
within the chart volume are displayed if you are looking at a
relatively small region, twenty light-years across.
</p>
<p> The dim magnitude limit may also be changed manually, either
via the button bar or in the Chart->Star Filter dialog.
</p>
<p> As of StarPlot version 0.95.5, a second number is shown in parentheses
after the dim magnitude limit. This number is the greatest (dimmest)
<em>apparent</em> magnitude, as seen from Earth,
of any star which could hypothetically
appear on the chart, given the requirement that all displayed stars
have an <em>absolute</em> magnitude brighter than the dim magnitude
limit. This parenthesized value is useful if you know that you are
using a star catalog with a hard limit on its <em>apparent</em>
magnitude. For instance, the SKY2000 catalog excludes stars dimmer than
apparent magnitude 8.0. If the number that appears in parentheses
is greater (dimmer) than the apparent-magnitude cutoff of your
star catalog, be aware some stars that should in principle appear on
the display may be missing due to their absence from the catalog.
</p>
<p> Finally, note that if the dim magnitude limit is smaller (brighter)
than the bright magnitude limit, your chart cannot display
any stars! If this condition occurs, likely due to a mistake,
both limits will be displayed in <span style="color: red;">red</span>
to draw attention to the problem. You can fix it either by
continued pressing of the
<img src = "images/button-dimmer.png"
alt = "increase magnitude limit (show more stars)" /> button,
or by manually setting one or both magnitude limits in the
Chart->Star Filter dialog.
</p>
<h4>2.2.2. The chart legend</h4>
<img src = "images/legend.png" alt = "[The chart legend]" style = "float: left;" />
<p> Shown at left is the chart legend, located at the upper right of
the main StarPlot window. The legend serves three functions.
It depicts the colors used for star symbols to represent each spectral
type; this is self-explanatory. It shows which spectral types are
currently being displayed; and finally, it allows you to select the
spectral types to be displayed.
</p>
<p> The spectral types of star currently permitted to be displayed are
shown with a filled circle in the legend, while those not permitted
to be displayed are shown with a hollow circle. For instance, the
legend at left corresponds to a chart in which type G and type K stars
(those having about the same surface temperatures as our Sun) are not
displayed. Note that the legend is prescriptive, not descriptive.
Even though Wolf-Rayet and type O stars are permitted to be displayed
in this case, most probably none of these rare stars exist within the
boundaries of the chart.
</p>
<p> Toggling whether or not a particular spectral type of star may be
displayed is very easy; simply left-click once on the appropriate
star symbol in the chart legend. That is, clicking on the filled
red circle would prevent type M stars from being shown, and clicking
on the hollow yellow circle would permit type G stars to be displayed.
(This function may also be performed through the
Options->Star Filter dialog.)
</p>
<p> In the legend, "wd" stands for White Dwarf, a very hot yet small
(and therefore dim) type
of star formed near the end of many stars' life cycles. "Non-stellar"
indicates an object which is not a star; for instance, a nebula.
Finally, "unknown" indicates a star whose spectral type is unknown.
</p>
<a name = "sec23"></a>
<h3>2.3. The Button Bar</h3>
<p> The button bar includes buttons for ten commonly used functions
that could otherwise be accessed only through the StarPlot menu
system. From top to bottom, they are as follows. Note that the
first five buttons all, in some way, change the set of stars displayed
on the chart. The last five buttons, on the other hand, only change
the angle at which the chart is viewed. (Prior to StarPlot version
0.95.5, these buttons were in a different order.)
</p>
<p> <img src = "images/button-open.png" alt = "[Open star database...]" />
Open a StarPlot data file. Also available from
File->Open Star Database.
<br /> <img src = "images/button-zoomin.png" alt = "[Zoom in]" />
Zoom in (decrease the chart radius) by a factor of two.
The chart radius may also be set in the
Chart->Define Chart dialog.
<br /> <img src = "images/button-zoomout.png" alt = "[Zoom out]" />
Zoom out (increase the chart radius) by a factor of two.
<br /> <img src = "images/button-brighter.png" alt = "[Decrease magnitude limit (Show fewer stars)]" />
Decrease the greater (dimmer) limit on absolute magnitude. This
will show you fewer dim stars. This limit may also be set in
the Chart->Star Filter dialog.
<br /> <img src = "images/button-dimmer.png" alt = "[Increase magnitude limit (Show more stars)]" />
Increase the greater (dimmer) limit on absolute magnitude. This
will show you more dim stars.
<br /> <img src = "images/button-cw.png" alt = "[Rotate chart clockwise about its axis]" />
Rotate the chart clockwise about its axis.
The chart orientation may also be set in the
Chart->Orientation dialog.
<br /> <img src = "images/button-ccw.png" alt = "[Rotate chart counterclockwise about its axis]" />
Rotate the chart counterclockwise about its axis.
<br /> <img src = "images/button-up.png" alt = "[Tilt chart north pole towards you]" />
Tilt the north pole of the chart more towards you. If you press
this button enough, you will view the chart from directly above
its north pole.
<br /> <img src = "images/button-down.png" alt = "[Tilt chart south pole towards you]" />
Tilt the south pole of the chart more towards you. If you press
this button enough, you will view the chart from directly below
its south pole. This may be confusing since pressing one of the
rotation buttons will make the chart appear to rotate in the opposite
sense from that expected.
<br /> <img src = "images/button-earthview.png" alt = "[Set chart orientation as seen from Earth]" />
Orient the chart so it is viewed from the same angle at which the
corresponding sphere of space in reality is seen from Earth. (In case
you're wondering, the button icon is supposed to represent
the Big Dipper.) This is not a permanent effect; each time you
change the location of the chart center, you will have to click this
button again in order to make the view be "as seen from Earth."
This button is disabled while the Sun is within the bounds of the
chart. Note that this button is new in StarPlot version 0.95.5.
</p>
<a name = "sec24"></a>
<h3>2.4. The File Status Bar</h3>
<img src = "images/filebar.png" alt = "[The file status bar]" />
<p> The file status bar, at the bottom of the StarPlot main window,
shows a little information about the currently open file(s).
If just one file is being viewed, it displays the number of stars
shown out of the total number available in the file, and gives the
file name. If more than one file is being viewed, it still displays
the number of stars shown and available, but only the number of
open files is given ("Viewing 17 of 3811 stars in 2 files"), not
the name of each file.
</p>
<h2><a href = "ch3.html">Continue to Chapter 3...</a></h2>
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