StarPlot - 2. The Program Display

# StarPlot Documentation

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## 2. The Program Display

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 Chapter 3.

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.

### 2.1. The Chart Display

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.

#### 2.1.1. The chart grid

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."

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.)

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.

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.

#### 2.1.2. The star symbols

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.)

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.

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.

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.

#### 2.1.3. Interacting with the star symbols

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.

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.

### 2.2. The Chart Status and Legend

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.

#### 2.2.1. The chart status information

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.)

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.

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.

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.

The dim magnitude limit may also be changed manually, either via the button bar or in the Chart->Star Filter dialog.

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) apparent magnitude, as seen from Earth, of any star which could hypothetically appear on the chart, given the requirement that all displayed stars have an absolute 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 apparent 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.

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 red to draw attention to the problem. You can fix it either by continued pressing of the button, or by manually setting one or both magnitude limits in the Chart->Star Filter dialog.

#### 2.2.2. The chart legend

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.

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.

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.)

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.

### 2.3. The Button Bar

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.)

Open a StarPlot data file. Also available from File->Open Star Database.
Zoom in (decrease the chart radius) by a factor of two. The chart radius may also be set in the Chart->Define Chart dialog.
Zoom out (increase the chart radius) by a factor of two.
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.
Increase the greater (dimmer) limit on absolute magnitude. This will show you more dim stars.
Rotate the chart clockwise about its axis. The chart orientation may also be set in the Chart->Orientation dialog.
Rotate the chart counterclockwise about its axis.
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.
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.
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.

### 2.4. The File Status Bar

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.

## Continue to Chapter 3...