Contents:

• Overview
• Collaboration Policy
• The Goal
• Data File Format
• The Matlab Figure
• Your Task
• Extra Credit

Topic: Stars and Constellations
Related Reading: file I/O, plotting
Due: Wednesday, 15 April 2009, 11:59pm

Collaboration Policy

For this assignment, you must work individually in regard to the design and implementation of your project.

Please make sure you adhere to the policies on academic integrity in this regard.

Our goal for this problem is to create an image based upon underlying data about the stars and constellations. Our version of the project is a variant of a nifty assignment designed by Karen Reid at the University of Toronto.

As an example, here is a figure that shows a view of the night sky (click on the image for a larger version):

Here is a second version of the image that also includes the display of several major constellations.

The raw data for this assignment is provided in the form of two different text files:

• stars.txt
  The raw data for each individual star.

  This file has one line for each star, with each line containing six pieces of information, separated by spaces. Those six fields are respectively

  • x-coordinate
    This describes the horizontal position of the displayed star on a scale from -1 to +1.
  
  
  • y-coordinate
    This describes the vertical position of the displayed star on a scale from -1 to +1.
  
  
  • z-coordinate
    This describes the relative distance to the star on a scale from -1 to +1. For this assignment, we will not use the z-coordinate data.
  
  
  • Henry Draper number
    For the purpose of cataloging astronomical data, each star has been assigned a unqiue identification number, known as its Henry Draper number.
  
  
  • magnitude
    This is a floating-point number that descibes the apparent magnitude of a star. It is a logarithmic scale with smaller numbers denoting brighter stars (with Sirius as the brightest star with a magnitude of -1.44). We portray brightness in our image by drawing all stars in white, but using bigger size for brighter stars. We use the formula brightness = 15 * 10^(mag/-2.512)
  
  
  • Harvard Revised number
    This is another identification number for cataloguing stars. For this assignment, we will not use the Harvard Revised numbers.


• constellations.txt
  The constellation file has the following format. A single file has information about multiple constellations. Each individual constellation begins with a line containing its name. The following line contains a single integer n, denoting the number of line segments to be drawn for the constellation. Following that are n lines, one for each segment, containing a pair of Henry Draper identification numbers for the two stars to be connected by the segment.

  For example, the data corresponding to the Big Dipper appears as follows:

  Big Dipper
  7
  120315 116656
  116656 112185
  112185 106591
  106591 103287
  103287 95418
  95418 95689
  106591 95689

To get the basic star image in Matlab, we rely upon the scatter command. We use the form scatter(X,Y,S,C,'filled') where X is a vector of x-values, Y is a vector of y-values, S is a corresponding vector of scales (brightnesses for our application), and C designates color. For this assignment, we use 'w' as the color for all stars. The final parameter 'filled' designates that the circles should be drawn as filled rather than the default outline.

To plot white stars against a black background, we need to configure the basic figure window. As with previous plots, it is also important that we force the two axes to be drawn with an equivalent scale so that the image is circular. You may use the following function in order to setup the figure window.

function plotSky
   % This sets up a basic figure window for our plot of the sky.
   % It sets the background to black and sets the axes to be
   % equal but undisplayed.
   %
   % This function leaves the size as the default, but the user
   % can manually resize the figure window for a larger view.
   clf;
   hold off;               % start a new figure
   axis equal;             % ensure equal treatment of x and y axes
   axis off;               % do not display the actual axes
   set(gcf,'Color','k');   % set background color to black
   hold on;

The constellation line segments are drawn based on the use of the plot command for each individual line segment, using the x and y coordinates for the two designated stars. To get all segments from the same constellation in the same color, but different constellations in different colors, we use the following hack. We have a string 'ymcrgb' which contains six character codes for the colors yellow, magenta, cyan, red, green, and blue. For each constellation, we pick one of those characters and use it when calling plot to control the color. Then when we move to a new constellation, we advance one character in that control string (wrapping around to the beginning if necessary).

Although we expect that in completing this assignment, you should start by working on the function to plot the stars, and then the function to plot the constellation, the final image will look nicer if you plot the constellations first and the stars second, because then the stars will be drawn on top of the constellation lines (making it easier to see the stars that define the constellations).

As af final note, our image includes a legend to identify the names of the constellations. Providing that feature is considered extra credit for the sake of this assignment.

To divide up the work more clearly, you are to write four different functions, described as follows.

• stars = parseStarFile(filename)
  We recommend that you return an "array of structures", so that each entry of the array is a structure having fields such as s.x, s.magnitude, s.draper, and so on. If not using a structure, then you will want a two-dimensional table where you rely on a convention that column 1 is the x-coordinate, column 2 the y-coordinate, and so on.

  Note: As an example of the structure format, we have saved our stars array in MATLAB's binary file format (download: stars.mat). If you would like to explore our structure, save this file in your MATLAB directory and then type load stars.mat from the MATLAB command window. This will add our stars variable to your current workspace, and then you can examine it with the workspace editor.



• plotStars(stars)
  This function should take the data returned by parseStarFile and plot it on the current figure (presumably one that was already initialized with our plotSky function).

  Note: If you are having trouble reading the star data yourself, you can use the stars.mat binary provided above to get our stars structure, and then test plotStars on that version.



• constellations = parseConstellationFile(filename)
  We recommend that each individual constellation be represented using either a cell array or a structure, with the first entry being the name of the constellation and the second entry being an n x 2 array with a row for each line segment in the constellation, and the columns of that row being the two Henry Draper identification numbers for those stars.

  Note: As an example of the structure format, we have saved our constellation array in MATLAB's binary file format (download: constellations.mat). If you would like to explore our structure, save this file in your MATLAB directory and then type load constellations.mat from the MATLAB command window. This will add our constellation variable to your current workspace, and then you can examine it with the workspace editor.



• plotConstellations(constellations, stars)
  This function should plot all the constellations on the current figure. Note that the function must be sent both the constellation structure and the stars data, because the constellation data makes references to Draper numbers of stars, and you will need to look up the x- and y-coordinates of the stars in order to be able to plot each line segment.

  Note: If you are having trouble reading the constellation data yourself, you can use the constellation.mat binary provided above to get our constellation structure, and then test plotConstellations on that version (perhaps even usng our stars structure as well).

Generate the legend when plotting the constellations.