Expansion of the Crab Nebula

The Crab Nebula is the result of a supernova explosion which was seen by
astronomers in Asia in the year 1054 AD. The explosion blew the outer layers
of gas off a star, and the ejecta has been flying outwards through space at
high speed ever since. Here are a couple of pictures showing the expansion.

First, a picture taken in 1973:

[Image]

Next, a picture taken in 2000:

[Image]

Note the two stars marked "A" and "B". To measure the distance between them
(or other features), do the following:

   * close the three toolbar sections of the Netscape browser window, so
     that you can display enough of a picture to show both stars A and B on
     the screen at once
   * type "Shift-PrtScrn" -- this saves the current screen contents into the
     clipboard
   * start the Microsoft Photo Editor program
   * choose "Edit" -> "Paste as New Image"
   * choose "View -> Measurement units -> pixels"
   * choose the "dashed-square" tool; it will show the cursor location in
     pixels down in the lower-left-hand corner of the screen
   * use the "magnifying-glass" tool to zoom in/out as desired

You can note the pixel position (x, y) for each star (or any other feature),
and then calculate distances between features by using the Pythagorean
Theorem.

Here's an animated image which blinks back and forth between the two. Note
how the stars (mostly) remain fixed between the exposures, but the gas
moves.

[Image]

You will need to use the spectrum of the gas to estimate the speed with
which it is expanding. The spectrum is show below. The units of wavelength
are Angstroms: one Angstrom is 10^(-10) meters.

[Image]