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:

Photograph taken by Bill Schoening with the 4-m Mayall Telescope at Kitt Peak on Ektachrome film

Next, a picture taken in 2000:

Image taken by Adam Block with the Kitt Peak Visitor Center's 0.4 m telescope and a CCD camera. Courtesy of Adam Block, NOAO and NSF.

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

• arrange the 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.

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.