Starship Asterisk*

After looking at the APOD photo of a time-exposure of the night sky (08Dec07) which produced star trails, I was wondering if a cross-section of the trails would produce a method of identifying the stars as well as comparing what is in the sky from night to night.

By cross-section I mean a high-resolution narrow strip image perpendicular to the star trails which would look like a long row of dots. If you enlarged
these you would you be able to identify the object associated with each dot? I wondered what the difference would be between these "dot-strip" images taken at different times of the year? Also, would the spectrum of star trails change as you measured along a star's trail?

I hope this makes some sense.

Hey nycpaull,

To identify the objects making the trails, one could just look at a single ordinary photograph. The star trail APOD http://antwrp.gsfc.nasa.gov/apod/ap071208.html was produced from multiple short exposures (over a single night), so any one of those would show identifiable stars. Taking "cross-sections" wouldn't be needed and wouldn't help with identification, if I understood what you meant. To produce dots perpendicular to star trails, you'd have to point your camera closer to the north celestial pole for each exposure as well as compensate for Earth's rotation, which seems more trouble than it's worth.

As for the star spectra along the trails, they do indeed vary! You can see in the APOD that the trails fade out near the horizon, and not just because of the clouds. Just as the Sun appears dimmer and redder near the horizon, so do stars, although I don't see reddening in the photo. "Extinction" is the term for this dimming and reddening due to absorption and scattering of light by atmospheric molecules and aerosols. "Airmass" is a relative measure of how much atmosphere you're looking through at a given zenith distance. high airmass = observing close to the horizon = not good (for astrometry at least).

"First-order" extinction dims starlight. "Second-order" extinction is colour-dependent; blue light fades faster than red light as airmass increases, which is consistent with what we see happen to the Sun. Having to compensate for atmosphere is a royal pain in the &$#... at least until we find the "atmosphere" check box and clear it!