Classical Cepheid variable stars have been important indicators of
extragalactic distance and Galactic evolution for over a century. The Spitzer
Space Telescope has opened the possibility of extending the study of Cepheids
into the mid- and far-infrared, where interstellar extinction is reduced. We
have obtained photometry from images of a sample of Galactic Cepheids with the
IRAC and MIPS instruments on Spitzer. Here we present the first mid-infrared
period-luminosity relations for Classical Cepheids in the Galaxy, and the first
ever Cepheid period-luminosity relations at 24 and 70 um. We compare these
relations with theoretical predictions, and with period-luminosity relations
obtained in recent studies of the Large Magellanic Cloud. We find a significant
period-color relation for the [3.6]-[8.0] IRAC color. Other mid-infrared colors
for both Cepheids and non-variable supergiants are strongly affected by
variable molecular spectral features, in particular deep CO absorption bands.
We do not find strong evidence for mid-infrared excess caused by warm (~500 K)
circumstellar dust. We discuss the possibility that recent detections with
near-infrared interferometers of circumstellar shells around delta Cep, l Car,
Polaris, Y Oph and RS Pup may be a signature of shocked gas emission in a
dust-poor wind associated to pulsation-driven mass loss.Comment: Accepted by The Astrophysical Journal on Nov 11, 200