entire focal plane. The IPC was sensitive to x-ray emissions between 0.4 and 4 keV and capable of ~1 arcmin spatial resolution. The Solid State Spectrometer (SSS) was a cryogenically cooled silicon drift detector. The SSS was sensitive to x-ray emissions between 0.5 and 4.5 keV. The cryogen keeping the SSS at its operational temperature ran out, as expected, in October 1979. Bragg Focal Plane Crystal Spectrometer (FPCS) was a Bragg crystal spectrometer. The FPCS was sensitive to x-ray emissions between 0.42 and 2.6 keV. Additionally, the Monitor Proportional Counter (MPC) was a non-focal plane, coaxially-mounted proportional counter that monitored the x-ray flux of the source being observed by the active focal plane instrument. Two filters could be used with the imaging detectors: The Broad Band Filter Spectrometer consisted of aluminum and beryllium filters than could be placed into the x-ray beam to change the spectral sensitivity. The Objective Grating Spectrometer transmission gratings. Riccardo Giacconi was the principal investigator for all of the experiments on board Einstein. Scientific results Einstein discovered approximately five thousand sources of x-ray emission during its operation and was the first x-ray experiment able to resolve an image of the observed sources. X-ray background Surveys by early x-ray astronomy experiments showed a uniform diffuse background of x-ray radiation across the sky. The uniformity of this background radiation indicated that it originated outside of the Milky Way Galaxy, with the most popular hypotheses being a hot gas spread uniformly throughout space, or numerous distant point sources of x-rays (such as quasars) that appear to blend together due to their great distance. Observations with Einstein showed that a large portion of this x-ray background originated from distant point sources, and observations with later x-ray experiments have confirmed and refined this conclusion. Stellar x-ray emissions Observations with Einstein showed that all stars emit x-rays. Main sequence stars emit only a small portion of their total radiation in the x-ray spectrum, primarily from their corona, while neutron stars emit a very large portion of their total radiation in the x-ray spectrum. Einstein data also indicated that coronal x-ray emissions in main sequence stars are stronger than was expected at the time. Galaxy clusters The Uhuru satellite discovered x-ray emissions from a hot, thin gas pervading distant clusters of galaxies. Einstein was able to observe this gas in greater detail. Einstein data indicated that the containment of this gas within these