FSC15307+3253 is an Ultra-Luminous InfraRed Galaxy (ULIRG), with a luminosity between 8 and 1000 µm of approximately 2×1013 L⊙, possibly the largest known at this time. The "FSC" refers to Faint Source Catalogue, one of the source catalogs produced by the IRAS infrared survey mission. The emission is believed due to some combination of starburst activity and accretion onto a super-massive black hole, producing primary radiation at shorter wavelengths which is mostly blocked by obscuring dust, which is in turn heated and re-radiates in the infrared. The redshift of the source is, indicating a distance of the order of 7 billion light years.