Black Body Radiation

A black body is a theoretical object that absorbs all of the radiation that hits it – none is reflected. Of course the radiation on is re emitted – a perfect absorber of radiation is also a perfect emitter, since if absorbs radiation of all wavelengths, if must also emit at all wavelengths - and when it is re emitted, the graph of intensity against wavelength is the shape of a block body curve characteristic of the temperature of that body.
Objects at around room temperature emit mainly infra-red radiationwhich is invisible. The sun emits most of its radiation at visible wavelengths, particularly yellow .A simple example of a black body radiator is a furnace. If there is a small hole in the door of the furnace heat energy can enter from the outside. Inside the furnace this is absorbed by the inside walls, and re emitted at wavelengths defined by the temperature of the walls. The radiation bounces about inside furnace until it eventually escapes through the hole. The radiation that escapes contain radiation of all wavelengths with intensities defined by the black body curve.

Stars are also approximate black body radiators. Most of the light directed at a star is absorbed. It is therefore capable of absorbing all wavelengths of electromagnetic radiation, so is also capable of emitting all wavelengths of electromagnetic radiation.