What is spectrum emission?

Spectrum emission is when an excited body releases radiation in a variety of wavelengths.

In this case, the body can be anything: a table, a star, a hot piece of metal, or a cloud of gas. Any matter can be considered a body in this definition. As an easy example, I prefer a hot bar of metal as it is easy to visualize.

When you heat up a bar of metal, it begins the color of the metal; for iron this is grey, for gold the color is yellow. As it heats, however, the colors change. Iron will progress from a deep red to a yellow, and eventually a bright white. These colors are important, as these emitted photons are in decreasing wavelengths.

The color you see is the peak wavelength of the light in your visual range. When the bar is cool, the grey light you see is only being reflected by the bar. There is light you do not see, however, in much longer wavelengths being emitted by the bar as radiation. When the bar is white, there are photons being emitted in shorter wavelengths than you can see.

This means that the bar is emitting light in a range of frequencies, or a spectrum.

All elements release photons in specific wavelengths more than others, on what are called emission lines. For example, the first spectrum line for hydrogen is called h-alpha, and is at 656.2 nm, and is used in astronomy frequently.