Objects begin to glow when they become incandescent. Incandescence is a name given when objects begin emitting thermal energy in the visible part of the electromagnetic spectrum. This usually starts at about 500-525 degrees C.
As an object heats up two things happen. Firstly they emit more radiated energy. Secondly, the peak wavelength of the radiated energy begins to shift towards the shorter wavelength. So physically the distribution of the radiation increases AND shifts. When it has shifted sufficiently towards the visible portion of the spectrum, it begins to radiated in the visible part of the spectrum. When an object emits enough energy in the visible spectrum, then it will be seen by the human eye as glowing red. If it heats up enough its peak wavelength shifts into the middle of the visible band, then it will appear to glow white. The sun is an example of an abject with its peak emission in the middle of the visible band. Some objects are not efficient at radiating energy, these objects may need to be at a higher temperature before they begin glowing.
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