The Sun's spectrum appears mostly continuous because it is a result of the combined emission from a vast number of atomic and molecular processes occurring within the Sun. While individual atomic and molecular processes do emit light at specific wavelengths, the overall spectrum of the Sun is broadened due to several factors:
Thermal radiation: The Sun's surface, known as the photosphere, has a temperature of about 5,500 degrees Celsius. At such high temperatures, atoms and molecules in the Sun are in constant motion, and this thermal motion generates a continuous range of wavelengths through a process called blackbody radiation. The spectrum resulting from this thermal radiation is known as a blackbody spectrum and appears continuous.
Broadening mechanisms: Various mechanisms in the Sun's atmosphere cause the emission lines produced by specific atomic and molecular processes to broaden. These mechanisms include thermal broadening, which is caused by the Doppler effect due to atoms and molecules moving at different speeds, and pressure broadening, which arises from the interactions between particles in a dense environment. As a result, the sharp emission lines are smeared out, contributing to the continuous appearance of the spectrum.
Absorption and re-emission: The Sun's atmosphere contains numerous atoms and molecules that can absorb specific wavelengths of light. These absorbed photons are then re-emitted in random directions, leading to scattering and redistribution of the energy across a range of wavelengths. This scattering and re-emission process also contributes to the continuous nature of the solar spectrum.
Overall, the Sun's spectrum appears mostly continuous because it is a combination of emissions from countless atomic and molecular processes, thermal radiation, and broadening mechanisms. These factors result in a broad spectrum that appears continuous, even though individual processes within the Sun emit light at specific wavelengths.