What Are Spectral Lines?

Spectral lines are bright or dark lines on a uniform and continuous spectrum, caused by photons in a narrow frequency range being more or less than other nearby frequencies.

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Right!

Spectral lines are uniform and continuous

In the radiation spectrum of various celestial bodies, there are often many spectral lines, some are emission lines, and some are absorption lines. Spectral lines are formed by transitions between discrete energy levels in a system.

If E1 and E2 are two discrete energy levels of a certain system, and E2> E1, when the system transitions from E2 to E1, the emission frequency is V = (E2 E1) / h; on the other hand, when the system starts from When E1 transitions to E2, it absorbs radiation with frequency v.

If the emission process has an advantage over the absorption process, an emission line will be generated; otherwise, an absorption line will be generated.

In stellar spectra, spectral lines are caused by transitions between the discrete energy levels of atoms, ions, and molecules. For example, the D1, D2, and H and K lines in the solar spectrum are caused by the transitions between the sodium and calcium ions at discrete energy levels, respectively.

In the radio band, there are also spectral lines. For example, the 21 cm line of neutral hydrogen is caused by the transition between the ultrafine structure energy levels of hydrogen atoms. Ultrafine structure energy levels are generated by the coupling between the atomic nucleus' spin and the total angular momentum of the electrons (see Atomic Ultrafine Structures). Many millimeter-wave spectral lines have been found in interstellar clouds. Most of the radio spectral lines are formed by the transitions of the rotational energy levels of various interstellar molecules.

Spectral lines have also begun to be found in the high-energy bands of X-rays and gamma rays. For example, a line of 58 kilovolts was found in the X-ray spectrum of Capricorn X-1, which may be formed by the transition between the Landau energy levels of electrons moving in a strong magnetic field.

No spectral line is infinitely narrow, but always has a certain width. This width is partly due to the limited resolution of observation instruments, and partly due to the astronomical radiation itself. There are many reasons for this broadening of the spectral lines, but they can be roughly divided into two categories: one is because the energy levels of the microscopic systems that form the spectral lines are not infinitely narrow, but have a certain width. Spectral lines generated by a certain level of energy level must also have a certain width. This width is called the natural width of the spectral line. This effect is called radiation damping. The other type is caused by superposition, because the radiation we observe is the superposition of the radiation of each emission or absorption system.

Generally speaking, the motion state of each emission or absorption system and the state of interaction with surrounding matter are different, and the frequencies they emit or absorb are also different, which results in the broadening of the spectral lines. The thermal Doppler effect, collision damping, statistical widening, rotation, expansion, and turbulence can all be used to widen the spectral line through the superposition effect.