Why is an ultraviolet space telescope needed to observe the interstellar absorption lines of most chemical elements, while the absorption lines of the same elements, but located in the atmospheres of stars, can be studied using a conventional optical ground-based telescope?
The density of the interstellar medium is very small, so atoms collide with each other very rarely and are in this ground state, in which the electron is located in the lowest orbit. Therefore, interstellar absorption lines are formed during the transition of electrons from a lower level to higher ones. In most of the most common atoms in nature (H, He, C, N, O), these transitions have high energy and therefore the absorption lines lie in the far ultraviolet region of the spectrum, accessible only to a space telescope.
In stellar atmospheres, density and temperature are high, and atoms are constantly excited as a result of frequent collisions. Therefore, they absorb light when the electron transitions from the second, third levels to higher levels. The energy of these transitions is low and the absorption lines lie in the visible region of the spectrum.
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