Describe three ways in which astronomers can distinguish between interstellar absorption lines and stellar absorption lines

Question

What will be an ideal response?

Answer 1

Some stellar spectra contain absorption lines that obviously don't belong to the stars because they represent the wrong ionization state. For example, you would expect the spectrum for an O star to have no lines of once-ionized calcium (Ca II) because that ion cannot exist at the high temperatures found in the atmosphere of such a hot star. These lines must have been produced in cool material between the star and Earth.

The widths of interstellar absorption lines also give their hidden identities. In the atmosphere of a star, the gas is so dense that the atoms collide with each other often and disturb the atomic energy levels.That blurs the spectral lines and makes them broader. Also in a hot gas, the atoms move so rapidly that they produce Doppler shifts, some red and some blue, which also broaden the spectral lines. In contrast, interstellar gas is cold and has a very low density, so atoms rarely collide. That means interstellar absorption lines are extremely narrow.

Lastly, interstellar lines are often split into two or more components. The multiple components have slightly different wavelengths and must have been produced when the light from the star passed through different clouds of gas on its way to Earth. Because individual clouds of gas have slightly different radial velocities, they produce absorption lines with slightly different Doppler-shifted wavelengths.