PX282 - B2 - typical spectral lines

• typical spectral lines include $H$, $He$, ionized metals, and metals

  • in astronomy, metals are everything but $H$ or $He$

• $H$ and metals occupy most of the spectrum

• $He$ and ionized metals occupy the hot end, ie: shorter wavelength

• when stars are too hot:

  • $H$ is ionized, therefore there are no absorption lines from it (no electron)
  • $He$ has very tightly bound electron, which needs high temperature to raise from the ground state, which allows absorption at visible wavelengths
  • metals get ionized at high temperatures, and are very prevalent for sun-like stars

• when stars are too cold:

  • $H$ has a few electrons in $n=2$ state
  • balmer lines, which are in the visible range, need electrons in $n=2$ state to be visible
  • lyman lines, which are formed, are in UV
  • molecules are seen only in the coolest stars, as hotter stars break them up

• key points:

  • temperature controls the lines seen, through excitation and ionization
  • if lines are not seen, it does not mean that an element is not present

image: byju's