Editing Extinction (astronomy) (section)

==Measuring extinction towards an object==

To measure the extinction curve for a [[star]], the star's spectrum is compared to the observed spectrum of a similar star known not to be affected by extinction (unreddened).<ref>{{Cite journal
  | last = Cardelli
  | first = Jason A.
  | author-link = Jason A. Cardelli
  | author2 = Sembach, Kenneth R.
  | author3-link = John S. Mathis
 | author3 = Mathis, John S.
  | title = The quantitative assessment of UV extinction derived from IUE data of giants and supergiants
  | journal = Astronomical Journal
  | date = 1992
  | volume = 104
  | issue = 5
  | pages = 1916–1929
  | bibcode = 1992AJ....104.1916C
  | doi = 10.1086/116367
  | issn = 0004-6256
| author2-link = Kenneth R. Sembach
 }}</ref>  It is also possible to use a theoretical spectrum instead of the observed spectrum for the comparison, but this is less common. In the case of [[emission nebula]]e, it is common to look at the ratio of two [[Spectral line|emission lines]] which should not be affected by the [[temperature]] and [[density]] in the nebula. For example, the ratio of [[hydrogen-alpha]] to [[Balmer series|hydrogen-beta]] emission is always around 2.85 under a wide range of conditions prevailing in nebulae. A ratio other than 2.85 must therefore be due to extinction, and the amount of extinction can thus be calculated.