Editing Redshift (section)

=== Doppler blueshift ===
[[File:Redshift blueshift.svg|thumb|Doppler redshift and blueshift]]
[[Doppler effect|Doppler]] blueshift is caused by movement of a source towards the observer. The term applies to any decrease in wavelength and increase in frequency caused by relative motion, even outside the [[visible spectrum]]. Only objects moving at near-[[Relativistic speed|relativistic speeds]] toward the observer are noticeably bluer to the [[naked eye]], but the wavelength of any reflected or emitted photon or other particle is shortened in the direction of travel.<ref>{{cite book|title=In Quest of the Universe | first1=Karl F. | last1=Kuhn | first2=Theo | last2=Koupelis |year= 2004|publisher=[[Jones & Bartlett Publishers]]|isbn=978-0-7637-0810-8|pages=122–3}}</ref>

Doppler blueshift is used in [[astronomy]] to determine relative motion:
* The [[Andromeda Galaxy]] is moving toward our own [[Milky Way]] [[galaxy]] within the [[Local Group]]; thus, when observed from Earth, its light is undergoing a blueshift.<ref>{{cite book |last=Woodhouse |first=Chris |chapter=M31 (Andromeda Galaxy) |date=2017-12-04 |title=The Astrophotography Manual |pages=308–313 |edition=2nd |publisher=Routledge |language=en |doi=10.4324/9781315159225-42 |isbn=978-1-315-15922-5}}</ref>
* Components of a [[binary star]] system will be blueshifted when moving towards Earth
* When observing spiral galaxies, the side spinning toward us will have a slight blueshift ''relative to'' the side spinning away from us (see [[Tully–Fisher relation]]).
* [[Blazar]]s are known to propel [[relativistic jet]]s toward us, emitting [[synchrotron radiation]] and [[bremsstrahlung]] that appears blueshifted.<ref>{{Cite book |arxiv=2412.11565 |doi=10.69646/aob104p029 |chapter=Monitoring Blazar Variability to Understand Extragalactic Jets |title=Publications of the Astronomical Observatory of Belgrade |date=2024 |last1=Maria Raiteri |first1=Claudia |volume=104 |pages=29–38 |isbn=978-86-82296-11-9 }}</ref>
* Nearby stars such as [[Barnard's Star]] are moving toward us, resulting in a very small blueshift.
* Doppler blueshift of distant objects with a high ''z'' can be subtracted from the much larger [[Hubble's law|cosmological redshift]] to determine relative motion in the [[Metric expansion of space|expanding universe]].<ref name="Aoki2005">{{cite journal | title = The Largest Blueshifts of the [O III] Emission Line in Two Narrow-Line Quasars | journal = Astrophysical Journal | date = January 2005 | first1=Kentaro | last1=Aoki | first2=Toshihiro | last2=Kawaguchi | first3=Kouji | last3=Ohta | volume = 618 | issue = 2 | pages = 601–608 |arxiv = astro-ph/0409546 |bibcode = 2005ApJ...618..601A |doi = 10.1086/426075 | s2cid = 17680991 }}</ref>