Editing Parsec (section)

=== Megaparsecs and gigaparsecs ===
<!-- Template:Convert/Mpc & Template:Convert/Gpc link here. -->
Astronomers typically express the distances between neighbouring galaxies and [[galaxy cluster]]s in megaparsecs (Mpc). A megaparsec is one million parsecs, or about 3,260,000 light years.<ref>{{cite web |url=https://astronomy.com/magazine/ask-astro/2020/02/why-is-a-parsec-326-light-years |title=Why is a parsec 3.26 light-years? |website=Astronomy.com |date=1 February 2020 |access-date=20 July 2021 }}</ref> Sometimes, galactic distances are given in units of Mpc/''h'' (as in "50/''h''&nbsp;Mpc", also written "{{nowrap|50 Mpc ''h''<sup>−1</sup>}}"). ''h'' is a constant (the "[[dimensionless Hubble constant]]") in the range {{nowrap|0.5 < ''h'' < 0.75}} reflecting the uncertainty in the value of the [[Hubble constant]] ''H'' for the rate of expansion of the universe: {{nowrap|1=''h'' = {{sfrac|''H''|100&nbsp;(km/s)/Mpc}}}}. The Hubble constant becomes relevant when converting an observed [[redshift]] ''z'' into a distance ''d'' using the formula {{nowrap|''d'' ≈ {{sfrac|''[[Speed of light|c]]''|''H''}} × ''z''}}.<ref>{{Cite web |title=Galaxy structures: the large scale structure of the nearby universe |url=http://pil.phys.uniroma1.it/twiki/bin/view/Pil/GalaxyStructures |url-status=dead |archive-url=https://web.archive.org/web/20070305202144/http://pil.phys.uniroma1.it/twiki/bin/view/Pil/GalaxyStructures |archive-date=5 March 2007 |access-date=22 May 2007}}</ref>

One gigaparsec (Gpc) is [[1000000000 (number)|one billion]] parsecs — one of the largest [[Orders of magnitude (length)|units of length]] commonly used. One gigaparsec is about {{Convert|1|Gpc|e9ly|sigfig=3|abbr=unit|disp=out}}, or roughly {{sfrac|14}} of the distance to the [[Cosmological horizon#Practical horizons|horizon]] of the [[observable universe]] (dictated by the [[cosmic microwave background radiation]]). Astronomers typically use gigaparsecs to express the sizes of [[Large-scale structure of the cosmos|large-scale structures]] such as the size of, and distance to, the [[CfA2 Great Wall]]; the distances between galaxy clusters; and the distance to [[quasar]]s.

For example:
* The [[Andromeda Galaxy]] is about {{Convert|0.78|Mpc|e6ly|abbr=unit}} from the Earth.
* The nearest large [[galaxy cluster]], the [[Virgo Cluster]], is about {{Convert|16.5|Mpc|e6ly|abbr=unit}} from the Earth.<ref>{{Cite journal |last1=Mei |first1=S. |last2=Blakeslee |first2=J.&nbsp;P. |last3=Côté |first3=P. |display-authors=etal |date=2007 |title=The ACS Virgo Cluster Survey. XIII. SBF Distance Catalog and the Three-dimensional Structure of the Virgo Cluster |journal=The Astrophysical Journal |volume=655 |issue=1 |pages=144–162 |arxiv=astro-ph/0702510 |bibcode=2007ApJ...655..144M |doi=10.1086/509598|s2cid=16483538 }}</ref>
* The galaxy [[RXJ1242-11]], observed to have a [[supermassive black hole]] core similar to the [[Milky Way]]'s, is about {{Convert|200|Mpc|e6ly|abbr=unit}} from the Earth.
* The [[galaxy filament]] [[Hercules–Corona Borealis Great Wall]], which is since November 2013 the [[List of largest known cosmic structures|largest known structure]] in the universe, is about {{Convert|3|Gpc|e9ly|abbr=unit}} across.
* The [[particle horizon]] (the boundary of the [[observable universe]]) has a radius of about {{Convert|14|Gpc|e9ly|abbr=unit}}.<ref>{{Cite journal |last1=Lineweaver |first1=Charles H. |last2=Davis |first2=Tamara M. |date=1 March 2005 |title=Misconceptions about the Big Bang |url=http://www.scientificamerican.com/article.cfm?id=misconceptions-about-the-2005-03&page=5 |url-status=dead |journal=Scientific American |volume=292 |issue=3 |pages=36–45 |bibcode=2005SciAm.292c..36L |doi=10.1038/scientificamerican0305-36 |archive-url=https://web.archive.org/web/20110810231727/http://www.scientificamerican.com/article.cfm?id=misconceptions-about-the-2005-03&page=5 |archive-date=10 August 2011 |access-date=4 February 2016|url-access=subscription }}</ref>