Editing Andromeda Galaxy (section)

=== Distance estimate ===
[[File:Milky Way and Andromeda in space, to scale.jpg|thumb|left|Illustration showing both the size of each galaxy and the distance between the two galaxies, to scale]]
At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy. In 2003, using the infrared [[surface brightness fluctuation]]s (I-SBF) and adjusting for the new period-luminosity value and a metallicity correction of −0.2 mag dex<sup>−1</sup> in (O/H), an estimate of {{convert|2.57|+/-|0.06|e6ly|e9AU|lk=on|abbr=unit}} was derived. A 2004 Cepheid variable method estimated the distance to be 2.51 ± 0.13&nbsp;million light-years (770 ± 40 kpc).<ref name="Karachentsevetal2006"/><ref name="karachentsevetal2004"/>

In 2005, an eclipsing [[binary star]] was discovered in the Andromeda Galaxy. The binary{{efn|name=M31VJ}} is made up of two hot blue stars of [[stellar classification|types]] O and B. By studying the eclipses of the stars, astronomers were able to measure their sizes. Knowing the sizes and temperatures of the stars, they were able to measure their [[absolute magnitude]]. When the [[apparent magnitude|visual]] and absolute magnitudes are known, the distance to the star can be calculated. The stars lie at a distance of {{convert|2.52|+/-|0.14|e6ly|e9AU|abbr=unit}} and the whole Andromeda Galaxy at about {{convert|2.5|e6ly|e9AU|abbr=unit}}.<ref name="Ribas2005"/> This new value is in excellent agreement with the previous, independent Cepheid-based distance value. The [[tip of the red-giant branch|TRGB]] method was also used in 2005 giving a distance of {{convert|2.56|+/-|0.08|e6ly|e9AU|abbr=unit}}.<ref name="McConnachieetal2005"/> Averaged together, these distance estimates give a value of {{convert|2.54|+/-|0.11|e6ly|e9AU|abbr=unit}}.{{efn|name=avg dist}}