Editing Perseus Arm (section)

==Overview==
The Milky Way is a [[barred spiral galaxy]] with two major arms and a number of minor arms or spurs.<ref name=Churchwell2009/> The Perseus Spiral Arm, with a radius of approximately 10.7&nbsp;[[kiloparsecs]], is located between the minor [[Cygnus Arm|Cygnus]] and [[Carina–Sagittarius Arm]]s.<ref name=Churchwell2009/> It is named after the [[Perseus (constellation)|Perseus constellation]] in the direction of which it is seen from Earth.

Recently, scientists in two large radio astronomy projects, the Bar and Spiral Structure Legacy (BeSSeL) Survey and the Japanese VLBI Exploration of Radio Astrometry (VERA), have made great efforts over about 20 years to measure the trigonometric parallaxes toward about 200 water vapor ({{chem|H|2|O}}) and [[methanol]] ({{chem|C|H|3|O|H}}) masers in massive star-forming regions in the Milky Way. They have employed these parallax measurements to delineate the forms of spiral arms from the Galactic longitude 2 to 240 degrees and extended the spiral arm traces into the portion of the Milky Way seen from the Southern Hemisphere using tangencies along some arms based on carbon monoxide emission. The image clearly presents the Milky Way as a barred spiral galaxy with fairly symmetric four major arms and some extra arm segments and spurs.<ref name=reid2009/><ref>{{cite journal | url=https://doi.org/10.1093/pasj/psaa018 | doi=10.1093/pasj/psaa018 | title=The First VERA Astrometry Catalog | year=2020 | last1=Hirota | first1=Tomoya | last2=Nagayama | first2=Takumi | last3=Honma | first3=Mareki | last4=Adachi | first4=Yuuki | last5=Burns | first5=Ross A. | last6=Chibueze | first6=James O. | last7=Choi | first7=Yoon Kyung | last8=Hachisuka | first8=Kazuya | last9=Hada | first9=Kazuhiro | last10=Hagiwara | first10=Yoshiaki | last11=Hamada | first11=Shota | last12=Handa | first12=Toshihiro | last13=Hashimoto | first13=Mao | last14=Hirano | first14=Ken | last15=Hirata | first15=Yushi | last16=Ichikawa | first16=Takanori | last17=Imai | first17=Hiroshi | last18=Inenaga | first18=Daichi | last19=Ishikawa | first19=Toshio | last20=Jike | first20=Takaaki | last21=Kameya | first21=Osamu | last22=Kaseda | first22=Daichi | last23=Kim | first23=Jeong Sook | last24=Kim | first24=Jungha | last25=Kim | first25=Mi Kyoung | last26=Kobayashi | first26=Hideyuki | last27=Kono | first27=Yusuke | last28=Kurayama | first28=Tomoharu | last29=Matsuno | first29=Masako | last30=Morita | first30=Atsushi | journal=Publications of the Astronomical Society of Japan | volume=72 | issue=4 | display-authors=1 | arxiv=2002.03089 }}</ref>

The Perseus Arm is one of the four major arms. The arm is the length of more than 60,000 lr and the width of about 1,000 lr and the spiral extension in the pitch angle near 9 degree.<ref name=reid2009/>

There is speculation that the local spur known as the [[Orion–Cygnus Arm]], which includes the [[Solar System]] and [[Earth]] and is located inside of the Perseus Arm,<ref name=Churchwell2009/> or is a branch of it, but this is unconfirmed.

The Perseus Arm contains the [[Double Cluster]] and a number of [[Messier objects]]:
*The [[Crab Nebula]] (M1)
*[[Messier 36|Open Cluster M36]]
*[[Messier 37|Open Cluster M37]]
*[[Messier 38|Open Cluster M38]]
*[[Messier 52|Open Cluster M52]]
*[[Messier 103|Open Cluster M103]].