local standard of rest

{{Short description|Astronomical reference frame defined by motion of near-Sun stars around Galactic Centre}}

{{Technical|date=July 2023}}

In astronomy, the local standard of rest or LSR is a reference frame which follows the mean motion of material in the Milky Way in the neighborhood of the Sun (stars in radius 100 pc from the Sun),{{cite book |title=The Physical Universe |author= Frank H Shu |page= [https://archive.org/details/physicaluniverse00shuf/page/n284 261] |url=https://archive.org/details/physicaluniverse00shuf |url-access=registration |isbn=0-935702-05-9 |date=1982 |publisher=University Science Books}} on average sharing the same velocity around the Milky Way as the Sun.{{cite web | title=LOCAL STANDARD OF REST definition and meaning | website=Collins English Dictionary | date=November 25, 2023 | url=https://www.collinsdictionary.com/dictionary/english/local-standard-of-rest | access-date=November 25, 2023}} The path of this material is not precisely circular.{{cite book |title=Galactic Astronomy |author1=James Binney |author2=Michael Merrifield |url=https://books.google.com/books?id=arYYRoYjKacC&pg=PA536 |page= 536 |date=1998 |isbn=0-691-02565-7 |publisher=Princeton University Press}} The Sun follows the solar circle (eccentricity e < 0.1) at a speed of about 255 km/s in a clockwise direction when viewed from the galactic north pole at a radius of ≈ 8.34 kpc{{cite journal | author = Reid, M. |display-authors=etal | title = Trigonometric Parallaxes of High Mass Star Forming Regions: The Structure and Kinematics of the Milky Way | date = 10 March 2014 | journal = The Astrophysical Journal | doi = 10.1088/0004-637X/783/2/130 | pages = 130 (14pp) | issue = 2 | volume = 783 | bibcode=2014ApJ...783..130R | arxiv=1401.5377|s2cid=119186799 }} about the center of the galaxy near Sgr A*, and has only a slight motion, towards the solar apex, relative to the LSR.{{cite book |chapter-url=https://books.google.com/books?id=bP9hZqoIfhMC&pg=PA19 |title=Mapping the Galaxy and Nearby Galaxies |editor1=F. Combes |editor2=Keiichi Wada |date=2008 |publisher=Springer |isbn=978-0-387-72767-7 |author= Mark Reid |display-authors= etal |chapter=Mapping the Milky Way and the Local Group |pages=19–20}}The Sun's peculiar motion relative to the LSR is 13.4 km/s. See, for example, {{cite book |author1=Binney, J. |author2=Merrifield, M. |name-list-style=amp |title=op. cit. |isbn=0-691-02565-7 |chapter=§10.6|date=6 September 1998 |publisher=Princeton University Press }} or {{cite journal |title=On the distance to the Ophiuchus star-forming region |journal=Astron. Nachr. |volume=AN 329 |doi= 10.1002/asna.200710827 |arxiv=0709.0505 |date=2008 |page=12; §2.3 |author=E.E. Mamajek|issue=1 |bibcode = 2008AN....329...10M |s2cid=14027548 }}

LSR could be understood by analogy to a group of cars traveling at similar speed on a highway i.e. at LSR. If a faster car passes by or they pass a slower car then the faster and slower cars could be considered at not traveling at LSR. Typically a large variation of speed in astronomical bodies could be considered as indicator of their extraterrestrial nature. This analogy was used by theoretical physicist Avi Loeb in his 2021 book Extraterrestrial: The First Sign of Intelligent Life Beyond Earth.

The LSR velocity is anywhere from 202–241 km/s.{{cite journal |title=Precision Astrometry, Galactic Mergers, Halo Substructure and Local Dark Matter |author=Steven R. Majewski1 |journal=Proceedings of IAU Symposium 248 |arxiv=0801.4927 |date=2008|bibcode = 2008IAUS..248..450M |doi = 10.1017/S1743921308019790 |volume=3 |pages=450–457 |s2cid=18252986 }} In 2014, very-long-baseline interferometry observations of maser emission in high-mass star-forming regions (HMSFR) placed tight constraints on combinations of kinematic parameters such as the circular orbit speed of the Sun (Θ₀ + V_☉ = 255.2 ± 5.1 km/s). There is significant correlation between the circular motion of the solar circle, the solar peculiar motion, and the predicted counterrotation of star-forming regions.{{cite journal | author = Reid, M. |display-authors=etal | title = Trigonometric Parallaxes of High Mass Star Forming Regions. VI. Galactic Structure, Fundamental Parameters, and Noncircular Motions | date = 20 July 2009 | journal = The Astrophysical Journal | doi = 10.1088/0004-637X/700/1/137 | pages = 137–148 | issue = 1 | volume = 700 | bibcode=2009ApJ...700..137R | arxiv=0902.3913|s2cid=11347166 }} Additionally, local estimates of the velocity of the LSR based on stars in the vicinity of the Sun{{cite journal | author = Dehnen, W. |author2=Binney, J.J. | title = Local stellar kinematics from HIPPARCOS data | date = October 1998 | journal = Monthly Notices of the Royal Astronomical Society | doi = 10.1046/j.1365-8711.1998.01600.x | pages = 387–394| issue = 2 | volume = 298 |doi-access=free | bibcode=1998MNRAS.298..387D | arxiv=astro-ph/9710077|s2cid=15936627 }} may potentially yield different results than global estimates derived from motions relative to the Galactic Center.{{cite journal | author = Bovy, J. |display-authors=etal | title = The Milky Way's Circular-velocity Curve between 4 and 14 kpc from APOGEE data | date = 10 November 2012 | journal = The Astrophysical Journal | doi = 10.1088/0004-637X/759/2/131 | pages = 131 (20pp) | issue = 2 | volume = 759 | bibcode=2012ApJ...759..131B | arxiv=1209.0759|s2cid=119279938 }}