S2 (star)

The designation S0–2 was first used in 1998. S0 indicates a star within one arc-second of Sgr A*, indicating the galactic centre, and S0–2 was the second closest star seen at the time of the measurements. The star had been catalogued simply as S2 a year earlier, the second of eleven infrared sources near the galactic centre, numbered approximately anti-clockwise. It is a coincidence that the star is numbered "2" in both lists; other catalogues number it differently.

The highly eccentric orbit of S2 will give astronomers an opportunity to test for various effects predicted by general relativity and even extra-dimensional effects.{{cite news |url=https://www.sciencenews.org/blog/on-the-scene/black-hole-peephole |title=Black hole as peep-hole |website=ScienceNews}} These effects reached a maximum at closest approach, which occurred in mid-2018.{{cite AV media |people=Ghez, Andrea M. (speaker) |date=19 April 2016 |title=Black Holes @ 100 Workshop: Galactic Center |publisher=Black Hole Initiative |medium=video lecture |url=https://www.youtube.com/watch?v=7z5jBJXbuuo#t=31:55 |archive-url=https://ghostarchive.org/varchive/youtube/20211220/7z5jBJXbuuo |archive-date=2021-12-20 |url-status=live|time=31:55 |time-caption=Remarks beginning at |location=Harvard University |quote=... we are ‘2018 or bust’ these days, because at that moment your orbital determination becomes so much better}}{{cbignore}}{{cite web |url=http://www.syfy.com/syfywire/a-star-is-about-to-plunge-head-first-toward-a-monster-black-hole-astronomers-are-ready-to |title=A star is about to plunge head first toward a monster black hole. Astronomers are ready to watch |date=2018-03-07 |df=dmy-all}} Given a recent estimate of {{solar mass|4.31 million|link=yes}} for the mass of the Sgr A* black hole and S2's close approach, this makes S2's the fastest known ballistic orbit, reaching speeds exceeding 5,000 km/s (11,000,000 mph, or {{frac|1|60}} the speed of light).{{cite press release |url=http://www.eso.org/public/news/eso0226/ |title=Surfing a black hole |publisher=European Southern Observatory}}

The motion of S2 is also useful for detecting the presence of other objects near to Sgr A*. It is believed that there are thousands of stars, as well as dark stellar remnants (stellar black holes, neutron stars, white dwarfs) distributed in the volume through which S2 moves. These objects will perturb S2's orbit, causing it to deviate gradually from the Keplerian ellipse that characterizes motion around a single point mass.{{cite journal

|first1=Nadeen |last1=Sabha

|first2=Andreas |last2=Eckart

|first3=David |last3=Merritt |author-link3=David Merritt

|first4=Mohammad |last4=Zamaninasab

|first5=Gunther |last5=Witzel

|first6=Macarena |last6=García-Marín

|first7=Behrang |last7=Jalali

|first8=Monica |last8=Valencia-S.

|first9=Senol |last9=Yazici

|first10=Rainer |last10=Buchholz

|first11=Banafsheh |last11=Shahzamanian

|first12=Christian |last12=Straubmeier

|display-authors=8

|date=September 2012

|title=The S-Star Cluster at the Center of the Milky Way: On the nature of diffuse NIR emission in the inner tenth of a parsec

|journal=Astronomy and Astrophysics

|volume=545 |page=A70

|doi=10.1051/0004-6361/201219203 |arxiv=1203.2625 |bibcode=2012A&A...545A..70S|s2cid=118358113

}} So far, the strongest constraint that can be placed on these remnants is that their total mass comprises less than one percent of the mass of the supermassive black hole.{{cite journal |last1=Gillessen |first1=S. |display-authors=etal |year=2009 |title=Monitoring stellar orbits around the massive black hole in the galactic center |journal=The Astrophysical Journal |volume=692 |issue=2 |pages=1075–1109 |doi=10.1088/0004-637X/692/2/1075 |arxiv=0810.4674 |bibcode=2009ApJ...692.1075G|s2cid=1431308 }}

In 2018, S2 made its closest approach to Sgr A*, reaching 7,650 km/s or almost 3% of the speed of light, while passing the black hole at a distance of just 120 AU or about 1,400 times its Schwarzschild radius.{{Cite journal|last=Witze|first=Alexandra|date=2018-07-26|title=Milky Way's black hole provides long-sought test of Einstein's general relativity|journal=Nature|language=en|volume=560|issue=7716|pages=17|doi=10.1038/d41586-018-05825-3|pmid=30065325|bibcode=2018Natur.560...17W|doi-access=free}}{{cite news|last=Devlin|first=Hannah|date=26 Jul 2018|title=Star spotted speeding near black hole at centre of Milky Way|newspaper=The Guardian|url=https://www.theguardian.com/science/2018/jul/26/star-spotted-speeding-near-milky-way-black-hole-for-first-time|access-date=2021-01-18}} S2 reached its pericenter on May 19, 2018, while its velocity in the line of sight from Earth peaked in April, and later hit its minimum in late August and early September.

Independent analyses by the GRAVITY collaboration{{cite journal

|url=https://www.aanda.org/articles/aa/full_html/2018/07/aa33718-18/aa33718-18.html

|title=Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole

|first1=Reinhard |last1=Genzel |author-link1=Reinhard Genzel

|display-authors=etal

|collaboration=GRAVITY collaboration

|journal=Astronomy & Astrophysics

|doi=10.1051/0004-6361/201833718 |doi-access=free

|type=Letter to the editor

|date=2018-07-26 |df=dmy-all

|volume=615 |page=L15

|arxiv=1807.09409 |bibcode=2018A&A...615L..15G

}}{{Cite web|last=|first=|date=|title=Tests of General Relativity|url=https://www.mpe.mpg.de/7260308/Tests-of-General-Relativity|access-date=2021-01-17|website=Max Planck Society|language=en}}{{Cite web|title=First Successful Test of Einstein's General Relativity Near Supermassive Black Hole - Culmination of 26 years of ESO observations of the heart of the Milky Way|url=https://www.eso.org/public/news/eso1825/|access-date=2021-01-17|website=www.eso.org|language=en}} (led by Reinhard Genzel) and the KECK/UCLA Galactic Center Group{{Cite journal|last1=Do|first1=Tuan|last2=Hees|first2=Aurelien|last3=Ghez|first3=Andrea|last4=Martinez|first4=Gregory D.|last5=Chu|first5=Devin S.|last6=Jia|first6=Siyao|last7=Sakai|first7=Shoko|last8=Lu|first8=Jessica R.|last9=Gautam|first9=Abhimat K.|last10=O’Neil|first10=Kelly Kosmo|last11=Becklin|first11=Eric E.|date=2019-08-16|title=Relativistic redshift of the star S0-2 orbiting the Galactic center supermassive black hole|url=https://www.science.org/doi/10.1126/science.aav8137|journal=Science|volume=365|issue=6454|pages=664–668|language=en|doi=10.1126/science.aav8137|issn=0036-8075|pmid=31346138|arxiv=1907.10731|bibcode=2019Sci...365..664D|s2cid=198901506}}{{Cite web|last=Siegel|first=Ethan|date=2019-08-01|title=General Relativity Rules: Einstein Victorious In Unprecedented Gravitational Redshift Test|url=https://medium.com/starts-with-a-bang/general-relativity-rules-einstein-victorious-in-unprecedented-gravitational-redshift-test-7ab4076bcd61|access-date=2021-01-17|website=Medium|language=en}} (led by Andrea Ghez) revealed a combined transverse Doppler and gravitational redshift up to 200 km/s/c, in agreement with general relativity predictions.

Additional analysis has revealed a Schwarzschild precession of 12 arcminutes (0.2 degrees) in S2's orbit caused by the close passage, fully consistent with general relativity.{{Cite journal|last1=Abuter|first1=R.|last2=Amorim|first2=A.|last3=Bauböck|first3=M.|last4=Berger|first4=J. P.|last5=Bonnet|first5=H.|last6=Brandner|first6=W.|last7=Cardoso|first7=V.|last8=Clénet|first8=Y.|last9=Zeeuw|first9=P. T. de|last10=Dexter|first10=J.|last11=Eckart|first11=A.|date=2020-04-01|title=Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole|url=https://www.aanda.org/articles/aa/abs/2020/04/aa37813-20/aa37813-20.html|journal=Astronomy & Astrophysics|language=en|volume=636|pages=L5|doi=10.1051/0004-6361/202037813|arxiv=2004.07187|bibcode=2020A&A...636L...5G|issn=0004-6361|doi-access=free}}{{Cite web|last=|first=|date=April 16, 2020|title=Dance around the heart of our Milky Way|url=https://www.mpg.de/14692117/detection-of-schwarzschild-precession-in-the-orbit-of-star-s2|access-date=2021-01-18|website=Max Planck Society|language=en}}{{Cite news|last=Sai Zhai|date=Aug 4, 2020|title=The first detection of the Schwarzschild precession in the orbit of the star S2|url=https://astrobites.org/2020/08/04/the-first-detection-of-the-schwarzschild-precession-in-the-orbit-of-the-star-s2/|access-date=2021-01-18|newspaper=Astrobites|language=en-US}}

{{main|S0–102}}

In 2012, a star called S0–102 (or S55) was found to be orbiting even closer to the Milky Way's central supermassive black hole than S0–2 does. At one-sixteenth the brightness of S0–2, S0–102 was not initially recognized because it required many more years of observations to distinguish it from its local infrared background. S0–102 has an orbital period of 12.8 years, even shorter than that of S0–2. Of all the stars orbiting the black hole, only these two have their orbital parameters and trajectories fully known in all three dimensions of space.{{Cite journal |last1=Meyer |first1=L. |last2=Ghez |first2=A.M. |last3=Schödel |first3=R. |last4=Yelda |first4=S. |last5=Boehle |first5=A. |last6=Lu |first6=J.R. |last7=Do |first7=T. |last8=Morris |first8=M.R. |last9=Becklin |first9=E.E. |last10=Matthews |first10=K. |doi=10.1126/science.1225506 |title=The shortest-known-period star orbiting our galaxy's supermassive black hole |journal=Science |volume=338 |issue=6103 |pages=84–87 |year=2012 |pmid=23042888 |arxiv=1210.1294 |bibcode=2012Sci...338...84M|s2cid=6029405 }} The discovery of two stars orbiting the central black hole so closely with their orbits fully described is of extreme interest to astronomers, as the pair together will allow much more precise measurements on the nature of gravity and general relativity around the black hole than would be possible from using S0–2 alone.{{citation needed|date=July 2018}}

Artist’s impression of S2 passing supermassive black hole at centre of Milky Way - annotated.jpg|Artist's annotation of S2 passing supermassive black hole (note black hole is not to scale) at center of Milky Way, confirming gravitational red shift{{cite press release |title=First successful test of Einstein's General Relativity near supermassive black hole – Culmination of 26 years of ESO observations of the heart of the Milky Way |url=https://www.eso.org/public/news/eso1825/ |publisher=European Southern Observatory |access-date=26 July 2018}}

SO-102 Orbital Plot.jpg|Orbits of S0–2 and S0–102 around the Milky Way galaxy's super­massive black hole

Galactic centre orbits.svg|Inferred orbits of S2 and five other stars around supermassive black hole candidate Sgr A* at the Milky Way Galactic Center{{Cite journal |last1=Eisenhauer |first1=F. |last2=Genzel |first2=R. |last3=Alexander |first3=T. |last4=Abuter |first4=R. |last5=Paumard |first5=T. |last6=Ott |first6=T. |last7=Gilbert |first7=A. |last8=Gillessen |first8=S. |last9=Horrobin |first9=M. |last10=Trippe |first10=S. |last11=Bonnet |first11=H. |last12=Dumas |first12=C. |last13=Hubin |first13=N. |last14=Kaufer |first14=A. |last15=Kissler-Patig |first15=M. |last16=Monnet |first16=G. |last17=Strobele |first17=S. |last18=Szeifert |first18=T. |last19=Eckart |first19=A. |last20=Schodel |first20=R. |last21=Zucker |first21=S. |doi=10.1086/430667 |title=SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light-Month |journal=The Astrophysical Journal |volume=628 |issue=1 |pages=246–259 |year=2005 |arxiv=astro-ph/0502129 |bibcode=2005ApJ...628..246E|s2cid=122485461 }}

Orbit of S2.jpg|Observations showing the discovery of the orbit of S2 about the Galactic Center

{{clear}}

• Lists of stars

{{reflist|25em|refs=

{{cite journal |doi=10.1086/306528 |bibcode=1998ApJ...509..678G |arxiv=astro-ph/9807210 |title=High Proper-Motion Stars in the Vicinity of Sagittarius A*: Evidence for a supermassive black hole at the center of our galaxy |journal=The Astrophysical Journal |volume=509 |issue=2 |pages=678–686 |year=1998 |last1=Ghez |first1=A.M. |last2=Klein |first2=B.L. |last3=Morris |first3=M. |last4=Becklin |first4=E.E.|s2cid=18243528 }}

{{cite journal |doi=10.1093/mnras/284.3.576 |bibcode=1997MNRAS.284..576E |title=Stellar proper motions in the central 0.1 pc of the Galaxy |journal=Monthly Notices of the Royal Astronomical Society |volume=284 |issue=3 |pages=576–598 |year=1997 |last1=Eckart |first1=A. |last2=Genzel |first2=R.|doi-access=free }}

}}

• {{cite press release |url=http://www.eso.org/public/news/eso0226/ |title=Star orbiting massive Milky Way centre approaches to within 17 light-hours |publisher=European Southern Observatory |date=16 October 2002}}
• {{YouTube|uVlcIb-rClI|Stars Orbiting around the Black Hole at the Center of the Milky Way}}
• {{cite web |url=http://www.galacticcenter.astro.ucla.edu/animations.html |publisher=University of California – Los Angeles |department=Galactic Center Group |title=animations page |access-date=2016-04-12 |archive-date=2018-09-03 |archive-url=https://web.archive.org/web/20180903214918/http://www.galacticcenter.astro.ucla.edu/animations.html |url-status=dead }}

{{Stars of Sagittarius}}

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Category:B-type main-sequence stars

Category:Sagittarius (constellation)

Category:Tests of general relativity

Category:Galactic Center