S55 (star)

{{short description|Star orbiting close to the supermassive black hole in the center of the Milky Way}}

{{Use dmy dates|date=June 2020}}

{{use Australian English|date=January 2020}}

{{Starbox begin

| name = S55

}}

{{Starbox observe

| epoch = J2000

| ra={{RA|17|45|40.0409}}

| dec={{DEC|−29|0|28.118}}

| constell = Sagittarius

| appmag_k = 17.2

}}

{{Starbox character

| class=

| variable =

}}

{{Starbox astrometry

| dist_pc = {{val|8178|13|fmt=commas}}

| dist_ly = {{val|26674|42|fmt=commas}}

}}

{{Starbox visbin

|reference=

| name=Sagittarius A*

| period={{val|12.8|0.1}}

| axis={{val|0.1078|0.0010}}

| eccentricity={{val|0.721|0.008}}

| inclination={{val|150|2}}

| node={{val|325|4}}

| periastron={{val|2009.34|0.04}}

| periarg = {{val|332|4}}

}}

{{Starbox catalog

| names = [GKM98] S0–102.

}}

{{Starbox reference

| Simbad=%5BGKM98%5D+S0–102

}}

{{Starbox end}}

S55 (also known as S0–102) is a star that is located very close to the centre of the Milky Way, near the radio source Sagittarius A*, orbiting it with an orbital period of 12.8 years. Until 2019, when the star S62 became the new record holder, it was the star with the shortest known period orbiting the black hole at the centre of the Milky Way. This beat the record of 16 years previously set by S2. The star was identified by a University of California, Los Angeles team headed by Andrea M. Ghez.{{cite web|url=http://newsroom.ucla.edu/portal/ucla/ucla-astronomers-discover-star-239172.aspx|title=UCLA astronomers discover star racing around black hole at center of our galaxy|author= Stuart Wolpert|date=4 October 2012}} At its periapsis, its speed reaches 1.7% of the speed of light. At that point it is 246 astronomical units (34 light hours, 36.7 billion km) from the centre,{{cite journal|last=Gillessen|first=S.|display-authors=et al.|title=An Update on Monitoring Stellar Orbits in the Galactic Center|journal=The Astrophysical Journal|date=February 2017|volume=837|issue=1|pages=30|doi=10.3847/1538-4357/aa5c41|arxiv=1611.09144|bibcode=2017ApJ...837...30G|s2cid=119087402 |doi-access=free }}{{cite web|url=http://folk.uio.no/sigurdkn/s_stars|title=Galactic center S-star orbital parameters|last=Næss|first=S.|date=October 4, 2019}}{{dead link|date=July 2022}} while the black hole radius is only a small fraction of that size (the Schwarzschild radius is about {{val|0.082|u=au}}).{{cite journal|author=The GRAVITY collaboration|date=April 2019|title=A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty|journal=Astronomy & Astrophysics|volume=625|pages=L10|doi=10.1051/0004-6361/201935656|url=https://www.aanda.org/articles/aa/full_html/2019/05/aa35656-19/aa35656-19.html|arxiv=1904.05721|bibcode=2019A&A...625L..10G|s2cid=119190574}} It passed that point in 2022 and will be there again in 2034.

Its position in the sky has been monitored from 2000 to 2012 using the W. M. Keck telescope and from 2002 to 2016 with the VLT. One complete orbit has been observed. From Earth's current perspective, it travels in a clockwise direction. Having observed two stars orbiting through complete periods around the centre (S55 and S2), the gravitational potential of SgrA* could be established. It is also possible that there is a great deal of dark matter around the orbits of these stars.{{cite journal |last1=Dokuchaev |first1=V.I. |last2=Eroshenko |first2=Yu.N. |last3=Klimkov |first3=K.S. |title=Precession of Fast S0 Stars in the Vicinity of Supermassive Black Hole in the Galactic Center |journal=Physics Procedia |date=2015 |volume=74 |pages=292–296 |doi=10.1016/j.phpro.2015.09.243|bibcode=2015PhPro..74..292D |doi-access=free }} Also general relativistic effects due to gravitational redshift should become observable.{{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 }}