S2 (star)

Short description: Star orbiting close to the supermassive black hole in the center of the Milky Way

Image of the Galactic Center showing the position of S2
Credit: ESO / MPE / Gillessen et al.

Observation data
Epoch J2000.0 Equinox (celestial coordinates)
Constellation | Sagittarius
Right ascension |  17h 45m 40.0442s[1]
Declination | −29° 00′ 27.975″[1]
Characteristics
Spectral type | B0-2 V[2]
Astrometry
Distance| 7,940±420[3] pc
Orbit[3]
Companion| Sagittarius A*
Period (P)| 16.0518[4] yr
Semi-major axis (a)| 0.12540 ± 0.00018″
Eccentricity (e)| 0.88466 ± 0.00018
Inclination (i)| 133.818 ± 0.093°
Longitude of the node (Ω)| 227.85 ± 0.19°
Periastron epoch (T)| 2018.37974 ± 0.00015
Argument of periastron (ω)
(secondary)| 66.13 ± 0.12°
Other designations

[CRG2004] 13, [GKM98] S0–2, [PGM2006] E1, [EG97] S2, [GPE2000] 0.15, [SOG2003] 1, S0–2.

Database references
SIMBAD| data

S2, also known as S0–2, is a star in the star cluster close to the supermassive black hole Sagittarius A* (Sgr A*), orbiting it with a period of 16.0518 years, a semi-major axis of about 970 au, and a pericenter distance of 17 light hours (18 Tm or 120 au) – an orbit with a period only about 30% longer than that of Jupiter around the Sun, but coming no closer than about four times the distance of Neptune from the Sun. The mass when the star first formed is estimated by the European Southern Observatory (ESO) to have been approximately 14 M☉.[5] Based on its spectral type (B0V ~ B3V), it probably has a mass of 10 to 15 solar masses.[citation needed]

Its changing apparent position has been monitored since 1995 by two groups (at UCLA and at the Max Planck Institute for Extraterrestrial Physics) as part of an effort to gather evidence for the existence of a supermassive black hole in the center of the Milky Way galaxy. The accumulating evidence points to Sgr A* as being the site of such a black hole. By 2008, S2 had been observed for one complete orbit.[6] In 2020, partway through its next orbit, the GRAVITY collaboration released an analysis showing full agreement with Schwarzschild geodesics.[7]

A team of astronomers, mainly from the Max Planck Institute for Extraterrestrial Physics, used observations of S2's orbital dynamics around Sgr A* to measure the distance from the Earth to the Galactic Center. They determined it to be 7.94 ± 0.42 kiloparsecs, in close agreement with prior determinations by other methods.[3][8]

S2 was precisely tracked during its May 2018 close approach to Sgr A*, with results in accord with general relativity predictions.[9]

## Contents

* 1 Nomenclature
* 2 Orbit
* 2.1 2018 pericentre passage
* 3 S0–102
* 4 Image gallery
* 5 See also
* 6 References
* 7 External links

## Nomenclature

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.[10] 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.[11] It is a coincidence that the star is numbered "2" in both lists; other catalogues number it differently.[10]

## Orbit

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.[12] These effects reached a maximum at closest approach, which occurred in mid-2018.[13][14] Given a recent estimate of 4.31 million M☉ 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 ​1⁄60 the speed of light) and acceleration of about 1.5 m/s2 (almost one-sixth of Earth's surface gravity).[15]

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.[16] 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.[17]

### 2018 pericentre passage

In 2018, S2 made its closest approach to Sgr A*, reaching 7650 km/s or almost 3% of the speed of light, while passing the black hole at a distance of just 120 AU or about 1400 times its Schwarzschild radius.[18][19] 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.[18]

Independent analyses by the GRAVITY collaboration[20][18][21][22] (led by Reinhard Genzel) and the KECK/UCLA Galactic Center Group[23][24] (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.[25][26][27]

## S0–102

Main page: Astronomy: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 11.5 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.[28] 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]

A still closer star S62 has since been discovered with an orbital period of 9.9 years.[29]

## Image gallery

* 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[30]

* Orbits of S0–2 and S0–102 around the Milky Way galaxy's super­massive black hole

* Inferred orbits of S2 and five other stars around supermassive black hole candidate Sgr A* at the Milky Way Galactic Center[31]

* Observations showing the discovery of the orbit of S2 about the Galactic Center

## See also

* Lists of stars

## References

1. ↑ 1.0 1.1 Schödel, R.; Merritt, D.; Eckart, A. (2009). "The nuclear star cluster of the Milky Way: Proper motions and mass". Astronomy & Astrophysics 502 (1): 91–111. doi:10.1051/0004-6361/200810922. Bibcode: 2009A&A...502...91S.
2. ↑ Paumard, T.; Genzel, R.; Martins, F.; Nayakshin, S.; Beloborodov, A.M.; Levin, Y.; Trippe, S.; Eisenhauer, F. et al. (2006). "The two young star disks in the central parsec of the galaxy: Properties, dynamics, and formation". The Astrophysical Journal 643 (2): 1011–1035. doi:10.1086/503273. Bibcode: 2006ApJ...643.1011P.
3. ↑ 3.0 3.1 3.2 Eisenhauer, F. (2003). "A Geometric Determination of the Distance to the Galactic Center". The Astrophysical Journal 597 (2): L121–L124. doi:10.1086/380188. Bibcode: 2003ApJ...597L.121E.
4. ↑ Hees, A. (2017). "Testing General Relativity with stellar orbits around the supermassive black hole in our galactic center". Physical Review Letters 118 (21): 211101. doi:10.1103/PhysRevLett.118.211101. PMID 28598651. Bibcode: 2017PhRvL.118u1101H.
5. ↑ Habibi, M. (2017). "Twelve years of spectroscopic Monitoring in the galactic center: The closest look at S-stars near the black hole". The Astrophysical Journal 847 (2): 120. doi:10.3847/1538-4357/aa876f. Bibcode: 2017ApJ...847..120H.
6. ↑ A short documentary on Sagittarius A* on YouTube
7. ↑ Abuter, R.; Amorim, A.; Bauböck, M.; Berger, J. P.; Bonnet, H.; Brandner, W.; Cardoso, V.; Clénet, Y. et al. (16 April 2020). "Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole". Astronomy & Astrophysics 636 (L5): L5. doi:10.1051/0004-6361/202037813. Bibcode: 2020A&A...636L...5G.
8. ↑ "Galactic Center Research". Max-Planck-Institut für extraterrestrische Physik. http://www2011.mpe.mpg.de/ir/GC/papers/nature2002.pdf.
9. ↑ Do, Tuan (16 Aug 2019). "Relativistic redshift of the star S0–2 orbiting the Galactic center supermassive black hole". Science 365 (6454): 664–668. doi:10.1126/science.aav8137. PMID 31346138. Bibcode: 2019Sci...365..664D.
10. ↑ 10.0 10.1 Ghez, A.M.; Klein, B.L.; Morris, M.; Becklin, E.E. (1998). "High Proper‐Motion Stars in the Vicinity of Sagittarius A*: Evidence for a supermassive black hole at the center of our galaxy". The Astrophysical Journal 509 (2): 678–686. doi:10.1086/306528. Bibcode: 1998ApJ...509..678G.
11. ↑ Eckart, A.; Genzel, R. (1997). "Stellar proper motions in the central 0.1 pc of the Galaxy". Monthly Notices of the Royal Astronomical Society 284 (3): 576–598. doi:10.1093/mnras/284.3.576. Bibcode: 1997MNRAS.284..576E.
12. ↑ "Black hole as peep-hole". https://www.sciencenews.org/blog/on-the-scene/black-hole-peephole.
13. ↑ Ghez, Andrea M. (speaker) (19 April 2016). Black Holes @ 100 Workshop: Galactic Center (video lecture). Harvard University: Black Hole Initiative. Remarks beginning at 31:55. Archived from the original on 2021-12-20. "... we are ‘2018 or bust’ these days, because at that moment your orbital determination becomes so much better"
14. ↑ "A star is about to plunge head first toward a monster black hole. Astronomers are ready to watch". 2018-03-07. http://www.syfy.com/syfywire/a-star-is-about-to-plunge-head-first-toward-a-monster-black-hole-astronomers-are-ready-to.
15. ↑ "Surfing a black hole" (Press release). European Southern Observatory.
16. ↑ Sabha, Nadeen; Eckart, Andreas (September 2012). "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". Astronomy and Astrophysics 545: A70. doi:10.1051/0004-6361/201219203. Bibcode: 2012A&A...545A..70S.
17. ↑ Gillessen, S. (2009). "Monitoring stellar orbits around the massive black hole in the galactic center". The Astrophysical Journal 692 (2): 1075–1109. doi:10.1088/0004-637X/692/2/1075. Bibcode: 2009ApJ...692.1075G.
18. ↑ 18.0 18.1 18.2 Witze, Alexandra (2018-07-26). "Milky Way's black hole provides long-sought test of Einstein's general relativity" (in en). Nature 560 (7716): 17. doi:10.1038/d41586-018-05825-3. PMID 30065325. Bibcode: 2018Natur.560...17W.
19. ↑ Devlin, Hannah (26 Jul 2018). "Star spotted speeding near black hole at centre of Milky Way". The Guardian. https://www.theguardian.com/science/2018/jul/26/star-spotted-speeding-near-milky-way-black-hole-for-first-time.
20. ↑ "Detection of the gravitational redshift in the orbit of the star S2 near the Galactic centre massive black hole". Astronomy & Astrophysics 615: L15. 2018-07-26. doi:10.1051/0004-6361/201833718. Bibcode: 2018A&A...615L..15G. https://www.aanda.org/articles/aa/full_html/2018/07/aa33718-18/aa33718-18.html.
21. ↑ "Tests of General Relativity" (in en). https://www.mpe.mpg.de/7260308/Tests-of-General-Relativity.
22. ↑ "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" (in en). https://www.eso.org/public/news/eso1825/.
23. ↑ Do, Tuan; Hees, Aurelien; Ghez, Andrea; Martinez, Gregory D.; Chu, Devin S.; Jia, Siyao; Sakai, Shoko; Lu, Jessica R. et al. (2019-08-16). "Relativistic redshift of the star S0-2 orbiting the Galactic center supermassive black hole" (in en). Science 365 (6454): 664–668. doi:10.1126/science.aav8137. ISSN 0036-8075. PMID 31346138. Bibcode: 2019Sci...365..664D. https://www.science.org/doi/10.1126/science.aav8137.
24. ↑ Siegel, Ethan (2019-08-01). "General Relativity Rules: Einstein Victorious In Unprecedented Gravitational Redshift Test" (in en). https://medium.com/starts-with-a-bang/general-relativity-rules-einstein-victorious-in-unprecedented-gravitational-redshift-test-7ab4076bcd61.
25. ↑ Abuter, R.; Amorim, A.; Bauböck, M.; Berger, J. P.; Bonnet, H.; Brandner, W.; Cardoso, V.; Clénet, Y. et al. (2020-04-01). "Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole" (in en). Astronomy & Astrophysics 636: L5. doi:10.1051/0004-6361/202037813. ISSN 0004-6361. Bibcode: 2020A&A...636L...5G. https://www.aanda.org/articles/aa/abs/2020/04/aa37813-20/aa37813-20.html.
26. ↑ "Dance around the heart of our Milky Way" (in en). April 16, 2020. https://www.mpg.de/14692117/detection-of-schwarzschild-precession-in-the-orbit-of-star-s2.
27. ↑ Sai Zhai (Aug 4, 2020). "The first detection of the Schwarzschild precession in the orbit of the star S2" (in en-US). Astrobites. https://astrobites.org/2020/08/04/the-first-detection-of-the-schwarzschild-precession-in-the-orbit-of-the-star-s2/.
28. ↑ Meyer, L.; Ghez, A.M.; Schödel, R.; Yelda, S.; Boehle, A.; Lu, J.R.; Do, T.; Morris, M.R. et al. (2012). "The shortest-known-period star orbiting our galaxy's supermassive black hole". Science 338 (6103): 84–87. doi:10.1126/science.1225506. PMID 23042888. Bibcode: 2012Sci...338...84M.
29. ↑ "S62 on a 9.9 yr Orbit around SgrA*", The Astrophysical Journal (American Astronomical Society) 889 (1): 61, 2020, doi:10.3847/1538-4357/ab5afd, Bibcode: 2020ApJ...889...61P
30. ↑ "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" (Press release). European Southern Observatory. Retrieved 26 July 2018.
31. ↑ Eisenhauer, F.; Genzel, R.; Alexander, T.; Abuter, R.; Paumard, T.; Ott, T.; Gilbert, A.; Gillessen, S. et al. (2005). "SINFONI in the Galactic Center: Young Stars and Infrared Flares in the Central Light‐Month". The Astrophysical Journal 628 (1): 246–259. doi:10.1086/430667. Bibcode: 2005ApJ...628..246E.

## External links

* "Star orbiting massive Milky Way centre approaches to within 17 light-hours" (Press release). European Southern Observatory. 16 October 2002.
* "animations page". University of California – Los Angeles. http://www.galacticcenter.astro.ucla.edu/animations.html.

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Stars of Sagittarius

Bayer|

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* V3903 (11)
* V3961
* V3984
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* V4745
* V4998
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* V5116
* V5558
* V5652
* V5668

HR|

* 6617
* 6679
* 6680
* 6691
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* 6693
* 6704
* 6716
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* 6748
* 6762
* 6766
* 6769
* 6777
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* 6798
* 6801
* 6835
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* 6842
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* 7334
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* 7643
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* 7658
* 7659
* 7703
* 7706
* 7725
* 7765
* 7799
* 7801
* 7808

HD|

* 163296
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* 164604
* 164816
* 165052
* 169142
* 170657
* 171238
* 175390
* 175754
* 180902
* 181342
* 181720
* 183577
* 187085
* 187150
* 190647
* 313846
* 316285

Gliese|

* 745

Other|

* A1742-294
* Ap 1-11
* AS 295
* BD -18 5550
* Bursting Pulsar
* GGD 27
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* GRS 1758-258
* GS 1826-24
* GX 3-1
* GX 5-1
* GX 9-1
* GX 13+1
* Herschel 36
* HETE J1900.1-2455
* IRAS 17423-1755
* IRAS 18162−2048
* GCIRS 3
* GCIRS 7
* GCIRS 8*
* GCIRS 13E
* GCIRS 16SW
* LBV 1806-20
* LS IV -12 111
* MACHO-1997-BLG-41
* MACHO-98-BLG-35
* 2MASS J19513587-3510375
* MOA-2007-BLG-192L
* MOA-2007-BLG-400L
* MOA-2009-BLG-387L
* MOA-2010-BLG-477L
* Nova Sagittarii 1898
* Nova Sagittarii 1982
* OGLE-2003-BLG-235/MOA-2003-BLG-53
* OGLE-2005-BLG-169L
* OGLE-TR-10
* OGLE-TR-56
* Pistol Star
* PSR B1744−24A
* PSR B1749−28
* PSR B1757−24
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* PSR B1800−21
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* PSR B1857−26
* PSR J1747−2958
* PSR J1748−2446ad
* Ross 154
* S0-102
* S2
* Sakurai's Object
* SAX J1747.0−2853
* SAX J1808.4−3658
* SAX J1810.8−2609
* SGR 1806−20
* SWEEPS J175853.92−291120.6
* SWEEPS J175902.00-291323.7
* SWEEPS J175902.67−291153.5
* Swift J1745-26
* SWIFT J1756.9−2508
* Th 4-4
* 4U 1755-33
* 4U 1820-30
* W33A
* WASP-67
* WR 102
* WR 102ea
* WR 102ka
* WR 104
* WR 110
* WR 111
* WR 112
* XTE J1748−288
* XTE J1807−294
* XTE J1810−197
* XTE J1814−338

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