Sagittarius A*

Where the numbers come from

There is no catalogue of black holes as such, as far as we know. A hole goes by the name of something we can see: the star orbiting it, the X-ray source, the galaxy around it. When two black holes merge into one, the new hole is named for the date the ripple in space from the merger (its gravitational waves) reached the detectors on Earth that listen for them. SIMBAD is astronomers' online catalogue of objects beyond the Solar System. Known as: in SIMBAD, Sgr A* and Sagittarius A*.

• Mass, in M☉ (the Sun's mass): 4.3 × 10⁶ ± 12 000 M☉ (4.297 ± 0.012 million, the error from the fit alone; the method's own uncertainty adds about 0.04 million), from the orbits of four stars round it (S2, S29, S38 and S55), tracked by GRAVITY, an instrument that joins the four big telescopes of the VLT in Chile into one. GRAVITY Collaboration, “The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits”, Astronomy & Astrophysics 657, L12 (2022), Table 1. doi:10.1051/0004-6361/202142465, arXiv:2112.07478
• Spin, a* (0 is still, 1 is the fastest a hole can turn): < 0.1, from stars orbiting it in flat discs, which a faster spin would have tipped over by now. G. Fragione and A. Loeb, “An upper limit on the spin of SgrA* based on stellar orbits in its vicinity”, The Astrophysical Journal Letters (2020). doi:10.3847/2041-8213/abb9b4, arXiv:2008.11734
• Spin: 0.90 ± 0.06, from the outflow method, from its X-ray and radio light. R. A. Daly et al., “New Black Hole Spin Values for Sagittarius A* Obtained with the Outflow Method” (2023), preprint. arXiv:2310.12108
• Spin: No value. Models of the Event Horizon Telescope image with no spin fit it less well. Where a spin has several published values, each one is listed with its method. The picture is drawn at the middle of the range of published values. Event Horizon Telescope Collaboration, “First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way”, The Astrophysical Journal Letters 930, L12 (2022). doi:10.3847/2041-8213/ac6674, arXiv:2311.08680
• Spin drawn: 0.48, the middle of the range of published values (spin assumed).
• Feed: 1.9 × 10⁻¹⁰, from its observed light in quiet, about 10³⁵ erg/s, over the Eddington luminosity for this mass, 5.2 × 10⁴⁴ erg/s. Event Horizon Telescope Collaboration, “First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole”, The Astrophysical Journal Letters 930, L16 (2022). doi:10.3847/2041-8213/ac6672, arXiv:2311.09478
• Angle: Not measured. The Event Horizon Telescope image rules out views more than 50° from face-on, and the models that fit it best are within 30°. Event Horizon Telescope Collaboration, “First Sagittarius A* Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole in the Center of the Milky Way”, The Astrophysical Journal Letters 930, L12 (2022). doi:10.3847/2041-8213/ac6674, arXiv:2311.08680 and Event Horizon Telescope Collaboration, “First Sagittarius A* Event Horizon Telescope Results. V. Testing Astrophysical Models of the Galactic Center Black Hole”, The Astrophysical Journal Letters 930, L16 (2022). doi:10.3847/2041-8213/ac6672, arXiv:2311.09478
• Distance: 26 996 ± 29 light years (8 277 ± 9 parsecs (pc; a parsec is 3.26 light years), the error from the fit alone; the method's own uncertainty adds about 30 parsecs), from the same fit to the stars' orbits. GRAVITY Collaboration, “The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits”, Astronomy & Astrophysics 657, L12 (2022), Table 1. doi:10.1051/0004-6361/202142465, arXiv:2112.07478
• S2: once round every 16.0 years, 1 034.21 ± 0.33 (0.12495 ± 0.00004 arcseconds on the sky, at 8 277 parsecs) au halfway between nearest and farthest, eccentricity 0.88441 ± 0.00006, tilted 134.70 ± 0.03° (over 90° is clockwise as we see it), its rising node 228.19 ± 0.03° east of north, its nearest point 66.25 ± 0.03° on from that, reached on 19 May 2018, from GRAVITY's four-star fit (Table B.1). GRAVITY Collaboration, “The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits”, Astronomy & Astrophysics 657, L12 (2022), Table B.1. doi:10.1051/0004-6361/202142465, arXiv:2112.07478
• S29: once round every 91.0 years, 3 290 ± 13 (0.3975 ± 0.0016 arcseconds on the sky, at 8 277 parsecs) au halfway between nearest and farthest, eccentricity 0.9693 ± 0.0001, tilted 144.37 ± 0.07° (over 90° is clockwise as we see it), its rising node 7.00 ± 0.33° east of north, its nearest point 205.79 ± 0.33° on from that, reached on 30 May 2021, from GRAVITY's four-star fit (Table B.1). GRAVITY Collaboration, “The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits”, Astronomy & Astrophysics 657, L12 (2022), Table B.1. doi:10.1051/0004-6361/202142465, arXiv:2112.07478
• S38: once round every 19.5 years, 1 179.80 ± 0.33 (0.14254 ± 0.00004 arcseconds on the sky, at 8 277 parsecs) au halfway between nearest and farthest, eccentricity 0.8145 ± 0.0002, tilted 166.65 ± 0.40° (over 90° is clockwise as we see it), its rising node 109.45 ± 1.00° east of north, its nearest point 27.17 ± 1.02° on from that, reached on 15 September 2022, from GRAVITY's four-star fit (Table B.1). GRAVITY Collaboration, “The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits”, Astronomy & Astrophysics 657, L12 (2022), Table B.1. doi:10.1051/0004-6361/202142465, arXiv:2112.07478
• S55: once round every 12.3 years, 864.12 ± 0.41 (0.10440 ± 0.00005 arcseconds on the sky, at 8 277 parsecs) au halfway between nearest and farthest, eccentricity 0.7267 ± 0.0002, tilted 158.52 ± 0.22° (over 90° is clockwise as we see it), its rising node 314.94 ± 1.14° east of north, its nearest point 322.78 ± 1.13° on from that, reached on 11 September 2021, from GRAVITY's four-star fit (Table B.1). GRAVITY Collaboration, “The mass distribution in the Galactic Centre from interferometric astrometry of multiple stellar orbits”, Astronomy & Astrophysics 657, L12 (2022), Table B.1. doi:10.1051/0004-6361/202142465, arXiv:2112.07478
• S2's turn, measured, as a multiple of relativity's: 1.10 ± 0.19, from the turn of S2's orbit, tracked through its 2018 closest approach, as a multiple of general relativity's (1 is relativity, 0 is Newton). GRAVITY Collaboration, “Detection of the Schwarzschild precession in the orbit of the star S2 near the Galactic centre massive black hole”, Astronomy & Astrophysics 636, L5 (2020). doi:10.1051/0004-6361/202037813, arXiv:2004.07187

Every number on this page is quoted from a published paper or a catalogue, with the method it came from. No image or data file from these sources is loaded, so there is nothing to licence; the figure is our own drawing from their numbers.