Sagittarius A*

</math>; some examples are Fragione & Loeb (2020) <math>a_{*} < 0.1</math>, Belanger et al. (2006) <math>a_{*}\sim 0.22

</math>, Meyer et al. (2006) <math>a_{*} > 0.4</math>, Genzel et al. (2003) <math>a_{*} \sim 0.52</math>,

Daly (2019) <math>a_{*} = 0.93 \pm 0.15</math>,

and Daly et al. (2023) <math>a_{*} = 0.90 \pm 0.06</math>.

Orbiting stars

thumb|left|Inferred orbits of six stars around supermassive black hole candidate Sagittarius A* at the Milky Way's center

thumb|Stars moving around Sagittarius A*, 20-year timelapse, ending in 2018

thumb|Stars moving around Sagittarius A* as seen in 2021

There are a number of stars in close orbit around Sagittarius A*, which are collectively known as "S stars". These stars are observed primarily in K band infrared wavelengths, as interstellar dust drastically limits visibility in visible wavelengths. This is a rapidly changing field—in 2011, the orbits of the most prominent stars then known were plotted in the diagram at left, showing a comparison between their orbits and various orbits in the Solar System. but later observations of the star have found this not to be the case.

The high velocities and close approaches to the supermassive black hole makes these stars useful to establish limits on the physical dimensions of Sagittarius A*, as well as to observe general relativity associated effects like periapse shift of their orbits. An active watch is maintained for the possibility of stars approaching the event horizon close enough to be disrupted, but none of these stars are expected to suffer that fate.

, S4714 is the current record holder of closest approach to Sagittarius A*, at about , almost as close as Saturn gets to the Sun, traveling at about 8% of the speed of light. These figures given are approximate, the formal uncertainties being and . Its orbital period is 12 years, but an extreme eccentricity of 0.985 gives it the close approach and high velocity.

An excerpt from a table of this cluster (see Sagittarius A* cluster), featuring the most prominent members. In the below table, id1 is the star's name in the Gillessen catalog and id2 in the catalog of the University of California, Los Angeles. a, e, i, Ω and ω are standard orbital elements, with a measured in arcseconds. Tp is the epoch of pericenter passage, P is the orbital period in years and Kmag is the infrared K-band apparent magnitude of the star. q and v are the pericenter distance in AU and pericenter speed in percent of the speed of light.

{|class="wikitable sortable" style="text-align:right"

! id1

! id2

! a

! e

! i (°)

! Ω (°)

! ω (°)

! Tp (yr)

! P (yr)

! Kmag

! q (AU)

! v (%c)

| S1

| S0-1

| 0.5950

| 0.5560

| 119.14

| 342.04

| 122.30

| 2001.800

| 166.0

| 14.70

| 2160.7

| 0.55

| S2

| S0-2

| 0.1251

| 0.8843

| 133.91

| 228.07

| 66.25

| 2018.379

| 16.1

| 13.95

| 118.4

| 2.56

| S8

| S0-4

| 0.4047

| 0.8031

| 74.37

| 315.43

| 346.70

| 1983.640

| 92.9

| 14.50

| 651.7

| 1.07

| S12

| S0-19

| 0.2987

| 0.8883

| 33.56

| 230.10

| 317.90

| 1995.590

| 58.9

| 15.50

| 272.9

| 1.69

| S13

| S0-20

| 0.2641

| 0.4250

| 24.70

| 74.50

| 245.20

| 2004.860

| 49.0

| 15.80

| 1242.0

| 0.69

| S14

| S0-16

| 0.2863

| 0.9761

| 100.59

| 226.38

| 334.59

| 2000.120

| 55.3

| 15.70

| 56.0

| 3.83

| S4714

| 0.102

| 0.985

| 127.7

| 129.28

| 357.25

| 2017.29

| 12.0

| 17.7

| 12.6

| 8.0

Discovery of G2 gas cloud on an accretion course

First noticed as something unusual in images of the center of the Milky Way in 2002, the gas cloud G2, which has a mass about three times that of Earth, was confirmed to be likely on a course taking it into the accretion zone of Sgr A* in a paper published in Nature in 2012. Predictions of its orbit suggested it would make its closest approach to the black hole (a perinigricon) in early 2014, when the cloud was at a distance of just over 3,000 times the radius of the event horizon (or ≈260 AU, 36 light-hours) from the black hole. G2 has been observed to be disrupting since 2009, In addition to the tidal effects on the cloud itself, it was proposed in May 2013 that, prior to its perinigricon, G2 might experience multiple close encounters with members of the black-hole and neutron-star populations thought to orbit near the Galactic Center, offering some insight to the region surrounding the supermassive black hole at the center of the Milky Way.

The average rate of accretion onto Sgr A* is unusually small for a black hole of its mass and is only detectable because it is so close to Earth. It was thought that the passage of G2 in 2013 might offer astronomers the chance to learn much more about how material accretes onto supermassive black holes. Several astronomical facilities observed this closest approach, with observations confirmed with Chandra, XMM, VLA, INTEGRAL, Swift, Fermi and requested at VLT and Keck.

Simulations of the passage were made before it happened by groups at ESO and Lawrence Livermore National Laboratory (LLNL).

As the cloud approached the black hole, Daryl Haggard said, "It's exciting to have something that feels more like an experiment", and hoped that the interaction would produce effects that would provide new information and insights.

Nothing was observed during and after the closest approach of the cloud to the black hole, which was described as a lack of "fireworks" and a "flop".

Andrea Ghez et al. suggested in 2014 that G2 is not a gas cloud but rather a pair of binary stars that had been orbiting the black hole in tandem and merged into an extremely large star.

Notes

References

External links

Is there a Supermassive Black Hole at the Center of the Milky Way? (arXiv preprint)
2004 paper deducing mass of central black hole from orbits of 7 stars (arXiv preprint)
ESO video clip of orbiting star (533 KB MPEG Video)
The Proper Motion of Sgr A* and the Mass of Sgr A* (PDF)
NRAO article regarding VLBI radio imaging of Sgr A*
Peering into a Black Hole, 2015 New York Times video
Image of supermassive black hole Sagittarius A* (2022), Harvard Center for Astrophysics
(NSF; 12 May 2022)