Sagittarius A*

## General information

Sagittarius A\* is the Milky Way's Galactic Center's supermassive black hole. It is situated around 5.6° south of the ecliptic, on the boundary between the constellations Sagittarius and Scorpius. Visually, it is near what is known as the Butterfly Cluster (M6) and Lambda Scorpii.

The object is a very compact and bright radio emitter in astronomy. Sagittarius A\* derives its name from historical precedent. The radio sources detected by John D. Kraus, Hsien-Ching Ko, and Sean Matt in 1954 at 250 MHz using the Ohio State University radio telescope are mentioned. These sources were grouped by constellation, and each constellation's brightness was then represented by a capital letter, with A standing for the constellation's brightest radio source.

The mass and upper bounds on the radius of the object have been ascertained by measurements of many stars around Sagittarius A\*, in particular star S2. Astronomers came to the conclusion that Sagittarius A\* had to be the center supermassive black hole of the Milky Way based on mass and more accurate radius restrictions. Its mass is 4.154±0.014 million solar masses at this moment.

Utilizing the Event Horizon Telescope, a global network of radio observatories, astronomers were able to establish that Sagittarius A\* is a black hole by releasing the initial image of the accretion disk surrounding its horizon in May 2022. Following the supermassive black hole imaging of Messier 87 in 2019, this is the second verified image of a black hole. It is only adjacent objects whose behavior is affected by the black hole that are visible, not the black hole itself. The gas and particles that fall into the black hole are heated to millions of degrees, which is where the observable radio and infrared radiation originates from.

### History

One of the pioneers of radio astronomy, Karl Jansky, found evidence of a radio signal traveling into the core of the Milky Way in April 1933 from a point toward the constellation Sagittarius. Later on, Sagittarius A was the name given to the radio source. The Galactic Center, as we currently know it, is not quite as far south as his observations went. A discrete and bright "Sagittarius-Scorpius" radio source was found by Jack Piddington and Harry Minnett utilizing the CSIRO radio telescope at Potts Hill Reservoir in Sydney. After additional observations with the 80-foot (24-meter) CSIRO radio telescope at Dover Heights, the source was identified in a letter to Nature as the likely Galactic Center.

Subsequent observations revealed that Sagittarius A is actually made up of several overlapping sub-components; astronomers Bruce Balick [de] and Robert Brown used the National Radio Astronomy Observatory's baseline interferometer on February 13 and 15, 1974, to discover Sgr A\*, a bright and extremely compact component. Because the radio source was "exciting" and excited states of atoms are indicated with asterisks, Brown came up with the moniker Sgr A\* in a 1982 publication.

It has been clear since the 1980s that Sgr A\*'s center component is most likely a black hole. A Berkeley team led by Nobel Prizewinner Charles H. Townes and afterwards winner of the Nobel Prize Reinhard Genzel conducted infrared and submillimeter spectroscopic experiments in 1994 that revealed the mass of Sgr A\* to be densely concentrated and of the order of 3 million Suns.

#### Observation

On May 12, 2022, the Event Horizon Telescope Collaboration published the first photograph of Sagittarius A\*. Based on data from the radio interferometer acquired in 2017, the picture verifies that the object is home to a black hole. This is the black hole's second picture. Processing this picture required five years of mathematics. Eight radio observatories at six different locations gathered the data. Aperture synthesis is used to create radio pictures from data, often from overnight observations of steady sources. The investigation is complicated by the fact that the radio emission from Sgr A\* changes on the order of minutes.

Their calculation yields an overall 51.8±2.3 μas angular size for the source. This results in a diameter of 51.8 million kilometers (32.2 million miles) at a separation of 26,000 light-years (8,000 parsecs). For instance, during perihelion, Mercury is 46 million km (0.31 AU; 29 million mi) from the Sun, whereas Earth is 150 million km (1.0 AU; 93 million mi) from the Sun. Regarding the right ascension, Sgr A\*'s proper motion is around −2.70 mas per year, and for the declination, it is about −5.6 mas per year. Einstein's theory of relativity was put to the test more thoroughly than before by the telescope's measurement of these black holes, and the findings matched exactly.

In 2019, measurements using the High-resolution Airborne Wideband Camera-Plus (HAWC+) installed in the SOFIA aircraft revealed that the surrounding ring of gas and dust flows into an orbit around Sagittarius A\* due to magnetic fields. The temperatures of this gas and dust range from -280 to 17,500 °F (99.8 to 9,977.6 K; -173.3 to 9,704.4 °C)[24]. This keeps black hole emissions low.

Due to 25 magnitudes of extinction caused by dust and gas between the source and Earth, astronomers have not been able to view Sgr A\* in the optical spectrum.

### Properties

It was shown in a report released on October 31, 2018, that Sagittarius A\* is unquestionably a black hole. Scientists have spotted clumps of gas traveling at around 30% of the speed of light by creating a virtual telescope with a diameter of 130 meters using the GRAVITY interferometer and the four telescopes of the Very Large Telescope (VLT). Three distinct brilliant flares were observed, indicating the emission of very energetic electrons in close proximity to the black hole. These precisely match theoretical expectations for hot regions in the vicinity of a 4,000,000 solar mass black hole. Magnetic interactions in the extremely hot plasma circling very near to Sagittarius A\* are assumed to be the source of the flares.

Leading up to the pericenter approach in May 2018, at around 120 AU (18 billion km; 11 billion mi) (about 1,400 Schwarzschild radii) from Sgr A\*, it was announced in July 2018 that S2 circling Sgr A\* had been measured at 7,650 km/s (17.1 million mph), or 2.55% the speed of light. The gravitational redshift was detected, in agreement with the general relativity (GR) prediction within the 10 percent measurement precision. GR predicts that S2 would exhibit a noticeable gravitational redshift in addition to the typical velocity redshift at that close distance to the black hole.

Under the assumption that general relativity can still accurately describe gravity in the vicinity of the event horizon, the Sagittarius A\* radio emissions originate from a bright spot near the event horizon, possibly in the accretion disc, or from a relativistic jet of material ejected from the disc, rather than from the center of the black hole itself. Due to the black hole's gravitational lensing, Sagittarius A\* may appear to be amplified beyond its actual size if it were perfectly centered on the black hole. This would produce, in accordance with general relativity, a ring-shaped structure with a diameter approximately 5.2 times the Schwarzschild radius (10 μas) of the black hole.

The symmetrical nature of the radio source of Sagittarius A\* was discovered by recent lower resolution measurements. Alternative theory of gravity simulations show outcomes that could be hard to differentiate from general relativity. In contrast, a 2018 research predicts a picture of Sagittarius A\* that fits with recent data; specifically, it explains the source's symmetrical form and modest angular size.

The low intensity of the radio and infrared emission lines, together with the relatively tiny mass of this supermassive black hole, suggest that the Milky Way is not a Seyfert galaxy.

In the end, findings compatible with the presence of a black hole around Sgr A\* are seen rather than the black hole itself. The detected radio and infrared radiation in the event of such a black hole originates from gas and particles that fall into the black hole and are burned to millions of degrees. The black hole itself is believed to produce solely Hawking radiation at a temperature of around 10−14 kelvin, which is extremely low.

#### Stars

The term "S stars" refers to a group of stars that are in close orbit around Sagittarius A\*. Interstellar dust severely restricts vision in visible wavelengths, hence K band infrared wavelengths are the main wavelengths in which these stars are viewed. This is an area that is changing quickly; in 2011, the graphic on the left depicted the orbits of the most well-known stars at the time, allowing viewers to compare their trajectories with other solar system orbits. Since then, it has been discovered that S62 approaches those stars even closer.

These stars are helpful in determining the physical dimensions of Sagittarius A\* and in observing general relativity-related phenomena such as the periapse shift of their orbits due to their high velocity and close approaches to the supermassive black hole. Nothing is predicted to happen to any of these stars, but a vigilant watch is kept for the chance that they will get near enough to the event horizon to cause disruption. Less than 10% of the theoretical maximum value of Sagittarius A\*'s spin may be attributed to the observed dispersion of the planes of orbits of the S stars.

At around 12.6 AU (1.88 billion km), or roughly as near as Saturn gets to the Sun at 8% of the speed of light, S4714 holds the record for the closest approach to Sagittarius A\* as of 2020. With formal uncertainties of 12.6±9.3 AU and 23,928±8,840 km/s, these are approximations. Although it has a 12-year orbital period, its severe eccentricity of 0.985 allows it to approach closely and travel quickly.

a screenshot of a table showing the most notable members of this cluster (see Sagittarius A\* cluster). The star's name in the University of California, Los Angeles collection is listed as id2 in the table below, whereas its name in the Gillessen catalog is id1. Standard orbital elements are a, e, i, Ω, and ω; an is measured in arcseconds. Tp is the pericenter passage epoch, P is the orbital period expressed in years, and Kmag is the star's apparent magnitude in the K-band infrared spectrum. Pericenter velocity in percentage terms and pericenter distance in astronomical units are represented by the values q and v.