Sagittarius A*, often abbreviated as Sgr A* and pronounced “Sagittarius A Star”, is a supermassive black hole located at the center of our spiral galaxy, the Milky Way.
Sagittarius A* is mostly dormant and only occasionally absorbs gas or dust, but nevertheless has a mass estimated to be millions of times that of our sun. Many mysteries still surround the supermassive black hole, but experts aren’t completely in the dark, and further observations from the Event Horizon Telescope promise to reveal more.
Stellar-mass black holes and intermediate-mass black holes form when massive stars cease nuclear fusion and can no longer sustain themselves against complete gravitational collapse, but the mechanism that forms supermassive black holes like Sagittarius A*n It’s not clear because there are no stars big enough to collapse directly into a black hole of this size.
Two possible mechanisms include smaller black holes growing to huge sizes by swallowing gas and dust from their surroundings or by hierarchical mergers of smaller black holes.
Sagittarius A*: Size
In 2008, astronomers Reinhard Genzel and Andrea Ghez determined that Sagittarius A* had a mass 4.3 million times that of the sun.
Astronomers have also calculated that the diameter of the supermassive black hole in the Milky Way is about 14.6 million miles (23.5 million kilometers). It’s tiny compared to the Milky Way itself, which is 100,000 light-years wide and 1,000 light-years thick.
Dwarf Sagittarius A* is also a surrounding disk of gas that extends between 5 and 30 light-years away, occasionally supplying matter to Sagittarius A*, causing faint X-ray flashes. This accretion disk is also linked to X-ray emissions caused by friction resulting in temperatures in the disc of up to 18 million degrees Fahrenheit (10 million degrees Celsius).
There’s still a lot to learn about Sagittarius A*, but the first image of the Milky Way’s central black hole taken by the Event Horizon Telescope (EHT) could reveal other secrets held by the cosmic object that shaped our galaxy. .
Sagittarius A*: Observations
Everything in our 13.6 billion year old galaxy orbits Sagittarius A*, including our solar system, located 26,000 light years away.
Black holes are notoriously difficult to spot, usually only inferred by the effects they have on their surroundings. Indeed, not only do they not emit light, but black holes also trap photons behind a boundary called the event horizon, which makes studying them directly in optical light almost impossible.
Observing Sagittarius A* from Earth is made even more difficult by the fact that it is shrouded in a thick intervening dust screen.
Fortunately, astronomers have developed other ways to better understand Sagittarius A*. For example, the mass of a central body and its radius can be determined by observing the gravitational influence it has on objects orbiting it.
To observe Sagittarius A*, astronomers monitored the star S2, which orbits Sagittarius A* at a distance of 11 billion miles (18 billion km) and at a speed of 17.1 million mph (11 .4 km/h). The star also has a highly elliptical 16-year orbit.
How was Sagittarius A* discovered?
Theories surrounding Sagittarius A* and its massive occupant date back to the early 1930s when Karl Jansky discovered a radio signal radiating from a location in the direction of the constellation Sagittarius pointing toward the center of the Milky Way.
The compact radio source of the galactic center Sagittarius A* was then identified in February 1974 by astronomers Bruce Balick and Robert L. Brown. It was during the 1980s that astronomers formulated the idea that the central compact object was likely to be a black hole of a size – until then – unimaginable.
In 1994, Reinhard Genzel and a team at UC Berkeley used infrared and submillimeter spectroscopy to deduce a compact object with a mass 3 million times that of the sun in the region.
Over the next decade, astronomers continued to rule out other possible candidates for this object – including closely clustered stars – which reinforced the idea that Sagittarius A* is a supermassive black hole.
Conclusive evidence that the compact object Sagittarius A* is a supermassive black hole was provided in 2018 when emissions caused by magnetic interactions from hot gas clusters near the black hole moving at about 30% the speed of the light were observed by astronomers using the European Southern Very Large Telescope (VLT) at the Observatory (ESO).
These observations matched exactly the theoretical predictions for hotspots orbiting near a black hole with a mass of four million solar masses.
Will Sagittarius A* swallow the Milky Way?
It’s a common misconception that black holes suck in matter. Instead, they simply capture in their gravity any matter that ventures too close. Thus, if the sun were replaced by a black hole, the Earth would remain stable and safe in its orbit, although deprived of heat and light.
Many black holes consume matter, gas and dust, or material stripped from nearby stars, surrounding it in an accretion disk that gradually feeds matter toward the center.
When this happens, the violent conditions within the accretion disk create powerful emissions and jets from an area called the active galactic nucleus (AGN). Sagittarius A* currently does not swallow enough matter to power an AGN and a star or other object would have to hit it almost directly to be consumed.
That could change in about 4 billion years, however, when the Milky Way collides and merges with the disturbing stars of Andromeda and provides another, potentially much larger, black hole into which stars can fall.
For more information on black holes, see “Introduction to Black Hole Physics” by Valeri P. Frolov and “Death by Black Hole – and Other Cosmic Quandaries” by Neil Degrasse Tyson.
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