Astronomers have unveiled the first image of the supermassive black hole at the centre
of our own Milky Way galaxy.
This result provides overwhelming evidence that the object is indeed a black hole and
yields valuable clues about the workings of such giants, which are thought to reside at
the centre of most galaxies.
The image was produced by a global research team called the Event Horizon Telescope (EHT)
Collaboration, using observations from a worldwide network of radio telescopes.
The image reveals a glowing, donut-shaped ring at the Milky Way’s heart.
Black holes are invisible by nature.
Their pull is inescapable, forever trapping any light that falls into their gravitational
abyss.
But just beyond a black hole’s point of no return, light persists, and its patterns,
like a photo negative, can reveal a black hole’s lurking presence.
Now an international team of astronomers, including researchers at MIT’s Haystack
Observatory, has captured the light around our own supermassive black hole, revealing
for the first time, an image of Sagittarius A* , the black hole at the heart of the Milky
Way galaxy.
The image was created by the Event Horizon Telescope (EHT) — a global network of radio
telescopes whose movements are choreographed so they function as one virtual, planet-sized
telescope.
The researchers focused the EHT array on the center of our galaxy, 27,000 light years from
Earth, cutting through our planet’s atmosphere and the turbulent plasma beyond our solar
system.
The resulting image reveals SgrA* for the first time, in the form of a glowing, donut-shaped
ring of light.
This ring structure lies just outside the event horizon, or the point beyond which light
cannot escape, and is the result of light being bent by the black hole’s enormous
gravity.
The bright ring encircles a dark center, described as the black hole’s “shadow.”
The ring’s white-hot plasma is estimated to be 10 billion Kelvin, or 18 billion degrees
Fahrenheit.
Judging from the ring’s dimensions, SgrA* is roughly 4 million times the mass of the
sun and incredibly compact, with a size that could fit within the orbit of Venus.
The image is the first visual confirmation that a black hole indeed exists at the center
of our galaxy.
Astronomers previously have observed stars circling around an invisible, massive, and
extremely dense object — all signs pointing to a supermassive black hole.
The image revealed today provides the first visual evidence that the object is a black
hole, with dimensions that agree with predictions based on Einstein’s theory of general relativity.
The image and accompanying analyses are presented in a number of papers appearing in a special
issue of The Astrophysical Journal Letters.
The findings are the result of work by more than 300 researchers from 80 institutions,
including MIT, which together make up the Event Horizon Telescope Collaboration.
The new image of SgrA* follows the first-ever image of a black hole, which was obtained
by the EHT in 2019.
That groundbreaking image was of M87*, the supermassive black hole at the center of Messier
87, a galaxy located 53 million light years from Earth.
To capture a clear image of SgrA*, astronomers coordinated eight radio observatories around
the world to act as one virtual telescope, which they pointed at the center of the Milky
Way over several days in April 2017.
Each observatory recorded incoming light data using high-speed recorders developed at Haystack
Observatory.
These recorders were designed to process an enormous amount of data at rates of 4 gigabytes
per second.
After collecting a total of 5 petabytes of data, encompassing observations of both SgrA*
and M87*, hard drives full of recorded data were shipped, half to Haystack, and the other
half to the Max Planck Institute for Radio Astronomy in Germany.
Both locations house correlators — massive supercomputers that worked to “correlate”
the data, comparing the data streams between different observatories, and converting that
data into signals that a planet-sized telescope would see.
They then calibrated the data — a meticulous process of weeding out noise from sources
such as instrumentation effects and the Earth’s own atmosphere, to effectively focus the virtual
telescope’s “mirror” on signals specific to SgrA*.
Then imaging teams took on the task of translating the signals into a representative image of
the black hole — a far trickier challenge than imaging M87*, which was a larger, steadier
source, changing very little over several days.
In coming years, the scientists expect to collect more data of SgrA* and other black
holes as the EHT expands, adding more telescopes to its virtual array.