Black hole hunters cast gaze at centre of the Milky Way galaxy

WASHINGTON: Residing at the centre of our spiral-shaped Milky Way galaxy is a beast — a supermassive black hole possessing 4 million times the mass of our sun and consuming any material including gas, dust and stars straying within its immense gravitational pull.

Scientists have been using the Event Horizon Telescope (EHT), a global network of observatories working collectively to observe radio sources associated with black holes, to study this Milky Way denizen and have set an announcement for Thursday that signals they may finally have secured an image of it. The black hole is called Sagittarius A*, or SgrA*.

The researchers involved in this international collaboration have declined to disclose the nature of their announcement ahead of scheduled news conferences but issued a news release calling it a "groundbreaking result on the centre of our galaxy."

In 2019, the EHT team unveiled the first-ever photo of a black hole. The image — a glowing ring of red, yellow and white surrounding a dark centre — showed the supermassive black hole at the centre of another galaxy called Messier 87, or M87.

The researchers also have focused their work on Sagittarius A*, located about 26,000 light-years — the distance light travels in a year, 5.9 trillion miles (9.5 trillion km) — from Earth.

"One of the objects that we hope to observe with the Event Horizon Telescope... is our own black hole in our own backyard," Harvard–Smithsonian Center for Astrophysics astrophysicist Sheperd Doeleman, the former EHT project director, said during a July 2021 scientific presentation.

Black holes are extraordinarily dense objects with gravity so powerful that not even light can escape.

There are different categories of black holes. The smallest are so-called stellar-mass black holes formed by the collapse of massive individual stars at the ends of their life cycles. There also are intermediate-mass black holes, a step up in mass. 

And finally, there are the supermassive black holes that inhabit the centre of most galaxies. These are thought to arise relatively soon after their galaxies are formed, devouring enormous amounts of material to achieve colossal size.

The EHT project was begun in 2012 to try to directly observe the immediate environment of a black hole. A black hole's event horizon is the point of no return beyond which anything — stars, planets, gas, dust and all forms of electromagnetic radiation — gets dragged into oblivion.

The fact that black holes do not permit light to escape makes viewing them quite challenging. The project scientists have looked for a ring of light - super-heated disrupted matter and radiation circling at the tremendous speed at the edge of the event horizon — around a region of darkness representing the actual black hole. 

This is known as the black hole's shadow or silhouette.

Known as a spiral galaxy, the Milky Way viewed from above or below resembles a spinning pinwheel, with our sun situated on one of the spiral arms and Sagittarius A* located at the centre. The galaxy contains at least 100 billion stars.

The M87 black hole is far more distant and massive than Sagittarius A*, situated about 54 million light-years from Earth with a mass 6.5 billion times that of our sun. In disclosing the photo of that black hole, the researchers said that their work showed that Albert Einstein, the famed theoretical physicist, had correctly predicted that the shape of the shadow would be almost a perfect circle.

Thursday's announcement will be made in simultaneous news conferences in the United States, Germany, China, Mexico, Chile, Japan and Taiwan. Netherlands-based radio astronomer Huib Jan van Langevelde is the current EHT project director.

Doeleman emphasized the size scale of supermassive black holes.

"There are big things out there and we are small," Doeleman said. "But that's also kind of uplifting in a certain way, too. We've got a lot to explore in the universe."