Astronomy - Three giants of the universe - comprehending Black Holes

Black holes are often imagined as a single type of cosmic object, however in reality they exist across an enormous range of masses and environments. At the extreme end are supermassive black holes, found at the centres of galaxies and containing millions or even billions of times the mass of our Sun. The concept of scale has to be fully re-imagined when examining the universe. 

Three remarkable examples are TON 618, M87* and Sagittarius A*. All three are supermassive black holes, but they differ enormously in size, distance and activity.

TON 618 — The Colossus

TON 618 is among the most massive black holes known thus far and it lies at the heart of an extraordinarily luminous quasar—a phenomenon produced when matter falling towards a supermassive black hole releases enormous amounts of energy.

The black hole itself cannot be seen. Instead, the brilliant environment surrounding it as material is drawn into the black hole's gravitational influence is what can been viewed.

TON 618 — essential facts

• Category: Extremely massive supermassive black hole powering a quasar
• Estimated mass: Approximately 40 billion times the mass of the Sun
• Distance: Approximately 18.2 billion light-years in present-day comoving distance
• Light-travel time: Approximately 10.8 billion years
• Estimated event-horizon diameter: Approximately 160 billion kilometres
• Equivalent scale: About 1,070 times the Earth–Sun distance
• Significance: One of the most massive black holes known

It's estimated event-horizon scale is itself difficult to comprehend. If placed at the centre of Earth's Solar System, it would extend far beyond the orbit of Pluto.

M87* — The Black Hole that has been imaged

At the centre of the giant elliptical galaxy Messier 87 lies another extraordinary supermassive black hole: M87*. M87* became famous in 2019 when the Event Horizon Telescope collaboration produced the first image of a black-hole shadow. The image showed the glowing material surrounding the black hole and the dark region created by its extreme gravitational field. M87* is also associated with a spectacular relativistic jet, extending thousands of light-years into space.

 M87* — essential facts

• Category: Supermassive black hole at the centre of a giant elliptical galaxy
• Estimated mass: Approximately 6.5 billion times the mass of the Sun
• Distance: Approximately 55 million light-years
• Estimated event-horizon diameter: Approximately 38 billion kilometres
• Equivalent scale: About 257 astronomical units
• Special significance: First black hole to have its shadow directly imaged
• Additional feature: Powerful relativistic jet extending thousands of light-years

M87* demonstrates an important principle of black-hole astronomy: although the black hole itself is relatively compact compared with its host galaxy, its influence can extend across enormous distances.

Sagittarius A* — The Black Hole at the Heart of Earth's Galaxy

Sagittarius A*, usually abbreviated to Sgr A* is the supermassive black hole at the centre of the Milky Way, approximately 26,000 light-years from Earth. Compared with TON 618 and M87*, Sagittarius A* is surprisingly small, yet four million solar masses concentrated into such a compact region still produce an extraordinary gravitational environment. Unlike TON 618, Sagittarius A* is currently relatively quiescent. It is not behaving as a brilliant quasar, although gas, dust and stars are constantly interacting with its powerful gravitational field.

 Sagittarius A* — essential facts

• Category: Supermassive black hole at the centre of the Milky Way
• Estimated mass: Approximately 4.3 million times the mass of the Sun
• Distance: Approximately 26,000 light-years
• Estimated event-horizon diameter: Approximately 25 million kilometres
• Equivalent scale: About 0.17 astronomical units
• Activity: Relatively quiescent compared with an active quasar
• Special significance: Close enough for astronomers to study individual stars orbiting around it 

Sagittarius A* is therefore the closest opportunity for Earth's astronomers to study a supermassive black hole in detail.

Three Black Holes — One Extraordinary Phenomenon

The differences between these three objects are staggering.

• Sagittarius A* contains approximately 4.3 million solar masses.

• M87* contains approximately 6.5 billion—more than 1,000 times the mass of Sagittarius A*.

• TON 618, with an estimated 40 billion solar masses, contains roughly 10,000 times the mass of Sagittarius A*.

All three belong to the same fundamental astronomical category: supermassive black holes. Sagittarius A* is the comparatively quiet giant residing at the centre of our own galaxy. M87* is a much larger black hole whose environment produces a spectacular relativistic jet—and whose shadow humanity has now imaged. TON 618 represents the extreme end of the scale: a colossal black hole powering one of the Universe's most luminous quasars.

These three objects offer three extraordinary perspectives on the same phenomenon. They are among the most extreme objects produced by nature—places where gravity becomes so powerful that space, time and light itself behave in ways that challenge the understanding of reality.