TON 618 is a hyperluminous quasar located in the constellation Canes Venatici, making it one of the most distant and powerful objects observed. Determining its size is complex because the name refers both to the ultramassive black hole at its core and the immense system of gas and radiation it powers. Measuring its vastness requires two separate calculations: the physical boundary of the black hole itself and the total extent of its surrounding influence. This breakdown clarifies what “size” means for this cosmic entity and provides the measurements in light-years.
Defining the Ultramassive Black Hole
The tremendous power of TON 618 originates from an ultramassive black hole at its center. This classification is reserved for black holes exceeding 10 billion solar masses, a threshold TON 618 surpasses significantly. Current estimates place its mass at approximately 66 billion solar masses, making it one of the most massive black holes known.
This immense mass drives the entire quasar system by consuming matter and releasing energy. For perspective, the entire Milky Way galaxy, including all its stars and dark matter, has a total mass only slightly greater than this single black hole. The gravitational dominance of such an object defines the structure and behavior of its host galaxy.
TON 618 is so far away that the light we observe today left the quasar when the universe was only a fraction of its current age. Its lookback time is approximately 10.8 billion years, and its distance due to the expansion of space is about 18.2 billion light-years. This extreme distance, determined by its redshift of z=2.219, means we are seeing the system as it existed in the distant past.
The Calculated Diameter of the Event Horizon
When considering the black hole itself, the only measurable physical “size” is the event horizon, also called the Schwarzschild radius. This boundary represents the point of no return, where gravity is so strong that nothing, not even light, can escape. The diameter of this boundary is directly proportional to the mass of the black hole.
Based on its estimated mass of 66 billion solar masses, the event horizon of TON 618 has a diameter of approximately 2,600 Astronomical Units (AU). One AU is the average distance between the Earth and the Sun. This diameter converts to approximately 0.041 light-years, which is the physical size of the black hole’s ultimate boundary.
To visualize this diameter, compare it to the size of our solar system, which extends roughly to the orbit of Neptune at about 30 AU. The TON 618 event horizon is more than 85 times the distance from the Sun to Neptune. If this black hole were placed at the center of our solar system, its event horizon would engulf the orbits of all eight major planets and extend far into the outer Kuiper Belt.
The event horizon diameter of 0.041 light-years is minuscule compared to the vastness of interstellar space, but it is the maximum physical extent of the black hole’s surface. This measurement represents the size of the object itself, distinct from the significantly larger light and gas structures it generates. The ultramassive size allows the black hole to operate as an intensely luminous quasar.
The Scale of the Quasar System’s Influence
TON 618 is a hyperluminous quasar, meaning it is an active galactic nucleus surrounded by energetic structures. The black hole’s colossal gravitational force draws in vast amounts of gas and dust, forming a rotating accretion disk superheated by friction. This accretion disk and the resulting radiation field constitute the larger quasar system.
The intense radiation from the accretion disk illuminates and ionizes surrounding gas clouds, creating components that extend far beyond the event horizon. One feature is the Broad Line Region (BLR), a dense area of fast-moving gas clouds orbiting the black hole that can span several light-weeks or light-months. The light from the black hole’s immediate vicinity causes these clouds to glow, which allows astronomers to measure their distance.
The influence of TON 618 extends much further, manifesting as an enormous cloud of gas known as a Lyman-alpha blob. Observations reveal that this nebula surrounding the quasar and its host galaxy has a diameter of at least 330,000 light-years. This single gas cloud is more than twice the size of the entire Milky Way galaxy.
This staggering diameter of 330,000 light-years represents the true scale of the TON 618 system’s energetic output and gravitational reach. It demonstrates a profound contrast between the black hole’s physical boundary (0.041 light-years across) and the massive nebula it has energized. The quasar’s intense radiation excites the hydrogen gas in this nebula, causing it to glow brightly across hundreds of thousands of light-years.
Determining the Size of Distant Cosmic Objects
Determining the size and mass of an object as distant as TON 618 requires indirect methods, since even the largest telescopes cannot resolve the black hole’s tiny event horizon. The object’s great distance is established by measuring its redshift, which is the stretching of light waves as the source moves away due to the expansion of the universe. The redshift value of z=2.219 enables the calculation of its 10.8 billion light-year lookback time.
The mass of the ultramassive black hole is estimated primarily through Reverberation Mapping. This method uses the speed of light to measure the size of the Broad Line Region (BLR) and the velocity of the gas within it. The luminosity of the accretion disk varies, and this change in light is seen as an “echo” in the BLR after a measurable delay.
By measuring the time delay between the initial light flare from the accretion disk and the light response from the BLR, astronomers calculate the distance to the BLR gas clouds. Combining this distance with the measured velocity of the gas (inferred from the width of spectral lines) allows for a calculation of the central black hole’s gravitational pull and mass. This mass figure is then used to calculate the physical size of the event horizon.