The question of whether Ton 618 is bigger than the Milky Way is a common point of confusion, as the comparison pits a complex stellar system against a single, immense engine of light and gravity. These two objects represent fundamentally different cosmic structures. The answer depends entirely on the metric used—physical size, total mass, or energy output—and understanding their individual natures is necessary to clarify this scale mismatch.
Defining the Milky Way’s Scale
The Milky Way is classified as a barred spiral galaxy, a massive collection of stars, gas, dust, and dark matter held together by gravity. Its visible structure is dominated by a flat disk containing spiral arms and a central bulge. The stellar disk is estimated to have a diameter of approximately 100,000 light-years, though the fainter, extended dark matter halo may stretch up to two million light-years across.
The galaxy is home to an estimated 100 to 400 billion stars. The entire galactic system, including its dark matter component, is thought to contain a total mass of around 1.15 to 1.5 trillion solar masses. The total mass of all the stars alone is estimated to be around 64 billion solar masses, which is a useful point of comparison for the central black hole of Ton 618.
Ton 618: An Ultramassive Black Hole and Quasar
Ton 618 is not a galaxy itself, but rather a hyperluminous quasar, which is a type of active galactic nucleus (AGN) powered by an ultramassive black hole (UMB). A quasar is the extremely bright core of a distant galaxy where the central black hole is actively consuming surrounding matter. The intense light originates from the superheated material spiraling into the black hole, forming a brilliant accretion disk.
The power source of the Ton 618 quasar is one of the most massive black holes ever found, with an estimated mass ranging from 40 billion to 66 billion times the mass of the Sun. This exceptional mass places it in the category of ultramassive black holes, far exceeding the typical supermassive black holes found at the centers of most galaxies. Its sheer gravitational pull drives gas to speeds up to 10,500 kilometers per second, a measurement used to determine its immense mass.
The black hole’s boundary, the event horizon, is defined by the Schwarzschild radius, the point beyond which nothing can escape. For a black hole of this mass, the event horizon is enormous, measuring about 1,300 astronomical units across, which is over 40 times the distance from the Sun to Neptune. However, this is still a tiny fraction of the surrounding quasar structure. The quasar is thought to be surrounded by an enormous Lyman-alpha nebula—a cloud of gas—that is estimated to be at least 330,000 light-years in diameter.
The Scale Mismatch: Comparing Different Cosmic Objects
The comparison between Ton 618 and the Milky Way is essentially between a black hole/quasar system and an entire galaxy, highlighting the mismatch in their fundamental nature. When looking at physical diameter, the Milky Way is vastly larger than Ton 618’s core components. The Milky Way’s stellar disk spans about 100,000 light-years, far greater than the Ton 618 black hole’s event horizon, which is only about 0.02 light-years in diameter.
While the black hole itself is incredibly small, the surrounding gas cloud of the Ton 618 system, the Lyman-alpha nebula, is estimated to be around 330,000 light-years across. This makes the nebula more than three times the diameter of the Milky Way’s visible disk. Therefore, the galaxy that hosts Ton 618, including its surrounding gas, is likely physically larger than our own galaxy.
Considering mass, the Ton 618 black hole is a single object significantly more massive than the combined mass of every star in the Milky Way (approximately 64 billion solar masses). However, the black hole’s mass of up to 66 billion solar masses is still only a small fraction of the Milky Way’s total mass, which is estimated to be over a trillion solar masses when including dark matter.
The most dramatic difference is in luminosity, or energy output. The active quasar of Ton 618 shines with a staggering luminosity of about 140 trillion times that of the Sun. This single object produces light thousands of times greater than the entire light output of the Milky Way galaxy, making it one of the brightest objects in the known universe.
Placing Ton 618 in the Universe
The immense scale of Ton 618 is compounded by its extreme distance and age, placing it as a relic of the early universe. Its observed redshift of 2.219 indicates that the light we see left the object approximately 10.8 billion years ago. When the light began its journey toward Earth, the universe was only a few billion years old, and our own solar system had not yet formed.
The current comoving distance to Ton 618 is estimated to be about 18.2 billion light-years, a measurement that accounts for the expansion of space since the light departed. The existence of such an enormous black hole in the early cosmos challenges current theories of black hole formation and growth. Astronomers hypothesize that these ultramassive black holes must have grown incredibly quickly, possibly through the direct collapse of massive gas clouds or the merging of many smaller black holes.
Ton 618 stands as a powerful example of an active galactic nucleus, offering insight into a period when galaxies were undergoing rapid, energetic growth. Its study is important for understanding the processes that shape the evolution of galaxies and the conditions that exist around the largest black holes.