TON 618

TON 618 first appeared in 1957 as entry number 618 in a Mexican catalogue of faint blue stars from the Tonantzintla Observatory. It was not a star. Radio surveys and spectra in the 1970s showed it to be a quasar: the blazing centre of a galaxy so far away that its light has travelled for about 10.8 billion years to reach us.

A monster at the centre

A quasar shines because gas spirals into a supermassive black hole and heats up to millions of degrees. TON 618 is among the brightest, at roughly 140 trillion times the Sun’s output. To estimate the black hole’s mass, astronomers measure how fast the glowing gas around it is moving. A 2004 study gave 66 billion solar masses. A 2019 analysis using a different spectral line revised that to about 40.7 billion. Both are among the largest black hole masses ever estimated, and both carry real uncertainty.

How big is that?

A black hole’s size is set by its event horizon, the boundary nothing can escape. For 40.7 billion Suns, that radius works out to about 800 times the distance from Earth to the Sun. Neptune orbits at about 30. Drop TON 618 into the middle of the Solar System and every planet and the Kuiper Belt would be deep inside it.

We will never see TON 618’s black hole directly. What reaches us is light from the gas falling in, emitted when the universe was a fraction of its present age.