Question 1

How heavy do they get?

Accepted Answer

The largest weighed so far run to tens of billions of solar masses, and they sit at the centres of the largest elliptical galaxies. That is around ten thousand times the mass of the hole at the centre of the Milky Way.

Question 2

Does more mass mean a bigger object?

Accepted Answer

Yes, and in the simplest possible way: the horizon radius is proportional to the mass. Doubling the mass doubles the radius, so the heaviest holes here are wider than the orbit of Neptune while still being, in every other respect, the same shape as the lightest.

Question 3

How is a mass measured at that distance?

Accepted Answer

By watching something else move. The orbit of a companion star, the speed of gas near the centre of a galaxy, or the waveform of a merger all fix the mass at the focus — nothing here is weighed directly, and every figure is the output of a model fitted to motion.

Question 4

How firm is the order?

Accepted Answer

Less firm than a ranked list looks. Mass estimates for the heaviest objects carry uncertainties of tens of per cent, and two adjacent rows are often the same mass as far as the measurements can tell. The object's own page carries the error bar.

Question 5

Why are some black holes missing?

Accepted Answer

Because no mass has been published for them. A mass needs a dynamical measurement, which needs something to watch, and most catalogued objects have never had one.

Question 6

Is there an upper limit?

Accepted Answer

There appears to be one around fifty billion solar masses. Above roughly that, a disc of infalling gas fragments into stars instead of feeding the hole, so growth by accretion stalls — a hole can still gain mass by merging with another.

Question 7

How is such a mass measured?

Accepted Answer

From the motion of what is nearby: individual stars where the galaxy is close enough to resolve them, otherwise the width of emission lines from gas orbiting the centre. Both are model-dependent, which is why the published figures for one object can differ by a factor of two.

Question 8

Is the biggest black hole the biggest thing in the universe?

Accepted Answer

Not by size. The heaviest known are a couple of thousand astronomical units across — wider than the planetary part of the Solar System, and still millions of times smaller than the galaxy each one sits at the centre of. What a black hole holds is mass, not volume.

The most massive black holes

COMMON QUESTIONS