Question 1

How close is close?

Accepted Answer

Not close in any sense that matters. The nearest known black hole is thousands of light years away, which is far enough that its gravity has no measurable effect here and near enough that its companion star can be watched from the ground.

Question 2

How is a nearby one found?

Accepted Answer

By the star going round it. A visible star on an orbit whose other focus holds nothing luminous gives away both the presence and the mass of the dark companion, and that is how the closest ones on this list were identified.

Question 3

How is the distance measured?

Accepted Answer

By parallax where the object is near enough — the shift in its apparent position as the Earth crosses its orbit, which is geometry and needs no assumptions. Beyond that, distance is inferred from how bright something of known luminosity appears, and the further out the ladder goes the more the uncertainty grows.

Question 4

How firm is the order?

Accepted Answer

Firmer at the top than at the bottom. Neighbouring rows often sit inside each other's error bars, so a swap between two adjacent entries is a revision rather than a correction. Each object's own page carries the figure with its uncertainty and the method it came from.

Question 5

Why are some black holes missing?

Accepted Answer

Because this catalogue holds no distance for them. A row with an empty distance column cannot be ranked on it, so the list is the nearest of those measured rather than the nearest that exist.

Question 6

Could the nearest black hole ever reach us?

Accepted Answer

No. It is bound to the galaxy on its own orbit, thousands of light years away, and nothing about a black hole makes it move towards anything — it falls the way any mass falls. At that separation its gravitational pull on the Solar System is smaller than that of many ordinary stars.

Question 7

Are there closer ones nobody has found?

Accepted Answer

Almost certainly. A black hole with no companion to disturb and no gas to swallow gives off nothing at all, and the models suggest the galaxy holds a hundred million of them. Only the ones doing something detectable are on any list.

Question 8

Why do the distances change between publications?

Accepted Answer

Because they are inferred from the companion star, and a revision to the star's distance or mass moves the black hole with it. Several entries here have shifted by tens of per cent since they were announced.

The nearest black holes to Earth

COMMON QUESTIONS