Research

The sky behind the hole

Behind every black hole on BlackHoles.fyi is a sky of stars and the glow of the Milky Way, its light bent on the way to you.

Most of the site's pictures show the sky as it looks from Earth. The site also draws ten real black holes that astronomers have measured, and for two of them the sky is drawn as it would look from the hole.

Each part carries a label saying how real it is: worked out from the physics, following a model another team published, chosen for the picture, or partly worked out and partly chosen.

The stars and the Milky Way behind the hole

Partly worked out from the physics

The hole's gravity bends their light, as a lens does.

The bending is traced, and each bright star sits at its catalogued place with its measured colour. The Milky Way's glow is NASA's map of the sky from Earth, stretched for contrast, and the stars' brightness is chosen so you can see them. Nine of the real black holes draw the sky from where they are instead: five in our Galaxy (Cygnus X-1, Gaia BH1, Gaia BH3, V404 Cygni and GRS 1915+105) from Gaia's stars, Sgr A* from the star cluster round it and the dust of the Galaxy's centre, and three outside it (M87*, TON 618 and 3C 273) from their host galaxy's glow and its own stars, modelled from measurements (for TON 618, whose host has never been seen, from scaling alone).

The sky as seen from a real black hole

Partly worked out from the physics

Some of the real black holes show the sky from where they are, not from Earth.

Seen from the hole, the Milky Way is not Earth's view. The stars come from Gaia, the European Space Agency's star-mapping satellite, moved to this hole's place. The glow between them is a model fitted to Earth's sky. Over it we lay the dark bands of dust that cross the Milky Way, as they look from Earth. Where Gaia cannot see, we use Earth's sky itself. A panel beside the hole's picture lists which parts are measured, modelled, chosen or borrowed. The four holes outside our Galaxy have no Gaia stars to move; for now their sky is dark, which the panel says, and the glow of their own galaxy is a later step.