A Stef and Oren Lewandowski production

Credits

BlackHoles.fyi is a project by Oren and Stef Lewandowski, a father and son team in the UK. Oren, who is 17, co-writes the words and checks the physics. Stef makes the pictures.

The site draws black holes from the physics of how light and gas behave near them. You can explore one on the homepage, look at ten real black holes in the gallery, each drawn from its published numbers, or read the Concepts pages, which take one idea at a time with a figure you can change.

Why we made it

It started with a book. Oren read Black Holes: The Key to Understanding the Universe by the physicists Brian Cox and Jeff Forshaw, and wanted to see two things in it. One was a Penrose diagram, a map that fits the whole of space and time onto one page, with someone on it you could move about. The other was what it looks like to fly into a black hole, from outside and from inside.

Stef went looking for pictures of those. He found plenty of beautiful computer-drawn pictures, but very few that explain what you are seeing. The best teacher we have found on the web is still Falling into a Black Hole, a site of real diagrams for a general reader that the astrophysicist Andrew Hamilton made around 2000. As far as we know it hasn't been rebuilt since, so we started drawing our own.

The site is ours: Brian Cox and Jeff Forshaw had no part in it.

Cast and crew

Pictures
Stef Lewandowski
Words
Oren Lewandowski
Physics checking
Oren Lewandowski
Spacetime
Albert Einstein, 1915
The size of a black hole's dark silhouette
James Bardeen, 1973
First picture of the glowing gas round a black hole
Jean-Pierre Luminet, 1979
Render farm
Freckle and Auburn, the two Macs that render the stills and films
Typefaces
Literata, by TypeTogether, and Public Sans, from the US Web Design System

How we made it

AI agents do the building

Stef directs a team of AI coding agents: programs that write and test code from a written brief. Each agent takes one piece of work, writes it, tests it and adds it to a private test version of the site once every check passes. Stef decides what looks right, and nothing goes public until Oren has signed it off.

Fibers keeps track of the work

We used Stef's AI engineering toolkit, Fibers, to build this project. Fibers coordinates many AI coding sessions working on one project at once. Each session claimed a piece of work on a shared board before starting it, logged its progress there as it went, and recorded each decision with who made it and why. That kept two agents from picking up the same job, and it means a decision made weeks ago can still be traced back to its reason.

Research comes first

Before we draw a part of the picture, it gets a research note tied to the papers it rests on. We also keep a note of what we tried and why it looked wrong.

Every number has a paper

Each number in the code records what it rests on, usually a paper, and how far that paper was read. Each part of the picture says how real it is: worked out from the physics, following a published model, or chosen for the picture.

Design choices are written down

Stef keeps a written record of the design choices he has accepted and rejected, each dated and in his own words. The agents read it before they design anything, so a look he has turned down is less likely to come back.

Sites we learned from

Sources

The same parts, with a page for each area of the picture, are in Research.

Where each part of the picture comes from (13 parts)

Most of each picture is made by tracing light. For every pixel, the computer follows a ray of light back from your eye, bending it as the black hole's gravity would, to find where it came from. Some parts are worked out from the physics in full, some follow a model another team published, some are chosen for the picture where nothing more exact is known or practical, and many are partly worked out and partly chosen.

The dark patch at the centre

Worked out from the physics

Astronomers call it the black hole's shadow.

Each pixel is traced along the light ray that reaches it, so the dark patch is traced, not drawn. For a spinning hole, its outline matches the shape the physicist James Bardeen worked out in 1973 to within 1%, at spins up to 0.99, where 1 is the fastest a black hole can spin.

  • Bardeen 1973, Timelike and null geodesics in the Kerr metric, in Black Holes (Les Houches lectures)

The thin bright ring round the dark patch

Worked out from the physics

This is the photon ring.

It is light that circled the hole at least once before reaching you. It comes out of the same traced rays as the shadow, and nothing is drawn on top of it.

The shape of the glowing disc

Partly worked out from the physics

Gas falling into a black hole circles it in a flat disc, called an accretion disc, and glows as it heats up.

The inner edge sits at the innermost stable orbit for the hole's spin: the closest that gas can circle without falling in, which moves inward the faster the hole spins. The gas circles at the rate for that orbit. M is a unit of distance set by the hole's mass, so the same drawing fits every black hole at its own scale. The disc is drawn out to 1 500 M, where the disc's own gravity would break it into clumps (Goodman 2003).

How hot the disc is

Follows a published model

The disc's colour and brightness come from its temperature.

The temperature follows the standard model of a thin disc, one much thinner than it is wide, corrected for the strong gravity near the hole (Novikov and Thorne 1973). The black hole on the site's front page is 3C 273, a real quasar: a feeding black hole at the heart of a distant galaxy. Its mass, 260 million suns, and how fast it feeds, 1.2 times the Eddington rate, are its published values. The Eddington rate is the rate at which the light from the falling gas starts to push the rest of it away.

Why one side of the disc is brighter

Worked out from the physics

The disc's light changes in colour and brightness on its way to you.

For each point of the disc we work out how far its light is shifted towards blue or red: by the gas moving towards or away from you, by its light climbing out of the hole's gravity, and by the hole's spin. Gas coming towards you looks brighter and bluer, so one side of the disc outshines the other.

How thick the disc is

Partly worked out from the physics

The disc is much thinner than it is wide, but not flat.

The thickness follows the standard thin-disc model (Shakura and Sunyaev 1973). Above and below its middle the gas thins out in the shape that gas held up by its pressure takes.

The threads of gas in the disc

Partly worked out from the physics

Fine streaks of brighter and fainter gas that curve round the disc.

The threads move at the gas's real orbital speed, so the faster inner gas draws each one into a trailing arc, as it does in a real disc, where the inner gas outruns the outer. We drew their shape to match the gas in published simulations, rather than simulating it ourselves.

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.

The circles drawn round the hole

Partly worked out from the physics

They mark the horizon (the point of no return), the photon sphere (where light can circle the hole) and the innermost stable orbit (the closest that gas can circle without falling in).

Their sizes are 2 M, 3 M and 6 M, the ones the physics gives. M is a unit of distance set by the hole's mass, so the same drawing fits every black hole at its own scale. They are drawn as if light travelled straight, so they show where each sphere is, not how it would look.

    The object you can drop in

    Partly worked out from the physics

    A small object falling towards the hole, so you can see how its fall looks from far away.

    Its fall, the time on its own clock and on yours, the delay of its light and its fading are worked out. It is drawn as a dot whose size and colour are chosen, and the fainter images its light makes by circling the hole are left out, so a real one would fade more slowly.

    The paths of light

    Partly worked out from the physics

    Lines that show the routes light takes past the hole on its way to you.

    Each path is found the same way the picture is: by following light back from your eye and bending it as the hole's gravity would. It is then drawn where it really runs in space, as if the hole's gravity did not bend your view of it. Paths stop at the middle of the glowing disc of gas that circles the hole. The picture gives that disc some thickness, so near its rim a path can end a little off what you see.

      The black hole cut in half

      Partly worked out from the physics

      A view with the near half of the hole and its disc of gas cut away. Lines mark each region on the cut: the ergosphere, where nothing round a spinning hole can stay still; the corona, very hot gas just above the disc; the jet, a beam of gas fired out along the spin axis; the magnetic field threading the gas; and, for a hole that takes in only a little gas, the cloud of hot gas it has in place of a disc. Inside the horizon sit a second, inner horizon and the singularity, a ring where the maths of gravity gives no answer.

      The gas on your side of the cut is left out, so the cut face is lit by the disc's gas, not painted on. The lines sit where each region is, as if light travelled straight. The horizon, the ergosphere and the disc's inner edge follow from the maths for the spin the hole is drawn with. The disc's thickness, the cloud's width, the corona's reach and the jet's edge come from the models the rest of the picture is drawn with, and the magnetic field is a pattern we drew, not one worked out. The inner horizon and the ring come from the idealised maths.

      More parts of the pictures are added here as each is checked against the code that draws it.

      The real black holes (10 black holes)

      Black holes that astronomers have measured, each drawn from its published numbers, with the count of papers those numbers come from.

      Data (4 sources)

      The star maps and catalogues the pictures use, credited as each source asks.

      • Deep Star Maps 2020. NASA/Goddard Space Flight Center Scientific Visualization Studio; Gaia DR2: ESA/Gaia/DPAC. Used for the Milky Way behind the hole.
      • The Hipparcos and Tycho-2 star catalogues. van Leeuwen 2007 and Høg and others 2000, from NASA's HEASARC archive. Used for the bright stars, placed one by one.
      • The SXS catalogue of black hole mergers. The SXS Collaboration (Simulating eXtreme Spacetimes), under the CC BY 4.0 licence, which lets anyone reuse it with credit. Used for the Concepts page where two black holes spiral together and merge.
      • SIMBAD. The reference database of named objects in the sky, run by CDS, Strasbourg. Used for the real holes' names and places in the sky.
      Software (12 projects)

      The open-source code the site is built with, and the licence each is shared under, which says how others may reuse it.

      Further reading (2 works)

      The book this site began with, and the site that showed us black holes can be explained with real diagrams.

      After the credits

      Read next: Why a black hole looks black