Seeing the Unseen
The mystery of what a black hole looks like has puzzled scientists and artists alike for decades.
A black hole is, by definition, unseeable — a region of space with a gravitational pull so intense light cannot travel outward to bring us an image. But, using the laws of physics and our understanding of the cosmos, scientists are able to recreate what a black hole should look like to a reasonable degree of accuracy.
The resulting images depend on a number of factors, for example, whether the black hole is active or not, or whether it's spinning, but they do share a number of features. Where many of these images end up diverging, ultimately, is in how scientifically accurate they actually are.
This image is a quite accurate depiction of what scientists think a black hole with an accretion disk of hot gas and dust around it might look like. The two rings we see are actually just one; gravity bends the light from the rear of the disk around the black hole on both top and bottom so that it appears perpendicular to the real thing.
Because the disk is spinning so quickly from left to right, the Doppler effect comes into play as well. Light on the left side appears brighter as it comes toward us and dimmer as it moves away.
The black hole itself, of course, is the black sphere in the middle, defined by the absence of light. The bright line that divides light from dark on the inner ring demarcates the inner edge of the photon sphere, the innermost region where we can still see particles of light escaping from the black hole's pull.
A different vision of a black hole, this time without the gravitational bending of light included. The resulting image doesn't capture relativistic effects, but it does illustrate the sometimes massive clouds of material that can surround the tiny pinprick of space that is a black hole.
This particular back hole is emitting what scientists call a "relativistic jet," a powerful beam of energy some very active black holes will emit as they suck matter inwards. Exactly how these jets are produced isn't quite known, but they can be some of the most powerful phenomena in the universe, sometimes extending for thousands, or even million of light years beyond the black hole.
Yet another conception of a black hole, this time without any material around it and backed by a dense field of stars.
Such objects do exist, in the form of quiet black holes that aren't currently sucking anything in. It's not clear whether such an object would look exactly like this, but the image does capture the way that a black hole would bend any starlight coming from behind it into a circle.
The ring of colorful stars results from light passing near the black hole curving around its gravitational well — the nearer to the black hole, the more the light gets bent.
In addition to an accretion disk, some black holes are also surrounded by what appears to be a large donut. The structure is called a torus, and scientists have just recently managed to image one with the ALMA radio telescope array in Chile
This galaxy is known as NGC 1068, and it's located about 50 million light years away. This picture is an artist's representation of what the very heart of the galaxy may look like, a shining accretion disk surrounded by a much larger torus of gas. How the structures form and behave is still largely unknown, and it's a topic of interest for researchers, as it may give new insights into how even larger things like galaxies come to be.
Black holes are sometimes found accompanied by a companion, as in this drawing of a stellar mass black hole being fed by a nearby star.
The unlucky star happens to be situated near enough to the black hole to begin losing matter to it, but not near enough to fall in wholesale. The result is that a steady stream of material will be siphoned from the star and fed into the singularity, creating a constant accretion disk and powering a pair of jets as excess material is shot into space.
Watching how different types of matter falls into and is absorbed by a black hole is another topic of fascination for researchers. The supermassive black hole at the center of our galaxy, Sagittarius A* (pronounced A star) and seen here, will swallow up a large cloud of gas, G2, at some point in the near future.
With the Event Horizon Telescope (EHT), scientists hope to watch the event as it happens, cataloguing the process of annihilation from beginning to end. Will the gas fall straight in at once, or will some peel off to spin around the singularity's edge? How will the massive meal impact the black hole's jets?
These are open questions right now, but the EHT is poised to give us answers. When it does, we may finally be able to move beyond fanciful renderings such as this one and into the realm of actual data.
Another image from 1978, though this one has bit more grounding in actual data. French astrophysicist Jean-Pierre Luminet made this drawing of a black hole based on calculations of general relativity done with the help of an early IBM punch-card computer.
Luminet took the resulting data points and marked them by hand onto photographic negative paper with blackink, and took a negative of that to get the image seen below. Though it was the first realistic rendering of a black hole, the image has held up over time. Space artists today, from Discover Magazine to the graphics team behind Interstellar, have created images that look similar.
What a black hole might look like as we zoom closer, as depicted on the cover of Astronomy Magazine in October 1978. This series of images shows the x-ray binary system Cygnus X-1, which contains a massive blue giant star and a black hole about 14.8 times as massive as our sun.
We begin on the upper left, about 10 million miles away. Streams of gas blowing from the star are captured by the black hole and spun into an accretion disk. 100,000 miles away, structures in the accretion disk begin to emerge, while at 1,000 miles the white-hot temperatures become apparent and the edge of singularity itself emerges.
Finally, just 10 miles from the black hole, the event horizon dominates what appears to be a massive, staring eye. Gas from the star streams x-rays as it falls into oblivion.