Brotherton, whose book Spider Star has creatures living in an accretion disk, points to a couple papers from the 1990s which say that it's possible that civilizations really could develop in that environment. One paper, "Accretion disk civilization 1: Habitable zone around accretion disks at galactic nuclei" by J. Fukue et al., notes that the density of stars and planets in the accretion disk is expected to be high, and wonders what sort of planets might develop there. Fukue's second paper, "Accretion disk civilization 2: From sunhook to photon floater," gets even more speculative, talking about civilizations that could harness the energy of an accretion disk using a "photon floater":

An accretion disk surrounding a supermassive black hole at the active galactic nuclei radiates tremendous energy. In order to utilize energy of the accretion disk system, the author investigates the configuration and stability of a floating platform - photon floater - above the accretion disk, which is supported by the radiation pressure of the disk radiation. In the case of the far-floater, which is located far from the disk, there exists a critical floating angle, where the gravitational force of the central black hole is balanced with radiation pressure. In the case of the near-floater, which is located very close to the disk, there exists a critical floating height, where the gravity is balanced with radiation. It is demonstrated that this floating height is dynamical stable. Finally, in the case of the axis-floater, which is located on the axis of the disk, the photon floater is unstable.

Accretion disk image from Science Clarified.

[Mike Brotherton]

Louis Crane and Shawn Westmoreland of Kansas State University propose a way to use black holes as fuel that is entirely within the bounds of physics and technology as we know them, but would take phenomenal amount of engineering.

The crux of their idea involves using using a laser to form a micro black hole, which could be used as an energy source. This would be a Schwarzschild, or non-rotating, black hole which outputs Hawking Radiation, and the smaller the black hole, the more energetic.

Of course, making a black hole isn't the world's most easy undertaking. It takes a huge amount of power to build one in the first place. To make one of these mini black holes, Crane and Westmoreland propose a 370km2 solar panel, at an orbit one million km from the surface of the sun, which, if perfectly efficient, would gather enough energy per year to make one black hole. This power would be fed to a spherically converging gamma laser, with a lasing mass of around 10^9 tonnes. However, after you make a few black holes, you can use them as a power source to make more.

According to the authors, a black hole to be used in space travel needs to meet five criteria:

1. has a long enough lifespan to be useful,
2. is powerful enough to accelerate itself up to a reasonable fraction of the speed of light in a reasonable amount of time,
3. is small enough that we can access the energy to make it,
4. is large enough that we can focus the energy to make it,
5. has mass comparable to a starship.

Fortunately, black holes have a sweet spot in terms of size, power and lifespan which is almost ideal. If you take a trip to Alpha Centauri, with an acceleration of 1g to the half way point, and then decelerate at 1g for the remainder of the journey, the trip takes a relativistic 3.5 years. A black hole that would survive the entire trip would have a radius of 0.9 attometers, would have a mass of 606,000 tonnes, and a power output of 160 petawatts. The lifespan of the black hole could be extended by feeding it mass, too.

For longer trips, you could use larger but weaker holes, and smaller and more powerful ones for short trips.

Getting the black hole to act as a power source also requires a bit of work. One potential method involves placing the hole at the focal point of a parabolic reflector attached to the ship, creating forward thrust. A slightly easier, but less efficient method would involve simply absorbing all the gamma radiation heading towards the fore of the ship, and let the rest shoot out the back to push you onwards.

Of course, there are potential problems with Crane and Westmoreland's ideas. According to Govind Menon, Professor of Physics at Troy University, most views on extracting energy from black holes involve using ones that rotate. "With non-rotating black holes, this is a very difficult thing...we typically look for energy almost exclusively from rotating black holes. Schwarzschild black holes do not radiate in an astrophysical, gamma ray burst point of view. It is not clear if Hawking radiation alone can power starships." Menon adds that extracting energy from black holes is highly problematic. "Given [this type] of black hole, it is not clear to me how someone would go about extracting energy."

Another issue is what to do with the black hole when it reaches the end of its life span, as they tend to explode. "Such an explosion is powerful by terrestrial standards, but not by astronomical standards", say Crane and Westmoreland, so it's merely a matter of dropping the black hole around 1 AU away from anything too important, and letting it detonate.

With a set of four machines: black hole generator, black hole drive, power plant, and a self perpetuating black hole powered black hole generator, the potential is enormous. As Crane and Westmoreland say:

A civilization equipped with our four machine tool set would be almost unimaginably energy rich. It could settle the galaxy at will.

Article available on ArXiv
Found via Next Big Future

The diffuse X-ray light suffusing the image comes from gas that has been heated by stellar explosions, massive young stars — and outflows powered by the supermassive blackhole at the heart of the galaxy, Sagittarius A. Scientists believe Sagittarius A gave off giant X-ray flares 50 years and 300 years earlier. (So when we finally visit the center of the galaxy searching for the mythical planet Sha-Ka-Ree in order to meet God and ask him why he wants a spaceship, we should time our visit to avoid one of those irregular X-ray bursts.) [Chandra]

The black hole, designated HLX-1, was discovered by a team of astronomers led by the University of Leicester's Sean Farrell. They found the black hole while searching for white dwarfs and neutron stars. HLX-1 is located just outside a galaxy some 290 million light-years away.

Previously, all black holes have fallen into one of two categories. The more commonly observed kind of black hole, the stellar-mass black hole, is the ultimate result of a massive star dying. The other kind, the supermassive black hole, can be anywhere from tens of thousands to a billion times larger than their smaller counterparts. Such black holes are generally thought to be found only at the center of galaxies.

The size of black holes can be determined using the Eddington limit. Named for the astrophysicist Sir Arthur Eddington, who devised the first version of this particular metric, the Eddington limit is the point at which the amount of material entering the black hole equals the amount of x-ray radiations it expels. This implies a direct relationship between the amount of radiation emitted and the black hole's gravitational force, which in turn can be used to determine the size of the black hole.

Farrell's team has measured the emitted radiation of HLX-1 to be ten times that of a normal stellar-mass black hole. That suggests its mass roughly 500 times bigger than our sun. Stellar-mass black holes are only about thirty times heavier than our sun. This places HLX-1 in an unprecedented intermediate stage between stellar-mass and supermassive black holes, and none of the currently theorized mechanisms for the formation of black holes can explain its existence.

There are two leading hypotheses to explain how HLX-1 came to be, and either would greatly expand and complicate our understanding of the cosmos. The preferred theory holds that HLX-1 is the result of multiple black holes being pulled together and then fusing to create one relatively large black hole. This would require HLX-1 to be located in what was once a densely packed globular cluster. Such clusters are generally extremely distant from their galaxy's center, which is where HLX-1 currently is.

Although that's strong circumstantial evidence to favor the above theory, I must admit I find the second possible explanation much more intriguing. It actually is possible that HLX-1 is the result of just one star collapsing, as long as the star in question was much, much bigger than any seen today. There is some thought that the most ancient stars were indeed considerably bigger than their modern counterparts, and so it is possible that HLX-1 is a remnant of the oldest generation of stars. Either way, HLX-1 has given astronomers yet another object to look for while scanning the skies, as it is the first of the newly dubbed intermediate-mass black holes.

[ScienceNOW]

Scientists now believe that the supermassive black holes at the center of young galaxies throw off immense heat as matter falls into them. This heat illuminates the surrounding gas, as seen in the picture above. Interstellar gases, drawn by gravity, cool off and condense, forming new stars. But eventually, the "superblobs" of gas surrounding the galaxy get too hot, and this forces the growth of the galaxy to slow down, scientists say. Here are some stills from a new animation illustrating the process, to go along with a paper in the July 10 issue of The Astrophysical Journal. The stills also have more info in the captions:

[National Geographic]

Black holes are objects with such incredible mass and strong gravitational fields that visible light can't escape them. Often, astronomers can perceive black holes because they are sucking gas from neighboring stars, and then spitting energy back out in the invisible, super-hot x-ray region of the spectrum. While we understand black holes far better than we did even a decade ago, there is still no conclusive theory that explains why black holes exist or how, exactly, they function.

Their mystery is part of what makes them such a tantalizing subject of science illustration - as well as science fiction film and art.

Black holes look even cooler when we can see them in action. Here are a couple of movies people have made that attempt to reconstruct what a black hole would look like - from a scientific perspective, not a Hollywood one.

Here's a black hole eating a star of roughly the same size as our sun:



Here's the simulation of a collision between two black holes:

Illustration via NASA/ARCADE

Here's a new image from the Hubble Space Telescope, which reveals that a black hole 40,000 times the mass of our own sun may be at the center of Omega Centauri. (That's intermediate size for a black hole, and these findings are important proof that black holes come in all sizes.) You can see the increased brightness from stars bunching up around the center of the cluster, drawn by the gravitational pull of the black hole. This ancient star cluster, one of 200 in or near our galaxy, is 17,000 light years from Earth.

Images by Spitzer Space Telescope and the Hubble Space Telescope.