Many people experience the blues because of the darkness of December. But at least earthlings in the Northern Hemisphere who feel overwhelmed by dread of the dark can look forward to the refreshing light of early spring.
There are, however, places in the universe that never get any light. Among them are black holes. These supermassive entities are always dark, swallowing any source of light around them.
“Black holes are an extraterrestrial weirdity, a ubiquitous oddity,” says Paul Hertz, senior scientist in the astronomy and physics division of the National Aeronautics and Space Administration.
Two black holes, discovered by Chandra, the X-ray observatory NASA launched in July 1999, were featured recently in national news as being on a collision course. The collision would occur, oh, several million years from now.
One of the reasons these black holes are “attracted” to each other is because of their intense gravitational pull. The two are about 3,000 light-years apart, which in astronomical terms is not astronomical.
The galaxy in which these two holes exist, known as NGC 6240, is so far away several hundred million light-years that a collision won’t mean a thing to the earthlings of A.D. 200,000,000.
In NGC 6240, it could trigger the creation of new stars and flush the galaxy with high-energy radiation.
Black holes are all over the place in our expanding universe. Even the Milky Way, Earth’s home galaxy, has one. This black hole is in the center of our galaxy, about 20,000 light-years away from our own planet.
“As far as we can tell, every galaxy has a black hole in its center,” Mr. Hertz says.
Our black hole most likely will collide with the black hole in our nearest galaxy, Andromeda. This could affect our future relatives. The collision might lead to a coalition in which Andromeda and our Milky Way turn into one big galaxy.
“In about four or five billion years, they will collide,” Mr. Hertz says. “It will be a spectacular event and be visible to whoever might live on Earth then. There may be colorful lights in the sky.”
Black holes do not emit light, however. They are completely, well, black.
True, Mr. Hertz says, but gases and particles that float around in the universe do emit light. As they get pulled into the black hole, they sometimes flare up, right at the edge of the hole, and give off colorful fountains of light, he says.
Nothing that comes within a certain distance of the black hole this distance is called the “event horizon” can escape getting pulled in.
Another consequence of a collision is that discs of gases and particles that circle around the black holes and haven’t yet been sucked into them might collide when the holes merge.
A disc collision might cause particles to blow out from the black holes at very high speeds, says Wallace Tucker, astronomer and Chandra spokesman.
“This could have effects on the rest of the galaxy, such as triggering new stars or bathing parts of the galaxy in high-energy radiation,” Mr. Tucker says.
Even if a collision of black holes might release tremendous amounts of energy, it is not likely to warp the entire galaxy.
“I don’t think that it will change the shape of the galaxy,” Mr. Tucker says.
Black holes mostly “take” from their surroundings. They devour stars and other space matter. Sometimes, though, they also “give back” a little:
The discs of gas and dust around a black hole are heated to millions of degrees. They contain intense electromagnetic fields that can eject high-energy particles away from the black hole at near the speed of light and extend over a million light-years.
Black holes come in two or three size categories:
The smaller black holes the ones that are between five and 50 times the mass of the sun probably came about during a star’s “evolution.”
When a massive star runs out of fuel, it collapses. The collapsed star can turn into a black hole, Mr. Tucker says. A black hole of this type grows and becomes even more massive over millions, or billions, of years as it continues to absorb material around it.
The black holes that are billions of times as massive as our sun must have developed in some other fashion, Mr. Tucker says.
“We don’t know,” he says. “That’s why black holes are so fascinating. Are they [the] result of a collapse of a whole galaxy? Or do they occur when entire galaxies run into each other? Which came first, the black hole or the galaxy?”
Because black holes swallow everything that reaches their event horizon through their incredible gravitational pull, they consist of all kinds of cosmic material (like the bottom of a clogged drain).
“They can be made of anything, entire stars or planets or individual particles,” Mr. Tucker says. “This would include hydrogen, helium, carbon, nitrogen and oxygen.”
Scientists also are studying what happens to material that falls into black holes.
“Everything that ventures too close to the black hole is doomed to fall in,” Mr. Tucker says. “It never comes out the same way it went in but where does it go next?”
The simple answer is that it goes nowhere, Mr. Tucker says, but maybe it does. Time-travel enthusiasts say black holes might enable people in the future to time-travel.
“Maybe it pops up somewhere else in the universe,” Mr. Tucker says. “Or even in another universe.”
Because a black hole cannot be observed directly, astronomers use circumstantial evidence to prove the holes’ existence.
They watch the motions of the stars in the vicinity of a hole. If they are orbiting around a massive dark object, astronomers deduce that it is a black hole, Mr. Tucker says.
They also watch for strong sources of radiation, which can be produced from gases and particles that are falling into the hole.
NASA has three, soon to be four, observatories that study black holes: the Hubble Space Telescope, the Compton Gamma Ray Observatory and the Chandra X-ray observatory. Spring 2003 will see the launch of an infrared observatory.
Each observatory is tuned into different wavelengths of radiation, which allows each to see something unique and a little different from the others, Mr. Tucker says.
For example, Hubble measures temperature and velocity as a way to study the motions of the stars around black holes. Chandra, by detecting short wavelengths of light, studies the discs of hot matter that are spiraling into black holes.
Using these tools, astronomers hope to solve the mystery of how the supermassive black holes are created. When they figure that out, they may understand more about how galaxies are formed, Mr. Tucker says.
“How these supermassive black holes formed is still a mystery, but observations indicate that they form when galaxies are very young,” he says.
“So, if we can understand how these first black holes were formed, we should know a lot more about how galaxies are formed.”