What happens to giant stars when they die? Do they crumble to nothingness, forming black holes that slowly gnaw away at their neighbors?
The answers hang high overhead this month in the constellation Cygnus, the Swan. There a distant blue star, viewable through binoculars, whirls furiously around a seemingly empty spot. X-ray telescopes have revealed the vacancy is actually a brilliant source of radiation, called Cygnus X-1. Most astronomers believe they are watching gas from the visible star falling toward a black hole, emitting a blare of X rays before it reaches the point of no return.
Day by day, researchers are learning more about these ultradense objects whose extreme gravity folds space-time like a taco. This past spring, the orbiting Rossi X-ray Timing Explorer observed odd flickerings from another likely black hole with the catchy name GRO J1655-40. Like many distant quasars, this object sends geysers of atom fragments spewing at right angles to a disk of hot, spinning gas. But GRO J1655-40 is much smaller and lies just 10,000 light-years away, inside our own galaxy.
A black hole seven times the mass of the sun seems responsible for this sci-fi-like scene. One clue is GRO J1655-40's X-ray emission, which varies in brightness 300 times each second. That's just what physicists would expect from debris orbiting 40 miles above a black hole's event horizon, the point where even light cannot escape. The big news is that Rossi's sensors picked up a second tone at 450 cycles per second, indicating the presence of additional flotsam just 30 miles from the horizon. For gas to orbit so close to a black hole, the entire space-time continuum in that area must be rotating, which means the black hole must be rotating as well. This is the first sign that black holes don't just make our heads spin, they do it themselves.
Swirling gas plunges in toward the black hole GRO J1655-40 before vanishing into Einsteinian limbo.Illustration courtesy of A. Hobart/Chandra X-ray Observatory
In an earlier effort, researchers teamed Rossi with three other space observatories— the Hubble Space Telescope, the Extreme Ultraviolet Explorer, and the Chandra X-ray Observatory— and aimed all four at another presumed black hole, XTE J1118+480, which pivots around a sunlike star. The results showed that the surrounding disk of spiraling gas halts mysteriously far from the black hole. Instead of circling at a point 25 miles from the event horizon, as scientists expected, the disk's edge lies 600 miles out. Intense radiation apparently heats the disk, swelling it into a huge bubble. Such observations let us visualize something nobody will ever see in person: the physical violence surrounding these cosmic rattraps.
Yet we may still misunderstand the entity driving all this action. Tarun Biswas, a relativity expert at the State University of New York, argues that black holes don't exist the way astronomers normally describe them and may not exist in any form at all. Although black holes are a logical end point according to Einstein's theory of relativity, the theory does not take other interactions into account, Biswas says. The strong nuclear force, in particular, becomes potent at small scales and could prevent a collapsing object from shrinking down to black-hole size.
Even if black holes can form, they never really do, at least not in our reality. The reason for this is another weird but established aspect of relativity. As a massive object collapses, its gravitational field grows more intense. But gravity slows the flow of time, and the stronger gravity becomes, the more time turns to molasses. "From an outside perspective— and we're all outside these objects— it takes infinitely long for a black hole to form," Biswas says.
In short, neither massive stars nor anything else can ever truly collapse into a black hole, and nothing can ever fall into one. Instead, everything ends in a spherical pileup just above the event horizon, frozen in time. A black hole is not a bottomless pit so much as a trash compactor, compressing everything it encounters into a thin membrane of eternity.
For the latest black hole news, see the Web sites for the Chandra X-ray Observatory (chandra.harvard.edu), the Rossi X-ray Timing Explorer (heasarc.gsfc.nasa.gov/docs/xte/learning_center), and the Hubble Space Telescope (hubble.stsci.edu).
NASA's Imagine the Universe project also has a nice section on black holes (imagine.gsfc.nasa.gov/docs/science/know_l1/black_holes.html). Or take a virtual trip to a black hole, courtesy of astronomer Robert Nemiroff (antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html).