Photo courtesy of Fulvio Melia

Within this decade, new telescopes will allow scientists to see this view of the supermassive black hole at the galactic center. The shadow in the middle is caused by its strong gravity, which absorbs all the light near it. The bright ring is due to light that has orbited once before escaping toward Earth. The region shown here is about the same size as the distance between the Earth and the sun.

Monday September 24, 2001

NASA telescope points to black hole in Earth's galaxy

There have been several clues pointing to the existence of black holes, but now scientists have the strongest evidence yet that a monster, millions of times more massive than our sun, lurks in the center of our galaxy.

The news came in the form of a burst of X-rays coming from the center of the Milky Way, detected by Chandra, NASA's $1.5 billion orbiting telescope.

These bursts appear to be directly related to this black hole, which "confirms what we had expected," said Fulvio Melia, a UA astronomy professor and associate head of the physics department whose research focuses on black holes.

Melia was invited to write a commentary on the discovery in the prestigious science journal Nature earlier this month. Both the research, presented by a team of American and Japanese scientists, and Melia's review appeared in the Sept. 6 issue.

This discovery "takes the proof into the threshold of certainty," that the black hole exists. The only thing missing is to get an actual photograph of it, Melia said.

A black hole, explained UA astronomy research associate Siming Liu, "is well known for its greedy characteristic of swallowing everything near it." However, before a star is sucked in entirely, it gives off a "death shine," a final, violent flash of radiation, which is what X-ray telescopes are able to see, he said.

However, an X-ray telescope like Chandra is unable to take a clear photograph of the black hole.

Telescopes that operate at its wavelength are often distorted by gas in the interstellar medium, making the images appear foggy. In order to get a clear picture, telescopes that operate with shorter wavelengths must be used.

"As you decrease the wavelength," Melia explained, "the fog lifts."

Some of these newer telescopes already exist, including the Hertz telescope, jointly operated by UA's Steward Observatory and the Max Planck Institute in Germany. But to get an accurate photograph of the black hole, scientists need at least a dozen of these instruments spread across the globe, working together in a large network.

Beyond that, scientists will someday combine those ground-based telescopes with space-based telescopes at shorter wavelengths, which "will give us an even better view," Melia said.

Liu, whose research is in the area of supermassive black holes, said the Chandra discovery gives physicists an "unprecedented chance to verify general relativity," a theory first presented by Emmy Noether and David Hilbert, and then five days later by Albert Einstein, in 1916.

General relativity was the first theory to mention black holes. Since that time, scientists have long believed black holes to exist, and the evidence for them has been steadily growing. But the discovery of a flare from the galactic center detected by Chandra is the most compelling evidence to date, Melia said.

Although other galaxies are believed to harbor black holes at their centers, the one in our own galaxy is special, Melia said. It's close enough to allow scientists to study it in detail, and it is the only one that we can one day expect to actually photograph.

The biggest X-ray telescope ever put into orbit, Chandra orbits the Earth at a distance one-third of the way between the Earth and the moon. Its creators boast that its accuracy is so high it can "read a stop sign at a distance of 12 miles," and its scope so large that it can "observe X-rays from clouds of gas so vast it takes 5 million light years to go from one side to the other."

The lasting importance of X-ray astronomy should not be underestimated, Liu said. "It opens a window for us to see and understand the universe, and a chance to appreciate the power of the cosmos." Sometimes, he added, "it is far beyond human imagination."