They are gigantic pillars of gas and dust -- star "hatcheries" that reveal the secrets of star formation. They have also provided one of the most dramatic photographs so far from the Hubble Space Telescope.
"We were blown away when we first saw these images," said astronomer Jeff Hester of Arizona State University, who snapped the picture while studying the way interstellar gas clouds -- called nebulae -- dissipate.
Hubble's view of the surface of the eerie formations reveals for the first time what appear to be dense knobs and blobs of gas emerging from the larger clouds.
Inside them, astronomers say, new stars are forming. They are calling the blobs EGGs, for "evaporating gaseous globules."
The EGGs are evaporating under a bombardment of powerful radiation from nearby stars, Dr. Hester said. And the Hubble image gives scientists their first strong evidence that such "photoevaporation" can cut off the growth of young stars by blowing away the gas clouds from which they are forming.
"I can show this image to anybody in the field, and in two minutes, they'll say, 'Yeah, that's what's going on here,' " Dr. Hester said.
Released yesterday by NASA, the photo shows three sinister-looking, reddish-brown columns in the Eagle Nebula, which is part of our own Milky Way galaxy.
The columns of mostly hydrogen and dust stand like thunderheads against a star-studded sky, surrounded by a bluish halo of hot gas. The whole scene seems to be illuminated by a light source beyond the top of the picture. The photo was taken in April by Hubble's Wide Field/Planetary Camera.
The Eagle Nebula is seen in the southern sky on summer evenings. Its gas clouds are 7,000 light years from Earth, and the tallest in the photo stands about one light year high from its top to the bottom of the picture. A light year is about 5.9 trillion miles -- the distance light, at 186,000 miles per second, travels in one year.
Dr. Hester said the golden fringe of light that gilds the top edges of the clouds is "about the diameter of Pluto's orbit" -- about 80 times the distance from the sun to the Earth.
The clouds in the Hubble photo project inward like stalactites from the inner wall of the much-larger Eagle Nebula. The nebula cloud is being hollowed out by radiation from massive and very hot young stars that formed within it in the last million years.
"As soon as these stars turned on and started producing these copious amounts of ultraviolet light, [the radiation] started dispersing gas from the cloud," Dr. Hester said.
It's a little like a car's defroster evaporating the "fog" from the inside of the windshield.
The fingerlike formations are regions of relatively dense gas that have been slower to "erode" under the bombardment of stellar radiation, like rock formations in the deserts that are left standing after softer rock around them has eroded.
Dr. Hester and Paul Scowen, his associate at Arizona State, hoped Hubble would show the process by which the ultraviolet radiation "boils away" the hydrogen from the dense cloud. The radiation heats the cloud's surface, he said. "That drives the pressure up and pushes the material away. So when you look at the pictures you see filaments, wisps or striations pointing away from the surface of those columns."
What startled the astronomers, however, was that the photographs also revealed tiny lumps, bumps, knobby fingers and free-floating blobs of gas that seem to persist at the surface of the cloud as the gas around them is driven off. They are like stones left exposed in the desert as the sand around them blows away.
"These blobs were especially dense little clumps of gas embedded in the molecular cloud," Dr. Hester said. "They are being uncovered as the columns themselves are evaporating away."
Inside the blobs, hydrogen gas is condensing into new stars. Some stars have been uncovered by the radiation, and shine at (( the tips of gaseous "stalks."
The blobs reveal a process never before witnessed.
"This observation takes us in one jump from no evidence that this process works, to very strong evidence," said Bruce Margon, chairman of astronomy at the University of Washington in Seattle.
The photo adds a new twist to the debate about what limits the size of stars. Stars like our sun form when clouds of hydrogen and helium condense with the pull of gravity. When the cores are compressed enough to trigger nuclear fusion, they begin to burn.
Radiation streaming outward from the fusion reaction then drives away the remnants of the gas cloud from which they formed, and the stars can grow no bigger.
In the Eagle Nebula, "We seem to be seeing a totally different process determining the size of stars," Dr. Hester said.
Infrared images show that stars much larger than our sun have ignited inside their EGGs, but "they seem to be just sitting there," he said. They have not driven their gas clouds away, and may still be growing.
Conversely, the process raises the possibility that an EGG, uncovered before its star is big enough to ignite, might become a "brown dwarf" -- an object made like a star but too small to light up.