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Eerie, dramatic new pictures from NASA's Hubble Space Telescope show newborn stars emerging from "eggs" – not the barnyard variety – but rather dense, compact pockets of interstellar gas called evaporating gaseous globules (EGGs). Hubble found the "EGGs," appropriately enough, in the Eagle nebula, a nearby star-forming region 6,500 light- years away in the constellation Serpens.
"For a long time astronomers have speculated about what processes control the sizes of stars – about why stars are the sizes that they are," said Jeff Hester of Arizona State University, Tempe, AZ. "Now in M16 we seem to be watching at least one such process at work right in front of our eyes."
Striking pictures taken by Hester and co-investigators with Hubble's Wide Field and Planetary Camera 2 (WFPC2) resolve the EGGs at the tip of finger-like features protruding from monstrous columns of cold gas and dust in the Eagle nebula (also called M16 – 16th object in the Messier catalog). The columns – dubbed "elephant trunks" – protrude from the wall of a vast cloud of molecular hydrogen, like stalagmites rising above the floor of a cavern. Inside the gaseous towers, which are light-years long, the interstellar gas is dense enough to collapse under its own weight, forming young stars that continue to grow as they accumulate more and more mass from their surroundings.
Hubble gives a clear look at what happens as a torrent of ultraviolet light from nearby young, hot stars heats the gas along the surface of the pillars, "boiling it away" into interstellar space – a process called "photoevaporation. "The Hubble pictures show photoevaporating gas as ghostly streamers flowing away from the columns. But not all of the gas boils off at the same rate. The EGGs, which are denser than their surroundings, are left behind after the gas around them is gone.
"It's a bit like a wind storm in the desert," said Hester. "As the wind blows away the lighter sand, heavier rocks buried in the sand are uncovered. But in M16, instead of rocks, the ultraviolet light is uncovering the denser egg-like globules of gas that surround stars that were forming inside the gigantic gas columns."
Some EGGs appear as nothing but tiny bumps on the surface of the columns. Others have been uncovered more completely, and now resemble "fingers" of gas protruding from the larger cloud. (The fingers are gas that has been protected from photoevaporation by the shadows of the EGGs). Some EGGs have pinched off completely from the larger column from which they emerged, and now look like teardrops in space.
By stringing together these pictures of EGGs caught at different stages of being uncovered, Hester and his colleagues from the Wide Field and Planetary Camera Investigation Definition Team are getting an unprecedented look at what stars and their surroundings look like before they are truly stars.
"This is the first time that we have actually seen the process of forming stars being uncovered by photoevaporation," Hester emphasized. "In some ways it seems more like archaeology than astronomy. The ultraviolet light from nearby stars does the digging for us, and we study what is unearthed."
"In a few cases we can see the stars in the EGGs directly in the WFPC2 images," says Hester. "As soon as the star in an EGG is exposed, the object looks something like an ice cream cone, with a newly uncovered star playing the role of the cherry on top."
Ultimately, photoevaporation inhibits the further growth of the embyronic stars by dispersing the cloud of gas they were "feeding" from. "We believe that the stars in M16 were continuing to grow as more and more gas fell onto them, right up until the moment that they were cut off from that surrounding material by photoevaporation," said Hester.
This process is markedly different from the process that governs the sizes of stars forming in isolation. Some astronomers believe that, left to its own devices, a star will continue to grow until it nears the point where nuclear fusion begins in its interior. When this happens, the star begins to blow a strong "wind" that clears away the residual material. Hubble has imaged this process in detail in so-called Herbig-Haro objects.
Hester also speculated that photoevaporation might actually inhibit the formation of planets around such stars. It is not at all clear from the new data that the stars in M16 have reached the point where they have formed the disks that go on to become solar systems," said Hester, "and if these disks haven't formed yet, they never will."
Hester plans to use Hubble's high resolution to probe other nearby star-forming regions to look for similar structures. "Discoveries about the nature of the M16 EGGs might lead astronomers to rethink some of their ideas about the environments of stars forming in other regions, such as the Orion Nebula," he predicted.
BACKGROUND INFORMATION: M16 – STAR BIRTH IN THE NEST OF THE EAGLE
Stars are born from the gas of interstellar space. When they eventually burnout and die, they bequeath their legacy back to the interstellar medium from which they formed. The signposts marking this ongoing cycle of birth, death, and renewal would be easily visible to any casual observer who had a bird's-eye view of our pinwheel-shaped galaxy. Spread across our galaxy such an observer would see majestic spiral arms, highlighted by bright young stars and the glowing clouds of gas that those stars illuminate.
On a clear, dark summer night earth-based observers can see these glowing clouds, called nebulae, scattered along the track of the Milky Way. Many can be found by looking in the direction of the great star clouds in the summer constellation, Sagittarius.
One of the most unique star-birth regions is the Eagle Nebula, (also called M16 because it is in the Messier Catalog of "fuzzy" permanent objects in the sky, that was compiled more than 200 years ago by French astronomer Charles Messier) it is visible in binoculars near the border between the constellations of Sagittarius and Serpens. The nebula is actually a bowl-shaped blister on the side of a dense cloud of cold interstellar gas.
Most of this cloud is so dense and cool that its hydrogen atoms are bound as molecules. This "molecular hydrogen" is the raw material for building new stars. The cloud contains microscopic dust particles of carbon (in the form of graphite), silicates and other compounds similar to those found in terrestrial and lunar rocks. Though this trace dust accounts for only a fraction of the nebula's mass, it's enough dust to absorb visible light – cloaking some of the visual details of star birth.
A cluster of about 100 newborn stars glitters inside the open "bowl" of the nebula. A few of these stars are much more massive than our Sun is, and so are tremendously hotter and brighter than the Sun. The brightest of these stars may be 100,000 times brighter than the Sun and have temperatures of nearly 90,000 degrees Fahrenheit (50,000 degrees Kelvin).
These young stars emit intense ultraviolet radiation which is so energetic it heats the surrounding gas, causing it to glow like the gas inside a fluorescent light bulb. When this ultraviolet light hits the bowl-shaped surface of the molecular cloud, it heats that gas, causing it to "evaporate" and stream away from the surface. If one could watch the process for more than a million years, they would see the bowl grow increasingly larger as the radiation from the stars eats deeper into the molecular cloud.
Unlike other stellar nebula which we see face-on – like the great Orion Nebula – M16 presents astronomers with a unique side view of the structure of a typical star-birth region: the cluster of hot, young stars in the center of the cavity, the evaporating surface of the cloud, and finally the great cold mass of the cloud itself.
The Eagle Nebula's name comes from its symmetrical appearance which is reminiscent of a bird of prey with outstretched wings and talons bared. The Eagle's "talons" are actually a series of dense columns of gas that protrude into the interior of the nebula. These columns form as a result of the same process that causes the bowl to grow. Because the columns are denser than their surroundings, they are not evaporating as rapidly as the surrounding gas, and so remain. The process is analogous to the formation of towering buttes and spires in the deserts of the American Southwest. These geological features formed when wind and rain eroded away softer ground, but places where the rock was harder resisted erosion and were left behind.
Inside these interstellar columns, the gas density can get so high that gravity takes over and causes the gas to start collapsing into ever-smaller clumps. As more and more gas falls onto these growing clumps they get further compressed by their own weight, until finally they trigger nuclear fusion reactions in their cores, and "turn on" as stars.
However, in M16 this process may not get a chance to go on to completion. If a forming star and the gas cloud that surrounds it are "uncovered" by photoevaporation before the star finishes growing, the mass of the young star may be "frozen." The star can't grow any more simply because the cloud from which it was drawing material is gone. In M16 Hubble Space Telescope's high resolution seems to have caught about 50 stars in this situation.
These are called EGGs "evaporating gaseous globules." The acronym is appropriate because these EGGs are objects within which stars are being born and are now emerging.
M16 is where the action is today, but it won't remain so forever. Within another few million years, star formation will have exhausted or dispersed the available raw material, and the massive stars that illuminate the Eagle will have lived out their short lives and died in spectacular supernova explosions. But even though the "birth cloud" nebula will be gone, most of the stars that formed there will remain. The offspring of the Eagle will "take wing" among the rest of the hundreds of billions of stars that make up our galaxy.
