Sunday, June 10, 2012

Why Pillars of Creation and not globes?

The famous Hubble photo of Eagle nebula
Larger image NASA
"Pillars of Creation" is a photograph taken by the Hubble Telescope of elephant trunks of interstellar gas and dust in the Eagle Nebula, some 7,000 light years from Earth. They are so named because the gas and dust are in the process of forming, or creating, new stars, while also being eroded by the light from nearby stars that have recently formed.

Taken April 1, 1995, it was named one of the top ten photographs from the Hubble by The astronomers responsible for the photo were Jeff Hester and Paul Scowen, at the time both of Arizona State University. In 2011, the region was revisited by ESA's Herschel Space Observatory.

Update: Hubble 25th year celebration HR photo of the famous Pillars of Creation in APOD

My question
Probably as a total amateur I understand the above explanation wrong. For me the text together with the brightly shining tops of the pillars in the image suggests that new stars are born at the top of the "elephant trunks" in some sort of pressurized movement of the interstellar cloud, front-ends of gas streamers if you wish.

But if that were the case common sense says that the cosmic clouds should be shaped as globes and not such pillars. For if the new star is the result of gravity collapsing a hydrogen molecule cloud one would expect a dense cloud with hot core, some kind of proto-star ball. So apparently my first impression is wrong.

What is going on in the photographed region of M16?

The answer from Hubble
The pillars are composed of cool molecular hydrogen and dust that are being eroded away by photoevaporation from the ultraviolet light of relatively close and hot stars. The leftmost pillar is about four light years in length.

The finger-like protrusions at the top of the clouds are larger than our solar system, and are made visible by the shadows of Evaporating Gaseous Globules (EGGs), which shields the gas behind them from intense UV flux. EGGs are themselves incubators of new stars.
Read more on this from wikipedia

EGGs, fingers, tear drops and icecream cones with cherries
So my question was not that far off since Evaporating Gaseous Globules are in fact dense centers of hydrogen gas and dust that act as incubators for new stars. Astronomers use creatively everyday language to describe to us the shapes the see in the emerging new stars and their incubators.

Jeff Hester explained soon after the publication of the famous photo in 1995:

Dr. Jeff Hester
Arizona State University
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."

Ass Prof Paul Scowen
Arizona State Uuniversity
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.

"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."
[As an archaeologist I love this explanation!]

"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.
Read more at Hubble site News center 1995

Even better answer from Herschel 
In 2011 Herschel Space Observatory captured a new image of Pillars of Creation in far-infrared wavelengths, which allows astronomers to look inside the pillars and structures in the region, and come to a much fuller understanding of the creative and destructive forces inside the Eagle nebula.

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