What causes EGGs ?

[dougettinger]

EGGs are evaporation gaseous globules which are compact pockets of interstellar gas. Some striking examples are seen in the Eagle nebula and in star cluster NGC 602 (referenced in APOD: 2010 April 3 - NGC 602 and Beyond). They are considered to be the location for the birth of new stars.

So why does the radiation pressure drive away the gases and dust from a broad region leaving behind a cone of opague dust and gases ? Supposely, a new star is forming at the tip of the cone ? How do we know this ? Why does this dark cone nebula remain opaque behind the forming star ? It does not make any sense. The forming star should create a cone that is transparent since this star birth is gathering all the dust that is passing by it.

How do astronomers know a new star instead of an old star is interupting the outward flow of dust and gases to form these EGGs ?

4/19/2011

[dougettinger]

Wikepedia does not have good answers to the above questions. I am still hoping for some response. Easter is around the corner; here is your chance to decorate some EGGs.

[neufer]

EGGs are evaporation gaseous globules which are compact pockets of interstellar gas.

EGGs are evaporating gaseous globules which are compact pockets of cold interstellar gas and dust strongly irradiated from the outside.

Some striking examples are seen in the Eagle nebula and in star cluster NGC 602 (referenced in APOD: 2010 April 3 - NGC 602 and Beyond). They are considered to be the location for the birth of new stars. So why does the radiation pressure drive away the gases and dust from a broad region leaving behind a cone of opaque dust and gases ?

"It's a bit like a wind storm in the desert," said Jeff 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."

Supposely, a new star is forming at the tip of the cone ? How do we know this ? Why does this dark cone nebula remain opaque behind the forming star ? It does not make any sense. The forming star should create a cone that is transparent since this star birth is gathering all the dust that is passing by it.

How do astronomers know a new star instead of an old star is interrupting the outward flow of dust and gases to form these EGGs ?

It is neither an old star nor a new star but rather a gravitationally condensed globule.

Explanation: It may look to some like a duck, but it lays stars instead of eggs. In the center of the above image lies Barnard 163, a nebula of molecular gas and dust so thick that visible light can't shine through it. With a wing span measured in light years, Barnard 163's insides are surely colder than its exterior, allowing conditions where gas can clump and eventually form stars.

These EGGS will probably never hatch however because the first born sibling hatch-lings will eventually evaporate them all away.

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."

Embryonic Stars Emerge from Interstellar "Eggs"

<<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.

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.">>

[dougettinger]

Wow, Art, you are indeed the master quotidian quotationist. You certainly did you homework. I believe I need to learn to do more googling. Thank you. Doug

I can comprehend the "elephant trunks" or "cone nebula" better by envisioning an intake fan at the tip of the cone that sucks in more dust and gas and creates a denser, opaque column of dust and gas in its discharge. The strange phenomenom is that no proto-star disk is observed at the tip of these elephant trunks. This means that other mechanisms can create stars other than the nebula hypothesis. Please make special note of this previous statement for future topics. The elephant trunk eventually breaks up due to surrounding or interior photo-evaporation and by the formation of more stars downstream of the first forming star at the tip of the column. Thanks for bearing with me my own summary statement.

I am still left with one nagging question. What happens if this shock front or cloud wall intersects an existing mature star? Many of these cloud fronts are many light years from their source and could easily cross the path of another star. What do you suppose happens ? What kind of view of this event would appear on the other end of our telescopes ?

Thanks to the Neufer. You made my Easter. Doug

[neufer]

What happens if this shock front or cloud wall intersects an existing mature star? Many of these cloud fronts are many light years from their source and could easily cross the path of another star. What do you suppose happens ? What kind of view of this event would appear on the other end of our telescopes

Even Bok Globules are more tenuous than the highest vacuums we can produce on earth.

Our sun is not threatened in any way by Zodiacal light for instance. It would be a non-event.

[dougettinger]

If these dark or opaque clouds or globules are so tenuous and non-threatening, what indeed does trigger star birth at the tip of an elephant trunk and create the concentration of gases and dust behind it ?

[neufer]

If these dark or opaque clouds or globules are so tenuous and non-threatening, what indeed does trigger star birth at the tip of an elephant trunk and create the concentration of gases and dust behind it ?

Shock waves do help to concentrate gas & dust into Bok Globules but only the most dense of these will then collapse into stars under their own gravity in a time frame of tens of millions of years. These new stars then, in turn, illuminate and evaporate the rest of the EGGs in the nest.

[dougettinger]

It is hard to believe that juggling shock waves and gravity for millions of years creates EGGs. I believe that EGGs occur almost spontaneously ( in cosmic time ) around a chunk of condensed higher metals similar to a rain droplet forming around a dust particle.

Art, have a happy Easter. Doug

[Chris Peterson]

I can comprehend the "elephant trunks" or "cone nebula" better by envisioning an intake fan at the tip of the cone that sucks in more dust and gas and creates a denser, opaque column of dust and gas in its discharge.

That is not at all what is happening.

The strange phenomenom is that no proto-star disk is observed at the tip of these elephant trunks. This means that other mechanisms can create stars other than the nebula hypothesis.

There are protostars inside many EGGs. Indeed, EGGs are one of the best observational examples supporting the idea that stars form from collapsing nebulas. I don't know of any proposed explanations for star formation that don't start with a collapsing nebula. There are many variation of this, however.

I am still left with one nagging question. What happens if this shock front or cloud wall intersects an existing mature star? Many of these cloud fronts are many light years from their source and could easily cross the path of another star. What do you suppose happens ?

Virtually nothing. The shock fronts are tenuous regions that would still be considered hard vacuums in most labs. A star would maintain a clear area around it the size of a solar system. It would be difficult to observe such a thing from here, given that we'd be seeing the star through a much larger region of dust, and the clear bubble would be impossible to resolve unless the star was fairly nearby. Our system could pass through such a region and we'd probably not be aware of it without high tech observational techniques.

[dougettinger@verizon.net]

I can comprehend the "elephant trunks" or "cone nebula" better by envisioning an intake fan at the tip of the cone that sucks in more dust and gas and creates a denser, opaque column of dust and gas in its discharge.

That is not at all what is happening.

I am trying to comprehend cone nebulae with some known analogy. What if I envisioned the dust and gases concentrating behind a forming star similar to eddy currents of water forming behind a rock in a river. Is this a better analogy ?

The strange phenomenom is that no proto-star disk is observed at the tip of these elephant trunks. This means that other mechanisms can create stars other than the nebula hypothesis.

There are protostars inside many EGGs. Indeed, EGGs are one of the best observational examples supporting the idea that stars form from collapsing nebulas. I don't know of any proposed explanations for star formation that don't start with a collapsing nebula. There are many variation of this, however.

I guess I don't know the difference between a protostar disk and a collapsing nebula. There was a statement, not mine, that no protostar disk have been observed at the tip of "elephant trunks". So what is the difference ?

I am still left with one nagging question. What happens if this shock front or cloud wall intersects an existing mature star? Many of these cloud fronts are many light years from their source and could easily cross the path of another star. What do you suppose happens ?

Virtually nothing. The shock fronts are tenuous regions that would still be considered hard vacuums in most labs. A star would maintain a clear area around it the size of a solar system. It would be difficult to observe such a thing from here, given that we'd be seeing the star through a much larger region of dust, and the clear bubble would be impossible to resolve unless the star was fairly nearby. Our system could pass through such a region and we'd probably not be aware of it without high tech observational techniques.

Thanks for your excellent explanation. However, if the gravity of an existing star and its planets do not trigger a more encompassing protostar disk with an intersecting cloud of dust and gases, then what does trigger a protostar disk or as you say, a collapsing nebula at the tip of an "elephant trunk" ?

Thanks for joining my Easter Egg Hunt. Doug