APOD: Thackerays Globules (2012 Jun 12)

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APOD: Thackerays Globules (2012 Jun 12)

Postby APOD Robot » Tue Jun 12, 2012 4:06 am

Image Thackerays Globules

Explanation: These are larger dust bunnies than you will find under your bed. Situated in rich star fields and glowing hydrogen gas, these opaque clouds of interstellar dust and gas are so large they might be able to form stars. Their home is known as IC 2944, a bright stellar nursery located about 5,900 light years away toward the constellation of Centaurus. The largest of these dark globules, first spotted by South African astronomer A. D. Thackeray in 1950, is likely two separate but overlapping clouds, each more than one light-year wide. Along with other data, the above representative color image from the 4-m Blanco telescope at Cerro Tololo, Chile indicates that Thackeray's globules are fractured and churning as a result of intense ultraviolet radiation from young, hot stars already energizing and heating the bright emission nebula. These and similar dark globules known to be associated with other star forming regions may ultimately be dissipated by their hostile environment -- like cosmic lumps of butter in a hot frying pan.

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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby Ann » Tue Jun 12, 2012 4:33 am

You wouldn't know it from the picture, but the bright stars here are actually very blue. At least four stars in this association (or spread-out cluster), HD 101113, HD 101190, HD 101205 and HD 101436, are O-type stars. Not only are these stars intrinsically very blue, but in spite of the great distance to them and the nebula surrounding them, their "apparent color" is nevertheless about as blue as Vega.

The reason why Thackeray's globules are evaporating so fast is because of the onslaught of intense ultraviolet radiation from these hot blue O-type stars.

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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby C0ppert0p » Tue Jun 12, 2012 5:17 am

Besides size, what is the difference between Thackery's globules and Bok globules?
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby bystander » Tue Jun 12, 2012 6:44 am

C0ppert0p wrote:Besides size, what is the difference between Thackery's globules and Bok globules?

Thackeray's Globules are just the set of Bok globules discovered in IC 2944 by South African astronomer A. David Thackeray in 1950.
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby K1NS » Tue Jun 12, 2012 10:10 am

Ann wrote:The reason why Thackeray's globules are evaporating so fast is because of the onslaught of intense ultraviolet radiation from these hot blue O-type stars.

Ann



I wonder if they are evaporating fast enough, and if there are enough historical images, to make a video? I'd like to see that.

For me, one of the fascinations of astronomy is that we can see change in the cosmos--in what the ancients considered a static "firmament."
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby saturno2 » Tue Jun 12, 2012 12:42 pm

Thackeray¨s Globules with many young and hot stars.
Indeed. Similar the dust under the bed.
I thought in dust home before to read the explanation.
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby saturno2 » Tue Jun 12, 2012 12:54 pm

Hi Ann
Please tell me what is O- type star
Thanks
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby orin stepanek » Tue Jun 12, 2012 1:01 pm

saturno2 wrote:Hi Ann
Please tell me what is O- type star
Thanks
The hottest stars! :)
http://en.wikipedia.org/wiki/Stellar_classification
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby flash » Tue Jun 12, 2012 1:15 pm

APOD Robot wrote: Along with other data, the above representative color image from the 4-m Blanco telescope at Cerro Tololo, Chile indicates that Thackeray's globules are fractured and churning as a result of intense ultraviolet radiation from young, hot stars already energizing and heating the bright emission nebula. These and similar dark globules known to be associated with other star forming regions may ultimately be dissipated by their hostile environment -- like cosmic lumps of butter in a hot frying pan.

I presume that the physical meaning of "dissipated by their hostile environment" is that the solids in the dust are being vaporized by the intense ultraviolet radiation, that the resultant gas molecules are then accelerated by the radiation away from the bulk of the globules, and that the globules are visible because they are opaque to the radiation due to their composition (solid dust particles), but that the resultant gas is transparent. Is this the case? And if so, are there locales where the vaporized gas loses it's energy and condenses back into solidness and visibility?

I'm confused by this because It seems to me that we see evidence of gas out there all the time: some of it blocks radiation when it becomes ionized and absorbs energy, and some of it glows when it gives this energy back as it becomes de-ionized. So the notion that solids block radiation and gasses transmit it seems too simple. What, exactly, is meant by "dissipated" in such descriptions, and what, exactly, is going on at the surfaces of these globules?
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby Chris Peterson » Tue Jun 12, 2012 2:27 pm

flash wrote:I presume that the physical meaning of "dissipated by their hostile environment" is that the solids in the dust are being vaporized by the intense ultraviolet radiation, that the resultant gas molecules are then accelerated by the radiation away from the bulk of the globules, and that the globules are visible because they are opaque to the radiation due to their composition (solid dust particles), but that the resultant gas is transparent. Is this the case?

No. Nothing is being vaporized. The dust remains dust, and the gas remains gas. The energy from the nearby stars is just dissipating the dust. Where it's dense, we don't see through it; where it's thin, we do.

I'm confused by this because It seems to me that we see evidence of gas out there all the time: some of it blocks radiation when it becomes ionized and absorbs energy, and some of it glows when it gives this energy back as it becomes de-ionized. So the notion that solids block radiation and gasses transmit it seems too simple. What, exactly, is meant by "dissipated" in such descriptions, and what, exactly, is going on at the surfaces of these globules?

Gases certainly absorb some radiation, especially in narrow bands that the atoms are "tuned" for. But they do not do this efficiently; most radiation will pass unaffected through many light years of gas clouds. Dense dust clouds, however, efficiently absorb continuum radiation over wide portions of the spectrum.

What is happening at the surface of the globules is similar to evaporation in a fluid (indeed, the dust regions act as fluids in many respects). Because the clouds are opaque, the incident energy is absorbed at the surface and does not have a significant effect on the interior. The energized outer layer is rapidly ejected by simple dissipation (the particles absorb energy which increases their momentum; those with natural velocity vectors away from the cloud are lost, since the particles are moving faster than the escape velocity of the gravitationally bound dust cloud).
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby Chris Peterson » Tue Jun 12, 2012 2:46 pm

Ann wrote:You wouldn't know it from the picture, but the bright stars here are actually very blue.

Of course, this is a representative color image, meaning that the colors as seen by the eye have been mapped differently. In this image, that shows up most prominently in the decision to map H-alpha (deep red) to yellow on the display. The original data were taken through three filters, including a blue one. So actually, the hot stars are blue... you just can't tell it easily because they are so bright they have saturated. In order to display color, you need to have a meaningful ratio between the different color channels. Once any of them saturate, the colors are distorted. Once they all saturate, you get white. Stars are always a challenge when imaging deep sky objects, because many are brighter than the dim targets by many orders of magnitude. So in order to capture the dim bits, you have to saturate the stars.

For looking at star colors, diffraction and scatter come to the rescue. Diffraction is what makes stars, which are optical point sources, spread out over multiple pixels. Bright stars can spread very widely indeed, and we see that in this image. Once you get away from the saturated core of the star image, you get back into the linear color region. And we see that in this image: around the bright stars (which appear white) are blue halos and diffraction spikes- clearly showing that the blue channel data is much stronger than the I or Ha- as you'd expect for these very hot stars.
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby Psnarf » Tue Jun 12, 2012 3:19 pm

My feeble imagination tries to impose a 3D aspect to such images. There is a globule close to the bright star (5 o'clock) that appears to be illuminated by glowing gas. Another (7 o'clock) more distant globule appears to have an extension also illuminated. Does the fact that the remainder of the globules appear dark suggest that they are beyond the glowing hydrogen gas with nothing to illuminate them? Do the colored bits suggest that the globules can reflect photons?
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby Chris Peterson » Tue Jun 12, 2012 4:33 pm

Psnarf wrote:My feeble imagination tries to impose a 3D aspect to such images. There is a globule close to the bright star (5 o'clock) that appears to be illuminated by glowing gas. Another (7 o'clock) more distant globule appears to have an extension also illuminated. Does the fact that the remainder of the globules appear dark suggest that they are beyond the glowing hydrogen gas with nothing to illuminate them? Do the colored bits suggest that the globules can reflect photons?

Certainly the globules reflect some light. No material is 100% absorbing of photons. But their albedo is low... I'm not sure we're actually seeing any reflected light visually in this image (neither am I sure we're not). Just by simple observation, it's got to be difficult to distinguish reflections from transmitted light through thin areas from emitted or scattered light in front of the globules. We could be seeing any one of these, or any combination.
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby ThePiper » Wed Jun 13, 2012 7:48 pm

APOD wrote:These are larger dust bunnies than you will find under your bed.

Hehe - APOD has no idea about the clots under my bed... :lol2:
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby Psnarf » Thu Jun 14, 2012 2:01 pm

Thank you, Mr. Snowman.
http://www.spacetelescope.org/static/ar ... o0201a.tif provides greater detail than the ground-based telescopes. The globules do look more like opaque dust clouds from Hubble's filters.
B 439 nm
V 555 nm
H-alpha 656 nm
R 675 nm
Two of the globues are each a light-year wide?! My brain just exploded, as so oftentimes happens in apod.com and its links. I'm shopping for a plexiglass dome to wear herein to make it easier to find and reassemble the pieces from such explosions. Good thing I don't have a larger brain.
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby Chris Peterson » Thu Jun 14, 2012 2:35 pm

Psnarf wrote:Two of the globues are each a light-year wide?! My brain just exploded, as so oftentimes happens in apod.com and its links. I'm shopping for a plexiglass dome to wear herein to make it easier to find and reassemble the pieces from such explosions. Good thing I don't have a larger brain.

Think about this, too: the density of dust and gas in those globules is less, by orders of magnitude, than the density of our atmosphere. If you were in the middle of a globule, you'd think that there weren't many stars, but you'd not see any dust. You'd still see great distances- many A.U.s before there was significant attenuation.
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Re: APOD: Thackerays Globules (2012 Jun 12)

Postby neufer » Thu Jun 14, 2012 3:04 pm

Chris Peterson wrote:
Psnarf wrote:
Two of the globules are each a light-year wide?! My brain just exploded, as so oftentimes happens in apod.com and its links. I'm shopping for a plexiglass dome to wear herein to make it easier to find and reassemble the pieces from such explosions. Good thing I don't have a larger brain.

Think about this, too: the density of dust and gas in those globules is less, by orders of magnitude, than the density of our atmosphere. If you were in the middle of a globule, you'd think that there weren't many stars, but you'd not see any dust. You'd still see great distances- many A.U.s before there was significant attenuation.

We actually live within a local "tiny globule of dust" whose optical depth is on the order of ~5 light years.

We only see our "tiny globule of dust" in (mostly forward scattered) reflected sunlight:
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