Starship Asterisk*

by **jt-3d** » Sun Jun 29, 2014 5:59 pm

The same effect....

by **Ann** » Thu Jun 26, 2014 7:38 am

I think the operative word here is "saturated". The Sun is so bright that it looks white. It could be that it is strongly reddened, but that saturation makes it look white.

Ann

by **geckzilla** » Thu Jun 26, 2014 5:38 am

Chris Peterson wrote:
geckzilla wrote:Interesting. Did not expect the sun to look like that at all.
What did you expect? This is a typical sunset/sunrise (enhanced by dust), with a saturated Sun looking white in the limited dynamic range image.

I expected more of a sunset appearance. I've only ever seen the sun behind thick smoke. Sandstorm...never been near one.

by **Nitpicker** » Thu Jun 26, 2014 4:49 am

Ann wrote:that red Earth sky, full of (reddish?) dust particles

Red ochre coloured dust. It took weeks for the last remnants of the dust to be swept/blown/washed away.

by **Chris Peterson** » Thu Jun 26, 2014 4:47 am

geckzilla wrote:Interesting. Did not expect the sun to look like that at all.

What did you expect? This is a typical sunset/sunrise (enhanced by dust), with a saturated Sun looking white in the limited dynamic range image.

by **Ann** » Thu Jun 26, 2014 4:25 am

: Click to view full size image

Image Credit: Mars Exploration Rover Mission, Texas A&M, Cornell, JPL, NASA

It is interesting that the Sun itself is looking white, while the sky is very orange. I can't help comparing that red Earth sky, full of (reddish?) dust particles, with the classic NASA photo of a white Sun creating a blue sunset in a reddish Martian sky, full of red dust particles.

Ann

by **geckzilla** » Thu Jun 26, 2014 3:15 am

Interesting. Did not expect the sun to look like that at all.

by **Nitpicker** » Thu Jun 26, 2014 2:57 am

Nitpicker wrote: I can't remember whether the sun was visible or not.

Here is another (perhaps more natural looking) image from that morning, showing the Sun rising over what I think is Long Reef, on the northern beaches of Sydney:

: Click to view full size image

http://webodysseum.com/wp-content/uploa ... orm_02.jpg

by **Nitpicker** » Thu Jun 26, 2014 1:21 am

Whilst that shot may have been taken shortly after sunrise, the lights are probably controlled by a sensor, so it could have been later. The storm made everything a lot darker. I can't remember whether the sun was visible or not.

by **geckzilla** » Thu Jun 26, 2014 1:12 am

The lights are on. That's the one thing with photography that makes it hard to place oneself in the moment. It has a tendency to make things look much brighter during low light situations. Was the sun visible as a red disc through the sand?

by **Nitpicker** » Thu Jun 26, 2014 1:08 am

I was in Sydney in 2009 when a very rare dust storm blew in from the outback. See: http://en.wikipedia.org/wiki/2009_Australian_dust_storm.

: Click to view full size image

http://img.izismile.com/img/img2/200909 ... orm_50.jpg
Everything looked progressively more red, the further away it was. The above photo is quite in keeping with my memory of the event.

This is what the scene looks like on a normal day:

: Click to view full size image

http://40pools.files.wordpress.com/2013 ... cn0616.jpg

by **Chris Peterson** » Wed Jun 25, 2014 11:54 pm

geckzilla wrote:I've heard of the sky turning completely orange or red during sandstorms. I'm curious about how accurate the photos are.

Pretty accurate. But the color isn't from scattering-based wavelength sorting, but simply the reflected color of the sand and dust. The Sun itself looks red through dust and smoke, however, because the shorter wavelengths are still scattered more than the longer ones.

by **geckzilla** » Wed Jun 25, 2014 10:14 pm

I've heard of the sky turning completely orange or red during sandstorms. I'm curious about how accurate the photos are.

by **Chris Peterson** » Wed Jun 25, 2014 5:07 pm

Ann wrote:More dust in the Earth's atmosphere would certainly make the sky less blue.

Not exactly less blue, but less saturated. I don't think you can add enough dust to make the sky red (unless the dust itself is red). With our Sun, the sky will always be blue, but as you add more particulates you'll add more of everything else, as well, meaning that the sky will move towards colorless. We see this with fog, and with heavy smoke.

by **MarkBour** » Wed Jun 25, 2014 4:53 pm

Fascinating! Thanks, Ann.

by **Ann** » Wed Jun 25, 2014 4:20 am

MarkBour wrote: Since the appearance of a blue nebula was stated to be similar to the reason our sky looks blue, I wonder if there is any analogous behavior in our atmosphere to what you're saying here. Would a progressively "dustier" atmosphere go through the same color progression that you're saying exists with increasing amounts of dust in a nebula?

The light at sunset, I guess. It shifts more towards red. If our atmosphere were twice as thick, would the sky appear red during the day?

More dust in the Earth's atmosphere would certainly make the sky less blue. I remember that one of the Soviet probes that landed on Venus saw that no (or extremely little) blue light made it through the extremely thick atmosphere to reach the ground of that planet.

How much dust it would take to make the Earth's sky look red, I don't know. There was a terrible volcanic eruption at the island of Krakatoa in 1883, possible the largest eruption known to humanity (but we haven't been able to record global effects of volcanic eruptions for that many centuries). Anyway, here are some things that Wikipedia says about the global effects of the eruption of Krakatoa:

http://en.wikipedia.org/wiki/1883_erupt ... al_climate wrote:
In the year following the eruption, average Northern Hemisphere summer temperatures fell by as much as 1.2 °C (2.2 °F).[9] Weather patterns continued to be chaotic for years, and temperatures did not return to normal until 1888.[9] The record rainfall that hit Southern California during the “water year” from July 1883 to June 1884 – Los Angeles received 38.18 inches (969.8 mm) and San Diego 25.97 inches (659.6 mm)[10] – has been attributed to the Krakatoa eruption.[11] There was no El Niño during that period as is normal when heavy rain occurs in Southern California,[12] but many scientists doubt this proposed causal relationship.[13]

The eruption injected an unusually large amount of sulfur dioxide (SO2) gas high into the stratosphere, which was subsequently transported by high level winds all over the planet. This led to a global increase in sulfuric acid (H2SO4) concentration in high level cirrus clouds. The resulting increase in cloud reflectivity (or albedo) would reflect more incoming light from the sun than usual, and cool the entire planet until the suspended sulfur fell to the ground as acid precipitation.[14]

What about the color of the sky? Apparently sunsets were strongly affected, but Wikipedia says little about a non-blue sky during daytime:

http://en.wikipedia.org/wiki/1883_erupt ... al_effects wrote:
The eruption darkened the sky worldwide for years afterward, and produced spectacular sunsets throughout the world for many months. British artist William Ashcroft made thousands of colour sketches of the red sunsets half way around the world from Krakatoa in the years after the eruption. The ash caused "such vivid red sunsets that fire engines were called out in New York, Poughkeepsie, and New Haven to quench the apparent conflagration."[15]

This eruption also produced a Bishop's Ring around the sun by day, and a volcanic purple light at twilight.
In 2004, an astronomer proposed the idea that the blood red sky shown in Edvard Munch's famous 1893 painting The Scream is also an accurate depiction of the sky over Norway after the eruption.[16]

Weather watchers of the time tracked and mapped the effects on the sky. They labeled the phenomenon the "equatorial smoke stream".[17] This was the first identification of what is known today as the jet stream.[18]

For several years following the eruption it was reported that the moon appeared to be blue and sometimes green. Blue moons resulted because some of the ash clouds were filled with particles about 1 µm wide—the right size to strongly scatter red light, while allowing other colors to pass. White moonbeams shining through the clouds emerged blue, and sometimes green. People also saw lavender suns and, for the first time, noctilucent clouds.[15]

Ann

by **Chris Peterson** » Wed Jun 25, 2014 12:44 am

hlwelborn wrote:Can anyone point me in the direction of efforts to increase astronomy studies in public schools?

In the U.S., at least, there's very little. We have the excellent Common Core Standards, but they are only advancing Math and Language Arts so far, not science. There are some rather isolated efforts to push STEM curricula, but anything actually aimed at astronomy is only happening at the local level because of individual dedicated teachers.

by **MarkBour** » Tue Jun 24, 2014 10:57 pm

Ann wrote:
neufer wrote:
Ann wrote: .. If there are too many dust particles between us and the star, there will either be no reflection nebula at all, or else there will be a reddened, non-blue reflection nebula. ...
Ann

Since the appearance of a blue nebula was stated to be similar to the reason our sky looks blue, I wonder if there is any analogous behavior in our atmosphere to what you're saying here. Would a progressively "dustier" atmosphere go through the same color progression that you're saying exists with increasing amounts of dust in a nebula?

The light at sunset, I guess. It shifts more towards red. If our atmosphere were twice as thick, would the sky appear red during the day?

by **hlwelborn** » Tue Jun 24, 2014 4:43 pm

Can anyone point me in the direction of efforts to increase astronomy studies in public schools?

by **starsurfer** » Tue Jun 24, 2014 4:08 pm

The central star of the Iris Nebula is also beginning to ionize some of the gas in the nebula, this is seen as purple edges to the cavity in some images but for some reason this is rarely seen. Adam Block's image is one of the few to show the full range of colours in one of my favourite reflection nebulae. It's interesting that nobody has ever done a HaLRGB composite image of this. Also Travis Rector discovered some Herbig Haro objects in the area, see here: http://arxiv.org/abs/1211.7190

Also this is a widefield image but no mention is made of the curved dusty nebula to the right, this is LBN 468. Near the left edge of this is a small white fan-shaped cometary reflection nebula. This is called Gyulbudaghian's Nebula, named after the Armenian astronomer Armen Gyulbudaghian and was also co-discovered by the Armenian astronomer Tigran Magakian. They published the GM catalogue of cometary reflection nebulae in 1977 and this particular nebula is also catalogued as GM 1-29. It is associated with the variable star PV Cephei and is also a variable nebula, changes have been reported in the past few years. It also has an associated giant Herbig Haro outflow, which was discovered 20 years later in 1997, read more here: http://adsabs.harvard.edu/abs/1997AJ....114.2708R
A closeup by Adam Block: http://www.caelumobservatory.com/gallery/PVCephei.shtml

Also I've just learnt that George Herbig passed away last year. He was known for his co-discovery of Herbig Haro objects in the 1950's and published hundreds of scientific papers. The last one he contributed towards that was published is about the discovery of a new variable nebula by the amateur astronomer Jim Thommes: http://adsabs.harvard.edu/abs/2012ApJ...748L...5R
Adam Block also released the only closeup image of it (can be seen near the left edge): http://www.caelumobservatory.com/gallery/lbn1022.shtml

by **Ann** » Tue Jun 24, 2014 3:37 pm

neufer wrote:
Ann wrote:
The brown tint of the pervasive dust comes partly from photoluminescence -- dust converting ultraviolet radiation to red light.
You mean the dust glows murkily reddish because of something similar to the process of ultraviolet light from hot stars ionizing nearby hydrogen, making it glow red? So that the reddish-brown dust clouds are their own sort of little "emission nebulas"?
http://www.starrywonders.com/irisst8300.html wrote:
Iris Nebula (LBN 487 / VDB 139) with associated open cluster NGC 7023
by Steve Cannistra

<<Although not officially designated as an emission nebula, closer examination ... will reveal a linear ridge on either side of SAO 19158 that represents HII emission. The peripheral regions are comprised of reddish dust that obscures light from background stars.

In the center of the nebula, there are several ruddy-colored wisps and filaments of dust that emit broad band red light, instead of reflecting the more typical blue light of a reflection nebula. These red regions represent extended red emission (ERE), which is a type of phospholuminescence associated with dust particles that are bombared by high energy UV radiation from SAO 19158.>>

More at: http://www.starrywonders.com/irisst8300.html

Thanks, Art. I'm well aware of those reddish filaments of the center of the nebula. They show up very well in this fine picture of NGC 7023 by Adam Block. But I wasn't referring to those faint filaments of emission nebulosity near the star itself, SAO 19158 (which I prefer to call HD 200775). I meant those dark reddish-brown nebulas far away from the blue reflection nebula. There are several of them in the upper right part of today's APOD, and some can be found below the Iris nebula, at about 8 and 9 o'clock. They are indeed murkily reddish in color. Does their color have anything to do with photoluminescence due to ultraviolet light? It would be the first thing I have heard about such a phenomenon.

zbvhs wrote:
Is SAO19158 behind the dust? It doesn't seem to be carving out a cavity.

SAO 19158 (HD 200775) is indeed carving out a cavity in the gas and dust surrounding it. Check out Adam Block's image again.

Generally speaking, stars that create blue reflection nebulas can't be too deeply "submerged" in dust. If a reflection nebula is blue, it means that dust particles are scattering blue light our way. But dust also scatters blue light away. If there are too many dust particles between us and the star, there will either be no reflection nebula at all, or else there will be a reddened, non-blue reflection nebula. This picture of the Trifid Nebula shows that there is a lot of dark dust at the dividing line between the blue reflection nebula and the red emission nebula, and here much of the blue light of the reflection nebula is scattered away. Parts of the blue reflection nebula are therefore greenish-blue or brownish-blue.

Ann

by **neufer** » Tue Jun 24, 2014 1:36 pm

zbvhs wrote:
Is SAO19158 behind the dust? It doesn't seem to be carving out a cavity.

http://www.robgendlerastropics.com/NGC7023text.html wrote:
<<The distinct hourglass shape of the [NGC 7023] nebula was formed by the interactions of the central star with the surrounding ambient dust cloud. HD 200775 is a B3 type pre-main sequence star known as a Herbig Be star. Herbig Ae/Be stars are intermediate mass (1-10 solar masses) pre-main sequence stars similar to their lower mass "solar size" counterparts, the T-Tauri stars. They are usually associated with dust and reflection clouds but unlike their more massive OB cousins they cannot form extended HII regions. That said, HD 200775 is one of the brightest Herbig Be stars known. With a temperature of 17000 degrees Kelvin it is hot enough to form a small HII region within a small radius of the central star. Studies of the star and nebula at radio wavelengths show the star is located within a bilobed cavity of gas whose walls delineate the exact borders of the optical nebula. The cavity is about 5 by 2.5 light years in dimension and lies in a northeast-southwest orientation. Bipolar outflows from the contracting pre-main sequence star most likely excavated the bilobed cavity which formed the template for the physical structure of the nebula. The lack of any high velocity gas within the nebula at present suggests that the outflows no longer exist.>>

More at: http://www.robgendlerastropics.com/NGC7023text.html

by **neufer** » Tue Jun 24, 2014 1:34 pm

Ann wrote:
The brown tint of the pervasive dust comes partly from photoluminescence -- dust converting ultraviolet radiation to red light.
You mean the dust glows murkily reddish because of something similar to the process of ultraviolet light from hot stars ionizing nearby hydrogen, making it glow red? So that the reddish-brown dust clouds are their own sort of little "emission nebulas"?

http://www.starrywonders.com/irisst8300.html wrote:
Iris Nebula (LBN 487 / VDB 139) with associated open cluster NGC 7023
by Steve Cannistra

<<Although not officially designated as an emission nebula, closer examination ... will reveal a linear ridge on either side of SAO 19158 that represents HII emission. The peripheral regions are comprised of reddish dust that obscures light from background stars.

In the center of the nebula, there are several ruddy-colored wisps and filaments of dust that emit broad band red light, instead of reflecting the more typical blue light of a reflection nebula. These red regions represent extended red emission (ERE), which is a type of phospholuminescence associated with dust particles that are bombared by high energy UV radiation from SAO 19158.>>

More at: http://www.starrywonders.com/irisst8300.html

by **zbvhs** » Tue Jun 24, 2014 1:15 pm

Is SAO19158 behind the dust? It doesn't seem to be carving out a cavity.

by **Ann** » Tue Jun 24, 2014 8:35 am

Nice picture!

I was fascinated by this:

The brown tint of the pervasive dust comes partly from photoluminescence -- dust converting ultraviolet radiation to red light.

You mean the dust glows murkily reddish because of something similar to the process of ultraviolet light from hot stars ionizing nearby hydrogen, making it glow red? So that the reddish-brown dust clouds are their own sort of little "emission nebulas"?

I didn't know that!

Ann

Starship Asterisk*

APOD: The Iris Nebula in a Field of Dust (2014 Jun 24)

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