Starship Asterisk*

I have been refraining from commenting on this picture because of the false colors (as you can imagine...), but even I think it is an impressive image. But I have been scratching my head to understand it. What drives the nebula? If it is a starforming region - and it is - then there must be a cluster of newborn hot stars providing the ultraviolet light necessary for the ionization of the nebula.

Well, where are the hot bright stars? Why are they so bashful? Why do they hide themselves?

There are a few bright stars scattered throughout the nebula. But they just don't look "right". For example, there is a bright star above the bright orange rim of the nebula, and the star is situated right above a finger-shaped pillar, too. But this star does not appear to have a retinue of smaller stars gathered around it at all, and look at the finger-shaped pillars below it! They aren't even lit up by the light from the star. No, this star has to be a foreground object. And indeed, according to my astronomy software, this bright-looking star is a modest K0 giant only about 300 light years away, ten times closer to us than the large nebula.

Well, you can see pillars almost all along the rim of the nebula, and they all seem to point at a small, unimpressive-looking cluster of small pinkish stars at the upper left. These stars are involved with some really dark nebulosity and aren't even able to light up this dust. Also, the stars all seem to be equally reddened - there is no "color gradient" among them to show which of them are more reddened by the dust than the others. Weird. But the pillars insist on pointing their fingers at this cluster as the ionizing source of the nebua, and the pillars are right. My astronomy software says that there is an O7 star among these little pinkies, and the star emerging out of the dust at the upper left of the cluster is a B0 star, if you are to believe my software.

So we've got a heck of a lot of impressive-looking nebulosity powered by a small group of unimpressive-looking stars. In reality I'm sure that the stars are very much brighter than the nebula.

Ann

This link is broken for hg7822 pix for10/22: http://hyperphysics.phy-astr.gsu.edu/hb ... vc.html#c1, at least from my PC.

To Chris - very helpful explanation - I've been wondering about the techniques and complexities of the software used to create these pictures. It is amazing that several pictures can be combined and keep the sharp definition of the various shapes. I wish somebody could make up a beginner's course on the use and coordination of the various programs involved.

Today's APOD is particularly beautiful - I'd love to see it as a background set in an opera scene. I can see the various lighting effects as the lights slowly move around, dim and brighten .....

tedw wrote:In looking at today's APOD, there are red dots throughout the photo.
I assume they are suns, but I can't imagine that they're red giants,
there too many and they're not big enough. Is this the result of a
filter?

Most red stars are not red giants, but cool, small dwarfs. And all stars are the same size in images: single points. The only reason they look different is because diffraction broadens them. And although they are all broadened the same amount, only bright ones let you see the wings of their profiles. So apparent size of stars in images is completely determined by apparent brightness, not by the size of the star.

That said, this image was constructed from data collected through multiple filters, and does not represent anything like natural colors. The red dots you see are stars, and they look red because they had a strong signal through the filters that were ultimately mapped to the red channel in the final image. Without precise details on the filters used, the exposure times through each, the quantum efficiency of the sensor over wavelength, and the process used to combine the data into the final RGB color channels, you can't make any assumption about what a particular star color really means.

Furthermore, this image was obviously processed with aesthetics in mind, which means that a non-linear transfer function was applied. As a result, you don't even know much about actual stellar intensities, other than saying that a given star is bright or dim relative to some other (assuming the transfer curve wasn't applied locally, which imagers sometimes do).

tedw wrote:
In looking at today's APOD, there are red dots throughout the photo.
I assume they are suns, but I can't imagine that they're red giants,
there too many and they're not big enough. Is this the result of a filter?

http://www.seinfeldscripts.com/TheRedDot.htm wrote:

• • The Red Dot
  George: How come this sweater is only 85 dollars?
  
  Woman: (showing the dot) Oh, here. This is why.
  
  George: What? I don't see anything.
  
  Woman: See this red dot?
  
  George: Oh yeah.
  
  Jerry: Oh it's damaged. (grabbing the sweater)
  
  George: (grabbing the sweater back) Well it's not really damaged. 85 dollars huh?
  
  Woman: There's no exchanges on this.
  
  George: You think she would care about the red dot?
  
  Jerry: It's hard to say.
  
  George: I don't even think she'd notice it. Can you see it?
  
  Jerry: Well I can see it.
  
  George: Yeah, but you know where it is.
  
  Jerry: Well what do you want me to do? Not look at it?
  
  George: Pretend you didn't know it was there. Can you see it?
  
  Jerry: It's hard to pretend because I know where it is.
  
  George: Well just take an overview. Can't you just take an overview?
  
  Jerry: You want me to take an overview?
  
  George: Please.
  
  Jerry: I see a very cheap man holding a sweater trying to get away with something. That's my overview.

In looking at today's APOD, there are red dots throughout the photo.
I assume they are suns, but I can't imagine that they're red giants,
there too many and they're not big enough. Is this the result of a
filter?

Congratulations to Neil Fleming, excellent photography and a well deserved APOD.
Greetings
César

eeryowl wrote:Sorry if this is a dumb question. Is this image what you would see if you were looking directly at this part of space with a powerful telescope, or is it color "enhanced" to bring out the cloud against the background? Also, if you were in a space ship traveling through the cloud, would you notice the gas swirling around you or is it too dispersed for that?

The colors represent different elements that are present. This nebula is nowhere near bright enough to stimulate human color vision. In a very real sense, its "true color" is gray; that is all you could ever see. That wouldn't change with a telescope, or with getting closer.

If you were within it, you would see almost nothing of the overall structure. If you were inside a dusty area, you would not see much of anything from outside, and neither would you see the dust itself. In a more open area, you would see the surrounding Universe, occluded in places by dust. Keep in mind that even the densest parts of nebulas like this are hard vacuum.

Hi "eeryowl" ; have you met Owlice?

eeryowl wrote:
Sorry if this is a dumb question.

There are no dumb questions here, just dumb answers.
(I apologize in advance if this turns out to be one of my many dumb answers.)

eeryowl wrote:
Is this image what you would see if you were looking directly at this part of space with a powerful telescope, or is it color "enhanced" to bring out the cloud against the background?

It is in "enhanced" false color
(; e.g., hydrogen emission is shown here in green whereas it really should be red).

eeryowl wrote:
Also, if you were in a space ship traveling through the cloud, would you notice the gas swirling around you or is it too dispersed for that?

If you were in a space ship traveling through the cloud,
it would look much as the Milky Way looks to us:
http://antwrp.gsfc.nasa.gov/apod/ap100913.html

Sorry if this is a dumb question. Is this image what you would see if you were looking directly at this part of space with a powerful telescope, or is it color "enhanced" to bring out the cloud against the background? Also, if you were in a space ship traveling through the cloud, would you notice the gas swirling around you or is it too dispersed for that?

It really is a quite nice picture! I always like the dust pillars.

I see the monster in today’s beautiful APOD…

gaping jaws
hungry to devour
life’s precious
life’s articulated flower
at bay at bay
you will not have the day
gaping jaw without paw
you are weak without utterance

I thought I was looking at the "Pillars of Creation" on the side. I guess not and it does not make it any less amazing.

Congratulations to Neil Fleming on his beautiful APOD! This image first appeared on Asterisk here, along with other fabulous images.

NGC 7822 in Cepheus

Explanation: Pillars of gas, dust, and young, hot stars seem to fill the gaping maw of NGC 7822. At the edge of a giant molecular cloud toward the northern constellation Cepheus, the glowing star forming region lies about 3,000 light-years away. Within the nebula, bright edges and dark shapes are highlighted in this colorful skyscape. The image includes data from both broadband and narrowband filters, mapping emission from atomic oxygen, hydrogen, and sulfur into blue, green, and red hues. The atomic emission is powered by the energetic radiation from the hot stars, whose powerful winds and radiation also sculpt and erode the denser pillar shapes. Stars could still be forming inside the pillars by gravitational collapse, but as the pillars are eroded away, any forming stars will ultimately be cutoff from their reservoir of star stuff. This field spans around 60 light-years at the estimated distance of NGC 7822.

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