APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Comments and questions about the APOD on the main view screen.
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Ann
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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by Ann » Wed Nov 10, 2010 1:12 am

Chris, I used an insolent word to describe your previous post, and I apologize for that. I had no right to say what I did. But I still don't agree with you.

If the color index of NGC 4452 is +0.89, then it is very considerally yellower than the Sun, and its light must be dominated by late class G and early class K stars. These stars are all yellower than the Sun. You called the Sun yellow, and yet you argue that a galaxy that is yellower than the Sun should properly be shown as brilliantly white, even blue-white?

I ask you compare the following two Hubble images of galaxies:

ImageImage

The galaxy on the right is starburst galaxy NGC 3310. Let's compare these two Hubble images with the SDSS image of NGC 4452 that I posted earlier:

Image

Chris, you said about the SDSS image of NGC 4452 that
there is a garish orange color that is seen in the galaxy halo
Look at the three pictures I have posted again. Where do you find the most striking "garish orange color"? There can be no doubt that the strongest orange color here is seen in the small bulge surrounding the bright nucleus in the Hubble image of NGC 3310. Now let us assume that the color of the Hubble image of NGC 3310 is well balanced, so that it really gives us a very good idea of the color of NGC 3310. But if that is the case, we must surely wonder what kind of stars make up the bulge of NGC 3310. In my opinion, the only way to get such a color in the bulge is that if it is made up of a mixture of stars like Antares and Betelgeuse (color index around +1.60) and blood red carbon stars (color indexes typically around +3 or more). To my knowledge, such bulges simply don't exist.

Take a look at the Hubble images of NGC 4452 and NGC 3310 again. Chris has told us that the color index of NGC 4452 is +0,89, which is considerably yellower than the Sun (+0.62). The color index of NGC 3310, however, is only +0.32, which is very blue for a galaxy.

Obviously the bulge of NGC 3310 is made up of yellow stars. Indeed, we can be sure that it is made up of the same kind of stars that make up all of NGC 4452. And yet, Hubble shows the late G and early K stars of all of NGC 4452 as brilliantly white, but in the image of NGC 3310 the same kind of late G and early K stars are shown as strikingly orange.

The SDSS picture gives you a good idea of the real color of a mixture of stars of spectral classes late G and early K. The Hubble picture of NGC 4452 shows these stars as brilliantly blue-white, while its picture of NGC 3310 shows the same kind of stars as garishly orange.

That is why the color balances of the Hubble pictures of NGC 4452 and 3310 are both lousy, either shockingly blue or garishly orange. Only the SDSS picture shows the approximately true color of a mixture of stars of late class G and early class K.

Ann
Last edited by Ann on Wed Nov 10, 2010 4:03 pm, edited 3 times in total.
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Chris Peterson
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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by Chris Peterson » Wed Nov 10, 2010 2:41 am

Ann wrote:I find Chris' post about the color of NGC 4452 nonsensical.
Sorry you feel that way. The fact is, though, there is lots of published information relating B-V indexes, color temperature, and RGB approximations. I think the analysis was solid, and would have hoped for a more substantive comment from somebody interested in color.
Chris

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Ann
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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by Ann » Wed Nov 10, 2010 3:35 am

Well, Chris, I was a bit too hasty here. I didn't actually read your whole post before I responded to it, and of course I should have read what you said before I had an outburst. :oops: I had no right to call that post nonsensical, and I apologize for that.

Let's try to make another comparison.

Image Image

Here are the two images of NGC 4452 again. I tried to quote your previous post to reproduce that color sample that you showed us, so that I could compare it with the galaxy pictures here, but your sample wouldn't show up as I quoted your post. I did notice, however, that the color you showed us did not represent 5000 Kelvin, but rather 5400 Kelvin, which is hotter and bluer than NGC 4452.

Bearing in mind that your color sample does represent a hotter and bluer temperature than the one representing NGC 4452, would you please quote my post so that we can compare the two images of NGC 4452 with your color sample (which, I repeat, is in fact bluer than NGC 4452)?

Ann
Last edited by Ann on Wed Nov 10, 2010 4:04 pm, edited 1 time in total.
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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by DavidLeodis » Wed Nov 10, 2010 11:01 am

Just what am I supposed to understand from the information brought up through the "thin" link in the "for example our own Milky Way Galaxy is thought to be about this thin" in the explanation? It is (to me) a very confusing and totally unnecessary link.

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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by owlice » Wed Nov 10, 2010 11:17 am

DavidLeodis wrote:Just what am I supposed to understand from the information brought up through the "thin" link in the "for example our own Milky Way Galaxy is thought to be about this thin" in the explanation? It is (to me) a very confusing and totally unnecessary link.
I liked it! It's another demonstration of how narrow something can appear when edge-on.

(The "this thin" text refers to NGC 4452 -- our galaxy is thought to be as "thin" as that galaxy.)

The links sometimes take us surprising places; yay! for that!
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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by DavidLeodis » Wed Nov 10, 2010 11:54 am

owlice wrote:
DavidLeodis wrote:Just what am I supposed to understand from the information brought up through the "thin" link in the "for example our own Milky Way Galaxy is thought to be about this thin" in the explanation? It is (to me) a very confusing and totally unnecessary link.
I liked it! It's another demonstration of how narrow something can appear when edge-on.

(The "this thin" text refers to NGC 4452 -- our galaxy is thought to be as "thin" as that galaxy.)

The links sometimes take us surprising places; yay! for that!
Thanks owlice. That has helped me.

The sentence with the link is poorly worded as I thought it was referring to the Milky Way (not NGC 4452) and that the Milky Way will look as thin as the rings of Saturn do when seen edge on, though from where and how far away the Milky Way will look that thin I don't know.

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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by zloq » Wed Nov 10, 2010 2:47 pm

Ann wrote: Let's try to make another comparison.
The HST image is described as the combination of a single blue exposure, colored blue, and a single near-IR exposure, colored red. Apparently they were combined in a way that left the galaxy looking gray since with only two channels, and one of them in the near-IR, it's really a pseudocolor image with no direct connection to the RGB appearance shown in the SDSS image. It's aesthetically interesting because the gray galaxy stands out against the blue and red stars/galaxies scattered in the background, but the gray color of the galaxy seems to be an artistic choice based on how best to make a compelling color image from two channels - one of them in the near-IR.

Maybe they just conjured up a synthetic color image from the two channels and then used Photoshop to set the white balance on the galaxy - and realized it was a kinda cool effect since it was so uniform and helped it stand out against the background.

zloq

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Ann
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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by Ann » Wed Nov 10, 2010 3:51 pm

zloq wrote:
Ann wrote: Let's try to make another comparison.
The HST image is described as the combination of a single blue exposure, colored blue, and a single near-IR exposure, colored red. Apparently they were combined in a way that left the galaxy looking gray since with only two channels, and one of them in the near-IR, it's really a pseudocolor image with no direct connection to the RGB appearance shown in the SDSS image. It's aesthetically interesting because the gray galaxy stands out against the blue and red stars/galaxies scattered in the background, but the gray color of the galaxy seems to be an artistic choice based on how best to make a compelling color image from two channels - one of them in the near-IR.

Maybe they just conjured up a synthetic color image from the two channels and then used Photoshop to set the white balance on the galaxy - and realized it was a kinda cool effect since it was so uniform and helped it stand out against the background.

zloq
Thanks, zlog! That's exactly what I think, too. I also agree that the finished HST picture is indeed compelling and beautiful, and the background galaxies look great. But for a person with my color hangup, the color of NGC 4452 was still so wrong that I needed to post the SDSS image to show people what the color of this galaxy is really like.

I don't mean to say that the Hubble picture isn't valuable; it is, in fact, superior to the SDSS image in every way, except when it comes to showing the color of the galaxy.

Ann
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Ken

Shared perspective

Post by Ken » Thu Nov 11, 2010 8:41 pm

I suppose if there are folks on a planet of a star in that galaxy studying their own version of APOD, then their photo of our Milky Way would look very similar to this (+/- dust lanes) due to a shared perspective.

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Re: Shared perspective

Post by Chris Peterson » Thu Nov 11, 2010 8:56 pm

Ken wrote:I suppose if there are folks on a planet of a star in that galaxy studying their own version of APOD, then their photo of our Milky Way would look very similar to this (+/- dust lanes) due to a shared perspective.
You might think so on first glance, but actually the view would be very different. NGC 4452 is located in a part of our sky nearly perpendicular to the galactic plane- only 15° from the north galactic pole. So from NGC 4452, the Milky Way appears nearly face on.
Chris

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Re: APOD: NGC 4452: An Extremely Thin Galaxy (2010 Nov 09)

Post by Rockstar » Fri Nov 12, 2010 3:19 am

looks like a galaxy without a black hole funnel.