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I was wondering what are the green objects that you see scattered in the Hubble Ultra Deep Field image (Warning possibly large/slow download : http://imgsrc.hubblesite.org/hu/db/images/hs-2004-07-a-full_jpg.jpg)

A crop of the image reduced in size :

enter image description here

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    $\begingroup$ Pretty much every thing you see in this image is a galaxy. $\endgroup$ – Phiteros Feb 6 '18 at 3:12
  • $\begingroup$ Yeah, but things that are red appear red due to "red shift". Why do some appear green, or are they green to begin with? $\endgroup$ – nmdr Feb 6 '18 at 3:33
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    $\begingroup$ Red shift isn't the only reason things are red. Some objects are simply red in color. Other objects are reddened because there is dust or gas between us and them. Some objects can just be green. Also, objects that are very blue can be redshifted to appear green. $\endgroup$ – Phiteros Feb 6 '18 at 3:36
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    $\begingroup$ @Ngm Redshift doesn't just make things appear red, it just moves the wavelengths towards the red end of the spectrum, i.e., towards longer wavelengths. It's very possible for something to be in the UV and end up shifting "red-ward" just enough that it now peaks in the green part of the visible spectrum. I'm not saying that's why some of these galaxies appear green, but that's how red-shifting works. $\endgroup$ – zephyr Feb 6 '18 at 14:08
  • $\begingroup$ @Zephyr Your comment could be turned into an answer. And actually OP should note that the same applies to whatever colour he/she can identify in a Hubble deep field picture. en.m.wikipedia.org/wiki/Redshift $\endgroup$ – Alchimista Feb 6 '18 at 14:50
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what are the green objects that you see scattered in the Hubble Ultra Deep Field image

The Hubble Ultra Deep Field (HUDF) has approximately 10,000 objects in it. Between 25 and 50 of them are identified as extremely faint stars. That means the remaining ~10,000 objects are galaxies. To answer your question literally then, the green objects are galaxies.

However, I believe what you probably really mean to ask is, why are these galaxies green? To answer that, there are actually a number of points to address.

  1. You seem to be under the belief that the concept of redshifting causes objects to appear red. While this is certainly a true statement, it is not exclusively true. Redshifting simply makes wavelengths longer (and thus shifted red-ward, towards the red end of the spectrum). It is certainly possible that an object which was once blue could be redshifted into the red and thus appear more red than it naturally is. However, an object that emitted most strongly in the UV could redshift into the green and thus appear more green than it naturally is. Some such effect may be going on in this image, but in actuality, I think there's other stuff going on.
  2. You have to account for image processing. It's very difficult to take a "real color" image of a galaxy. Very often (as was the case with the HUDF), images are taken in a few different color bands and combined together with some sort of image processing program. Its up to the discretion of the image processor to balance the colors correctly, get the right saturation, hues, luminosities, etc. There are actually a variety of different forms of the HUDF that include different bands and different forms of image processing where the images that appear green in your image don't appear green in those. An example is shown below of a rendition of the HUDF, taken directly from the same NASA/STScI Hubble website you found your image on. The coloration is significantly different than yours and many of the greenish galaxies from your image appear more blue.

enter image description here

  1. I'm not saying this is the case with the galaxies in the HUDF, but some galaxies actually are green! A few examples include the Pea Galaxy or the Green Bean Galaxy. When you plot up galaxies based on color (note that this is the astronomical definition of color which doesn't necessarily have a 1-1 correspondence with visual color) you get a Color-Magnitude diagram (CMD) wherein most galaxies are clumped into a red portion or a blue portion. However, there are a small group of galaxies in what is known as the Green Valley which do appear green for one reason or another. What actually causes their greenness, I think, is beyond the scope of this question and would be better suited for a new question.
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