Ah, color, color, color. I was surprised and, I must admit, very slightly horrified
, when I saw this picture.
As a Color Commentator and a lover of all blue things, I'm a dedicated lover of all blue galaxies, and NGC 4449 is one of the bluest galaxies in the nearby universe. And, hmmm, it doesn't look much like that at all
in today's APOD.
But then, filters, filters, filters. In my trusted and beloved The Color Atlas of Galaxies
by James D Wray, NGC 4449 is definitely one of the bluest-looking galaxies in the entire atlas. But then, James D Wray used U
filters, and the U exposures are shown as blue, the B ones are shown as green and the V ones as red.
In Wray's atlas, the inner parts of NGC 4449 are either bluish or white (because they are overexposed), and the light comes from brilliant OB stars. The outer parts of the galaxy are greenish in Wray's image, undoubtedly from A- and F-type stars. The stellar populations that would look red in Wray's image, made up of G- and K-type stars, are simply too faint to show up in Wray's not very highly resolved images. I wish I could show you James D Wray's UBV picture of NGC 4449, taken in 1979, but I can't!
NGC 4449. Photo: NASA, ESA, A. Aloisi (STScI/ESA), and The Hubble Heritage
No, but I can
show you a Hubble Space Telescope picture of NGC 4449. Most interestingly, the filters used for the image are F435W (B)
, F555W (V)
, F658N+(N II)
and F814W (I), which is quite invisible to the human eye, but in the processed image, it would be shown as red
. In the Hubble image, the B filter image would be shown as blue, the V filter image would be shown as green, and both the F658N and the F814W filter images would be shown as red. And thanks to the F814W filter, the populations of faint little K and M-type main sequence stars (and red giant stars) will look relatively bright in the Hubble image.
NGC 4449. Photo: R. Jay GaBany.
Photo: R. Jay GaBany.
Here you can see two pictures of NGC 4449, both taken by R. Jay GaBany. I'm sorry that one of them is so tiny, but the only other versions of that particular picture are huge. Here is - caution! - a 3MB version
of the image at right.
Both pictures, and some info on them, can be found on this page
. (There are nice mouseover effects there, too.) On this page we are told that the pictures have been made using one luminance filter, plus RGB filters. There has been an 1,080 minutes exposure through the luminance filter, and 240 minutes exposures through the red, the green and the blue filters.
But please note how, in the picture at left, the outer parts of NGC 4449 look relatively blue. In the picture at right, and certainly in the large version of it, the outer parts of the galaxy look beige. The beige stars definitely represent a non-blue population dominated by, most likely, red giants, most of them of spectral class K. They are (pretty much) the same color as the stars of the small stellar stream merging with NGC 4449.
Today's APOD is redder in overall color balance than any other version I have seen of this galaxy, at least in a picture that looks at least moderately "true color". I apologize for my horrible attachment, which is the first one I've ever done. But you can see how red today's APOD is, compared with the other pictures of NGC 4449 that I've shown you.
Edit: An interesting aspect of today's APOD is that in that picture, NGC 4449 appears to lack a bright nucleus. But that bright nucleus is very obvious in the Hubble picture of NGC 4449 near the top of my post.
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