It is probably a pity that I'm going to be the first to comment on this picture, since I'm going to be critical of its color balance.
APOD Robot wrote:
The colors of the Milky Way stars can also be found in the starlight from the large island universe. Its core is dominated by light from cool yellowish stars. Along its grand design spiral arms are the blue colors of hotter, young stars mixed with obscuring dust lanes and pinkish star forming regions.
Although it is true that M101 has a yellow core and blue spiral arms, it is also true that the overall colors of M101 are extremely blue
for such a large galaxy. The B-V index of M101 is 0.450, which is an extreme value. The second bluest large galaxy that I can think of offhand is M61
, whose B-V index is 0.530. For both M101 and M61, their striking overall blue colors are due to truly huge numbers of hot blue stars. The centers of these two galaxies, by contrast, are not remarkably large or yellow, certainly not the center of M101, although it definitely is
To get a better idea of the color balance of today's APOD, I want to focus on the dwarf companion of M101, NGC 5474, whose B-V index is 0.490, which is almost exactly the same as the color of Mirphak, Alpha Persei.
Mirphak is the the brightest star in the elongated group of stars above the "p" in the word "SCIENCEphotoLIBRARY" that is written across the picture at left. The color of Mirphak looks white, not yellow. In my opinion, however, NGC 5474 looks mostly yellow in today's APOD.
The image at right is a Hubble picture of NGC 5474. As you can see, the color balance isn't very yellow at all. In my opinion, and given the color index of NGC 5474, the colors of the Hubble image are realistic.
That said, anyone who isn't as blue-loving and nitpicking as I am - which is to say, 99,99999999999999999999999% of the human population - ought to be able to really enjoy today's APOD. Because it's a nice and interesting picture, which shows us the relative sizes of M101 and NGC 5474 and gives us a good idea of how they are interacting with one another. And there is nothing wrong
with the colors it shows us of M101 and its dwarf companion, NGC 5474. It's just that I personally love to see blue objects in space really portrayed as blue.