M81, M82 and tiny Holmberg IX, as well as galactic cirrus from the Milky Way.
Photo: Andre van den Hoeven.
Ah, good old M81. It is a red galaxy, with a huge yellow bulge. It is also a grand design galaxy, with two elegant and symmetrical spiral arms. Interactions with neighboring galaxies M82 and NGC 3077 might have enhanced the spiral pattern. We might notice, too, that M81, like the Andromeda galaxy, is unbarred.
An interesting object in today's APOD is dwarf satellite galaxy Holberg IX. I'm not too happy with the picture at right, because of the filters used for the image, a wideband green filter and a wideband near infrared filter. That's not so good for a galaxy like Holmberg IX, which is dominated by hot ultraviolet stars.
But the caption is better:
Of the more than 20,000 stars that can be resolved in this Hubble image, only about 10% are considered to be old stars with ages of billions of years. The rest are thought to be young stars with ages of only 10 - 200 million years. Due to the Advanced Camera for Surveys' resolution in this image, astronomers have noted that the old and the young stars have distinct spatial distributions which might be related to their origin.
I recommend that you take a look at this 1 MB picture of Holmberg IX
from Large Binocular Telescope Observatory. You can really see the difference in distribution between the young stars (at upper right) and the old stars (at lower left). Also note an object at upper center left, a faint circular nebula with what looks like a pair of blue stars inside it. You can actually see the nebula in today's APOD, where it looks like a faint, faint pink object at bottom center, seemingly detached from the blue "body" of the rest of the tiny galaxy.
The rejuvenation event of Holmberg IX likely happened 200 million years ago:
Simulations predict that the triplet M81, M82, and nearby NGC 3077 had a close passage 200-300 million years ago. This close encounter may have triggered the newer star formation that has occurred in Holmberg IX.
So Holmberg IX is totally dominated by the light of blue stars, which is why Holmberg IX looks so blue in today's APOD. Note that it looks bluer than the spiral arms of M81. Note, however, that no new star formation is taking place in this tiny dwarf galaxy. The gas that created all the star formation in Holmberg IX has been used up or dispersed. The faint pink nebula that can be just made out in the APOD might possibly be star formation, but my guess is that it is more likely the remnant of an outburst of a pair of massive stars in Holmberg IX. Something like Eta Carina
(image by NASA, ESA, and J. Hester), but on a much, much more modest scale.