Oh, that's so interesting! What a lovely APOD!
NGC 5101 and NGC 5078 are similar in some respects - their sizes and their B-V indexes - but they are also really quite different. NGC 5101 looks a bit similar to NGC 1398
, here portrayed by Adam Block. Both NGC 5101 and NGC 1398 have bright inner lenses, bars with bar-end enhancements, and rings surrounding their bars. The galaxies also have rather faint spiral arms. Even though NGC 1398 appears to have a little more star formation than NGC 5101, the different populations of NGC 5101 are beautifully apparent by their colors in today's APOD. The population inside the inner ring is really quite reddish and obviously very old, and some of the reddishness extends beyond the inner ring. But there is also an outer feature which looks like an outer ring (even though NGC 5101 clearly has spiral arms), and this outer feature is grayish-blue. This feature clearly contains young stars, but there are probably also stars of intermediate age and also perhaps old metal-poor stars.
The regular and beautiful shape of NGC 5101 is striking, and its beauty is enhanced by our nearly face-on view.
NGC 5078, by contrast, is seen almost perfectly edge-on. We can see its very thick central dust lane, whose "thick part" appears quite straight and undisturbed, while its "thinner parts" are stretched out, flared out and apparently elongated.
Personally I can see no obvious signs of star formation in NGC 5078, but its color indexes are very interesting and suggestive. Its B-V index is 1.040, which is really quite red, but its U-B index is 0.360, which is surprisingly blue in view of the galaxy's very red B-V index. I have to wonder if the U-B index of NGC 5078 has been "contaminated" by the obviously ultraviolet-bright dwarf galaxy IC 874, with which it is interacting.
The far infrared index of NGC 5078 is 10.384, which makes it almost two magnitudes brighter in far infrared than in blue light. This is often a signature of star formation, but I am uncertain if this is the case in NGC 5078's case. Clearly NGC 5078 is very
dusty, but I still wonder if its U-B index is "its own" or partly its neighbour's. If the U-B index has in fact nothing to do with the small neighbour, then NGC 5078 must indeed have a respectable amount of star formation.
The color indexes of 5101 are quite different from those of NGC 5078. The B-V index of NGC 5101 is 1.005, similar to NGC 5078, but the U-B index of NGC 5101 is 0.565, considerably redder than NGC 5078's value of 0.360. Even so we can actually see young blue clusters in NGC 5101, while no young stars are apparent in NGC 5078.
What about the far infrared magnitude of NGC 5101? Its far infrared magnitude is 12.803, more than a magnitude fainter than its B magnitude. NGC 5101 is dust-poor.
What a fascinating APOD this is! It is so beautiful and detailed, and its colors are so well-balanced that they in themselves give us many clues to the nature of these beautiful galaxies.