This is a very interesting "juxtaposition" of cosmic "objects".
M15 is about 35,000 light-years away. Comet Garradd is only about 13 light-minutes away! Well, M15 shines by the light of some 100,000 stars, and Comet Garradd is probably a small dark asteroid-like body with a supply of volatiles which are currently in eruption. (According to http://en.wikipedia.org/wiki/Comet
, comets are among the darkest, least reflective objects known in the solar system, because they are often covered with extremely dark remnants of previous outbursts.) Comets can be seen because the gas and dust that is erupting from them reflects the light of the Sun (the dust) and is ionized by it (the gas). If we were to calculate the absolute magnitude of Comet Garradd and globular cluster M15, we would see that we are talking about two very different kind of fish! A book I own, Sky Catalogue 2000.0, claims that the absolute luminosity of M15 is -8.91. I believe that corresponds to a luminosity about 300,000 times that of the Sun. I won't try to estimate the luminosity of the comet, but the comet is certainly a very
dim bulb by comparison!
M15 is a very interesting gobular. Like one of the links of today's APOD points out, the center of M15 has one of the densest concentrations of stars known, and it may contain a black hole!
To me, the color of things is always very interesting. According to Sky Catalogue 2000.0, the apparent, slightly dust-reddened B-V index of M15 is 0.68. This is very, very close to the color of the Sun, whose B-V index is 0.656±0.005. But the unreddened color of M15 is bluer, "only" 0.56. The reason for the relatively blue color is that the stars of M15 are made of gas that almost
consists of only hydrogen and helium. Such "metal-poor stars" will go through a stage when they turn into "blue horizontal stars", so that they can actually turn bluer when they get old than they were when they were young.
Take a look at the blue star to the left of M15. This is not a blue horizontal star, because such stars are rare in the midplane of the Milky Way, which is characterized by metal-rich stars. Instead, this star (HD 204862), is a young main sequence star, no more than a few hundred million years old, compared with 12 billion
years for M15. HD 204862 is of spectral class B9.5V, and its B-V index is -0.05. It is a little more than 400 light-years away and about 50 times the luminosity of the Sun. The blue horizontal stars are often both bluer and a bit fainter than HD 204862.
Metal-rich stars like the Sun can't turn blue with age, not until they run out of fuel entirely and cast off their atmospheres to become white dwarfs. The Sun will turn into a red giant, and the metal-poor stars do that, too. (They become red giants before and after they become blue horizontal stars.) The red "end stage" of metal-poor stars - before they turn into white dwarfs - is the so-called "asymtotic giant branch star", an orange and quite bright stage. Take a look at the orange-colored star to the lower right of M15 is today's APOD. That is a red giant, admittedly a rather moderate specimen which is only about 50 times brighter than the Sun. So it's intrinsic brightness is almost exactly the same as the intrinsic brightness of the blue star HD 204862, but far fainter than an asymtotic giant branch star. The mixture in M15 of red giants, red asymtotic giant branch stars, blue horizontal stars and faint Sun-colored hydrogen-fusing "dwarfs" like the Sun together produce an overall color which is intrinsically slightly bluer than the Sun. But the orange points of light that seems to dot the "halo" of M15 is today's APOD are bright asymtotic giant branch stars.
And what about the color of the comet? As you can see from this picture of Comet Holmes that I found at Wikipedia, the coma of the comet is almost always green, while the ion tail is often blue. The ion tail consists of ionized gas. Comets often have dust tails too, which are usually white.
Comet Garradd is not strongly colored in today's APOD, but in other images it has looked quite green. Check out all the links of today's APOD, and you'll find a gorgeous color picture by Michael Jäger of Comet Garradd and M15! The comet does not appear to have an ion tail, but it does have a green coma and a dust tail.