Starship Asterisk*

Today's APOD is a beautiful portrait of a galaxy taken with a relatively modest telescope which is in no way comparable to the Hubble Space Telescope. Should images taken with much less advanced telescopes than the Hubble be posted here at APOD at all? What would happen if only Hubble images or "Hubble quality images" were posted here?

As for the Hubble, there is an enormous demand on the limited time that can be offered by that one instrument. In other words, there are only so many pictures that the Hubble can take in the first place! If Hubble and "Hubble-like" images were the only ones that could be posted here, the number of images that could be posted here would be reduced most drastically. Anyway, if you are only interested in Hubble images, why don't you skip the APOD site altogether and just hang around the Hubble homepage, http://hubblesite.org/, and look at the Hubble gallery?

Second, the Hubble is not equally good at everything it does. Hubble has a very small field of view, and it is downright bad at photographing objects that "take up so much sky" as a galaxy like NGC 6946, whose apparent size is estimated at 14 x 14 arcminutes. To photograph all of NGC 6946 Hubble would have to image different parts of the galaxy separately and then fit the different images together like a mosaic. But photographing a deep sky object that way is very time-consuming, and if there is one thing that the HST hasn't got it is plenty of time.

Also, in my opinion Hubble isn't all that good at capturing the soft color gradient across the face of a galaxy. That, however, is something a photographer like Adam Block can do masterfully. His image contains color information that would not be so easily accessible in a Hubble image, and his image captures all of NGC 6946 and its surroundings, too. His picture is therefore not only beautiful but scientifically valuable, too, since it gives us a kind of "summary of the whole" of objects like NGC 6946, while the Hubble will mostly give us "details". And now that I have said these things about the relative worth of Hubble images (which are indeed marvellously resolved and extremely valuable) versus "complementary" images taken with much more modest telescopes, I will finish this post and discuss today's APOD in more detail in a second post.

Ann

NGC 6946 is a most interesting object, and Adam Block's image does it very fine justice.

I recommend today's APOD caption, which is very informative and interesting. I particularly recommend the link to http://dg-imaging.astrodon.com/gallery/ ... ?imgID=223, which contains some very interesting information and great pictures of the dust content in NGC 6946. NGC 6946 appears to be an unusually dusty galaxy. A good measure of a galaxy's dust content and star formation is the galaxy's far infrared magnitude versus its blue light magnitude. The brighter the galaxy is in far infrared compared with blue light, the dustier it is and the more star formation it usually contains. NGC is two full magnitudes brighter in far infrared than in blue light! For a face-on galaxy like NGC 6946, that is really a lot. NGC 6946 is famous for its remarkable production of supernovae, but two other face-on "supernova factories" that can be compared with NGC 6946 are M83 and M100. M83 is "only" one magnitude brighter in far infrared than in blue light (which is still a lot for a face-on galaxy) and M100 is barely 0.3 magnitudes brighter in far infrared than in blue light.

What does this mean? Well, NGC 6946 is a vigorously starforming galaxy. Dusty galaxies often are. It is worth remembering that the most vigorously starforming galaxies we can observe in the universe are distant so called ULIRGs, Ultra Luminous InfraRed Galaxies. These galaxies are blazingly bright in infrared light but faint in visible light, because the runaway star formation inside them produces copious amounts of dust that blocks most of the visible light from them. It is important to remember that star formation produces dust. Therefore, it is no wonder that a richly starforming galaxy like NGC 6946 is dusty and therefore bright in the far infrared.

But we must also remember that we see NGC 6946 through the constellation of Cepheus, which is a dusty a starforming part of our own galaxy. Rogelio Bernal Andreo recently produced a mosaic of the most prominent of the many emission nebulae in this constellation: http://apod.nasa.gov/apod/ap100909.html

So we see NGC 6946 through a veil of dust in our own galaxy. A galaxy that we see through an even thicker layer of Milky Way dust is another beautiful starforming face-on galaxy, IC 342: http://apod.nasa.gov/apod/ap101222.html

IC 342 is really extremely reddened, and it is almost 2.3 magnitudes brighter in far infrared than in blue light. But in IC 342's case, that is probably mostly because it is so very faint in blue light due to Milky Way reddening. (Interestingly, IC 342 and NGC 6846 are slightly similar in shape: both have a "thick" upper arm that ends somewhat abruptly, but IC 342's arm points to the right, wheras NGC 6946's arm points to the left.)

The color distribution of NGC 6946 is very interesting. It has a white or blue-white nucleus (whose color may be influenced by a burst of star formation) surrounded by a small and strongly orange lens. The orange color may be due to a rich population of of old red stars whose light may also be somewhat reddened by dust. Further out is a yellow-beige bulge, which is also obviously dusty. Most of the rest of the galaxy is muted blue or "dirty blue" or more beige than blue. Clearly the color of these blue parts of the galaxy are reddened by dust in our own galaxy, but NGC 6946 is probably also self-reddened by dust produced by many generations of star formation. NGC 6946 is very clearly a multi-generation galaxy: It obviously has a rich population of old red stars, its arms are probably a mixture of young, intermediate and old stars, and there are very many pink emission nebulae, maternity wards of newborn stars, lined up along the the dust lanes of the arms. And still the galaxy has all this dust left which has yet to condense to ice cold "stellar wombs" where stars that don't yet exist will start forming. This galaxy has clearly been producing stars for millions and billions of years, and it is still vigorously churning out stars. Its "dirty blue color" testifies to its rich and prolonged production of stars.

Ah, but there is one smallish part of this galaxy which is brilliantly, almost shockingly blue. It is a round blue region situated at four o'clock, or rather at three thirty, from the nucleus. This is a young, circa 15 million year old single-generation product of an incredible local starburst, comparable, when it was younger, to the Tarantula Nebula in the Large Magellanic Cloud, or even bigger. There exists a great Hubble picture of this small part of NGC 6946:

http://www.astro.uu.nl/siu/people/N6946PC.GIF

Here we can see that this region is made up of three components; one, gas clouds glowing red from emission, two, an extended association made up of individual small blue clusters and blue supergiants interspersed with the number of red stars that you would expect from a 15 million year old starburst, and finally an incredibly compact large bright cluster. This large cluster is a so called Super Star Cluster, a very young globular cluster. This is what some of the Milky Way globular clusters (the smaller ones) looked like when they were young. You can read a bit about this super star cluster here:

http://iopscience.iop.org/0004-637X/556/2/801

The "shockingly blue" color of this region of NGC 6946 is probably due to two factors: the single-generation nature of this region and its consequent lack of intermediate and old stars, but also the probable lack of dust here. The prodigious stellar winds of a multitude of O and B supergiants have likely blown all the dust out of this particular part of NGC 6946, allowing the light from this region to be brilliantly blue.

Ann

telescopenut wrote:Question about the image: at about the four-o-clock position, there is a circular ring of stars aroung a group of huge blue stars. Based on the size of the galaxy, this ring is huge! What could have created such a nearly circular structure? It seem way too big to be a super-nova remnant.

You may want to check out my second post here, where I discuss that large round blue object. You are right, it is not a supernova remnant. The closest analogy would be the Tarantula Nebula and the huge cluster inside it, R136, as it will look about ten million years from now. At that time most of the gas of the nebula may have been blown away, leaving the massive and incredibly impressive cluster "naked", but surrounded by an overabundance of bright mostly blue supergiants in the general vicinity.



The Tarantula Nebula. The giant cluster is at the center of the red nebula and a large number of bright blue stars are seen in the general vicinity. This is probably what the circular blue area in NGC 6946 looked like ten to twelve million years ago. What made the giant cluster form? Probably a collision between two massive dusty gas clouds, or even a violent compression of just one large gas cloud, perhaps caused by several supernovae.

Ann