cketter wrote:I love this exposure; it really contrasts the dark nebula against the rest of the image.
I wonder if the dark nebula has a depth as long as it's apparent width, does anyone know the answer to this?
Also, why do all the stars appear blue and orange in this visible light image? does this have something to do with the settings on the imaging sensor used?Chris
I doubt that the nebula is as wide as it is long. Note that much of the dark nebula is half "transparent", suggesting it isn't very wide. Besides, if such "sheet-shaped" nebulae were common in the Milky Way, a number of them ought to have been photographed by now.
Why do so many of the background stars appear blue and orange? Part of the answer is that photographer Tony Hallas, for aesthetic reasons, has chosen a fairly high level of color saturation for his image. But another and more interesting answer is that the stars probably really are
blue and orange, and that is because many of the stars are young. Here is how it works.
When a young cluster is born, its brightest members are blue. These are typically of spectral classes A and B. Take a look at this
great image by Enzo Santin of the very young starforming region NGC 7129. The brightest stars hatching out of this molecular cloud are blue stars of spectral classes A and B. The hottest (and therefore certainly brightest) of these very young stars are of spectral class B3, according to http://iopscience.iop.org/0067-0049/154 ... .text.html
, a paper discussing the infrared properties of NGC 7129:
The primary sources of photoionization, the B3 stars BD +65°1637 and BD +65°1638, are not detected directly [in the infrared], but extended halos of warm dust are seen at their positions.
However, the authors of this paper have found 39 likely young proto-stars which have not yet reached the main sequence. Many of them are very faint.
Take a look at Enzo Santin's NGC 7129 image again. Three bright sources can be seen which are deeply embedded in dust and very orange. Otherwise, the brightest stars in the cluster appear to be blue or bluish.
These stars are less than a million years old, according to http://en.wikipedia.org/wiki/NGC_7129
. As the cluster ages, most or all of the dust will be blown away. The initial effect of this will likely be to make the overall light of the cluster bluer. You can see for yourself that much of the reddening of this cluster is clearly caused by dust.
However, after perhaps a hundred million years or so, the brightest blue stars, those of spectral class B3, will have used up the hydrogen in their cores. When this happens they will expand and turn into red giants, probably red giants that are very large, bright and reddish as giants go. Perhaps they will turn into M-class giants like, say, Mirach, Beta Andromedae, a bright M0III giant about 450 times as bright as the Sun in visual light. But the former blue class B3 stars will not stay in their M0III stage for long: after less than an additional one hundred million years they will shed their outer atmospheres, turning into small hot compact stars surrounded by planetary nebulae.
Now the brightest stars of NGC 7129 will be gone, or at least they will have become very faint compared with what they used to be. By now, however, other formerly blue stars of NGC will have turned into red giants, probably of spectral class K. These formerly blue stars were never as blue or as bright as the stars of class B3, and the red giants they turn into will similarly not be as bright or as red as the red giants that the B3 stars turned into. Nevertheless, these "lesser blue stars" will become red giants too, perhaps similar to Aldebaran, the bright K-type star that seems to be superimposed on the Hyades cluster.
So first the class B3 stars will turn into red giants. Then the stars of late class B will do the same. Then the stars of early class A like Vega and Sirius will become red giants like Arcturus and Dubhe. Then the stars of late class A, like Altair, will follow suit. Then the stars of class F, like Procyon. After perhaps ten billion years, even the stars of early class G like the Sun will turn into red giants.
So the youngest, brightest stars are blue. Many of the not quite so bright young stars are blue, too. The not so bright stars stay blue longer than the very hot ones. But eventually, they all turn into orange stars, the red giants.
What do we see in today's APOD? We see a star field dominated by blue and orange stars. My guess is that a majority of the blue stars are main sequence class A (or early F) stars, whereas a majority of the orange stars are orange-colored red giants of spectral class K. The blue stars are likely all younger than, say, a billion years old (and most of them are probably younger than 500 million years). That's fairly young. Some of the orange stars are probably fairly young, too, while others may be older than the Sun.
What about the stars that are like the Sun? Main sequence stars of spectral class G? We likely see few of them in the field of today's APOD. Why is that? It's because they are so much fainter than the main sequence A-type stars and the K-type red giants. That doesn't mean that our Sun is faint as stars go, rather the opposite. In fact, we have reason to believe that our Sun belongs to the top 5% echelon in the brightness competition league of our galaxy! In other words, 95% of the stars of the Milky Way are likely fainter than the Sun! But these stars are so faint that we are not likely to see more than, at best, two or three in a rich background field like the one in today's APOD.
After all, what stars dominate the skies of the Earth? Of the 25 visually brightest stars in the sky, six belong to spectral class A and seven to spectral class B. Three belong to spectral class K and three to spectral class M. The K and M stars are all either giants or supergiants. All are intrinsically brighter than the Sun. All of them, with the exception of Alpha Centauri, are likely to be younger than the Sun. (Though it has to be said that it is not so easy to determine the age of K-type giants.)
But if we look at the most nearby stars, 18 of the 29 nearest stars are tiny red main sequence stars of spectral class M. They are all so faint that it is impossible to see even a single one of them with the naked eye, even though they are so comparatively nearby.
In the same way as the Earth's sky is dominated by intrinsically bright young blue or orange stars, in the same way a distant starry background like the one in today's APOD is likely to be dominated by intrinsically relatively bright blue and orange stars.
So to summarize: The crowded background in today's APOD looks the way it does because there is a large fraction of young bright stars in it.