When we look at an astrophotography image, what are the different features and how are they related to each other?
Let's begin by looking at my image of Messier 101, a big and bright "grand design" spiral galaxy. This image (one of my favourites) was captured at Lac Teeples on March 18, 2021.
Let's consider the overall composition.
The black background is black because there is no visible light coming from those directions. Yes, that is what we call a "tautology". In day time the sky is blue from Rayleigh scattering of sunlight, but at night the sun is conveniently out of the way and there is much less scattering. There is some scattering of light all over the universe from stars and dust, but it is comparatively very dim.
Why is there no visible light when there are billions and billions of stars in our galaxy AND billions and billions of galaxies spread over the universe? This is known as Olbers' Paradox, which doesn't seem to have a conclusive solution. Perhaps the most important factor is the expansion of the universe: The light from many distant stars has been shifted to such different frequencies that they are not visible (red shift due to the expansion of the universe). Another factor is that the universe's scale is effectively reduced by the finite speed of light: some light hasn't reached us yet.
Edgar Allen Poe wrote about this in a poem called Eureka in 1848:
Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the Galaxy – since there could be absolutely no point, in all that background, at which would not exist a star.
Let's now consider the individual stars we see covering the entire image. Our Milky Way galaxy is a disk galaxy, and our Sun is within that disk. This view of the sky is away from the Milky Way band of light we see in the night sky. By looking away from the disk we see fewer nebulae and stars, and can see more clearly out to distant objects like the galaxy M101 in the center.
The thickness of the Milky Way disk is roughly 2000 light years. If we assume our Sun is in the middle of the disk then most of the stars will be within 1000 light years of us. Some will be much closer (the nearest star to our Sun is only 4 light years away) and others will be further than 1000 light years since there is no hard cutoff for the disk.
Next, let's look at the M101 Galaxy in the center of the image, a collection of billions of stars at a distance of 21 million light years. Yes, that galaxy is VASTLY more distant than the stars in the foreground, approximately 20 thousand times more distant! The center of the galaxy is very bright due to the light of the core, which is basically a very dense collection of stars. It also has diffusely glowing spiral arms which are the result of a large collection of stars and various nebulae that are also emitting or reflecting light. There are also some individual stars visible in the spiral arms: These stars are so extremely bright that they can be separated from the others at such a great distance. In some cases there will be Milky Way stars right in front of our view of the distant galaxy and often it is hard to tell whether they are near to us or part of the distant galaxy.
Finally, there are other galaxies in the frame. In viewing M101 we are looking out in space away from the Milky Way itself and have a clear view to the depths of space. We know that the universe is full of galaxies, and this image has many. In the bottom right there is an obvious companion galaxy, near to M101 in space. At the upper left we see the edge of a galaxy's disk that is smaller and farther away than M101. A careful close-up look also shows two tiny galaxies in the bottom left corner. These galaxies are all irregularly shaped in this image, but others will appear very much like stars. Some of the spots of light we see (that appear to be stars) may in fact be the cores of galaxies at vast distances from our Milky Way galaxy.
In this image M101 is the nearest large galaxy to our own Milky Way and therefore it dominates the image. Between the nearby stars of the Milky Way's disk and the M101 galaxy there is virtually nothing, empty space. Behind M101, reaching out great distances to where those other galaxies lie, there is the same nothingness. There is an inter-galactic medium, but is so incredibly thin that "void" is a good description of it.
The image shown of M101 was captured on March 18, 2021 from Lac Teeples using the RASA-8 telescope and the ZWO ASI6200 camera. Over 4 hours of observations were used in producing this image.
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