Nebulae and star clusters

Over time, the densest parts of the regions of dust in galaxies collapses into smaller and smaller regions due to self-attraction by gravity, thereby getting denser and denser. These are dark nebulae, visible only by the fact that they completely block our view of anything beyond them, so they look like black spots in the sky. In the center of these dark nebulae, very dense regions collapse to form new stars. The largest of these new stars give up such intense light that they blow away most of the nearby dust, and brightly light up the blown-away dust so long as it remains somewhat nearby. Two such bright nebulae are visible with the nakid eye, and look spectacular in telescopes.

M42

M42 is the great nebula in Orion. First an image from the Anglo-Australian telescope:

(Copied from: http://www.seds.org/messier/m/m042.html)
Notice how the deepest part of the blown-out cavity is hidden behind the near edge of the cavity, but some regions of dust in that near edge are thin enough that light from the inside is leaking through.

Here's an image by Robert Gendler showing just the central part of the nebula:

(Copied from: http://antwrp.gsfc.nasa.gov/apod/ap020213.html)

M8

M8 is the "Lagoon" nebula, located near Sagittarius. First an image from the Anglo-Australian telescope:

(Copied from: http://antwrp.gsfc.nasa.gov/apod/ap980707.html)

Next, an image by Barba, showing the central part in higher resolution:

(Copied from: http://antwrp.gsfc.nasa.gov/apod/ap010820.html)
Notice that the cavity is facing us nearly directly, so the deepest part is directly visible instead of hidden behind an edge of the cavity. Also notice the small very dark spots where the dust is collapsing to form new stars or small clusters of stars.

Finally, here's a very high resolution image of the centermost part of the nebula, taken by the Hubble Space Telescope:

(Copied from: http://hubblesite.org/newscenter/newsdesk/archive/releases/1996/38/)

M44

After a very long time, nearly all the dust in a nebula has either formed stars or been blown completely away, so all we see in that region are the long-lived stars that remain. Here's an image of such a star cluster, M44, the "Beehive" cluster, also known as "Praesepe", taken by Kopernik:

(Copied from: http://www.kopernik.org/images/archive/m44.htm)
Most of the bright stars in the central region are actually members of the cluster, whereas most of the stars elsewhere in the image are unrelated stars in the background, but it's difficult to be sure exactly which are which by viewing such an image. We need to measure the actual distances to the individual stars to be sure which stars are really in that region of space, hence probably members of the cluster, and which are much closer or further away hence surely unrelated.

Col399

Here's an uncredited image showing a higher resolution view of just the coathanger and nearby stars:

(Copied from: http://www.seds.org/messier/xtra/ngc/brocchi.html)
So what a strange looking star-cluster that seems to be! But alas, it's not a real cluster. The Hipparcos satellite measured the distances to most of the nearby stars, including the stars in the coathanger, and each is at a very different distance from any other, so the stars aren't really near each other at all. They are merely aligned, by chance, nearly along a line-of-sight from Earth out into space, so they only look like they are related when viewed from Earth or anywhere else along that particular line, but when viewed from any other direction they'd be strewn across the sky so that nobody would even notice them.