Nebulae and star clusters

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)

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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/)

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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.

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Col399

Here's a wide-angle image modified by adding lines to use as a star chart, showing Vega in Lyra at the top, and the easy-to-recognize (but hard to find unless you know where to look) Sagetta (arrow shape) near the bottom. With binoculers it's easy to sweep this region and see all the stars shown here. If you sweep upward from the rightmost end of Sagetta, to the next semi-bright star, then to the right and slightly up from there, you see a strange shape like an upside wooden coathanger, with the stars of the hook brighter than the stars of the wooden bar. Can you see it in this image?
(Copied from: http://www.slivoski.com/astronomy/vulpecul.htm)

(In several recent Sky & Telescope issues there are pictures showing a wide region of the sky, but at high resolution. In most of these images, it's possible to see the coathanger right there in the image if you know where to look up-right from Sagetta. Look below Cygnus to find Sagetta.)

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.

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