Non-asterisms
In the formal sense used here, asterisms are groups of stars that have not been categorized as something else. Objects which do not fall within the bounds of this definition include the Milky Way, nebulae, and open clusters.
Dividing the night sky into two roughly equal hemispheres, the Milky Way appears as a hazy band of white light arching across the entire celestial sphere. Many cultures have myths about "the broad white road in heaven." That the glow originates from innumerable faint stars and other materials which lie within the galactic plane was one of Galileo's early telescopic discoveries. Similarly, the Magellanic Clouds are not asterisms, but galaxies in their own right.
Nebulae, clouds of gas and dust that dot the galaxy, whether emission, such as the Pelican, or dark, such as the Horsehead, are clearly not asterisms as they are not composed of stars.
Open clusters are groups of stars that are physically related — gravitationally bound together and moving through the galaxy in the same direction and speed. As these groupings are not human constructs, but real phenomena, they do not count as asterisms. Among the best-known and closest are the Pleiades (M45) and the Hyades in Taurus and the Beehive (M44) in Cancer. (It may be noted that, with the addition of Aldebaran, which is in the same line of sight, the Hyades open cluster forms a V-shaped sectional asterism in Taurus).
The fine point of what constitutes an asterism may be seen in two examples. Theta Orionis (θ Ori) is embedded in, and illuminates, the Orion Nebula (M42). Looked at telescopically, it resolved into four stars arranged in a trapezoid, and they were nicknamed the Trapezium. The asterism retained this name even when it was discovered that there were yet more stars in the group. However, it has since been determined that the Orion Nebula is a stellar nursery and that the Trapezium is actually an Open Cluster. Thus it is no longer an asterism. On the other hand, M73 in Aquarius, which was thought to be an Open Cluster, turns out to be composed of unrelated stars, and may now be considered to be an asterism.
Read more about this topic: Asterism (astronomy)