The distribution over the sky is almost isotropic, as shown in Fig. Some apparent clustering is due to non-uniformity of the search procedure. Among distant objects which are readily detected across the galaxy and which are related to stars of moderate mass are the planetary nebulae, the remnants of old red supergiants. The ones older than a few thousand years expanded so much that they are too diffuse to see. The nuclei of the nebulae, former cores of red supergiants, are on their way to become white dwarfs.
Planetary nebulae are very bright, and readily discovered out to large distances.
Notice the strong concentration toward the galactic plane of these galactic disk objects. Notice the strong concentration toward the galactic center of these typical galactic halo objects. These blow huge bubbles in the surrounding interstellar medium forming nebulae known as supernova remnants. Their distribution in the sky is shown in Fig. 3, revealing a striking concentration toward the galactic plane. There are also galactic halo objects in the astronomical inventory.