The blue region is centered on another bright star but this region's dust reflects light to us. The two regions are thus called an emission nebula and reflection nebula, respectively. The Trifid Nebula can be seen in the constellation Sagittarius with a small telescope. A nebula is a localized conglomerate of the gaseous and finely divided dust particles that are spread throughout interstellar space. As a result, many objects now known to be star clusters or galaxies were called nebulas.
Nebulas exist within other galaxies as well as in our own Milky Way galaxy.
They are classified as planetary nebulas, supernova remnants, and diffuse nebulas, including reflecting, emission, and dark nebulas. Small, very bright nebulas known as Herbig-Haro objects are found in dense interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of formation. Planetary nebulas, or planetaries, are so called because many of them superficially resemble planets through telescopes. They are actually shells of material that an old average star sheds during a late, red giant stage in its development, before becoming a white dwarf.