Most stars are not isolated. Binary and multiple systems share gas and interact in ways that can produce spectacular effects. Any inventory of the Milky Way galaxy must consider the material that is found in the environment of stars. Some of this material is close to a star and is lit up by a star's energy. Some of this material is thinly distributed through the gulfs of space between stars.
Charles Messier. Click here for original source URL.
Peering into the heart of the Crab Nebula. Click here for original source URL.
Dumbbell planetary nebula as seen by the Hubble Space Telescope. Click here for original source URL.
Looking down a tunnel of gas at a doomed star. Click here for original source URL.
False color composite image of the Orion Nebula as seen by the Hubble and Spitzer Space Telescopes. Click here for original source URL.
Stars are surrounded by a thin but chaotic medium of gas, dust, and radiation. They form from this thin material, interact with it, recycle it, and expel it to form new interstellar material. Each individual cloud of gas and dust is called a nebula. A nebula is material that is concentrated by gravity and lit up by a nearby star. The French astronomer Charles Messier published a catalog of the brightest and most vivid examples in 1781. They are thus known by their Messier numbers, or M numbers. Messier was a comet-hunter, and he wanted a list of the nebulous objects in the sky that did not move with respect to the stars so he would know which objects to avoid in his searches. The well-known Orion Nebula, for example, is known as M 42. Others have numbers from the more recent New General Catalog (NGC) of William Herschel and from the Index Catalog (IC). By tradition, astronomers have named most bright nebulae according to their appearance in small telescopes; examples include the Crab Nebula (M 1), the Dumbbell Nebula, and the Ring Nebula.
We sometimes casually say that "space is a vacuum," but this is not quite true. While space is a better vacuum than can be achieved in labs, the space between stars is filled with diffuse gas and dust. The material between stars is called the interstellar medium. What significance can this thin material have for us? For one thing, these vast clouds of dust and gas are landmarks in the night sky; some are twisted into beautiful wispy forms, some are dark, and some glow with different colors. For another thing, our solar system, the Earth, and we ourselves are formed from atoms that were once part of the interstellar gas and dust. More provocatively, recent discoveries have demonstrated that interstellar material contains complex organic molecules. These discoveries demonstrate the universal aspects of chemistry that make organic molecules common both in living things on Earth and in inanimate objects such as comets and interstellar clouds. Lastly, the thin material between stars dims and reddens starlight.
Another important realization is that there is an interplay between stars and their environment. Most stars lose mass late in their lives, ejecting gas into the interstellar medium. Sometimes this mass loss is steady, and sometimes it is violent, as in a nova or a supernova. When stars form, they congeal by gravity from the same interstellar material. So gas is continually flowing out of old stars and into new stars, a cycle of birth and death that has been going on in the Milky Way for over ten billion years.