Description: A nebula is a distant celestial object that has the appearance of a cloud. Usually, a nebula consists of interstellar gas and dust. Historically, the term nebulae included any extended fuzzy-looking object, including what we today recognize as galaxies – distant star systems like our own Milky Way galaxy. Today, the term nebula is restricted to gas and dust clouds that are part of the interstellar medium – the gas and dust between stars within a galaxy. The category includes a number of different kinds of object: Molecular clouds are comparatively cold and dark, and predominantly consist of molecular hydrogen; inside clouds like this is where new stars are formed. Giant molecular clouds can contain up to a few millions of solar masses' worth of hydrogen gas. Young stars frequently emit narrow jets of ionized gas; when those jets excite the surrounding gas, the result is a type of nebula called a Herbig–Haro object. When massive stars have formed, their intense radiation makes the surrounding gas emit characteristic reddish light; the results are nebulae of hot and ionized hydrogen gas which are called HII regions. Other types of nebula are associated with the death of stars: low-mass stars leave behind expanding shells of gas that are (somewhat confusingly) called planetary nebulae. When a high-mass star explodes as a supernova, the ejected gas forms a type of nebula called a supernova remnant.

Related Terms:

• Dust
• Galaxy
• Gas
• Planetary Nebula
• Supernova
• Interstellar Medium

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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