The large disk-shaped aggregation of stars, gas, and dust in which our solar system is located. The Milky Way Galaxy is a large, spiral-shaped galaxy that hosts approximately 300 billion stars. The term "Milky Way" refers to the diffuse band of light visible in the night sky emanating from millions of stars located inward of our solar system toward the galactic core (Fig. 1). Besides this nighttime glow, "Milky Way" can also refer to our Galaxy as a whole. See also: Galaxy; Solar system; Star; Universe
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Molecular cloud
A large and relatively dense cloud of cold gas and dust in interstellar space from which new stars are born. Molecular clouds consist primarily of molecular hydrogen (H2) gas, have temperatures in the range 10–100 K, and contain 1031–1036 kg of mass (for comparison the mass of the Sun is 2 × 1030 kg). Molecular clouds are among the most massive gravitationally bound objects in the Milky Way Galaxy.
Article
Nebula
A concentration of interstellar matter that is largely gaseous but can contain almost all states of matter, including solid particles, and that is a constituent of the Milky Way Galaxy or another galaxy. The term “nebula” was originally used to refer to any fixed, extended, and usually fuzzy luminous celestial object. With increased angular resolution of telescopes, astronomers learned that nebulae can be separated into two classes: those that are stellar systems made up of individual stars, and those that are gaseous in nature and diffuse in appearance. Examples of stellar systems include galaxies (which contain billions of stars and are located outside our own Milky Way Galaxy) and star clusters such as open clusters and globular clusters (which contain thousands of stars and are within the Milky Way Galaxy). This article is restricted to the modern definition of nebulae, which are gaseous objects usually located within the Milky Way Galaxy, although with increasingly powerful telescopes gaseous nebulae can now be observed in external galaxies. See also: Galaxy, external; Milky Way Galaxy; Star
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Orion Nebula
The brightest emission nebula in the sky, designated M42 in Messier's catalog. The Great Nebula in Orion consists of ionized hydrogen and other trace elements (Figs. 1 and 2). The nebula belongs to a category of objects known as H II regions (the Roman numeral II indicates that hydrogen is in the ionized state), which mark sites of recent massive star formation. Located in Orion's Sword at a distance of 460 parsecs or 1500 light-years (8.8 × 1015 mi or 1.4 × 1016 km), the Orion Nebula consists of dense plasma, ionized by the ultraviolet radiation of a group of hot stars less than 100,000 years old known as the Trapezium cluster (Fig. 3). The nebula covers an area slightly smaller than the full moon and is visible with the aid of binoculars or a small telescope. See also: Orion
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Planetary nebula
The ejected outer layers of stars born with mass between approximately 1 and 8 times the solar mass. Toward the end of their lives, such stars go through the asymptotic giant branch phase. At the end of this phase they suffer a dramatic increase in mass loss rate, which reduces the star to its bare core. By then these stars have grown to a few hundred times the radius they had at the start of their lives and have lost some to most of their mass as they become white dwarf stars. The material that is lost during this phase slowly expands outward until the inner stellar core is hot enough to ionize it. The ionization fires up the ejected gas, which shines as a visible planetary nebula. These nebulae have various symmetric and usually striking shapes (Fig. 1). See also: Stellar evolution