Nebula | Definition, Types, Size, & Facts | Britannica
On the basis of appearance, nebulae can be divided into two broad classes: dark nebulae and bright nebulae. Dark nebulae appear as irregularly shaped black patches in the sky and blot out the light of the stars that lie beyond them.
Planetary nebula | Definition, Temperature, Size, & Facts - Britannica
Planetary nebula, any of a class of bright nebulae that are expanding shells of luminous gas expelled by dying stars. They have a relatively round compact appearance rather than the chaotic patchy shapes of other nebulae—hence their name, which was given because of their resemblance to planetary
Nebula - Astronomy, Formation, Gas & Dust | Britannica
Most are clusters of stars, 35 are galaxies, and 11 are nebulae. Even today many of these objects are commonly referred to by their Messier catalog number; M20, for instance, is the great Trifid Nebula, in the constellation Sagittarius. The work of the Herschels
Galaxy | Definition, Formation, Types, Properties, & Facts | Britannica
2024年11月22日 · What are the types of galaxies? Galaxy classification is based on the scheme proposed by the American astronomer Edwin Hubble . Hubble’s scheme is based on the optical appearance of galaxy images and divided into three general classes: ellipticals, spirals, …
Nebula - Gas, Dust, Radiation | Britannica
Many characteristics of nebulae are determined by the physical state of their constituent hydrogen, by far the most abundant element. For historical reasons, nebulae in which hydrogen is mainly ionized (H + ) are called H II regions , or diffuse nebulae; those in which hydrogen is mainly neutral are designated H I regions ; and those in which ...
Galaxy - Elliptical, Spiral, Irregular | Britannica
2024年11月25日 · Know how galaxies categorized according to their shape, such as elliptical, spiral, or irregular Learn about the three general types of galaxies: spiral, elliptical, and irregular. (more) See all videos for this article
nebula - Students | Britannica Kids | Homework Help
On the basis of appearance, nebulae can be divided into two broad classes: dark nebulae and bright nebulae. Dark nebulae block the light of the stars that lie beyond them and appear as irregularly shaped black patches in the sky. The Coalsack, in the southern sky, is a well-known example of a dark nebula.
Black hole | Definition, Formation, Types, Pictures, & Facts
2024年11月25日 · Details of the structure of a black hole are calculated from Albert Einstein’s general theory of relativity.The singularity constitutes the centre of a black hole and is hidden by the object’s “surface,” the event horizon.Inside the event horizon the escape velocity (i.e., the velocity required for matter to escape from the gravitational field of a cosmic object) exceeds …
Nuclear fusion | Development, Processes, Equations, & Facts
2024年11月8日 · Fusion reactions are of two basic types: (1) those that preserve the number of protons and neutrons and (2) those that involve a conversion between protons and neutrons. Reactions of the first type are most important for practical fusion energy production, whereas those of the second type are crucial to the initiation of star burning.
Galaxy - Irregular, Star Clusters, Nebulae | Britannica
2024年11月25日 · Galaxy - Irregular, Star Clusters, Nebulae: Most representatives of this class consist of grainy, highly irregular assemblages of luminous areas. They have neither noticeable symmetry nor an obvious central nucleus, and they are generally bluer in colour than are the arms and disks of spiral galaxies.