Stars, Galaxies, and the Universe

By Kurklind Kretsch


Eventually, a main sequence star burns through the hydrogen in its core, reaching the end of its life cycle. At this point, it leaves the main sequence. Stars smaller than a quarter the mass of the sun collapse directly into white dwarfs. White dwarfs no longer burn fusion at their center, but they still radiate heat. A Red Giant is a very large star of high luminosity and low surface temperature. Red supergiants are thought to be in a late stage of evolution when no hydrogen remains in the core to fuel nuclear fusion.

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light.


A supernova is an astronomical event that occurs during the last stellar evolutionary stages of a massive star's life, whose dramatic and catastrophic destruction is marked by one final titanic explosion.

A neutron star is a type of compact star that can result from the gravitational collapse of a massive star after a supernova. Neutron stars are the smallest and densest star known to exist in the Universe; with a radius of only about 11–11.5 km (7 miles), they can have a mass of about twice that of the Sun.

A black hole is a region of spacetime exhibiting such strong gravitational effects that nothing—including particles and electromagnetic radiation such as light—can escape from inside it. A globular cluster is a spherical collection of stars that orbit a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers.

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud, and are still loosely gravitation ally bound to each other. In contrast, globular clusters are very tightly bound by gravity.


A galaxy is a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction. A spiral galaxy is a galaxy in which the stars and gas clouds are concentrated mainly in one or more spiral arms. An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. Unlike flat spiral galaxy with organization and structure, they are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center. A galaxy that does not have the clearly defined shape and structure of typical elliptical, lenticular, or spiral galaxies. Irregular galaxy typically contain large amounts of gas and dust, and their stars are often young. A nebula (Latin for "cloud"; is an interstellar cloud of dust, hydrogen, helium and other ionized gases. Originally, nebula was a name for any diffuse astronomical object, including galaxies beyond the Milky Way. A stellar nebula is the nebulosity surrounding a star : a star's shell or envelope of nebulosity. A planetary nebula , often abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from old red giant stars late in their lives. Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies.