NGC 4151

NGC 4151 is a nearby spiral galaxy in Canes Venatici whose active nucleus makes it a type 1.5 Seyfert galaxy and a prominent target for studying black-hole accretion.

Active galactic nucleus: A compact, luminous region at a galaxy’s center powered by matter accreting onto a supermassive black hole. This central engine produces NGC 4151’s Seyfert activity.
Reverberation mapping: A method that estimates the size and structure of an active nucleus by measuring delays between changing emissions. Variations in NGC 4151’s continuum and emission lines helped establish this method.
William Herschel: An astronomer who discovered Uranus and cataloged thousands of nebulae and star clusters. Herschel discovered NGC 4151 in 1787 during his surveys of the northern sky.
Type 1 Seyfert galaxy: A Seyfert galaxy whose spectrum shows both broad and narrow emission lines from its active nucleus. NGC 4151 shows broad-line features, but its classification is intermediate rather than purely type 1.
Supermassive black hole: A black hole with a mass of millions to billions of times the Sun’s mass. Its central black hole supplies the gravitational engine for the galaxy’s active nucleus.
Hubble Space Telescope: A space-based observatory that observes ultraviolet, visible, and near-infrared light above Earth’s atmosphere. Hubble resolved structures around NGC 4151’s nucleus that ground-based images could not separate.
Seyfert galaxies: A class of galaxies identified by unusually bright nuclei and distinctive emission-line spectra. NGC 4151 became a key example of the class Carl Seyfert defined in 1943.
Type 2 Seyfert galaxy: A Seyfert galaxy whose observed spectrum lacks the prominent broad permitted lines seen in type 1 objects. Its contrast with type 2 spectra helps explain the mixed features behind NGC 4151’s subtype.
Accretion disk: A rotating structure of gas and dust that spirals toward a central object while releasing energy. In NGC 4151, disk emission helps power the variable nuclear radiation.
Very Large Array: A radio interferometer in New Mexico composed of 27 movable antennas. Its radio images revealed extended emission associated with NGC 4151’s nucleus.

Linked from 2 pages