Astrophysics & Astronomy Glossary

The fundamental physical constant representing the maximum speed at which all conventional matter and information in the universe can travel. Defined as exactly 299,792,458 m/s in vacuum.

A unit of astronomical distance equal to the distance that light travels in vacuum in one Julian year (365.25 days), approximately 9.461 trillion kilometers.

In spacetime physics, the boundary formed by all light rays emitted from a given event. It defines the temporal boundary of events that can be causally influenced by that event.

A physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source radiating energy uniformly in three dimensions.

The background radio noise thermal threshold created by Cosmic Microwave Background Radiation (~2.725K) and interstellar synchrotron noise, below which raw electromagnetic signals become indistinguishable from static.

The scattering of light by particles smaller than its wavelength. Scattering efficiency varies inversely with the fourth power of wavelength, causing visible light to scatter while long infrared waves penetrate dust.

The lengthening of electromagnetic radiation wavelengths traveling across expanding metric space. Light emitted billions of years ago in visible spectra is stretched into the infrared band by the time it reaches Earth.

A standard unit of distance used to express the size of stars equal to the current radius of the Sun (~696,340 km), roughly 109 times the volumetric radius of Earth.

An enormous, highly luminous star exhibiting massive stellar wind mass loss rates. Hypergiants like Stephenson 2-18 represent the most massive and volumetrically expansive stellar class in astronomy.

The acceleration due to gravity experienced at the surface of a celestial body, determined by universal gravitational constant $G$, total mass $M$, and body radius $R$.

The minimum speed required for a non-propelled ballistic body to escape from the gravitational influence of a primary celestial body and reach infinite distance without further acceleration.

The problem of predicting the individual motions of a group of celestial objects interacting with each other gravitationally. Unlike 2-body Keplerian systems, N-body systems exhibit chaotic dynamics requiring numerical integration.

A planetary defense technique involving crashing a high-mass spacecraft into a hazardous asteroid at hypersonic velocity to transfer kinetic momentum and alter its orbital trajectory away from Earth.