Glossary

Plain-language definitions for the terms and abbreviations used across these docs. On any page, hover over, focus or tap a dotted-underlined term to see its definition.

Active galactic nucleus (AGN)

A galaxy's central region when a supermassive black hole is feeding and outshining the stars around it.See also: BH

Astronomical unit (AU)

The average distance from Earth to the Sun, about 150 million kilometres. Used to measure distances inside a star system.

Asymptotic giant branch (AGB)

A late stage for stars of low and middle mass, when they are very large and bright and shed their outer layers into space.See also: RGB, SGB

Black hole (BH)

An object so dense that nothing, not even light, can escape its gravity once it passes a boundary called the event horizon.

Coronal mass ejection (CME)

A huge burst of hot, magnetised gas thrown out by the Sun. If it reaches Earth it can disturb satellites and power grids.See also: DONKI

Database Of Notifications, Knowledge, Information (DONKI)

NASA's public record of space weather events such as solar flares and eruptions, with the reports and forecasts about them.See also: CME, SWPC

Digital Shape Kernel (DSK)

A SPICE data file describing the detailed three-dimensional surface shape of a body such as an asteroid or moon.See also: SPICE, SPK

Distributed compute

Splitting one large job across many computers working at the same time, so very large simulations finish in a practical amount of time.

Gigayear (Gyr)

One billion years. Used for stellar ages and the timescales of galaxies.See also: Myr

Habitable zone (HZ)

The range of distances from a star where a planet could be warm enough for liquid water on its surface.

Initial mass function (IMF)

The spread of masses among newly formed stars: many small stars for each massive one.See also: IFMR

Initial-final mass relation (IFMR)

A rule linking a star's mass at birth to the mass of the white dwarf or other remnant it leaves behind when it dies.See also: IMF

Interstellar medium (ISM)

The thin gas and dust that fills the space between stars in a galaxy, and the raw material for new stars.See also: SFR

Just-in-time (JIT)

Doing work at the moment it is needed instead of in advance, so only what you are looking at is computed.See also: LOD

Large language model (LLM)

An AI system trained on very large amounts of text that can read, summarise and write language.

Level of detail (LOD)

Showing distant or small objects in less detail and close ones in more, so a huge scene stays smooth to explore.See also: JIT

Line of sight (LOS)

The straight path between an observer and a target, for example between a ground station and a satellite.

Many-body gravity (N-body)

A calculation that follows how many objects, such as stars in a cluster or moons around a planet, pull on one another through gravity over time.

Megayear (Myr)

One million years. Used for short stellar lifetimes and stages of a star's life.See also: Gyr

NASA's Navigation and Ancillary Information Facility (NAIF)

The NASA group that builds and publishes SPICE and the reference data used to plan and analyse space missions.See also: SPICE

Pseudo-random number generator (PRNG)

A formula that produces numbers that look random but are repeatable: the same starting value always gives the same sequence.

Red giant branch (RGB)

The stage in which a star has swollen to many times its original size and cooled to a reddish colour.See also: SGB, AGB

Space Weather Prediction Center (SWPC)

The US government forecasting office, run by NOAA, that issues public space weather alerts and forecasts.See also: DONKI

Spacecraft and Planet Kernel (SPK)

A SPICE data file recording where a planet, moon or spacecraft is over a span of time.See also: SPICE, DSK

Spacecraft, Planet, Instrument, C-matrix, Events (SPICE)

NASA's published toolkit and data format for working out where planets, moons and spacecraft are, and how they are oriented, at any moment in time.See also: NAIF, SPK, DSK

Star formation history (SFH)

How quickly a galaxy made new stars at each point in its past.See also: SFR

Star formation rate (SFR)

How much gas a galaxy turns into new stars in a given period, usually measured in solar masses per year.See also: SFH, ISM

Subgiant branch (SGB)

A brief stage in a star's life after it runs out of hydrogen fuel in its core and begins to swell.See also: RGB, AGB

Two-line element set (TLE)

A compact, widely used text format describing a satellite's orbit, from which its position at a given time can be estimated.

Ultraviolet (UV)

Light with a shorter wavelength than violet, invisible to the eye. It can strip or damage planetary atmospheres.See also: XUV

X-ray and extreme ultraviolet (XUV)

The most energetic light a star gives off short of gamma rays. It heats planetary atmospheres and can slowly erode them.See also: UV