Glossary

Absolute magnitude: A measure of a star’s intrinsic brightness — the apparent magnitude it would have if observed from a standard distance of 10 parsecs. Unlike apparent magnitude, absolute magnitude does not depend on how far away the star is (see also apparent magnitude, distance modulus, magnitude scale).

Apparent magnitude: A measure of how bright a star appears from Earth, which depends on both the star’s intrinsic luminosity and its distance. Measured in magnitude units, where lower values correspond to brighter objects (see also absolute magnitude, magnitude scale).

Asymptotic giant branch (AGB): A late stage of stellar evolution in which a star has exhausted the helium in its core and is left with an inert carbon/oxygen core surrounded by helium- and hydrogen-burning shells. The star’s luminosity increases and its surface temperature falls, and its evolutionary track on the HR diagram runs roughly parallel to the red giant branch, converging toward it asymptotically from the left (see also red giant branch, horizontal branch).

Binary star system: Two stars gravitationally bound and orbiting their common centre of mass. When the two stars in a binary system are too close together to be resolved as separate points of light, they are called an unresolved binary, and their combined light causes them to appear brighter — and sometimes redder — than a single star of the same colour, producing scatter above the main sequence in a cluster’s HR diagram (see also main sequence, blue straggler).

Blackbody radiation: Thermal electromagnetic radiation emitted by an object in thermal equilibrium. Stars emit radiation that approximates a blackbody spectrum, meaning the distribution of light across wavelengths depends primarily on the star’s surface temperature — hotter stars emit more light at shorter (bluer) wavelengths, and cooler stars at longer (redder) wavelengths (see also colour index, surface temperature).

Blue straggler: A star in a cluster that appears above the main sequence turnoff point, seemingly too blue and luminous to have remained on the main sequence for the cluster’s full age. Blue stragglers are thought to result from mass transfer in binary star systems or stellar collisions, which replenish a star’s core hydrogen and effectively reset its main sequence clock. They are not well captured by isochrone models and should be excluded when fitting (see also binary star system, main sequence turnoff point).

Bok globule: A dense, dark cloud of interstellar gas and dust that may collapse under gravity to form new stars. Bok globules appear as dark patches against bright emission nebulae in star-forming regions (see also emission nebula, star-forming region).

Colour index: The difference in magnitude values measured for the same star in two different photometric filters — for example, B−V or BP−RP. Since the magnitude scale is inverted, a star that is brighter in the blue filter than the red will have a negative colour index. Colour index is a quantitative proxy for surface temperature, with hot blue stars having lower (bluer) values and cool red stars having higher (redder) values (see also apparent magnitude, blackbody radiation, surface temperature).

Colour-magnitude diagram: A graph of stellar apparent magnitude against colour index. For a star cluster, a colour-magnitude diagram displays the same information as an HR diagram but in observed (rather than intrinsic) units, before corrections for distance and interstellar reddening are applied (see also HR diagram, colour index, apparent magnitude).

Distance modulus: The difference between a star’s apparent magnitude and its absolute magnitude, which depends only on the star’s distance. Used to convert between apparent and absolute magnitudes once the distance is known, or to estimate distance when the absolute magnitude can be independently determined (see also apparent magnitude, absolute magnitude).

E(B−V): The standard parameter measuring interstellar reddening — specifically, the difference between a star’s observed B−V colour index and its intrinsic B−V colour index. Because interstellar dust scatters blue light more than red, stars viewed through dust appear both dimmer and redder than they intrinsically are. E(B−V) quantifies this reddening effect; the corresponding extinction in a given filter band is proportional to E(B−V) (see also interstellar extinction, reddening).

Emission nebula: A cloud of ionised gas that emits light at specific wavelengths corresponding to electronic transitions in atoms such as hydrogen and oxygen. Emission nebulae are typically associated with regions of active star formation, where radiation from young, hot stars ionises the surrounding gas (see also Bok globule, star-forming region, Hα, OIII).

Field stars: Stars that appear in the same region of sky as a cluster but are not physically associated with it — they lie at different distances and have different space velocities. Field stars must be removed from cluster data before analysis, typically using proper motion and distance measurements from the Gaia mission (see also proper motion, field star removal).

Field star removal: The process of separating probable cluster members from unrelated foreground and background stars, using measurements of proper motion and distance to identify stars that are co-moving and at the same distance as the cluster (see also field stars, proper motion).

Gaia: A space mission operated by the European Space Agency (ESA), launched in 2013, that has measured the positions, distances, and proper motions of over a billion stars with extraordinary precision. In cluster analysis, Gaia data are used for field star removal — identifying probable cluster members by their shared distance and proper motion — and for constructing HR diagrams using Gaia’s BP and RP photometric bands (see also proper motion, field star removal, HR diagram).

Galactic halo: The roughly spherical region surrounding the disk of the Milky Way, extending well above and below the galactic plane. Globular clusters are found in the galactic halo, where they are less obscured by interstellar dust than objects in the galactic disk (see also globular cluster).

Globular cluster: A tightly gravitationally bound, roughly spherical collection of roughly 10⁵–10⁷ stars, typically among the oldest objects in the Milky Way. Globular clusters formed in the galactic halo within the first few billion years of the galaxy’s existence and have survived intact since then. Their HR diagrams are dominated by evolved giant stars (see also open cluster, red giant branch, horizontal branch).

Hα (hydrogen-alpha): An emission line produced when hydrogen atoms transition from the third to the second energy level, emitting red light at a wavelength of 656 nm. Hα emission is a tracer of ionised hydrogen gas in star-forming regions and emission nebulae (see also emission nebula, OIII).

Horizontal branch: A phase of stellar evolution in which a star is fusing helium in its core. Horizontal branch stars are less luminous than the brightest red giants but considerably hotter, and appear as a spread of moderately bright, relatively blue stars in a cluster’s HR diagram. The horizontal branch is not well reproduced by standard isochrone models due to pulsational instability during this phase (see also red giant branch, asymptotic giant branch).

HR diagram (Hertzsprung-Russell diagram): A graph of stellar luminosity (or absolute magnitude) against surface temperature (or intrinsic colour index). Named after Ejnar Hertzsprung and Henry Norris Russell, who independently studied these relationships in stellar populations. The HR diagram is the primary tool for interpreting cluster stellar populations and fitting isochrone models (see also colour-magnitude diagram, main sequence, isochrone).

Infrared: Electromagnetic radiation with wavelengths longer than visible red light. Infrared light penetrates interstellar dust more readily than optical light, making it useful for imaging star-forming regions and revealing embedded stars. Survey data from missions such as 2MASS and WISE provide infrared photometry used in cluster analysis (see also interstellar extinction, 2MASS, WISE).

Interstellar extinction: The dimming of starlight as it passes through interstellar dust, which absorbs and scatters photons. Extinction is stronger at shorter wavelengths, which is why dust also reddens starlight. Extinction is quantified per photometric band and is related to the reddening parameter E(B−V) by a proportionality constant that depends on the wavelength (see also E(B−V), reddening).

Isochrone: A theoretical curve on an HR diagram or colour-magnitude diagram showing the predicted positions of stars that all have the same age, chemical composition, and distance, but different masses. By fitting an isochrone model to a cluster’s observed data, astronomers can estimate the cluster’s age, metallicity, distance, and interstellar reddening (see also HR diagram, isochrone matching).

Isochrone matching: The process of adjusting the parameters of an isochrone model — age, metallicity, distance, and reddening — to find the best fit to a cluster’s observed colour-magnitude or HR diagram data. The isochrone should pass through the densest part of the main sequence (its bottom edge), through the turnoff point, and along the red giant branch (see also isochrone, main sequence turnoff point).

Luminosity: The total power radiated by a star across all wavelengths, typically expressed in solar luminosities or watts. Luminosity depends on a star’s size and surface temperature, and for main sequence stars is strongly correlated with mass (see also absolute magnitude, surface temperature).

Main sequence: The broad diagonal band running from the upper-left to the lower-right of an HR diagram, occupied by stars that are actively fusing hydrogen in their cores. Stars spend the majority of their lifetimes on the main sequence, and their position along it is determined primarily by mass — with the most massive, luminous, hot stars at the upper-left and the least massive, dim, cool stars at the lower-right (see also main sequence turnoff point, HR diagram).

Main sequence turnoff point: The location on an HR diagram or colour-magnitude diagram where stars begin to leave the main sequence and evolve into red giants, indicating that they have exhausted the hydrogen in their cores. The position of the turnoff point along the main sequence is a direct indicator of a cluster’s age — the further down the main sequence the turnoff point has migrated, the older the cluster (see also main sequence, red giant branch).

Metallicity: The total abundance of elements heavier than hydrogen and helium in a star, expressed relative to the solar value. In stellar physics, all elements heavier than helium are conventionally referred to as “metals.” Higher metallicity increases a star’s luminosity while decreasing its surface temperature, and affects the shape of the isochrone model (see also isochrone).

OIII (doubly ionised oxygen): An emission line produced by oxygen atoms that have lost two electrons, emitting blue-green light at 500.7 nm. OIII emission traces hot, highly ionised gas in star-forming regions and is commonly used in narrowband imaging alongside Hα to reveal nebular structure (see also emission nebula, Hα).

Open cluster: A loosely gravitationally bound group of typically a few hundred to a few thousand stars, found in the disk of the Milky Way. Open clusters formed from the same molecular cloud and share a common age, metallicity, distance, and reddening. They gradually disperse over timescales of roughly a billion years as their member stars escape the cluster’s weakening gravitational field (see also globular cluster, cluster evaporation).

Cluster evaporation: The gradual dispersal of an open cluster as its fastest-moving member stars escape the cluster’s gravitational pull. As stars escape, the cluster’s total mass decreases, weakening its gravitational field and making it easier for additional stars to escape — a runaway process that eventually dissolves the cluster entirely. Most open clusters evaporate within roughly a billion years (see also open cluster).

Parsec (pc): A unit of distance used in astronomy, equal to approximately 3.26 light-years or 3.09 × 10¹⁶ metres. One parsec is the distance at which a star would show a parallax angle of one arcsecond against the background sky as Earth orbits the Sun. Distances to star clusters are typically measured in parsecs or kiloparsecs (kpc), where 1 kpc = 1000 pc (see also proper motion).

Photometric filter: A device that transmits light only within a specific range of wavelengths, allowing astronomers to measure a star’s brightness in a defined band of the spectrum. Standard filters include B (blue), V (visual/green), R (red), and I (near-infrared); Gaia uses BP (blue-photometer) and RP (red-photometer) bands. By comparing a star’s brightness in different filters, astronomers obtain its colour index, which is a measure of surface temperature (see also colour index, apparent magnitude, photometry).

Photometry: The measurement of the brightness of astronomical objects in one or more photometric filter bands. In the context of star cluster analysis, photometry refers to measuring the flux of light from individual stars in images taken through different filters, which yields apparent magnitudes used to construct colour-magnitude diagrams (see also apparent magnitude, colour index, photometric filter).

Planetary nebula: The expanding shell of gas shed by a low- to intermediate-mass star near the end of its life, after it has exhausted both core hydrogen and core helium fusion. The hot stellar remnant at the centre ionises the ejected gas, causing it to glow. What remains of the star after the nebula disperses is a white dwarf (see also white dwarf).

Pre-main sequence star: A young star that has not yet reached the main sequence and is still contracting under gravity toward the conditions needed to ignite stable hydrogen fusion in its core. Pre-main sequence stars can appear in young star-forming regions as cool, luminous objects that lie above the main sequence in a cluster’s HR diagram (see also main sequence, star-forming region).

Proper motion: The apparent angular motion of a star across the sky, measured in milliarcseconds per year, resulting from the star’s true velocity through space relative to Earth. Stars in the same cluster share nearly identical proper motions, which allows them to be distinguished from unrelated field stars at different distances moving in different directions. Proper motion data used in this module come from the Gaia space mission (see also field star removal, Gaia).

Reddening: The shift of a star’s apparent colour toward redder wavelengths caused by preferential scattering of blue light by interstellar dust. Reddening and extinction always occur together — dust both dims and reddens starlight simultaneously. The degree of reddening is parameterised by E(B−V) (see also E(B−V), interstellar extinction).

Red giant branch (RGB): A phase of stellar evolution in which a star has exhausted the hydrogen in its core and is fusing hydrogen in a shell around an inert, contracting helium core. The star’s luminosity increases and its outer layers expand, lowering the surface temperature. Red giant branch stars occupy the upper-right region of the HR diagram (see also main sequence turnoff point, horizontal branch, asymptotic giant branch).

Signal-to-noise ratio (SNR): A measure of the strength of a detected signal relative to the background noise. In astronomical imaging, higher SNR corresponds to more reliable brightness measurements. Stars near the faint end of a cluster’s HR diagram have lower SNR and therefore less accurate photometry, which is why isochrone fitting should prioritise the bright end of the diagram (see also photometry).

Star-forming region: An area of a galaxy where interstellar gas and dust are actively collapsing under gravity to form new stars. Star-forming regions are associated with emission nebulae, Bok globules, and pre-main sequence stars, and often show variable extinction due to the inhomogeneous distribution of remaining gas and dust (see also Bok globule, emission nebula, pre-main sequence star).

Surface temperature: The effective temperature of a star’s outermost radiating layer, which determines the star’s colour and the shape of its blackbody spectrum. Hotter stars appear blue-white and cooler stars appear orange-red. Surface temperature, together with stellar radius, determines luminosity (see also blackbody radiation, colour index, luminosity).

Turnoff point: See main sequence turnoff point.

White dwarf: The dense, inert remnant left after a low- to intermediate-mass star has exhausted all its nuclear fuel and shed its outer layers as a planetary nebula. A white dwarf is composed primarily of carbon and oxygen, supported against further collapse by electron degeneracy pressure rather than fusion. White dwarfs appear in the lower-left of the HR diagram — hot but intrinsically faint — and gradually cool over billions of years (see also planetary nebula, blue straggler).

2MASS (Two Micron All Sky Survey): A NASA all-sky survey that imaged the entire sky in three near-infrared bands (J, H, and K). 2MASS photometry is used in cluster analysis to extend colour-magnitude diagrams into the infrared, providing additional constraints on isochrone fitting and revealing stars obscured by interstellar dust in optical wavelengths (see also infrared, WISE).

WISE (Wide-field Infrared Survey Explorer): A NASA space telescope that surveyed the entire sky in four mid-infrared bands (3.4, 4.6, 12, and 22 microns). WISE data can be combined with optical and near-infrared images to reveal warm dust, star-forming structures, and embedded stellar populations invisible at optical wavelengths (see also infrared, 2MASS).